Europe's Quiet Revolution in Microgravity Research

A custom publication produced in collaboration the European Space Agency. with Space Channel on behalf of Europe’s Quiet Revolution Quiet Revolution Europe’s Research in Microgravity

It’s All Relative All It’s of the limits Probing theoriesEinstein’s Soft matter study advances gravity from free when Microgravity can lower lower can Microgravity blood pressure Complexity The of Plasma Hope for Hypertension? Man-Made Meteorites the survival Testing of rocks microbes and

PRESENTS LOOKING UP LOOKING

SCIENTIFIC AMERICAN PRESENTS LOOKING UP: EUROPE’S QUIET REVOLUTION IN MICROGRAVITY RESEARCH IN SPACE FOR THE BENEFIT OF MANKIND

The Swedish Space Corporation designs, tests, launches and operates space and aerospace systems

A core business at SSC is to provide a complete microgravity service with the MASER and MAXUS sounding rocket programs. SSC provides payload development, service systems and launch services using the SSC Esrange Space Center including flight control and land recovery.

www.ssc.se PREFACE

Dear Reader,

stronauts are the heroes of our time. Wher- entifi c areas involved, such as crystal growth, alloy ever they show up, they attract the curiosity formation, soft matter physics, the wide fi eld of fl uid A and admiration of the public. On television, sciences, relativity physics, the highly important we see them fl oating through a space station; we see fi eld of human and plant physiology and fi nally the them exercising, eating and working outside the sta- fundamental studies of the behaviour and survival tion attaching or fi xing something. Sometimes they of biological systems at the cellular level, not only are stationed in front of a console, pulling a drawer under microgravity, but also under exposure to the or pushing a button, but frankly, we do not have any harsh space environment. idea what they are doing there or what is happening As I acquainted myself with this research, I also inside those racks. This Special Presentation of Sci- came to realise how in recent years this work had entifi c American, devoted to life acquired the mantle of top-class and physical sciences research in science, yet had not received the space, is meant to help remedy exposure at the popular science this defi ciency. level that I felt it deserved. As My own fi eld is space plasma Chairman of the Advisory Com- physics. I have been engaged in mittee on Human Spacefl ight, Mi- experiments exploiting the elec- crogravity and Exploration of the tromagnetic forces of near-Earth European Space Agency (ESA), I space acting on artifi cially inject- was therefore highly gratified ed plasma clouds. These experi- when Scientifi c American agreed ments are very different from the to produce a Special Presentation research discussed in this publica- devoted to this work by European tion. Here one exploits the near- scientists, often in collaboration weightless (microgravity) conditions found in orbit- with scientists from other countries. al fl ight to study the behaviour of physical and bio- Having seen the highly informative articles that logical systems in the absence of the effects of have resulted along with the fi ne and captivating art- gravity. The use of microgravity is what makes this work and the excellent editorial work of Scientifi c research unique compared with the more classic American, I can only wholeheartedly recommend space sciences of plasma and solar physics, astrono- this highly readable Special Presentation entitled my and the planetary sciences. “Looking Up: Europe’s Quiet Revolution in Micro- As a former Chairman of the European Space Sci- gravity Research” to anyone seeking a deeper under- ence Committee of the European Science Founda- standing of this fascinating area of scientific tion, I have had the privilege of being able to survey research. the whole gamut of European space sciences and consequently been able to gauge for myself its strengths and weaknesses. In the course of this, I Gerhard Haerendel gradually became familiar with the work of Euro- Chairman of the Advisory Committee pean scientists in the microgravity and related fi elds. on Human Spacefl ight, Microgravity and What struck me fi rst was the enormous range of sci- Exploration (ACHME) for ESA COVER ART: SPACE CHANNEL / PHIL SAUNDERS SCIENTIFIC AMERICAN PRESENTS Europe’s Quiet Revolution LOOKING UP in Microgravity Research

Preface...... 1 HUMAN HEALTH By Gerhard Haerendel Counteracting Hypertension INTRODUCTION with Weightlessness? ...... 16 A Letter to Readers ...... 4 By Peter Norsk and John M. Karemaker By John Rennie Many of us have been told to lose weight to lower our blood pressure, but going weightless? Studies of astronauts show that PERSPECTIVES gravity does contribute to cardiovascular stress. Life in Orbit ...... 6 By Thomas Reiter Clinical Immunology The personal and professional rewards of space exploration in New Frontiers...... 24 By Alexander Choukèr, Boris Morukov and Clarence Sams When thinking big, we must also remember to think small, especially when we need to fend off illness in space. As Victor Hugo once said, “Where the telescope ends, the microscope ASTROBIOLOGY begins; and who can say which has the wider vision?”

Meteorites: SPECIAL REPORT: Stones with Stowaways? ...... 8 Bone Loss in Space...... 32 By Frances Westall and Sticks and stones may break your bones, but Rosa de la Torre Noetzel underuse will make them brittle. Like extended Can organisms be transported from one planet to another bedrest, an environment without gravity reduces the and survive both space conditions and atmospheric entry to workload on bones and makes them weaker. impregnate the host planet with new forms of life? Scientists Because research in this area is extensive, explore whether Earth was colonised by microscopic hitchhikers. two articles are presented here to illustrate different approaches being taken to understand—and combat— these effects. 32 8 Zero Gravity: Bad to the Bones By Laurence Vico and Christian Alexandre Our Sensitive Skeleton By Rommel G. Bacabac, Jack J.W.A. Van Loon and Jenneke Klein- Nulend MATTER MATTERS 68 Breaking the Mould: Metallurgy in Microgravity ...... 68 By Hans J. Fecht and Bernard Billia Factories ﬂ oating in space? Not quite, but the next giant step in metal processing technology may come from an unexpected source: the ISS.

Shake, Rattle and Roll: Using Vibrations as Gravity ...... 74 By Daniel Beysens, Pierre Evesque and Yves Garrabos In space, you can’t toast your successes with Champagne or sweep sand into a pile to clean up. So how can you make matter behave?

The Complex Matter of Plasma . . . . . 82 PHYSICS FUNDAMENTALS By Gregor Morﬁ ll and Hubertus Thomas The 1993 discovery that complex plasma can exist as soft Timing Gravity ...... 50 matter changed the face of physics. Since then, research into its By Stefano Vitale, Christophe Salomon properties and behaviours has exploded, and experiments on the and Wolfgang Ertmer ISS have led to a number of discoveries that could not have been made in the presence of gravity. High-precision experiments performed in space will put some of Einstein’s theories to the test.

Moving in Flow-Motion ...... 60 By Michael Dreyer, Ilia Roisman, Cameron Tropea and Bernhard Weingartner A malfunctioning fuel pump and a harrowing near-disaster at PLANT BIOLOGY Heathrow Airport illustrates the necessities of understanding ﬂ uid ﬂ ow dynamics. Removing gravity from this complex equation allows for new and exciting research in this area. The Puzzle of Plants ...... 92 May the capillary force be with you! By Dieter Volkmann, Anders Johnsson and František Baluška Roots grow downward while stems shoot upward. Clearly, plants “sense” gravity, but how? Years of research in gravitational biology have yielded some remarkable new 50 insights—and striking contradictions.

Bibliography ...... 100

Looking up: Europe’s Quiet Revolution in Microgravity Research is published by Scientifi c American, Inc., 415 Madison Avenue, New York, NY 10017-1111. Copyright © 2008 by Scientifi c American, Inc., All rights reserved. No part of this issue may be reproduced by any mechanical, photographic or electronic process, or in the form of phonographic recording, nor may it be stored in a retrieval system, transmitted or otherwise copied for public or private use without written permission of the publisher. Subscription enquiries for Scientific American Magazine: US and Canada: 800-333-1199; Other countries: 515-248-7684. 4 Associate Publisher, Production: Art Director: Art Director: Advertisements &Sponsors: CUSTOM PUBLISHING Vice President, Finance, Vice President andPublisher: Vice President: SPACE CHANNEL SCIENTIFIC AMERICAN SCIENTIFIC AMERICAN Senior Writer: Scientifi PRESENTS Custom Publishing: Musser, ChristineSoares, Kate Wong P. Collins, MarkFischetti, SteveMirsky, George Circulation Director: Chairman: Copy Director: Chief NewsEditor: Chief ExecutiveOfficer; Director of Chairman; Copy Editors: and General Manager: SCIENTIFIC AMERICAN PROJECT TEAM LOOKING UP Business Manager: Editors: Executive Editor: Editor InChief: Editorial Advisor: Editor: Managing Editor: President: Production Manager: Prepress andQualityManager: Director of Ancillary Products: Project Director, Director ofGraphic Design: in MicrogravityResearch Europe’s QuietRevolution LOOKING UP: Europe’s UP: LOOKING Quiet Revolution in Microgravity Research on HumanSpacefl ight, Microgravity and Chairman ofthe Advisory Committee Special thankstoGerhardHaerendel, Paul Deans c Advisor: Steven Ashley, Peter Brown, Graham Exploration (ACHME) forESA. BrianNapack Steven Yee Edward Bell Sandra Salamony Heidi Moore, KatharinaRoesler Gary Stix Maria-ChristinaKeller Frances Newburg John Rennie MarietteDiChristina NormanLongdon Dr. Paul Clancy Ricki L. Rusting EuropeanSpaceAgency(ESA). Philip M. Yam LisaPallatroni Marie Maher Space Channelonbehalfofthe produced incollaborationwith Christian Dorbandt ChristinaHippeli Michael Florek PhilipSaunders Susan Saunders BruceBrandfon A custompublication Diane McGarvey SilviaDeSantis William Sherman F Thus we experience gravity everywhere, ways we often take for granted. forgranted. take ways weoften Nevertheless, compared to the other fun- other to the compared Nevertheless, powerful when you are, say, skydiving or in lives our rules gravity truth: found plenty of opportunity to appreciate apro- toappreciate of opportunity plenty being torecall Iseem forwhich parachute, time you take a step, you move your body you body move your astep, you take time tance in the way that the positive and and positive the way that the in tance fl itdramatic lends galaxies and tems the fact that it holds together solar sys- damental forcesdamental of nature—electromag- the alternating waves of terror and exhil- and of waves terror alternating the netism and the two nuclear ones that bind bind that ones nuclear two the and netism negative poles of electromagnetism do. of electromagnetism poles negative atomic nuclei—gravity is a piker. Every and farther than I ever had before, I had Ihad before, had I ever than farther and faster falling myself Ifound as aration, You air. the in havenodoubt miles a half cause its force never self-negates over dis- self-negates never force its cause correctly surmised that I was wearing a wearing Iwas that surmised correctly entire Earth. Gravity just seems strong be- strong seems just Gravity Earth. entire of the tug gravitational the against easily est force in the universe. True, it can feel feel itcan True, universe. the in force est extremely grateful at the time. Between Between time. at the grateful extremely struggling to push a stalled car uphill, and and uphill, car astalled topush struggling Fundamental Force Fundamental Further exploration into space requires a better grasp of gravity grasp intospace exploration ofgravity requiresabetter Further Strangely enough, gravity is the weak- the is gravity enough, Strangely ifteen years ago or so, I had the oc- the Ihad orso, ago years ifteen casion to step out of a perfectly good airplane while it was two and and two itwas while airplane good INTRODUCTION air. which for centuries impeded our ability to to ability our impeded forcenturies which wonderful overview ofscientifi such overview wonderful will need to know what life outside our less, free-fall environments. Scientists are are Scientists environments. free-fall less, familiar 1g means. possibly and of space, exploration its ues phenomena, and I hope you will enjoy you will Ihope and phenomena, related and microgravity into plorations instruments into space to study weight- tostudy space into instruments ies, and how it is involved in the complex complex the in involved how itis and ies, bod- own up our make that tissues the ing intellectual trip. Geronimo! technology. We can now send people and and people now We send can technology. it. derstanding the structure of various materials, includ- materials, of various structure the them. Your feet may stay planted on the onthe planted Your stay may feet them. discovering precisely how gravity shapes shapes how gravity precisely discovering dynamics of fl dynamics conduct experiments aimed at better un- at better aimed experiments conduct ground but you are still in for an exciting even its colonisation eventual of it, we smaller scales. As the human race contin- race human the As scales. smaller That has changed, thanks to modern The articles in this collection provide a John Rennie, Scientifi editor in chief c American owing fl owing uids at larger and and at larger uids c ex-

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Life in Orbit The personal and professional rewards of space exploration

BY THOMAS REITER

always experience déjà vu when telling children of my adven- each crew member to maintain the station effi ciently so that it tures in space. I remember myself as I watched Neil Armstrong can run as smoothly as possible. I walking on the moon and decided, “Yes, that is what I want to During my mission on the ISS, we took part in an experiment do.” Not that there were many obvious and immediate chances for called the Materials ISS Experiment or MISSE. We installed two Europeans to become astronauts. Building a suitable background platforms containing some specifi c materials that would then be by studying aerospace technology and getting into aviation came exposed to the conditions of low Earth orbit. These platforms fi rst, and then when the opportunity actually came, I didn’t have will be retrieved later, when they will be examined to see how to think for a second about what my answer would be. this environment affects these materials. We also installed a so- I am often asked why I took on such a risky life. For me, the called fl oating point measurement unit, which is a scientifi c de- benefi ts of a career in spacefl ight defi nitely outweigh the risks— vice on the S1 truss segment. Finally we set up one technology and those benefi ts include not only the scientifi c knowledge experiment that will use an infrared camera on future shuttle gained from our experiments, but also the cultural and personal fl ights to determine any defects on the exterior of the shuttle. rewards. I believe people are inherently curious, and this desire Most of the experiments on board are from the areas of life sci- to explore the unknown is something I very much feel and follow. ences, biology, physics, astrophysics and technology. So a pretty To me, exploration is worth the risks. wide band of science is covered. With the International Space Station nearing completion, all One interesting physics experiment involves plasma crystalli- of the agencies involved in the ISS programme are looking for- sation. A plasma is generated, dust particles are injected and then ward to the moment when we can really utilise the station and those particles inside the plasma behave like molecules in a crys- all the capabilities of its multifunctional research laboratory. tal. A very new area of research, these results will be important People want to know about life in orbit. Having experienced six in different areas of fl uid physics, solid-state physics as well as months on Mir as well as a long mission on the ISS, I can say other areas. In fact, we ran this experiment not only once but there is a general similarity in the daily routine. There are many repeated it during the mission. For the life sciences, we carried out differences, however, between the two stations. Everything is investigations related to the human vestibular system, the cardio- much more advanced aboard the ISS, much more modern. In- pulmonary system and the skeleton; and in biology we performed side we have quite a bit more space than we had on Mir. We now an experiment related to the generation of protein crystals. have two Mission Control Centres we can talk to, and we can Our research in life science serves two purposes: fi rst, to bet- talk to them almost 24 hours a day. On Mir we could talk to ter understand certain diseases that exist here on Earth and help the Russian control centre only at certain times. Last but not to fi nd cures for them; and second, in view of the continuing ex- least, from a personal point of view, the food has become a bit ploration of space, to prepare us for long-duration missions in more international. We have more items we can choose from, future decades, maybe to our neighbour planet Mars. Before we which becomes quite important if you are staying on the station attempt to travel that distance, we defi nitely need to understand for a long time. how to counteract the effects of weightlessness. One advantage of having several people on board the station Speaking of which, we astronauts are always asked about liv- is that all astronauts work together to increase the station’s sci- ing in a weightless environment. It’s indeed true that when you entifi c output. With more people, we have more time to perform get into space, the perception of weightlessness is simply very, the scientifi c programme. Opportunities will continue to arise to very nice. It’s a unique feeling not to perceive the weight of your install new scientifi c hardware both internally and externally, own body, to fl oat around and to use the whole space inside the and the operational capabilities of the station will be expanded station. You can work on the fl oor or on the ceiling without any

even further in the future. Of course, it is always the objective of difference. However, the drawback is you don’t use your muscles ESA

6 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research as much in space as you use them here on the ground—even when less conditions combined with cosmic radiation affects the cen- we are only sitting up, we still have to exercise some muscle con- tral nervous system and immune system. How and why these trol to hold ourselves up against gravity. Of course, when we are effects occur needs to be better understood. walking or standing, we use them even more. This is not the case As you can see, the ISS is not just a “plaything” in space; nor in weightlessness, however, where we lose muscle mass with lack is it an expensive “white elephant.” It is an essential part of the of use. Weightlessness also has a similar negative effect on bones. human endeavour to seek greater knowledge of the universe as Our bones immediately start to lose calcium once we get into well as to take advantage of microgravity to ﬁ nd a better under- orbit, which makes bones more brittle. In addition, the weight- standing of how gravity affects on our life on Earth. ■

www.spaceﬂ ight.esa.int 7 METEORITES: STONES WITH STOWAWAYS? Can organisms be transported from one planet to another and survive both space conditions and atmospheric entry to impregnate the host planet with new forms of life? Scientists explore whether Earth was colonised by microscopic hitchhikers. By Frances Westall and Rosa de la Torre Noetzel

n the summer of 1996, the equivalent of an earthquake shook the scien- tiﬁ c world. David McKay of the Johnson Space Centre (JSC) in Hous- I ton, Texas, and his colleagues announced in the lay press and in a scien- tiﬁ c paper published in the prestigious journal Science that they had found traces of Martian life in a meteorite from Mars. This announcement led to a spate of rebuttals, confrontations, and new studies, as well as provided an incredible stimulus to Mars research—in particular, the search for life on Mars.

Why from Mars? It was only when 1.3 billion-year-old meteorites of volcanic rock were found that scientists realised they must therefore have come from a geolog- ically active body. Analysis showed that these meteorites contained trapped gases identical to those in the Martian atmosphere. The rocks had been ejected from the surface of Mars when some large body of matter crashed into the planet. They then went into orbit around the sun for several million years before falling to Earth. More than 10 years after their initial announcement, the JSC team still maintains that they have evidence for life (although they have had to back down on a number of their original lines of evidence) … and the rebuttals continue. The present situation is a stalemate, and we probably won’t know if there is or was life on Mars until we get the RIGHT rock back from Mars.

8 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research

■ ■ ■ ■ GLANCE A AT ASTROBIOLOGY it through Earth’s atmosphere. Earth’s through it made yet havenot croorganisms mi- but meteorites, ofreal typical developed the dark fusion crust Some rock samples survived and spheric re-entry. test if they atmo- would survive to shield heat the on holders ple sam- in rocks sedimentary placed artiﬁcial meteorites, scientists create to capsule aspace Using meteorites? on Earth to carried have been microbes could and Mars, on life ever there was questions: two begs survive to ability Their in volcaniccracks rocks. diverse environments,including invery live can Microorganisms — The Editors The www.spaceﬂ ight.esa.int

9 SPACE CHANNEL / PHIL SAUNDERS Microbes, Please— planet of Mars fi rst consolidated. How could On the Rocks such an ancient rock contain traces of life when The rock that McKay and his group are still the planet had only just condensed? analysing in painstaking detail is the equivalent It later turned out that the minerals fi lling the of terrestrial basalt, a volcanic rock. In fact, all cracks in the rock where the supposed microbi- the meteorites from Mars (38 have been identi- al traces were found were deposited by low tem- fi ed to date) are igneous rocks, which are rocks perature fl uids some 3.9 billion years ago. This that have consolidated from magma, molten was a period when Mars had signifi cant quanti- Martian meteorite ALH 84001 (full rock ejected during volcanic eruptions. ties of water on its surface and when its surface name Allan Hills 84001) was found Since the 1996 announcement and the blos- was habitable. Later (between 3.8 and 3.5 bil- in 1984 in Allan Hills, Antarctica, soming of astrobiology (the search for the origin lion years ago) the surface of Mars became hos- by US meteorite hunters. Al- and existence of life in the universe), numerous tile to life; however, life may still survive in the though it appears absolutely normal with 80% of its surface cov- investigations have shown that microbes can live subsurface. ered by the standard dark fusion in the most diverse habitats and environments, crust, this meteorite made head- including cracks in cooled volcanic rocks. How- No Stone Unturned lines in 1996 when scientists an- ever, evidence against McKay and colleagues In the same year that McKay was making his nounced it contained traces of hardened when it was discovered that his rock announcement, René Demets, project scientist life—a claim still hotly debated. formed about 4.5 billion years ago when the for space biology at the European Space

A Hitchhiker’s Guide to the Galaxy

A meteorite’s journey from Mars to Earth was long and violent. First, an 2 asteroid or other large body crashed Mars into the Martian surface with enough force to eject chunks of the planet into space. After orbiting the sun for millions of years, these Martian meteorites then survived the heat of atmospheric entry to ﬁ nally land on Earth.

Earth

1 3 NASA / JPL-CALTECH (Asteroid); SPACE CHANNEL / PHIL SAUNDERS; ESA (FOTON capsule)

10 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Research and Technology Centre (ESTEC), the mentary rocks with a similar composition to technical heart of the European Space Agency, those found on Mars (such as sandstone) to sur- developed a plan with André Brack at the CNRS vive during entry into the Earth’s atmosphere. in France. Was there a way to fi nd out more As the experiments progressed, however, this A HISTORY about what might survive a journey through the goal has broadened to look at the wider possibil- SET IN STONE Earth’s atmosphere without waiting for another ity of microorganisms being carried between The series of Stone fl ights began suitable Martian meteorite to land? Their solu- planets. On the right, “A History Set in Stone” in 1999. Their objective was to tion: to expose mineral samples to the environ- gives a brief overview of the early Stone experi- simulate a meteorite entry into ment created on the heat shield of a recoverable ments 1–4, while this article concentrates on the the Earth’s atmosphere by plac- Foton capsule (a Russian spacecraft) when it re- two most recent experiments, Stone 5 and 6. ing sedimentary rocks in sample holders on the heat shield of the entered the Earth’s atmosphere. ESA scientists unmanned Foton Russian space- had used the unmanned Russian Foton cap- Stone 5 craft. Upon the craft’s return to sule’s heat shield to test materials before, but Led again by André Brack, Stone 5 was launched Earth, these “artifi cial meteors” never in the name of astrobiological research. in 2005 and proved a great success. Four rocks would then be recovered and This growing interest in Europe in astrobiol- were embedded in the heat shield of a Foton 12 analysed. ogy led to a completely new series of experi- capsule around the point where the spacecraft ments: the Stone series. The objective was, and is subjected to the highest heat stress upon STONE 1 is, to test the ability of different types of sedi- atmospheric entry. Launched 1999 with 3 rocks placed on heat shield

Control sample: Dolerite, a basalt with similar characteristics to stony meteors (formation of a black fusion crust conﬁ rms that re-entry speed was high enough to melt rock, as with real meteors)

Natural sediment: Carbonate sediment and mixed basalt

Artiﬁ cial sediment: Basalt and gypsum created to simulate Martian sediment

Results Control sample lost during ﬂ ight

Two test samples showed alteration due to high temperature; They produced CO2 and calcium oxide; Neither had a fusion crust

Loss of the control rock meant that the speed of re-entry could not be proved

STONE 2 Crash landed—rocks not recovered

STONE 3 An illustration of the Russian Foton capsule used to simulate atmospheric entry. Sedimentary Not ﬂ own rocks were placed on the heat shield, sent into space and then brought back to Earth to test their ability to survive a journey through our atmosphere. STONE 4 Exploded after launch

www.spaceﬂ ight.esa.int 11 One rock was the same type of igneous vol- We probably won’t showing that the speed of re-entry at 7.6 km/sec canic rock as a real meteorite—basalt—to act provoked a sufﬁ ciently high temperature to melt as a control. One of the most characteristic fea- know if there is or the exterior of the rock. tures of stony meteorites is their dark-coloured All four rocks in this experiment survived at- fusion crust of molten rock, which is created by was life on Mars mospheric entry. Apart from the basalt, they in- the intense heat of atmospheric entry. If a fusion until we get the cluded two sediments: a dolomite (limestone crust formed on the basalt sample, that would rich in magnesium carbonate) and a sandstone mean that the temperatures endured during at- RIGHT rock (quartz sand cemented by calcium carbonate). mospheric entry were high enough to melt the The fourth rock was rather unusual; it was an rock and thus the similarity with real meteorites back from Mars. impactite, which is a rock that has been impact- would be validated. The basalt control sample ed by a previous meteorite as it hit the ground. came back with a very acceptable fusion crust, It was brought from the Haughton Impact Cra-

Man-Made Meteorites

Top left: Stone sample holders and samples mounted on the Foton heatshield. Top right: To test their survivability, lichens were placed in the Biopan canister attached to the Foton capsule. At a certain altitude, the container opened, exposing the lichens to the extreme conditions of space for 16 days. Bottom: Recovery of the Stone and Biopan samples after returning to Earth.

12 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research ter on Devon Island in the Northern Territories ganisms did not survive entry into the atmo- of Canada. These rocks tend to be very porous sphere. Those on the surface of the impactite because they melt during impact and the intense disappeared as the surface was vaporised (ab- heat causes their components to alter, during lated) during entry, and those in the holes in the which gases are liberated (a process called de- backs of the rocks were killed by the intense volatilisation). Such rocks would be common on heat of entry. Two lessons were learned here. the surface of Mars. The fi rst is that fi ve millimetres of rock cover are To test the heat sensed during atmospheric not suffi cient to protect the microorganisms entry, Gabriel Bourrat of the University of Lau- from the intense heat produced during atmo- sanne in Switzerland ingeniously placed ther- spheric entry. The second is that the origin of mic probes consisting of minerals that melt at the photosynthetic—that is sunshine-using— specifi c temperatures inside the dolomite. Based microorganisms on Earth had to have been a on which materials melted within the dolomite terrestrial phenomenon because such microor- rock, temperatures went beyond 271°C but not ganisms that live close to the rock surface can- as high as 630°C. In addition, a fusion crust did not survive atmospheric entry. form on the surface of the dolomite (in contrast to Stone 1, see “A History Set in Stone,” page Ultraviolet-Resistant Plants 11), which corroborated the result of the forma- The same Foton capsule that carried Stone also tion of fusion crust on the basalt and indicated carried another experiment called Lichens that these experiments did simulate meteorite designed to test the panspermia hypothesis. entry, despite the lower entry speeds. Attached to the capsule was a canister, Biopan 5, which opened once the capsule had reached the Earth “Impregnated” with Life? required altitude to expose its contents to the Another aspect of the Stone 5 experiment was harsh conditions of space. Within the canister of particular relevance for astrobiology. The were live lichens that were exposed for 16 days experiment addressed not only the survivability to full solar and cosmic radiation as well as of rocks but also the survivability of living extreme dryness due to the vacuum conditions organisms within or on the stones, a concept of space. Lichens are not single organisms; they called “panspermia,” which poses the question: consist of a fungal element and either photosyn- “Can organisms be transported from one plan- thesising bacteria or algae that live together in a etary body to another and survive both space symbiotic relationship. Lichens are extremely conditions and atmospheric entry to impreg- resistant to UV radiation and can survive at high nate the host planet with new forms of life?” altitudes above 2000 metres where no other The idea of microorganisms living in rocks plants can survive; and like the endolithic pho- (called endolithic microorganisms) is not unusu- tosynthetic bacteria of the Stone 5 experiment, al—volcanic rocks can be an excellent source of they can also live at high latitudes in the Polar nutrients for certain types of microorganisms. regions. They were used as a test case in this One type of endolithic microorganism can experiment because of their UV resistance. live under the surface of a (relatively) transpar- Despite their known resistance to UV radia- ent rock and uses sunlight to produce energy. tion, it was still a surprise to discover that, after Called Chroococcidiopsis, their rocky habitat 16 days of exposure to space conditions and protects them from the intense harmful UV ra- harsh UV radiation, all the lichens showed the diation that is found at both the North and same photosynthetic activity after the fl ight as South poles, where these types of organisms are before exposure to space. The team wondered if, often found. The porous impactites described however, there had been some subtle damage to above are also ideal habitats for such organisms, the microbial cells as a result of this exposure. and the surface of the porous rock used in Stone They proceeded to make minute microscopic 5 was soaked in a solution containing the spores studies of the cells with powerful electron micro- of this type of endolithic bacterium. Further- scopes but could still detect no signs of degrada- more, all the rocks carried a mix of different mi- tion. It was only when they used a simple biolog- croorganisms, endolithic bacteria and fungi that ical test called a live/dead test, which can differ- had been placed inside tiny holes drilled into the entiate the number of live cells compared to dead back of each 1-cm thick rock sample. ones, that they were able to detect the slightest The idea to test the hypothesis of pansperm- sign that there were, not surprisingly, more dead

ESA ia was good, but unfortunately, the microor- cells after the ﬂ ight than before.

www.spacefl ight.esa.int 13 To Boldly Go we hypothesised that these ancient Earth sedi- Where Others Cannot ments with their traces of fossil life would be The sheer resistance of the fungi to space radia- ideal analogues for Martian rocks from its tion, a condition lethal to bacteria and other “warmer and wetter” Noachian period some 4.5 microorganisms, is astounding. How do they to 3.5 billion years ago. survive? It seems that the thick outer coating Another rock that “fl ew” as part of Stone 6 around the lichens, their cortex, provides them was a piece of ancient lake sediment, about 400 with adequate protection against solar radia- million years old from the Orkney Islands off tion, at least for 16 days. Scotland. The intriguing aspect of this rock is The lichens had yet another story to tell after that it is rich in the organic remains of life forms their space trip. Space is a high vacuum envi- that lived in and around the lake. These remains ronment and any matter containing water, such are the molecules of the degraded components as microorganisms, are subject to an extreme of the dead organisms. We wanted to test degree of moisture loss called desiccation. The whether this type of sediment containing only lichens were no exception. However, 24 hours the molecular organic remains of life could after returning to Earth and in the presence of make the ultimate test, survival of entry into CREDIT WHERE water, the lichens recovered their full metabol- Earth’s atmosphere, had it indeed made the CREDIT IS DUE ic activity. journey from Mars. The Stone 5 and the Biopan 5 experiments The last rock in the collection was a piece of Groups involved in the Stone/Biopan provided interesting information regarding the granite from the mountains of Spain that was experiments. survivability of microorganisms in space, but injected with lichens and placed on the heat ESA Project Scientist and they also highlighted the diffi culty of returning shield of the Foton capsule to see fi rst whether Project Manager the microorganisms alive into the Earth’s atmo- they survived in space, and then whether they R. Demets, P. Baglioni, ESA-ESTEC, sphere. Stone also confi rmed, once again, that could survive atmospheric entry as well. This Noordwijk, the Netherlands. sedimentary meteorites can survive atmospher- would be the ultimate step in the panspermia ic entry. However, it left us wondering about the hypothesis. Just to make sure that these mi- Stone experiments: types of sediments that can resist the trauma of crobes really did thrive well in space, a variety A. Brack, F. Westall, CNRS, Orléans, entering the atmosphere and if it were possible of lichen species communities and endolithic France. for microbes that could survive space condi- bacteria were placed in another Biopan canister G. Kurat, F. Brandstätter, Natural tions to get though this barrier alive. for inclusion in the same experimental fl ight. History Museum, Vienna , Austria. C. Cockell, J. Pillinger, S. Sancisi-Frey, In addition, Stone 6 built on the lessons I. Franchi; Open University, Milton Stone 6 learned from Stone 5 where the rocks had not Keynes, UK. Another experiment, Stone 6, (led by author been thick enough to protect the microbes dur- C.-A. Roten, G. Borruat, University Frances Westall) was designed to address these ing atmosphere re-entry. For this experiment, of Lausanne, Switzerland. outstanding questions. Apart from the basalt we engineered thicker 2-cm “chariots” for our H. Edwards, University of control, one type of sediment used as a poten- microscopic hitchhikers that were representa- Bradford, UK. tial Martian meteorite was volcanic sandstone tive of the type of rock on Mars at the time in J. Parnell, University of Aberdeen, UK. dating from an epoch on Earth when there was question. Endolithic bacteria were placed on the Biopan, Lichen and still water on Mars and the possibility that life rear of the rocks. Lithopanspermia could exist at its surface. experiments: This 3.5 billion-year-old volcanic sandstone Back on Earth R. de la Torre , M. Reina, J.M. Frías from the Pilbara in Australia was deposited in Although the basalt rock sample was lost, the (CAB), Spanish National Establish- environmental conditions that were similar to others had survived. The 3.5 billion-year-old ment for Aerospace, INTA, Madrid, those of early Mars, conditions that are consid- reconstituted volcanic sandstone from Australia Spain. ered extreme by present day standards (no oxy- had lost 12 millimetres in thickness but had L. Sancho, C. Ascaso, A de los Rios, gen, no ozone layer to protect the surface from gained a respectable fusion crust, albeit a creamy CSIC, Madrid, Spain. harmful UV radiation, much volcanic and hy- colour, unlike the dark fusion crusts of the J. Wierzchos, Universidad Lerida, drothermal activity and many impacts from as- basalts. Chemical changes in the space cement Spain. teroids, meteorites and comets). This sounds used in the reconstitution of this rock proved G. Horneck, Petra Rettberg, T. Berger, like a catastrophic scenario but it was on this that the temperatures during entry were in DLR-Institute of Aerospace Medicine, Cologne, Germany. early planet Earth (and maybe on Mars) that excess of 1700°C. The lake sediment full of the J.P. de Vera, S. Ott, University of life appeared and thrived. Traces of this early molecular organic remains of living organisms Düsseldorf, Germany. life occur in fossil form in the volcanic sands had turned into a rather un-appetising green C. Cockell, K., Olsson, Open Universi- that hardened into rock. Given the similarities glassy mess, but it had also partially survived. ty, Milton Keynes, UK. in the early environments of Mars and the Earth, To our disappointment, however, the bacte-

14 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Stone 6 Results

Basalt Control Volcanic Sandstone Lake Sediment Granite Biopan Canister

Igneous volcanic rock 3.5 billion years old from 400 million years old From the mountains of Various lichen species similar to a real the Pilbara in Australia from the Orkney Islands Spain and photosynthetic meteorite off Scotland cyanobacteria placed Was formed when Earth Injected with lichens in Biopan container Mounted on Foton conditions were similar to Contained degraded Only some pieces of capsule those on Mars organic remains of life All life survived molten rock turned to and live endolithic Formation of dark Reconstituted to be 2 cm glass survived bacteria fusion crust indicates thick, endolithic bacteria Lichens did not survive similar re-entry placed in rear of rock Partially survived, but temperature to a real became a green glassy Lost 12 mm in thickness, meteorite mess on re-entry but gained a creamy- Sample was lost coloured fusion crust Bacteria did not survive

Bacteria did not survive Study to determine if the fossil remains survived is ongoing

ria in the rear of the rocks had not. Obviously but the preliminary two centimetres of rock thickness are still not observations are very suffi cient to protect the microorganisms; or it encouraging. Foton-12Fotonon-12 StoStonene samplesamplep hoholderlder may simply be that the blazing heat during en- afteafterr rreturneturn to earth.earth. 3.5 billion-year-old Australian try penetrates behind the sample holder to Answers, and More Questions sandstone recovered from Stone 6. scorch any living cell. We may have to fi nd an- The Stone and Biopan experiments have provid- other experimental confi guration to test re-en- ed us with some of the answers we seek. Yes, sed- try survival of microorganisms. imentary rocks can survive atmospheric entry as What about the lichens? Well, as with the pri- meteorites. Unlike basalt, however, they do not or Biopan experiment, the lichens and photosyn- form dark fusion crusts. Both the dolomite from thetic cyanobacteria in the Biopan container eas- Stone 5 and the 3.5 billion-year-old silicifi ed vol- ily survived space conditions at 300 kilometres canic sand from Australia from Stone 6 produced above the Earth for 12 days. Within three days creamy-coloured fusion crusts—not ideal when of rehydration and re-activation, they showed searching for meteorites on the Antarctic ice or the same levels of photosynthetic behaviour as in sandy deserts. So, sedimentary meteorites will before the experiment. Unfortunately the lichens be challenging to find and identify. When it ABOUT THE AUTHORS in the protective layer of granite exposed on the comes to living microbes being transported from Frances Westall is currently at the heat shield of the Foton capsule did not survive, one planet to another inside or outside rocks, this Centre for Molecular Biophysics at CNRS in Orléans, France. and all that remained of the granite were some appears to be more of a problem. Whereas micro- pieces of molten rock turned into glass. So, the organisms like lichens can survive space condi- Rose de la Torre Noetzel is cur- exposed living organisms did not survive entry tions (for 16 days at least), neither they nor endo- rently at the Department of Earth Observation, Area of Research and into Earth’s atmosphere, even when protected by lithic microorganisms can survive entry into the Instrumentation at the National In- two centimetres of rock. What about the fossil Earth’s atmosphere, at least not when protected stitute for Aerospace Technology ■ ESA microorganisms? This study is still in progress by only two centimetres of rock. in Madrid, Spain.

www.spaceﬂ ight.esa.int 15 HUMAN HEALTH

Many of us have been told to lose weight to lower our blood pressure, but going weightless? Studies of astronauts show that gravity does contribute to cardiovascular stress. By Peter Norsk and John M. Karemaker

n this age of heightened health concerns, most people are acutely aware of their salt and fat intake, and the effect of I modern lifestyle on blood pressure. One factor that also affects circulation but that doesn’t often register, as it is—for the most part—an unchangeable constant, is gravity. When we are standing, walking or even sitting upright, an ap- preciable amount of blood settles in the lower parts of the body: the abdomen, buttocks and legs. If the body were not able to counteract these effects and ensure that the brain and other organs were suffi ciently supplied with blood at all times regardless of body position, we would be unable to stand or sit upright. It AT A GLANCE is our central nervous system which provides control over blood pressure (see Figure 1). ■ When upright, gravity If you’ve ever witnessed someone faint dead away from a pulls blood downward to the lower body. The heart standing position, this startling occurrence is the result of a fail- and nervous system must ure in the blood pressure control system. In such a case, the force counteract this effect to of gravity is strong enough to overpower the free-fl ow of blood. ensure adequate blood The most effi cient life saving procedure in this case is to place the supply and consistent sufferer immediately in a horizontal position so that blood can blood pressure. again fl ow more freely back to the heart and hence to the brain.

■ Studies of astronauts have In the near-weightless conditions of space, gravitational stress shown a decrease in blood is absent. Because gravity is not pulling fl uids into the lower ar- pressure in near-weight- eas of the body, these fl uids are distributed into the upper areas less conditions. of the torso more than is usual on Earth. Blood moves more freely from the lower to the upper parts of the body, (over-) fi lling the ■ Though multiple causes are likely, this decrease heart and lungs and straightening out the facial wrinkles that the has been partly attributed astronauts developed over years on Earth. This is what the astro- to dilation in the peripher- nauts call “puffy face and chicken legs” syndrome. al arteries, which indicates a relaxed cardiac state. Does Gravity

■ Strangely, the sympathetic Affect Blood Pressure? nervous system also shows How does the everyday stress of gravity affect blood pressure and increased activity in space, ﬂ uid volume control in the human body? Studies have shown that which is contradictory to a blood pressure and ﬂ uid volume control are sensitive to changes resting state. in gravitational stress. This background became the basis for

—The Editors experiments that took advantage of the weightless conditions on

the International Space Station to look at how gravity on Earth SPACE CHANNEL / PHIL SAUNDERS

16 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Counteracting HYPERTENSION with Weightlessness?

www.spacefl ight.esa.int 17 60 mm Hg FIGURE 1. GRAVITY AND BLOOD PRESSURE When upright, gravity drags blood toward the lower parts of the body. This is counteracted by the blood pressure refl ex system, which acts through nervous connections to accelerate the heart rate and constrict the small arterial resistance vessels. Values on the left of the fi gure indicate the variation in blood pressure from head to feet as caused by the gravity-induced hydrostatic pressure gradients.

inﬂ uences human blood pressure control as well 100 mm Hg as shed light on the integrated systems involved in blood pressure regulation. The results have been surprising and in some cases appear to contradict the expected responses. While they have provided a greater under- Heart Rate standing of the effects of gravity on blood pressure control, they have raised other questions surrounding fluid volume control and endocrine responses. Continued research to ﬁ nd the answers to these and other questions could someday indirectly contribute to the development of therapies for high blood pressure, or hypertension, here on Earth.

Blood Pressure on Earth In basic terms, blood pressure refers to the force exerted by circulating blood on the walls of blood vessels. The workhorse muscle of the heart pumps the blood from its ventricles through the arteries to all peripheral tissues of the body, where it feeds the tissues and then returns to the heart through the veins. The heart and all the interconnected vessels are dynamic and work in unison for efﬁ cient transportation of blood. When we are upright on Earth, the heart beats faster than when we are lying down, and the small peripheral blood vessels (small arter- Vasoconstriction ies or arterioles) in skin, fat, muscles and gastrointestinal organs are more contracted. This contraction increases the vascular resistance and compensates for a decreased cardiac output— the volume of blood pumped away from the heart by the ventricles. In this way, blood pressure at the level of the heart is unchanged or even increased despite the drag of gravity on the blood column.

Nervous Responses A fast-responding nervous control system is responsible for any acute adjustment of blood pressure that becomes necessary when we are upright, when we exercise or when we 200 mm Hg change posture or position. The control system

consists of a network of blood pressure sensors SPACE CHANNEL / PHIL SAUNDERS

18 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Central Nervous System (CNS) Internal carotid Internal carotid in the upper chambers of the heart (the atria), artery artery in the main artery (the aorta) and in the arteries in the neck just below the base of the skull Carotid sinus baroreceptors (see Figure 2). Carotid sinus baroreceptors Placing the fi ngers on the peripheral arteries allows us to feel the pulse, or the rhythm of the Common Common carotid artery carotid artery heart beat. When the pulse pressure is reduced or the mean blood pressure drops, a fl urry of neural signalling occurs. First, the intensity of nerve signals from the arterial pressure sensors Aortic arch FIGURE 2. BARORECEPTORS baroreceptors to the central nervous system decreases. In re- Blood pressure sensors (barore- sponse, the cardiovascular centre in the brain ceptors) to counteract the effects stem communicates back to the heart through of gravity on blood pressure are the vagus nerve, which is responsible for heart located in the large central artery, rate among other things. The function of the va- the aorta and the two neck arteries just below the base of the gus nerve is to restrain the nervous activity in the skull. Whenever these sensors are heart, to “put on the brakes.” In this case, the stretched or relaxed by changes in message from the brain orders the vagus to re- blood pressure, nervous signals lease the brake, thereby increasing heart rate and are transmitted to the central ner- simultaneously unleashing activity in the sympa- vous system, which communicates thetic nerves. with the heart and peripheral If the vagus nerve acts as the brake, the sym- arterial resistance vessels. pathetic nerves act as the accelerator. They strengthen each cardiac contraction and constrict the small arterial resistance vessels Decreased Gravitational Stress throughout the body. When stimulated, these To understand how the stress of gravity affects nerves help the body to counter stress by in- blood pressure in humans, controlled, long- creasing heart rate and decreasing blood fl ow to term bedrest is a useful investigational model the skin, muscles and gastrointestinal organs. to study what happens when that stress is This refl ex system functions quickly to keep reduced. If gravity does stress the cardiovascu- blood pressure within strict limits around a lar system and increases blood pressure, one given mean pressure, which is primarily set by would expect that a long-term decrease in grav- the amount of fl uid in the body and thus blood itational stress would decrease it. in circulation. The kidneys, then, play a domi- There are indications that this theory holds nant role in long-term blood pressure regulation true. Seven healthy men consented to six weeks because they regulate body fluid volume by of bedrest, where the beds were tilted head fi ltering and excreting water and minerals, pro- down at a 6º angle to simulate the fl uid shifts vided that intake of salt and water is not experienced under weightless conditions (see restricted. Figure 3). During the experiment, mean blood

6˚

FIGURE 3. SIX DEGREES OF RELAXATION During an experiment to test the effects of decreasing gravitational stress on the body, beds are tilted head down at a -6o angle. To maintain this position at all times, test subjects are transported to the various test laboratory settings on stretchers that are adapted for the occasion. SPACE CHANNEL / PHIL SAUNDERS

www.spacefl ight.esa.int 19 blood pressure for 24 hours during which the astronauts carried out their normal activities. Twenty-four-hour blood pressure measurements are used regularly in clinical work to establish whether an individual exhibits high blood pressure because blood pressure readings taken during daily activities are a more reliable predictor for development of cardiovascular disease than those taken during a few visits to a doctor’s surgery. As a control, 24-hour measurements were recorded both before and after the fl ight while the astronauts were on the ground. Experimental measurements were taken under weightless conditions during missions that varied from fi ve to 10 days. One set of measurements was performed on the fi rst or second day of fl ight and another during the last. On the ISS, Dutch cosmonaut An- pressure at heart level decreased during the day The investigators observed a reduction in dré Kuipers is delivering blood compared with a control period of normal bodi- daytime heart rate as well as the diastolic pres- samples before being fi tted with ly activity before and after bedrest. Interesting- sure, the lower value in a blood pressure read- the Portapres 24-hour blood pres- ly, the hypotensive effect was maintained at the ing that indicates the pressure between heart sure monitoring device. same level from the fi rst week of bedrest until beats when the heart is fi lling with blood. The the 38th day. Thus, giving the body periods of decrease in diastolic pressure was small but in- reprieve from holding itself erect leads to lower dicated that some degree of dilation of the small blood pressure than that seen when the body arterial vessels had occurred, and led the au- normally remains upright. Researchers also ob- thors to conclude that spacefl ight decreases the served that the output of blood from the heart stress on the human cardiovascular system. seemed almost unchanged; so they deduced that this decrease in blood pressure was primarily Cardiac Output caused by dilation of the smaller blood vessels, While the 24-hour monitor allowed investiga- decreasing the resistance against which the tors to record what happened, it does not heart pumps. explain how. To fi nd out whether a change in So can we blame high blood pressure on blood pressure is induced by a change in the out- good posture? Maybe not, but these observa- put of blood by the heart or by dilation of the tions indicate that the daily stress of gravity peripheral arteries, one needs to measure cardi- does raise blood pressure. Gravity thus partici- ac output. pates to some degree in setting blood pressure Cardiac output can be measured using a at heart level in humans. closed breathing system, whereby the subject in- hales and exhales a gas mixture to and from a Weightless Equals Stress-less rubber bag (see Figure 4). This gas mixture con- The weightless conditions in space offer a tains a tracer that is taken up by the blood fl ow- unique chance to observe how gravity modu- ing through the lungs. The amount of tracer relates human physiology because gravitational maining in the exhaled air is monitored. The stress is totally abolished. During simulation rate of uptake is proportional to the amount of experiments on the ground, the gravitational blood fl owing through the lungs, which is the load can be minimized but not nullifi ed. Head- same as cardiac output. down bedrest can decrease the effects of gravity, When this method was fi rst used in space, the but the gravitational force still affects the body rate of tracer uptake increased by some 18% from front to back, back to front, or side to side. during the initial days compared with the up- Thus, in the end, there is no equal for actual right standing position on the ground, indicat- weightless conditions to explore how the every- ing a corresponding increase in cardiac output. day effects of gravity affect the human body. This initial increase subsided a bit during the Thanks to a portable system worn by 12 as- following days; however, blood pressure was

tronauts, researchers were able to measure not measured during this ﬁ rst investigation. ESA

20 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Because cardiac output increased in weightless conditions and blood pressure was un- FIGURES 4 – 5. changed or slightly reduced, the small arterial Measuring Cardiac Output resistance vessels must have been dilated, because otherwise, blood pressure would have increased. We estimated a 14% decrease in resis- Rebreathing tance of the arterial vessels during a week of valve spaceﬂ ight as calculated by mean blood pressure divided by cardiac output, where the increase in cardiac output was 22% and the mean blood pressure remained unchanged (see Figure Rebreathing bag 5). The values in space were compared with values obtained on Earth while the subjects were Lungs sitting upright. Relatively speaking, a week in O2 N2O space is relaxing to the cardiovascular system.

Confounding Kidneys Heart When studying the reaction of an interconnect- Veins Arteries ed system such as circulation to an extreme change in environment such as weightlessness, however, the answer is never as simple as the adjustment of a single variable. The explanation Organs for the body’s reaction is a complicated one, and in some respects appears contradictory. Obser- vations made approximately 15 years ago indicated that the sympathetic nervous system is stimulated during space travel. As discussed FIGURE 4. Cardiac output can be measured non-invasively using a rebreathing previously, the sympathetic nerves are the technique, whereby a tracer gas (N2O) is taken up by the blood fl owing through the “accelerators,” and their activity would be lungs and the reduction of the gas detected by an infrared photo acoustic system. inconsistent with the relaxed cardiac state indi- The reduction rate of the gas is proportional to the amount of blood fl owing cated by dilated blood vessels and decreased through the lungs, which is equal to cardiac output. blood pressure. This discrepancy becomes apparent when FIGURE 5. Cardiac output as a mean ± we examine the kidney and hormonal respons- SEM measured in four astronauts uses. During a Spacelab mission in 1993, four of ing the rebreathing technique. Results the astronauts performed the fi rst and, until to- shown are after one week of spacefl ight compared with the upright seat- day, only intravenous saline infusion during a ed position on the ground before spacefl ight. The intention was to investigate fl ight. Cardiac output was elevated by MEAN PERIPHERAL VASCULAR how the renal output of salt and fl uid would be 1 L/min or by about 20%. Simultane- RESISTANCE OVER 4 HOURS affected by a week in space. Surprisingly, the ously, mean peripheral vascular resis- mm Hg * min/L rate of salt and urine excretion in space was sig- tance is decreased by about 14%, indi- 18 nifi cantly less than when laying prone on Earth cating less constriction of the smaller and was at the same level as when sitting up- blood vessels. 16 right. In addition, levels of noradrenaline, a 14

hormone transmitter of the sympathetic ner- MEAN CARDIAC OUTPUT 12 vous system, not only increase in space, at times OVER 4 HOURS 10 they are even higher than when a person is seat- L/min ed upright on the ground. Increased blood lev- 8 8 els of noradrenaline were also observed later by 6 6

other investigators during the Neurolab Mis- 4 4 sion that fl ew in 1998. 2 2 What makes these fi ndings surprising is that 0 0 usually, in a resting person, reduced excretion of 1 G O G 1 G O G salt and fl uid and an increased noradrenaline (Earth’s gravity, seated) (Weightless, spaceflight) (Earth’s gravity, seated) (Weightless, spaceflight)

SPACE CHANNEL / PHIL SAUNDERS (Figure 4); SOURCE DATA :AUTHORS (Figure 5) concentration in the blood go hand in hand with

www.spacefl ight.esa.int 21 an increased peripheral vascular resistance— weightlessness can be created for only 20–25 sec- rather than the decreased resistance seen in onds. In these fl ights, the aircraft follows the space conditions. Conversely, an increase in car- same path as an object in free-fall, such as a can- diac output during resting, non-exercising con- nonball fi red into the air. During this brief time, ditions would be accompanied by less sympa- we have shown that cardiac output increases by thetic nervous activity, meaning increased excre- as much as 29%, and the arterial resistance ves- tion of salt and fl uids and decreased release of sels dilate, which leads to a decrease in blood noradrenaline into the bloodstream. pressure. One hypothesis could be that thorax expan- Lung–Heart Interaction sion stimulates the blood pressure responses in One explanation for the above discrepancy an unusual way so that sympathetic nervous ac- could reside in the unique lung–heart interac- tivity is gradually stimulated instead of being tion seen in weightless conditions, which is suppressed. The continuous stretch of the ves- impossible to replicate on the ground. Without sels could cause a decrease in sensitivity of the the pull of gravity, the thoracic cage expands, chest cavity pressure sensors. Alternatively, the pulling the lungs and structures surrounding decreased extracellular fl uid volume in the legs the heart open like an accordion, thereby fur- might also trigger sympathetic activity. ther expanding the central vessels and the heart For now, however, these explanations re- itself. This mechanical expansion of the thorac- main speculative. We cannot explain the mech- ABOUT THE AUTHORS ic cage is maintained throughout spacefl ight, anisms for the body to be, physiologically irrespective of its duration, and thus contributes speaking, both relaxed and agitated at the same Peter Norsk is a professor in grav- to the increase in blood fl ow to the heart. time, but fi nding such an explanation is a high itational and space physiology at This reaction occurs during all weightless or priority. We are currently monitoring blood the department of Biomedical Sci- ences at the University of Copen- near-weightless conditions—including parabol- pressure and cardiac output over 24 hours in as- hagen, Denmark. The purpose of ic, free-fall trajectory aircraft flights where tronauts during missions on the ISS of at least his research is to understand how gravity affects blood pressure and volume regulation in humans, and whether it is a factor in the development of hypertension. For this Lessons from the Animal Kingdom purpose, chronic long-term weightlessness in space is a unique experimental model. Understand- n fi sh, blood pressure must remain low because their one- ing how blood pressure is regulat- chamber heart pumps blood through the gills before it can ed is important for treatment of I fl ow to the organs. Any substantial increase in blood pres- hypertension, which is the leading cause of death among the world’s sure and they would bleed to death through the delicate population. structures of the gills. The arteries supplying the gills must therefore respond strongly and quickly to high blood pres- John M. Karemaker is an associ- sure by slowing the heart beat. ate professor of Physiology at the Humans too have delicate structures, such as our lungs Academic Medical Centre of the and brain, that cannot tolerate too high a pressure. In fact, University of Amsterdam in The Netherlands. His interest in space the blood pressure in human lungs is about the same as that research is directly related to his in a fi sh. The arteries that in fi sh supply the gills with blood research in blood pressure control. have developed through evolution into similar sites in mam- Astronauts are the most healthy mals (and thus humans), where systemic and pulmonary and most often medically tested blood pressure is sensed. subjects that one can imagine. Still, Giraffes too can teach us something about pressure— when returning from space they and gravity. When compared with humans, giraffes exhibit very often experience spells of diz- ziness or outright fainting. This much higher blood pressures at heart level, but exhibit simi- condition is an important subject lar blood pressures in the arteries just below the brain. This of research in the AMC, since “nor- discrepancy indicates that the heart compensates for the mal people” may also be plagued varying distance between the head and the heart. Does that by this condition, which is very dif- mean tall people are at risk for higher blood pressures than fi cult to treat. It is his hope that shorter individuals? That has not been determined, but there the combination of both areas of are indications that this is the case. research, space and terrestrial, may benefi t both astronauts and

patients alike. SPACE CHANNEL / PHIL SAUNDERS

22 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Dutch cosmonaut André Kuipers is shown here (right, next to Thomas Reiter) on board ISS enjoying his meal while wearing the Portapres 24-hour monitoring device on his left hand. A small cuff remains continuously infl ated to track instantaneous changes in blood pressure. To prevent fi nger pain, the device switches measurement from one fi nger to the other every half hour.

three months. Simultaneously, we are collecting nary results indicate that the increase in cardiac blood to measure the noradrenaline concentra- output and dilation of peripheral arterial vessels tion in platelets as a refl ection of the long-term continues during these missions of several effects of weightless conditions on sympathetic months. If confi rmed, these results could mean nervous activity. that spacefl ight and weightlessness over long periods of time are healthy for the cardiovascular Perspectives system. Hypertension. On the ground, people normally Heart failure. Data from space and simula- spend most of their time upright, and, as we tion studies also have implications for under- have discussed, gravity takes its toll on a body standing how gravity stresses patients with continually in this position. When upright, heart failure. In heart failure, the pumping ca- blood pressure is higher in the lower portions of pacity of the heart is reduced and thus is more the body, which can induce constriction and sensitive to the pull of gravity. The insuffi cient structural thickening of the vessel walls of the pumping of the blood through the kidneys acti- dependent arteries and thus might contribute to vates the hormone system that helps regulate hypertension and cardiovascular disease. The long-term blood pressure, leading to constric- extent of gravity’s role in disease development tion of the blood vessels and fl uid accumulation and the potential ramifi cations of its removal in the body. are not clear. To alleviate the stress of gravity in heart fail- Current data indicate that spacefl ight may ure, we have immersed patients in thermo-neu- have positive effects on the circulation because tral water of 34.5ºC. Here, the outside water the small arteries are continuously dilated, and pressure compensates for the gravity-induced the uneven pressure levels between the upper pressure gradients in circulation, which leaves and lower body are abolished. However, inves- the cardiovascular system virtually weightless. tigations of the heart muscle in space have pro- When we compared study parameters in these duced confl icting results. One report showed patients with control patients sitting upright, that the ability of the heart to contract was com- water immersion improved the circulatory con- promised immediately after 129–144 days of dition in the heart failure patients, who had spacefl ight. In contrast, Russian scientists used been stabilized with standard medical drug ultrasound to perform cardiac scans of 15 cos- treatment prior to the intervention. monauts following a few months of spacefl ight Because the results seen during spacefl ight and concluded that the pumping of blood from validate those seen during water immersion, the left heart chamber was improved. it is fair to conclude that gravity is a constant Clearly the health of the astronauts demands burden for heart failure patients, which aggra- that we fi nd a defi nitive answer for these con- vates the condition. Further development and fl icting results. We are currently exploring the study of situations that alleviate the stress of effects on the cardiovascular system of extend- gravity may lead to future therapies for these

ESA ed missions of six months on the ISS. Prelimi- patients. ■

www.spaceﬂ ight.esa.int 23 HUMAN HEALTH

24 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Clinical Immunology in New Frontiers

When thinking big, we must also remember to think small, especially when we need to fend off illness in space. As Victor Hugo once said, “Where the telescope ends, the microscope begins; and who can say which has the wider vision?” By Alexander Choukèr, Boris Morukov and Clarence Sams

efore 1961, when Soviet cosmonaut Yuri Gagarin became the fi rst human in space, many considered our very sur- B vival in such circumstances to be impossible. Debates AT A GLANCE erupted about whether there would be fatal incidents of heart failure. No one knew what would happen to the body in the ■ Many factors contribute to absence of gravity. At no stage had the evolutionary process pre- immune system suppres- pared human beings to live without it, in isolation and darkness sion in space, including lack of gravity, cosmic ra- and in an overall hostile environment. diation and highly stress- Since that historic voyage, people have continued to push the ful living conditions. boundaries of space exploration and have survived the experience in increasing numbers. With the ensuing progress of tech- ■ Blood analyses have nology for international spacefl ight, two or more cosmonauts/ shown that both the innate and adaptive arms astronauts were routinely sent to space during the Soviet Salyut of the immune system are and Mir missions and the US Gemini/Apollo and Space Shuttle negatively affected by missions. While the concerns of fatal heart failure proved, thank- microgravity, and confi ne- fully, to be unnecessary, physiological changes as a result of space ment studies have shown travel did occur in other areas. inhibition in response Blood analysed from astronauts after the 1973 NASA Skylab to stress. Mission showed that immune function was altered. More than ■ Current study is focused 50% of the Apollo astronauts had experienced either bacterial or on using small monitoring viral infection during spacefl ight and then later upon return to devices to watch for how Earth. Although most prominent research of human life sciences and when immune chang- in space is usually focused in areas such as bone and muscle loss, es become a health risk to these results inspired more profound investigations in the fi eld of better target prevention fundamental immunology. Maintaining good health under the and therapy.

unique stresses of space travel requires intense study of the im- —The Editors

SPACE CHANNEL / PHIL SAUNDERS mune system. After all, the nearest hospital is hundreds of miles

www.spacefl ight.esa.int 25 away. Research into the body’s reaction to mi- ease-causing) bacteria, virus-infected cells and crogravity and cosmic radiation, as well as the other foreign substances. This second line of de- psychological stress of living in a hostile envi- fence is located in tissues such as the lung or in- ronment, is crucial before attempting longer testine, which can be vulnerable entry points for missions to Mars and beyond. germs. When under attack, the innate system triggers a generalized infl ammatory response, Lines of Defence which causes redness and swelling as well as the In broad terms, the immune system consists of fever and body aches associated with the fl u. layered mechanisms that protect the body from When this system overreacts, the aberrant in- disease. Containing a wide variety of cell types, fl ammation can damage cells and tissues, caus- if you were to add up its more than four trillion ing signifi cant clinical problems. cells, the human immune system weighs more This infl ammatory response can be initiated than the liver and brain put together. The fi rst by a vast re-circulating pool of innate immune lines of defence are those surface barriers cells in the blood or tissues that act through im- between the host and its environment, including portant and evolutionarily preserved receptors physical barriers such as skin and mucus as well on cell surfaces, the toll-like receptor family. as chemical barriers such as stomach acid and These receptors control the production of sig- enzymes in saliva or tears. nalling proteins called cytokines that induce in- Invading microbes that penetrate the surface fl ammation and activate immune cells in re- and enter the host body meet the second line of sponse to invading microbes. These immune defence—the so-called “innate” immune sys- cells are located in more than 500 lymph nodes tem—that includes phagocytes, white blood throughout the human body, waiting inside like cells that engulf and destroy pathogenic (dis- military troops on active reserve. Lymph nodes

A23187 PMA FIGURE 1. INNATE IMMUNE CHANGES IN SPACEFLIGHT In the course of fi ghting bacteria, killer white blood cells called phagocytes (which in Greek means “cell-eater“) produce hydrogen per- PREFLIGHT oxide (H2O2). In this study, blood was taken from astronauts and an immune reaction induced using either an ion-carrier with anti- biotic properties (A23187) or an organic compound used in research because it promotes tumor growth (PMA). Blood was tested before spacefl ight and then again one and seven days after return from a six-month mission. The Y-axis shows the signal intensity of H2O2 production, and the X-axis defi nes the 1 DAY population of white blood cells (granulocy- AFTER RETURN tes) when scanned through a laser beam (FSC = forward scatter).

A23187 RESULTS (left): The rate of H2O2 production was reduced by more than half on the ﬁ rst and seventh days after 6 months in orbit compared with normal preﬂ ight rates of about 700 units.

7 DAYS PMA RESULTS (right): In contrast to the ion AFTER RETURN carrier results, the capability to generate adequate H2O2 did not change after space- ﬂ ight when stimulated by PMA, which may indicate that cell functions can be restored. DATA SOURCE: ALEXANDER CHOUKÈR ALEXANDER SOURCE: DATA

26 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research act as sentries along the castle walls: they detect hind the efficacy of vaccination and the ac- the presence of invaders, process the threat and quired immunity from diseases such as chicken sound the alarm. The troops are released and pox that occur once, most often in childhood. when they recognize a pathogen, they trigger the cytokines that unleash both the general in- Immunity Altered by Stress fl ammation described above and a more specif- Germs and microorganisms are not the only ic, or adaptive, response described below. threats that provoke the immune system to leap ABOUT THE AUTHORS This generalized innate immunity is an an- into action. Environmental or situational Alexander Choukèr attended cient defense system found in orders of life from change can tax the body in other ways, creat- medical school at the Ludwig-Maxi- fungi to plants to starfi sh. Only higher verte- ing a physical response. Although the defi ni- milians-University (LMU) in Munich, brates like humans developed a third line of de- tion of “stress” is still a matter of debate in this Germany. He completed scientifi c fence that reacts specifi cally to the germ itself. context, stress research pioneer, endocrinolo- training at the LMU and at the NIH in Bethesda, Maryland, US. Current- After the fi rst exposure to a microorganism, gist Hans Selye, succinctly defi ned the adapta- ly he is based in the Department of “killer” B- and T-lymphocytes are able to recog- tion processes of any biological system to envi- Anaesthesiology at the LMU and is nize that microorganism so that on the second ronmental changes: associate professor and clinical encounter, the response is targeted to that spe- specialist in Aneasthesiology, In- “Stress is a non-specific response of the cifi c germ, allowing more rapid and more effi - tensive Care and Emergency Medi- body to any demand placed upon it … [it] cine at the LMU’s medical faculty. In cient action and elimination. results whenever we are faced with exter- 2003, he was awarded the John E. In their lifetime, lymphocytes may or may nal changes or demands and such demands Fogarty fellowship grant from not encounter the target they are capable of rec- NIAID at the NIH. In 2007, he re- include any variations in the individuals’ ognizing, but if they do they can be activated to ceived the Berblinger Award by the environment.” multiply into a large number of identical cells. German Academy of Aviation Medi- cine. Since 2007, he has been vice T-cells recognize the invaders by their specifi c In general terms, stress is a challenge: can we chairman of the ESA Life Sciences proteins or antigens and can either directly at- adapt to a new condition, regardless of what it Advisory Group (LSAG) and coordi- tack the invaders or activate fellow T- or B -cells. may be, as quickly and with the least damage to nator of the ESA topical team on The B-cells produce antibodies that either help ourselves as possible? Sweating is a simple exam- immunology. destroy the microorganisms or mark them for ple: faced with a rise in temperature, we sweat to Boris Morukov attended medical attack. Adaptive immunity is the principle be- cool off and maintain a constant temperature. school at the Second Moscow Medi- cal Institute in Moscow and received a PhD in space, aviation and FIGURE 2. ADAPTIVE IMMUNE CHANGES IN SPACEFLIGHT naval medicine from the Institute T-lymphocytes can remember a microbe after the fi rst exposure, a property called adaptive immunity. As part for Biomedical Problems in Moscow, of this function, T-lymphocytes produce chemicals called cytokines that aid in the immune response. Two cy- Russia. He completed scientifi c tokines were measured in this study: interleukin 2, which stimulates growth and differentiation of T-cells, training as a cosmonaut/researcher and interferon-y, which inhibits viral replication among other functions. and as a fl ight surgeon at Johnson Space Centre in Houston, Texas, US. Human T-cells were separated and grown in culture medium that induces cell reproduction. Prefl ight results Currently he is deputy director and showed normal production of interferon-y (green circle) and interleukin-2 (orange circle). After a long mis- head of Department of Biochemis- sion on the ISS, T-cells were almost completely unable to generate both cytokine messengers. try and Immunology at the State Research Centre RF-Institute for Biomedical Problems of the Russian PRE-FLIGHT POST-FLIGHT Academy of Sciences. Interferon-y Interferon-y 103 103 Clarence F. Sams received his PhD CD4/1FNg+ CD4/1FNg+ in 1983 in biochemistry from Rice 25.2% 4.2% University in Houston, Texas, US, and performed postdoctoral re- 102 102 search at the University of California, Berkeley. Dr. Sams served as project scientist for NASA, and is currently 101 101 the director and technical monitor of the Immunology Laboratory at John- son Space Centre in Houston. Dr. Sams was awarded the NASA Excep- 0 0 10 10 tional Service Medal in 1998. In ad- CD4/1L-2+ CD4/1L-2+ dition to his research activities, Dr. 74.4% 31.4% Sams currently serves as project sci- 0 0 entist for the ISS Medical Project, 100 101 102 103 100 101 102 103 which integrates fl ight-related hu- Interleukin-2 Interleukin-2 DATA SOURCE: CLARENCEDATA SAMS man life science research on the ISS.

www.spacefl ight.esa.int 27 The immune response is obviously much port and communication. Recently it has been more complicated. This system is not autono- shown that microgravity alters certain internal mous and several co-factors affect responses signalling pathways of immune cells, thus caus- when adapting to a stressful, new environment. ing those cells to function improperly. Space missions demand that humans cope with living conditions never experienced before: pri- Cosmic Radiation marily the absence of gravity plus varying de- Cosmic radiation is everywhere in space. When grees of cosmic radiation. In addition, astro- combined with other stress factors such as micro- nauts face mental stress that results from con- gravity, it is highly likely that radiation can have finement and uncomfortable and unnatural an aggravated effect on the immune system. living and working conditions including the When on extended missions, astronauts will lack of privacy, a heavy workload and an abnor- be exposed to stronger and more varied types of mal day–night cycle. In addition, the microbio- radiation than that experienced normally. These logical environment on the space station chang- radiation types consist primarily of solar ener- es over time and germs may evolve in response. getic particles, protons and highly charged energetic particles of galactic cosmic rays. The de- Effects of Microgravity gree of exposure to solar energetic particles will Depression of T-cell lymphocyte activation due increase during interplanetary missions as the to near-zero gravity was fi rst observed in an partial “shielding” that results from the Earth’s experiment conducted during the Spacelab 1 magnetic field is left behind when leaving Mission in 1983. The data triggered several oth- Earth’s orbit. er investigations that attempted to map the com- Although little is known about the conse- plex mechanisms of T-cell activation and sup- quences of cosmic radiation on immune cells, pression both in space under true microgravity many publications confi rm the impact of other and on the ground in the clinostat machine, forms of radiation on immune cells. Such radia- which is used to approximate microgravity. tion can kill cells, cause mutations, cause infl am- Results showed that these effects and the po- mation and malignancies and otherwise weaken tential consequences to our ability to fi ght off the immune system and induce immune system germs were indeed debilitating. The lympho- disorders and cancer. This damage, however, can FIGURE 3. INNATE AND ADAPTIVE cytes exposed to microgravity almost complete- take years to become apparent. So while an as- IMMUNE CHANGES ly lost their capability to react in their normal tronaut returning home from a long mission may Changes in immune cell activity of defensive role. Studies of animals conducted in appear to be healthy, is there a time bomb tick- both the innate (natural killer space also indicated reduced killer cell activities ing away somewhere within the body? cells) and the adaptive (T-lympho- and a higher susceptibility to viral infections. cytes) arms were measured within This suppression is thought to happen be- Mission-Associated Stress one week of returing from space cause the organization of the cells is geared to Like that of cosmic radiation, the consequences missions of 137–437 days. In both cell types, decreased function was gravity; without it, the cells become disoriented of mission-associated or mental stress are insidi- found in more than 40% of the and fail to function normally. Cell architecture ous and may not be immediately apparent. In cosmonauts, indicating suppres- consists of a cytoskeleton, which functions like addition, we may not immediately associate sion across both innate and ac- scaffolding. It maintains cell shape as well as stress with an immune system response. Condi- quired immunity. plays an important role in intracellular trans- tions such as post-traumatic stress disorder are often associated more concretely with psychological trauma than with long-term physical problems, as there is little conclusive research in that NATURAL KILLER CELLS T-LYMPHOCYTES Number of Cosmonauts (n) Number of Cosmonauts (n) area. To understand the relationship between the mind and the immune system, we must fi rst con- 60 60 sider the human body as a whole, with complex, interrelated systems. Immune system function depends on the informational signalling of both

30 30 the endocrine and nervous systems. The endocrine system is composed of small organs that produce hormones, chemicals that act as messengers to regulate not only immuni- 0 0 Normal immune Decreased immune Normal immune Decreased immune ty but metabolism, growth, puberty and other

activity after spaceflight activity after spaceflight activity after spaceflight activity after spaceflight “holistic” or whole-body functions. These hor- SOURCE:DATA BORIS MORUKOV

28 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research mones circulate in the blood from their place of stress hormones and suppressed immune cell release to their target cells, tissues and organs. activities, which resulted in the reactivation of These hormones can be produced and act local- dormant herpes simplex virus type 1 (cold sores) ly or be produced by a distinct gland such as the During extended as well as the Epstein-Barr virus. Other investi- pituitary, thyroid or adrenal gland and trans- periods of stress, our gations showed that psychological stress can ported through the blood to the active defence defences are kept on affect wound healing and responses to vaccina- cells. Once there, the hormones will stimulate, tion, supporting the hypothesis that stress does inhibit or maintain cell function depending on alert. Cortisol, often negatively affect immune responses with poten- the messages sent. referred to as the tial clinical consequences. Clinical investiga- During extended periods of stress, our de- “stress hormone,” tions have shown that patients under psycholog- fences are kept on alert. Cortisol, a glucocorti- ical or mental stress also exhibit signs of impaired coid that is often referred to as the “stress hor- circulates at higher and altered immunity. mone,” circulates at higher levels in response to levels in response to Other investigations have been conducted of stress and other potential dangers. It increases stress and other stressful living conditions, an important concern blood pressure, blood sugar levels and suppress- for astronauts. Earth-bound confi nement stud- es the immune response. Cortisol and other hor- potential dangers. ies have illustrated the effects of this type of stress mones are released into the blood in response to on regulation of the immune system. The ex- neural stimulus and their levels are controlled by treme living conditions studied were similar to brain activity. those experienced during spacefl ight. In different studies, subjects either were confi ned for 10 to Investigating Stress 240 days in modular chambers that mimicked While the physical processes behind the effects the living quarters in space or were confi ned in of stress on the body are complex, the real-life isolation for months in the Canadian Arctic. The translation can be unfortunately simple. When stress responses and specifi c immune changes under strain, we are more likely to get sick. In seen in these studies were quite similar to those studies of medical students during the highly observed in astronauts after spacefl ight. stressful time of exams, tests showed increased As with the medical students, confi ned sub-

Tracing Immunology

linical immunology is the study of diseases caused by disorders of the immune system (failure, C aberrant action and malignant growth of the cellular elements of the system) as well as diseases of other systems where immune reactions play a part in the pathology and clinical features. The results of blood tests from the 1973 NASA Skylab mission were a wake-up call for the fi eld. The astronauts’ immune functions were clearly altered following spacefl ight. Afterward, the resulting investigations fi rst used in vitro cell suspensions containing lymphocytes, a type of white blood cell, to estimate the effects of gravitational changes on specifi c immune cell responses. Emerging methodology in biomedicine has allowed immunologists to further study relationships between the organ systems and the immune responses. In addition to controlling infections and eliminating germs, immunologic responses are also responsible for eliminating non-functional or dysfunctional tissue-cells (e.g., tumor cells). Failure to maintain adequate immunity may result in autoimmune diseases (e.g., rheumatoid arthritis), acute and life threatening infections, over- whelming systemic immune responses (e.g., septicemia) or the development of cancer. Psychoneuroimmunology (PNI) is the fi eld of human physiology research dealing with the complex interactions between the central nervous system, endocrine and immune systems under conditions of stress. PNI researcher Kevin Tracey from the Feinstein Institute for Medical Research in Manhasset, New York, and Ronald Glaser from The Ohio State University Mind/Body Center in Columubus, Ohio, have shown that brain activity and the nervous system control infl ammation in a feedback manner where hormones are released into the blood in response to a neural stimulus, altogether affecting immune and health status. SPACE CHANNEL / PHIL SAUNDERS

www.spacefl ight.esa.int 29 jects showed a reactivation of the herpes virus, response is more well known, catecholamines which is usually dormant in healthy adults. This suppress the cell activation pattern of white virus also became active during spacefl ight. Un- blood cells. The most recognizable cate- der these conditions, the body’s ability to re- cholamine is epinephrine, commonly known as spond to a threat such as the herpes virus de- adrenaline. Along with norepinephrine, these creases. Stress causes cortisol to be released and “fi ght-or-fl ight” hormones not only increase suppress immune functions of both the innate heart rate and blood pressure but also inhibit and adaptive systems by inhibiting cytokines immune cells’ capability to ingest microbes. and T-cell production. After missions of several months, samples of the astronauts’ blood illus- Hidden Threats trated that the immune cell functions of both the How will an already suppressed immune system innate and adaptive immunity were indeed sup- cope with challenges from the microbiological pressed in the fi rst days after return to Earth (see environment in a closed space station or a Figures 1–3). remote habitat? Although no unusual microbial These tests and subsequent experiments also hazards have been indentifi ed to date, the Inter- revealed higher levels of catecholamines circu- national Space Station will inevitably house an lating in the astronauts’ blood than in non- unknown number of microorganisms that will stressed subjects. Like glucocorticoids, cate- increase with the duration of operation. Recent cholamines are chemicals released by the adre- surveys have identifi ed some potentially oppor- nal glands. Although the glucocorticoid stress tunistic pathogens, and the number and types of microorganisms may further increase with the length and number of missions and the number of visitors to the ISS. Not only is the microbiological load itself subject to changes, but the bugs’ ability to cause dis- Mars500 ease are also subject to change during spacefl ight. In fact, the virulence of specifi c bacteria has been he State Scientifi c Centre of the Russian Federation-Institute of Biomedical Prob- shown to intensify. Researchers tested this by T lems of the Russian Academy of Sciences and the European Space Agency have growing the bacteria Salmonella typhimurium scheduled a program in 2008–2010 designed to simulate travel to Mars and back on board the space shuttle and comparing this (each leg is 250 days), including a 3-week “landing” on Mars. Six subjects will live and growth with ground control cultures. Complex work in a newly built facility including living quarters, a medical module, a storage proteomic and genetic analyses revealed that container, a simulator of the Martian surface and a simulator of the Mars landing mod- changes in gene transcription and protein syn- ule. The last will house three of the six participants during the three-week landing thesis resulted from spacefl ight. These changes phase. The living conditions, as well as most of the mission-associated stressors, will were not only of interest for “academic” reasons be comparable to those currently experienced on the ISS—including the communica- but also because these bacteria increased infection delay. Normally, the delay in the communication with the control centre can be up tion in a mouse model. In summary, the variable to 20 minutes. Thus, the MARS500 crew will have to make independent decisions on microbiological environment together with the problems that arise. In the future, it is possible that the mission’s duration could be ex- greater disease-causing ability of bacteria may tended to 700 days to simulate a manned fl ight to Mars. increase health risks to the astronauts when the suppressed immune system is confronted with microbiological contamination.

An Ounce of Prevention Systematic investigation of the human immune system’s adaptation in space has recently begun on the ISS. Current scientiﬁ c projects include Immuno and Integrated Immune. The upgrad- ed research opportunities available on the ISS Columbus module will enable more intense investigations into the interaction of the human immune system with the speciﬁ c microbiological environment on the ISS. In addition, Earth-bound studies of control-

led bedrest, conﬁ nement in the Arctic or Ant- NASA / JPL / MALIN SPACE SCIENCE SYSTEMS

30 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research arctic or the simulation of long-duration space low, the oxygen deprivation would result in a conditions will provide important knowledge on condition called hypoxia similar to that seen in how and when these changes become a health climbers with altitude sickness. A subnormal A journey to Mars risk to the individual and to the overall mission. oxygen level in tissues leads to symptoms that Innovative and user friendly monitoring devices include headaches, shortness of breath and nau- and back could that use, for example, miniscule amounts of sea. The effects of prolonged hypoxia on im- become reality in our blood or exhaled air will enable repeated assess- mune cells could result in immune suppression children’s lifetime, ments, which would be used to decide the type as well as other challenges to the system. of intervention necessary as well as when it and one of the most should be administered. Interventions could in- Summary crucial requirements clude vaccination or drug treatment. More so- In recent years the scientifi c community became will be adequate life phisticated preventive methods lie in the remote aware that the combination of multiple sources future, such as the manipulation of gene se- of both physical and mental stress that occur support systems and quences that trigger gravity-dependent immune during spacefl ight can have detrimental effects living conditions. cell dysfunction as well as those that protect on the immune system. Understanding how from radiation. this system will cope with the even greater Currently, options to protect the crew of long- stress of extended duration missions such as term missions in space are threefold: pharma- interplanetary spacefl ight is absolutely essen- ceutical treatment, environmental manipulation tial to reduce the risk to those brave enough to (such as the use of artifi cial gravity) and protec- crew such a mission. tion from cosmic radiation by using shields or by An international research approach will open avoiding major solar events. Those space trav- new insight into the regulation of immune re- elers who are highly susceptible to immune or sponses and will address health concerns that stress-related health problems are most likely to may occur during these missions. In the newly benefi t from such protection. installed Columbus research laboratory and other modules of the ISS, researchers will use a Beyond the ISS multidisciplinary approach to understand bet- A journey to Mars and back was once thought ter how microgravity, cosmic radiation and mis- to be the realm of science fi ction, but explora- sion associated stress can infl uence immune tions of this kind are being actively discussed by function. In addition to the crucial task of main- scientists and engineers today (see “Mars500,” taining the astronaut’s health, this research will opposite page). These plans could become real- also benefi t patients here on Earth with the po- ity in our children’s lifetime, and one of the tential for targeted drug treatment, vaccina- most crucial requirements will be adequate life tions and other therapies. These achievements support systems and living conditions. could be implemented into the treatment of any Even with that issue addressed, however, the patient, of any age, at any location, from the lo- conditions on lunar or Martian surfaces and cal clinic to the ISS. ■ their consequences on human immunology are poorly investigated so far. In addition, the loss of visual contact with Earth or “Earth out of sight syndrome,” during such a long mission and the communication delay that will surely occur ACKNOWLEDGMENTS will add new mission-associated stress. Any exploration of Mars and, looking even The authors are grateful to the following agencies for the support of this research: further into the future, lunar or Martian colo- Roscomos, NASA, and ESA as well as the German National Space Program supported by the German Space Agency (DLR) and the Federal Ministry of Economics and Technology nisation program means facing technical chal- (BMWi) for providing all the logistic and fi nancial support. The authors would like to ex- lenges to defi ne and develop suitable living con- press their gratitude to the crew members of the shuttle and ISS missions and to all vol- ditions. Creating a robust habitat with the at- unteers of Earth-bound studies who generously spent their time and participated with mospheric pressure that the human body is enthusiasm and extreme professionalism. accustomed to on Earth requires an enormous For their signifi cant scientifi c input, support and assistance in the review we would amount of energy. Finding the lowest accepta- also like to especially express our gratitude to: Prof. Manfred Thiel (LMU, München), ble value of continuous atmospheric pressure Dr. Brian Crucian (NASA, USA), Dr. Marina Rykova (IBMP, RUS), Dr. Sarah Baatout that the body can tolerate will be critical to the (SCK-CEN, Belgium), Prof. Gustav Schelling, Dr. Ines Kaufmann and Mrs. Andrea effi cient running of a life support system. If the Boltendahl (all LMU, Germany). atmospheric pressure and oxygen levels are too

www.spaceﬂ ight.esa.int 31

32 32 Bone Loss Bone LOOKING UP: Europe’s UP: LOOKING Quiet Revolution in Microgravity Research SPECIAL REPORT: SPECIAL

CREDIT HUMAN HEALTH in Space

uringuring the manymany millions of yyearsears in which vertebrate aniani-- overall calcium losses, and a gradientgradient of loss that increases be-be- mals evolved, the human skeleton graduallygradually stood erect, ginningginning at the sspinepine throughthrough the lower bodbody.y. The authors also D against thethe Earth’sEarth’s gravity. As part andand parcelparcel ofof thisthis analyseanalyse datadata on whetherwhether losseslosses occurredoccurred in thethe hardhard outer layerlayer feat, the skeletal system also evolved to function within its bound- of bone or the more porous bone interior. aries—to withstand its force and the impacts caused by simple The second article, “Our Sensitive Skeleton,” by Rommel Ba- movements like walking. The skeleton’s primary role is locomo- cabac, Jack van Loon and Jenneke Klein-Nulend, looks at bone tion: it allows the body’s muscles to move the limbs. Secondary adaptation on the cellular level. The authors discuss what is known roles such as maintaining balance and supporting the body’s and unknown about how bone cells called osteocytes sense grav- shape are infl uenced by the demands of movement. Bone cells ity and mechanical stress. They also analyse the high level of sen- adapt to these variables—the total mechanical load or stress sitivity of bone cells to minute amounts of strain as well as the ef- placed on them—for more effi cient performance. fects of different types of strain or stress on bone formation. Despite its infl exible appearance, bone is a living tissue, and How this knowledge translates into prevention and recovery it must continually destroy and rebuild itself throughout a life- is the ultimate goal. Current strategies to counteract micrograv- time of use. This process, called remodelling or turnover, occurs ity’s effects consist mainly of exercise, but it has not been enough. in two phases. The rapid resorption phase takes two to three weeks and then the slower formation phase lasts two to three months. Different types of bone cells carry out these phases. Os- teoclasts degrade the bone surface and release calcium to be re- Sticks and stones may break your absorbed. Osteoblasts then fi ll in the pit with new bone. bones, but underuse will make them Bone tissue works to increase or decrease skeletal mass based on its use. This means that during times of increased exercise, for brittle. Like extended bedrest, an envi- example, skeletal mass is preserved or increased. Conversely, it ronment without gravity reduces the decreases with reduced exercise or movement, which is what happens to astronauts during spacefl ight. workload on bones and makes them Simply put, the skeleton doesn’t have to work as hard without weaker. Because research in this area is the pressure of gravity. In near-weightlessness, not only is less extensive, two articles are presented movement required and fewer impacts felt but less strength and power is required to execute the motion. The detrimental effects here to illustrate different approaches of this extreme “unloading” on astronauts have been seen since being taken to understand—and the earliest spacefl ights. Experiments have carefully documented bone loss in various parts of the body and in different types of bone combat—these effects. tissue, but research continues into exactly how and why these losses occur. Part of this research is focused on the bone cells themselves and their ability to “sense” the mechanical load placed on As we exlore further out into space, astronauts will have to spend them in order to adapt to it. How gravity and other loading factors more time in microgravity and risk increasing losses in bone den- are detected and then translated into a response is yet unknown. sity. Some research indicates that the rate of recovery for missions Because of this long history of intense research, two articles of several months can exceed twice the time spent in space, but are presented here that examine the effects of microgravity from no one knows how or if the body would recover after spending very different angles. The fi rst article, “Zero Gravity: Bad to the years there. Research into the dynamics of skeletal changes, cel- Bones,” by Laurence Vico and Christian Alexandre, reviews ev- lular communication and how to effectively increase the mechan- idence of bone loss during missions from the 1960s to today. Re- ical load on bones is necessary not only to protect the health of sults from these studies show markers of increased resorption, the astronauts but also to combat osteoporosis here on Earth. SPACE CHANNEL / PHIL SAUNDERS

www.spaceﬂ ight.esa.int 33 HUMAN HEALTH

Zero Gravity: Bad to the Bones

fter earlyearly explorationsexplorations into sspacepace when to combat such a widely experienced condition doctors announced that the astronauts on Earth. In addition, the microgravity environ- A had suffered bone loss, people all over ment also offers the means to study other factors the world took notice. For many, “bone loss” that are either diffi cult to observe and measure spelled “osteoporosis,” a disease associated on Earth or have not yet been investigated. with aging; not with the strong, robust young men who were sent into space. Literally trans- Early Indications lated to mean “porous bones,” osteoporosis is Two important parameters have marked the a common aging disorder that affl icts a large increasing level and speed of research. First, as portion of older people. The World Health the length of missions increased, researchers Organization estimates that the condition is were able to gather more useful data. Second, responsible for approximately 650,000 frac- the means to gather data improved with the tures a year in the European Union alone. In advent of more precise instruments. fact, one WHO report states, “Osteoporosis- After most of the early Gemini and Apollo induced fractures cause a great burden to soci- missions in the 1960s, astronauts showed both ety,” and adds that such hip fractures can affect increased urinary calcium levels and decreased 40% of women over 50 years old. bone mineral density (BMD) as measured by ra- While hormonal and other changes associat- diodensitometry. Urinary calcium is an indica- ed with aging are the main considerations for os- tor of bone remodelling, or turnover. As bone teoporosis on Earth, in space, lack of gravity re- tissue rebuilds itself, it fi rst destroys old bone AT A GLANCE sults in reduced mechanical use of bones. Both and then replaces those cells with new bone.

■ Bone loss in space seems to the decrease in strength or power necessary to Cells called osteoclasts degrade bone tissue by occur in a gradient that be- create movement and the decrease in the amount secreting substances that dissolve calcium and gins at the spine and be- of movement overall are thought to be the main other minerals that make up its structure, a pro- comes greater in the lower factors leading to bone loss in astronauts. Stud- cess called resorption. An increase in urinary limbs. ies have shown that the number and the ampli- calcium can indicate a corresponding increase

■ The rate of recovery after tude of various demands on the skeleton while in bone resorption and release of calcium from long missions can exceed in orbit are extremely low compared to the nu- bone into the bloodstream—in the absence of twice the time spent in mi- merous impacts during daily life on Earth. In other factors such as increased calcium intake. crogravity conditions. fact, near weightlessness reduces the stimuli on bone by approximately half of that experienced The Beginning of Long Flights ■ Future study should focus on the dynamics of bone during a typical work day on Earth. Beginning with the studies conducted during the loss, its relationship to oth- Little wonder then that doctors and scientists three Skylab missions in the 1970s: Skylab 2 (29) er body systems and more saw a two-pronged reason for delving further days, Skylab 3 (59 days), and Skylab 4 (84 days), effective exercise and/or into the cause and effect of bone loss in space. we could see greater bone loss in the heel com- prevention strategies. The ﬁ rst was to prevent the foreseen danger to pared with the upper limbs as a function of the astronauts’ health during long, interplanetary time spent in microgravity. The astronauts’ uri- —The Editors missions, and the second was to ﬁ nd new ways nary calcium output progressively increased to

34 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research SPECIAL REPORT: Bone Loss in Space

Bone loss in astronauts was ﬁ rst documented in the early ﬂ ights of the 1960s. Since then, longer missions and better equipment have enabled more detailed research and a wealth of data. By Laurence Vico and Christian Alexandreandre

80–100% above normal, where the levels even- nauts. These studies indicated that bone and tually plateaued. In addition to calcium, other overall calcium loss resulted from a combina- urinary markers of bone degradation increased: tion of reduced intestinal absorption of calci- the amino acid hydroxyproline increased up to um, increased calcium excretion and increased 33% and urinary phosphorus also increased. bone resorption. Markers of bone formation After the Skylab 3 and 4 missions, bone loss was decreased after 14 days of fl ight; but after 60 to found in the heel while no changes were observed 110 days of fl ight, levels were similar to pre- in the two forearm bones. fl ight values. One consistent result seen across Three months after the 84-day Skylab 4 fl ight, the different studies was an increased amount heel bone density was still signifi cantly lower of bone formation markers after the flight, than normal. Frozen urinary samples recently which indicates enhanced metabolic activity. analysed with specifi c blood tests developed to Although microscopic bone tissue samples measure biochemical markers of bone remodel- of astronauts were never tested, these analyses ling confi rmed the increased resorption. were done in studies of the effects of bedrest on bone mass. Here, biopsies of the iliac crest (part Peace in Space of the pelvis) indicated reduced bone formation In 1986 the Soviet Union launched the fi rst activity; however bone formation markers were functional, operating space station. Named not decreased. This seeming contradiction is an after the Russian term for peace or world, Mir important reminder to consider and test both not only provided a long-term crewed vehicle site-specifi c and systemic activity. for experiments, it also allowed scientists to gather invaluable data on what was happening Bone Structure to the astronauts’ bodies during previously Later missions incorporated more detailed unexplored lengths of time during a mission. study of how and where bone loss occurred After the fall of the Soviet regime, the Rus- within the structure of the bone itself. Bone sian space agency, NASA and ESA were able to consists of both compact and spongy tissue. expand the use of Mir for collaborative mis- Compact or cortical bone makes up the hard sions. Flight times of up to 312 days, with peri- outer layer of bones, and spongy or trabecular odic measurements, provided a general picture bone, which is porous like a sponge to allow of what was happening, while allowing for in- blood vessels and other cells to traverse the dividual metabolism, size and age of the astro- bone interior, makes up the inner cavity.

www.spaceﬂ ight.esa.int 35 FIGURE 1. GRADIENT OF LOSS Collating the results of many experiments points to a gradient of bone loss that begins at the lumbar spine and becomes greater in HEAD the lower limbs. Results are given as t-scores obtained from dual energy x-ray absorptiometry scans. ARM A score above -1 is considered nor- RIBS mal. A score below -2.5 is deﬁ ned THORACIC VERTEBRA as osteoporosis. LUMBAR VERTEBRA

PELVIS

LEG

–4.5 –4.0 –3.5 –3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0 0.5 1.0

The fi rst comprehensive documentation of Viewing these results as a whole, it seems bone loss by tissue type was taken from two cos- that there is a gradient of mineral loss begin- monauts who fl ew in one- and six-month mis- ning at the lumbar spine level and becoming sions, respectively, and was later confi rmed on progressively greater in the lower limbs, includ- another 15 cosmonauts. In addition to periph- ing the attachment point of the femur and dis- eral quantitative computed tomography (pQCT) tal tibia but excluding the heel. This progres- of the distal radius in the arm and the tibia in sive decline is also characterised by higher per- the leg, ultrasound measurement was performed centage loss of trabecular bone compared with on the heel bone. After the six-month fl ight, cortical bone. The upper limbs appear to be bone loss in the tibia and the heel was more protected from loss of bone minerals and there ABOUT THE AUTHORS marked than that seen after the one-month have even been surprising reports of increased fl ight, whereas no loss was observed at all in the BMD in the skull (see Figure 1). One reason Christian Alexandre was educat- radius. Although no change was seen in the ra- may be that a redistribution of minerals from ed at the Medical School of Lyon, France. He is currently dean of the dius regardless of mission duration, BMD loss the lower to upper body occurs in space as grav- medical school and a practicing in spongy bone of the tibia was already present ity no longer exists to pull fl uids to the lower rheumatologist based at St-Etienne after the fi rst month and continued to deterio- extremities. Hospital-University, where he spe- rate with mission duration. Loss of cortical Another explanation for the absence of bone cializes in bony diseases. While at St. bone tissue in the tibia occurred after a two- loss in the radius is that the arms play a greater Etienne, he developed the Inserm research laboratory dealing with month fl ight but was less pronounced than that role in locomotion and in overall movement in bone adaptation to stressful condi- for trabecular bone. space than the legs. So while the lower limbs ex- tions of intensive exercise and Later, in the year 2000, crew members on In- perience a decrease in use, the upper limbs ex- weightlessness. ternational Space Station missions were subject- perience an increase, which may contribute to Laurence Vico was educated in St- ed to BMD testing using QCT measurements of the skeletal adaptation in both instances. Test Etienne and Lyon Universities in the hip and spine in addition to the routine dual results varied widely between individual astro- France. She is the present director of energy X-ray absorptiometry (DXA) measure- nauts and no relation with potential confound- Inserm research laboratory for the ments. Before and after fl ight measurements ing factors, including previous time spent in biology of bone tissue at St-Etienne School of Medicine. conducted in 14 astronauts who performed four- space, was found. Measurements were also tak- to six-month missions showed that they experi- en during a recovery period equal to the mission Both C. Alexandre and L. Vico received the Philip Morris scientifi c enced substantial loss of both trabecular and length. During recovery, bone loss persisted in award in 1992 in life sciences in cortical bone in the hip and somewhat smaller the tibia; suggesting that the time needed to re-

space. losses in the spine. cover is longer than the mission duration. SPACE CHANNEL / PHIL SAUNDERS, SOURCE DATA: LAURENCE VICO

36 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Confounding Factors demineralization by increasing the urinary out- These reported losses in bone mass occurred put of water, sodium and calcium. despite physical exercise training. Although This increase in urine production and sodium these changes have been mainly attributed to excretion occurs soon after entering a microgra- decreased body movement and reduced weight vity environment and was thought to contribute bearing, there are other associated factors that to loss of astronauts’ body mass. However, contribute to bone loss to unknown degrees. continuous daily body mass measurements Magnetic resonance imaging (MRI) evaluation showed a gradual reduction over the entire mis- in 16 astronauts on four- to six-month missions sion instead of a rapid loss of 2 to 3 kg at the be- showed that muscle volume did not change in ginning of a mission. Studies of energy (caloric) the upper body but decreased in the back and intake versus energy expenditure have shown legs, with the greatest change in the lower leg. that astronauts are burning more calories than These regional changes in muscle mass seem to they are consuming, resulting in a negative ener- mirror those of bone. gy balance and a loss in body mass. Insuffi cient Researchers have attempted to associate this vitamin K intake (to the best of our knowledge, upper/lower body discrepancy with the afore- fresh vegetables were scarce on the Mir space mentioned fl uid redistribution caused by micro- station) seems to also contribute to bone loss. gravity. The loss of gravity-induced pressure During a 179-day mission, investigators admi- gradients in the lower body causes fl uids to shift nistered 10 mg of vitamin K to one astronaut to the upper body. The kidneys, which are the from day 86 to day 136. During and after being primary regulators of body fl uid volume and given the extra vitamin K, bone formation mar- composition, respond to the fl uid shift and bone kers increased signifi cantly. Other factors that

Stripped to the Bone

Osteoblasts

Osteoclasts Osteoblasts

hroughout life, the body continuously destroys and rebuilds bone solving enzymes, which carve out pits in the bone and release calci- Ttissue, a process called remodelling. Two types of cells perform um to be reabsorbed. Right: Osteoblasts then swarm into the pit and these functions: osteoclasts, the destroyers, and osteoblasts, the secrete calcium and proteins to ﬁ ll it with new bone builders. Left: Osteoclasts invade the bone’s surface and release dis- SPACE CHANNEL / PHIL SAUNDERS

www.spacefl ight.esa.int 37 may contribute to bone loss are disruption of cir- riosteal compensation in an adult would take a cadian rhythms as well as neuromuscular and signifi cant amount of time. The extent to which vestibular systems, but the potential impact on this compensatory effect protects against frac- the skeleton have not yet been investigated. ture, however, remains to be seen. Fracture is one of the key health risks of os- Rate of Recovery teoporosis, one that usually occurs after the One year after ISS astronauts returned to Earth, disease has quietly ravaged the body over time. bone mineral testing was again performed on the While it will take years for fractures to occur, femur to assess the effects of reexposure to other symptoms related to fl uid shifts are im- Earth’s gravity. Area and volumetric bone min- mediately observable when the astronauts re- eral density, bone volume, as well as bone min- turn to Earth. Changes in volume and compo- eral content (BMC) were evaluated. Results sition of fl uids surrounding muscle cells can showed that BMC of the femur had recovered, cause water retention in those cells, making but overall BMD results and estimated bone them swell and become painful. The doctor for strength showed only partial recovery. The the French National Space Agency noted that return of BMC to normal values can be attribut- returning astronauts experienced fl at feet as ed to an increase in bone volume and cross-sec- well as back pains several days after returning tional area while BMD remained below normal. from space. Tennis players also said that they Additional research results indicate that re- systematically hit under the ball on the two or covery of skeletal density after long-duration three days after landing. space missions may exceed twice the time spent in microgravity conditions. These relatively long Future Steps recovery periods may be due to the type of bone To make a better assessment of microgravity’s cells and mechanisms compensating for the over- impact on bone, the next research step is to all loss. The outer layer of bone is the periosteum, establish the dynamics of skeletal changes dur- and these cells regulate the general outer shape ing microgravity exposure as well as under dif- of bone, including length and thickness. Most ferent levels of microgravity. Obtaining a better periosteal expansion takes place during puberty understanding of these effects is especially when hormonal changes induce the growth spurt important in view of the planned Lunar and that determines our adult height among other Mars explorations, as these longer missions will things. Little periosteal growth occurs during require better prevention strategies. Evaluations advancing age when it serves to partially main- of these dynamics require not only measure- tain the cross-sectional area of bone, so any pe- ments of bone resorption and formation markers but also repeated assessments of site-specific changes during fl ight to bone volume, mineral content, density and so on. Because of limited space, a miniaturized peripheral QCT was developed to measure trabecular and cortical BMD separately. This appa- ratus has now evolved into a high resolution de- Trabecular area vice able to evaluate the three-dimensional structure of the skeleton (see Figure 2). A 3-D view will allow researchers to validate or refute Intermediate area whether bone resorption in space occurs equally in all planes of trabecular bone. Also unknown is whether the recovery of BMD is uniform or whether it is selective for the horizontal Cortical area

FIGURE 2. BONE STRUCTURE Representative 3-D reconstruction of the tibia with peripheral QCT showing the distinct regions for volumetric BMD evaluation. All planes of bone are visible: the outer cortical area and the two trabecular regions of interest: the intermediate area and inner trabecular area. SCANCO MEDICALSCANCO AG

38 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Alphabet Soup: DXA, BUA, QCT

everal techniques exist for measuring Sbone mineral density. Below is a brief explanation of what each method entails. How Results

Two X-ray beams with different energy levels DXA actually measures areal BMD. True are aimed at the patient’s body. One of these density is calculated using mass divided DXA: Dual energy X-ray beams is absorbed mainly by soft tissue and by volume. DXA results can be adjusted absorptiometry the other by bone. The soft tissue amount can by calculating volume based on the be subtracted from the total and what remains projected area. is a patient’s bone mineral density (BMD).

A bony extremity, usually the heel, is placed Attenuation as measured in decibels is between two ultrasound transducers, one used to calculate the BUA index, which BUA: Broadband ultrasound transmitting and one receiving. The amount of varies between normal subjects and attenuation power lost by various frequencies as they travel those with osteoporosis. through bone and soft tissue—called attenuation—is measured.

Tomography uses digital geometry processing to QCT is the only technique that can generate a three-dimensional image of the inside distinguish between cortical and QCT: Quantitative computed of an object from a series of two-dimensional cancellous bone. It measures volumetric tomography X-ray images taken around a single axis of BMD. New bone-dedicated machines are rotation. QCT devices are speciﬁ cally designed able to evaluate 3-D structure. for high-precision bone measurements.

or vertical spongy bones. These answers have that stimulus from exercise has been inadequate important implications for future bone health to maintain bone mass because of insuffi cient and risk for potential fractures, especially of load or duration. We know that the number and the vertebrae and hip. Even if bone density re- the amplitude of impacts in orbit are very low covers, the 3-D structure of the bone may be al- compared to those experienced daily on Earth, tered, which could compromise its architectur- so any prevention plan should mimic the im- al integrity. In addition, most of the present pacts of daily life on Earth in both number of data have been obtained in men. More data col- repetitions and magnitude of force applied. lection is needed not only in men but also in The “required impact dose” to prevent hip women because gender-based differences in bone mineral density loss has been estimated at bone loss exist. 100 impacts per day with a gravitational force Further studies are also needed to clarify the of 3.9g, greater than that of Earth at 1g. Future relationships among the different systems (mus- prevention strategies would ideally transiently culoskeletal, neurovestibular, nutritional, car- load the body at more than 1g because of the diovascular, etc). A greater effort toward a coor- technical diffi culties of loading at 1g through- dinated, multidimensional approach, with an ul- out the fl ight. Any programme must be sure to timate goal of prevention and rehabilitation, is maintain the natural operation of muscle and required in order to design strategies to counter- bone as a single unit working against the force act the effects or treat as needed, research that of gravity. Extreme situations, including not will also benefi t osteoporosis patients on Earth. only microgravity but conditions such as spinal Pharmacological interventions have not been cord injury or myopathy, can disrupt the nor- routinely used in space. Programmes designed mal muscle/bone function. Thus, the frequency to rectify the effects of microgravity have fo- and regimen of any prevention strategy have to cused mainly on exercise. However, it is clear be carefully studied and defi ned. ■

www.spaceﬂ ight.esa.int 39 SPECIAL REPORT: Some reports of in- Bone Loss in Space creased bone density.

Bone loss in No bone loss seen at hip greater all in forearm after 1- than loss in or 6-month missions. spine during 4- to 6-month missions.

Bone loss seen in the spine, femur neck, trochanter (bony crest at the top of the thigh bone) and pelvis after missions up to 7 months.

Loss of spongy bone seen in tibia after 1 month in space and continued to decrease with mission Bone loss seen duration. in heel after Skylab 3 and 4. BMD was still below normal 3 months after Bone loss in tibia and heel Skylab 4. was greater after 6-month ﬂ ight than 1-month ﬂ ight. SPACE CHANNEL / PHIL SAUNDERS; NASA IMAGES

40 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research A Brief History of Bones in Space

During the many million of years in which vertebrate animals evolved, the human skeleton gradually stood erect, against the force of earth’s gravity. The major role of the skeleton is to serve mechanical needs of the body to move.

With the start of manned spaceﬂ ight in the 1960s, changes in bones became apparent very quickly based on bone radiodensitometry measurements during the Gemini and Apollo ﬂ ights. Increased urinary calcium and decreased bone mineral density were observed.

In the 1970s, the Skylab missions were more focused on life science objec- tives, including the study of space-related bone metabolism. Numerous changes were observed: a progressive increase in urinary calcium and then a plateau at 80–100% above normal; calcium balances averaging -200 mg/day on longer Skylab missions; increases in other urinary markers of bone loss of up to 33%. Bone loss in the astronauts’ heels was also found. Months after the ﬂ ight was over, the density of the heel bone was still signiﬁ cantly lower than normal.

IIn the 1980s, better research tools and longer missions, such as on tthe Russian space station Mir, took bone research much further. CComputed tomography was used to measure vertebral BMD (bone mmineral density) in four cosmonauts after missions lasting up to sseven months; all lost vertebral bone density mainly from the posteri- oor vertebra. A dual absorption X-ray photon device showed loss in tthe spine, femur neck, trochanter (bony crest at the top of the thigh bbone) and pelvis of about 1–1.6% per month and 0.3–0.4% per mmonth in the legs and whole body.

SStarting in 2000, bone investigations were conducted in crew members Research results indicate that recovery oof the ISS missions using computed tomography measurements of the of skeletal density after long-duration hhip and spine in addition to the routine dual X-ray absorptiometry mea- space missions may exceed twice the ssurements. Pre- and post-ﬂ ight measurements were done in 14 subjects time spent in microgravity conditions. wwho performed 4- to 6-month missions. ISS crewmembers experienced Prevention strategies so far have fo- ssubstantial loss of both trabecular (inner layer) and cortical (outer layer) cused on exercise, but they have been bbone in the hip and somewhat smaller losses in the spine. Taken togeth- inadequate. Future study must focus eer, it seems that there is a gradient of mineral loss beginning at the lum- on the dynamics of skeletal changes bbar spine level and becoming progressively greater in the lower limbs. as well as the right type of exercise or preventive movement to combat the loss of gravity. www.spaceﬂ ight.esa.int 41 HUMAN HEALTH

Our Sensitive Skeleton

ou’reou’re pplayinglaying a gamegame of word associa-associa- Since the earliest space fl ights, the negative ef- tion.tion. Your partner says motmother;her; you say fect of prolonged weightlessness on astronauts’ Y father.fatherYourpartnersayscatyousay Your partner says cat; you say bones has been of critical concern. Spacefl ight dog. Your partner says skeleton; you say … produces a unique condition of skeletal “unload- what? Perhaps you respond with strong, or even ing” as a result of the near-weightless conditions. Halloween, but you probably do not say sensi- Bones are no longer subject to the gravitational tive. Although we rarely if ever associate it as pressure normally experienced on Earth, and, such, bone tissue is sensitive to its mechanical like bedrest, this results in bone loss. environment, a condition known as mechano- Experiments performed during spacefl ight sensitivity, which allows bone tissue to adapt have shown that microgravity acts directly on during the course of a lifetime for more effi cient skeletal tissue without the infl uence of systemic performance. This adaptation is a cellular pro- factors such as stress hormones (see Figure 1). cess by which bones alter their mass and struc- The exact mechanism of how bone loss occurs ture in response to the demands placed on them; in spacefl ight is still unknown and many ques- and to adapt, bone tissue needs a system that tions remain. How is the lack of gravity detect- senses the mechanical load or stress on bones. ed? Can near-weightless conditions act directly Then, this loading information must be commu- on bone cells? Could certain cells read the grav- AT A GLANCE nicated to the “effector” cells that have an active itational fi eld change directly? To answer these role in bone remodelling, or turnover. Bone cells questions, the fundamental properties of the ■ Bone cells can sense the continuously dissolve and then rebuild bone tis- way bone cells respond to loading in general load or stress placed on sue. This occurs throughout adult life, where have to be addressed. them and can adapt according to these mechanical bone tissue is resorbed by osteoclasts and new demands. bone matrix deposited by osteoblasts. Bearing the Load The ability of bones to adapt is part of the rea- Although adaptation is a general phenomenon ■ Studies suggest that the son why larger animals like elephants have cor- and not specifi c for bone tissue, it is intriguing rate of the strain or move- responding wider bones than say, humans. Gali- that such a hard and seemingly inert material ment is more important to building bone than the am- leo Galilei was probably the fi rst to describe the as bone can be gradually altered during life, plitude or power required role of gravity, i.e., weight, on living organisms. and in such a “sensible” or sensitive manner. to execute it. He observed the dimensions of bones from ani- All eukaryotic cells (cells having a nucleus) are mals of different weights and noted that the probably sensitive in some way and physical ■ These fi ndings have direct length-to-width ratio was remarkably different factors, including gravity, tension, compres- implications for the type of exercise or other preven- between light and heavy animals. The very act of sion and shear, infl uence growth and remodel- tive strategies that may carrying body mass around places a heavy me- ling in all living tissues. In vertebrates, bone is work to counteract the ex- chanical load on bones. The reverse is also true. the tissue best suited to cope with large forces treme “unloading” that oc- If the mechanical load is lower than normal due because of its hard but fl exible extracellular curs in space. to situations such as bedrest or immobilization, matrix, a tough composite material outside the bone mass decreases in a condition known as dis- cell walls that provides structural support to —The Editors use osteoporosis. the cells in addition to other important func-

42 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research SPECIAL REPORT: Bone Loss in Space

Galileo was the ﬁ rst to notice the effect of weight—or gravity—on bone size. But if bone tissue can sense and react to gravity, how are those proc-c- esses affected once gravity is removed? By Rommel G. Bacabac, Jack J.W.A. van Loon and Jenneke Klein-Nulendlein-Nulend

tions. Mechanical adaptation ensures effi cient FIGURE 1. GROWTH OF load bearing: the daily loads are carried by a EMBRYONIC MOUSE LONG BONES IN SPACE. surprisingly thin structure. Bones from 16-day-old mice were cultivated under The organisation of the cellular activity different gravitational conditions. (A) Growth imme- behind this adaptation is poorly understood, diately after isolation; (B) Cultured under Earth’s gravity of 1g on the centrifuge during fl ight; (C) Cul- but over the last few years signifi cant progress tured under microgravity conditions in the Biorack has been made. These studies emphasize the facility. During the 4-day culture period, calcifi cation role of the osteocytes, the most abundant cells starts in the centre of the embryonic bone, which is in adult bones, as the professional “sensor” clearly visible as a black spot in the centre (D). The cells of bone. calcifi ed section of the bone cultured in microgravity These load-sensing osteocytes are in contact was signifi cantly smaller than when cultured under with each other via their long slender cell “fi n- Earth’s gravity on the centrifuge. TL = total length gers” that reach through tiny canals within bone of bone. called canaliculi. The osteocytes themselves lie BC in a space called a lacuna, Latin for “pit” (see Fig- ures 2 and 3). The space between the cell body, A its “fi ngers” and the bone matrix is fi lled with fl uid. This network of interconnected cells sur- rounded by fluid can efficiently detect local changes in stress. When a heavy load is placed on

bone, the result is something like squeezing a wet D TL sponge. The bone becomes deformed and causes fl uid to fl ow through the canaliculi, which acti- vates the osteocytes (see Figure 4) and transports cell signalling molecules, nutrients and waste products. One of these signalling molecules is nitric oxide (NO). The quick-responding osteocytes react to the fl uid fl ow by activating the en-

REPRODUCED FROM J BONE MINER RES. 1995;10:550-557 WITH PERMISSION OF THE AMERICAN SOCIETY FOR BONE AND MINERAL RESEARCH zyme responsible for NO production.

www.spacefl ight.esa.int 43 FIGURE 2. OSTEOCYTES. Left: Osteocytes are arranged in concentric circles around a blood Mechanotransduction in Bone cells. In addition, the production of the bone vessel. Note the many cell exten- Exactly how a mechanical signal is detected and matrix protein osteopontin by osteocytes rap- sions (fi ngers), radiating from the converted into a chemical, intracellular idly increases the amount and timing of chang- osteocyte cell bodies (black), in response—processes called mechanosensing es to bones after acute disuse, which may medi- particular in perpendicular direc- and mechanotransduction, respectively (see Fig- ate bone resorption. tions. Right: Isolated osteocytes ure 4)—has yet to be established. The composi- Bone cells are not the only cells to exhibit this in culture form cell fi ngers in all tion of the bone matrix immediately surround- kind of stress response, and not the only cells to directions (original magnifi cation: ing the osteocytes and their attachment to that use NO as a signal. In the vascular system, x320-390). bone matrix appears to be very important. The changes in the diameter of arteries occur in re- bone matrix determines how porous the bone is, sponse to changes in rate of blood fl ow in order and thus the rate and pressure of the fl uid fl ow to ensure a constant vessel tone. (See “Counter- in response to stress. As stated above, the “fi n- acting Hypertension with Weightlessness?” on gers” of the osteocytes reach through bone to page 16.) Endothelial cells, which line not only detect and also amplify deformations that occur blood vessels but also other internal cavities, are with pressure. In addition, osteocytes are capa- widely recognized as the sensory cells in this case. ble of producing proteins and “fi ller” substances Like the osteocytes, endothelial cells respond to that exist between cells; so they might also be fl uid fl ow with the release of NO and signalling able to adapt the porosity of the matrix around molecules. While it is surprising that such differ- them as part of the loading response. ent systems seem to use a similar sensory mecha- Osteocyte receptors attached to the extracel- nism, this concept explains local gains and loss- lular bone matrix are prime candidates for es in bone as well as the remodelling that occurs mechanosensing. In addition, each osteocyte (as after damage. well as all other cells) has its own dynamic skeletal structure aptly named the cytoskeleton that Quality, not Quantity not only provides scaffolding for the cell, but Several studies suggest that the rate of the also contains “trans-membrane” proteins, mechanical strain is more important to bone which do as their name implies. They cross the formation than the magnitude of the strain. cell membrane to link the internal structure of Low-magnitude (<10 microstrain), high-fre- the cell to the bone matrix outside, and any sen- quency (10–100 Hz) loading has been shown to sation of force is likely registered through the stimulate bone growth and inhibit disuse osteo- communication of these connector proteins. porosis. These data make sense from an evolu- The rapid release of NO by bone cells under tionary standpoint. Low-amplitude, high-fre- stress makes NO an interesting candidate for quency stimuli such as that experienced when

communication within the network of bone walking are fairly common in daily life, where- AMOLF / VUMC

44 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research FIGURE 3. OSTEOCYTE MORPHOLOGY. as occasions for high-amplitude, static strain currence of high-amplitude strains in daily life. Left: Isolated osteocytes in cul- such as lifting heavy objects are more rare. In Further understanding of how bone cells re- ture form fi ngers in all directions. studies of rats with osteoporosis, bone mass spond to stress might provide a deeper insight Right: Osteocytes embedded in and strength both increased after jumping exer- on how living bone is able to sustain itself with calcifi ed bone. Note the many cell cises, indicating a frequency-related response in the occurrence of meagre strains, whereas cul- fi ngers, radiating from the osteo- bone formation. tured bone cells are known to require greater cyte cell bodies. Scanning elec- In another study, in vivo measurements were strain. One explanation may be that sporadic tron microscopy pictures (original taken of the human hip bone while walking and release of a signalling molecule is adequate for magnifi cation: x1000). jogging, and the results showed that the strains sustained bone health. experienced during these activities have a rich frequency spectrum. This implies that despite You Can Feel It in Your Bones the minute level of strain, the speed of the im- Although stiff, bone can feel very small levels of pacts when walking or jogging could account strain, possibly even from fl exing the muscles. It for the quality of the exercise. Studies of bone makes sense to have healthy muscles along with cells showed that the increase in NO correlated healthy bones. One study has shown that whole linearly to the rate of fl uid shear stress in the body bone mineral content as an indicator of canaliculi. That is, bone cells respond better with bone strength correlated to lean body mass as an faster stimuli. This supports the notion that indicator of muscle strength. This implies that bone formation is stimulated by dynamic rather muscle activity can influence healthy bone than static stresses, and that low-magnitude, growth and that bone tissue can be highly sensi- high-frequency exercise may be as stimulating tive to minute strains. As stated earlier, bone pre- as high-amplitude, low-frequency exercise. In fers the quality of stress rather than its quantity. studies of bone cell responses to vibration stress Vibrating plates and ultrasound have also using a wide frequency range of 5–100 Hz, the been used to create low-magnitude, high-fre- release of NO correlated with the maximum ac- quency stress, and both methods seem to effec- celeration rate. This correlation may be related tively stimulate bone cells; however, how the to oscillations of the nucleus, which is more forces are transferred to the effector cells dense than the cytoplasm and more apt to be through soft tissue barriers of skin, muscle and “felt,” providing a physical basis for how cells so on is not straightforward. In fact, the stimu- sense high-frequency loading. lation of bone cells may not directly result from The corresponding reaction of bone cells to the transfer of force to the cells themselves. If fl uid shear stress provides a physical rather than the intervening soft tissue serves to further evolutionary explanation as to why adaptive weaken an already meagre source of stress,

VUMC / AMOLF / VUMC bone formation occurs despite the sporadic oc- there may be other processes or cellular com-

www.spacefl ight.esa.int 45 MECHANOTRANSDUCTION IN BONE faintest levels of stress, such as that caused by Load muscular vibration, ultrasound or vibratory motion, and characterized by low-magnitude high-frequency strain. Mechanical load Response to Cell Deformation The response of the bone cells is not only corre- Osteocytes Fluid fl ow in canaliculi lated with the magnitude and frequency of the mechanical load, it also depends on the deformation of the bone cell under stress, the cytoskeleton and trans-membrane proteins Mechanosensing by osteocytes Flow linking the cell to its surroundings and even other cells. Now, studies have shown that, when Bone bone is “squeezed” like a sponge under heavy Bone remodelling by osteoclasts / osteoblasts loads, those cells deformed by exposure to fl uid fl ow release more of certain signalling mole- FIGURE 4. SIGNAL DETECTION cules compared with cells deformed by the When a heavy load is placed on bone, the bone tissue squeezes like a sponge and fl uid fl ows stretching of bony surfaces to which they are through pores called canaliculi. This fl uid fl ow is essential for osteocytes to “sense” the attached. Cellular deformation caused by fl uid load or stress and respond appropriately. flow is fundamentally different from that induced by surface stretching. Fluid shear stress munication at work to compensate for the slight has a greater effect on the bone cells, while the force transfer. effect of surface strain is focused on the cell– Part of the answer may lie in the relationship surface attachments. ABOUT THE AUTHORS between bone cells and the fl uid fl ow that re- We have observed the process of cell deforma- Rommel G. Bacabac is currently a sults from stress. Research has indicated that tion in a simple laboratory model. In the body, postdoctoral fellow in the Nether- the release of NO and other molecules reaches osteocytes are attached to the extracellular ma- lands, at the Stichting voor Funda- its peak once a certain threshold of fl uid shear trix, and they sense a load or force once that ma- menteel Onderzoek der Materie In- stitute for Atomic and Molecular stress is achieved. Bone cells do not react to ev- trix starts to deform under the stress of the load. Physics (FOM-AMOLF). He received ery single movement or stress by stimulating At this point, the osteocytes adapt by pulling on the S.M. Perren Research Award bone growth; rather, they can sense the level the attachment sites. In the lab, we attached a (2006) of the European Society for when it becomes necessary. Bone cells are single cell fl oating in medium to small spheres of Biomechanics for his work on the re- “turned on” when a specifi c threshold of stress 2.5 microns on its opposite ends. By wiggling lation of cell shape and elasticity to how cells sense mechanical stresses. is reached, which allows these cells to respond one of the spheres or beads, the cell is made to appropriately to a “noisy” stimulus, such as that experience minute forces of less than a tenth of Jack J.W.A. van Loon founded the Dutch Experiment Support Centre experienced in repetitive exercise. Reacting the cell’s weight. However, the traction force in- (DESC), in Amsterdam, which sup- when the proper amount of noise is present is a duced by the osteocytes themselves is relatively ports gravity-related research. He phenomenon called “stochastic resonance.” Just high—more than twice their own weight—and received his PhD in bone biology as a person can recognize a familiar face in a they release NO simultaneously with increasing while working on several experi- crowd of strangers, bone can recognize the force application. We also observed that the cells ments on Shuttle, Bion and Soyuz. He was experiment coordinator for “correct” stimuli during “noisy” exercise. It is adapt to the traction force by changing their the Dutch Soyuz mission DELTA in possible that bone-building cells adapt to low morphology from a spherical to an elongated 2004. He is currently president of levels of stress by using the constant background shape. These experiments demonstrate a strong the European Low Gravity Re- “noise” of weak, but repetitive stress as an addi- link between the processes involved in force search Association, ELGRA. tional stimulus when the intensity of stress is be- sensing and force induction as well as the release Jenneke Klein-Nulend is profes- low the threshold necessary for a response. of signalling molecules by cells. sor of oral cell biology at the Den- If osteocytes can tune their response for sig- tal Faculty, Vrije Universiteit in nalling molecules like NO to the type of stress Use It or Lose It Amsterdam, where she holds a special chair of the Skeletal Tissue En- encountered, that ability would explain the ben- Findings that the rate of mechanical loading is gineering Group and is a member efi ts of dynamic stress to bone formation. Fur- more important than magnitude for bone health of the Directorate of the research thermore, the possible role of noisy or stochas- have powerful implications on the local activity institute MOVE. In addition she is tic stress in bone adaptation provides a partial of osteocytes in directing adaptation. Under Chair of the Dutch Society for Bio- explanation as to why and how indirect stress extreme conditions of unloading such as near- materials and Tissue Engineering and secretary of the Dutch Society helps build bone. These properties indicate a ca- weightlessness, it might be possible to counter-

for Calcium and Bone Metabolism. pacity for an enhanced response to even the act the onslaught of bone loss with sporadic AMOLF / VUMC

46 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research bouts of rapid-impact exercise, since the “qual- model, reduced fl uid pressure leads to reduced ity” of exercise as recognized by bone would fl uid fl ow through the canaliculi, whereas in- depend on the speed of the strain, despite the creased fl uid pressure would increase fl uid fl ow. minute amount. Currently, bone loss prevention This is compatible with bone loss in the former is based on either exercise or pharmaceutical situation and bone gain in the latter, which pro- applications, but the approach must eventually vides indirect evidence for a relationship be- target the bone cells. As discussed, bone loss tween fl uid pressure and bone balance. Current- results from a disturbance in stress levels, the ly no direct studies are underway to link these amount of signalling molecules released, and two properties, but it may be worthwhile to an- their roles for directing local bone remodelling. alyze a possible relationship, and manned space- Since these parameters are closely related, it fl ight studies could offer unique conditions to might be possible to restore their stability test this hypothesis. despite extreme conditions of unloading. Scientists have considered whether bone cells Conclusion are able to read the changes in the gravitational Tremendous progress has been made toward force directly or detect these changes indirectly understanding the function of osteocytes since via contact stresses. It seems that the light the fi rst experiments on bone loss in astronauts weight of cells undermines the role of gravity in were performed. Information has increased their behaviour. At the molecular level, gravity regarding their potential functions and response has only a small role because movements are de- to strain. Molecular mechanisms and pathways termined by diffusion (Brownian motion) or by involved in osteocyte mechanosensation have specifi c molecules that induce much higher forc- been identifi ed and expanded signifi cantly. It es compared with the weight of the molecules remains to be determined whether the loss of themselves. Thus, the question remains as to gravity affects the way these cells sense forces, ACKNOWLEDGEMENTS how osteocytes might detect near weightless- leading to bone loss during spaceflight. The The authors would like to acknowl- ness directly. future of scientifi c study in this area looks excit- edge the Netherlands Institute for Studies have also shown that the organisation ing with great potential for discoveries surround- Space Research (SRON), who sup- of the cytoskeleton is gravity dependent and that ing the osteocytes’ role in sensing stress and their ported the work of Jack van Loon this organisation may affect particle transport function under microgravity. New insights will (MG-057) and Mel Bacabac (MG- within the cell. Any reorganisation of cellular help to develop treatment strategies to prevent 055). Dr. Bacabac also received fi - components has implications on properties like dangerous bone loss in astronauts. ■ nancial assistance from ESA. elasticity and viscosity, which would affect how the cells sense forces. If gravity affects the relation between cytoskeletal organisation and intracellular transport, then the gain or loss of Ebb and Flow those forces—and corresponding impact on structure and transport—may enable cells to directly detect gravitational changes. Thus, it is n April 19, 2004, the Dutch Soyuz Mission “DELTA” possible that the signal for gravity fl uctuations O(Dutch Expedition for Life Science, Technology and At- comes from inside the osteocytes and these cel- mospheric Research) was launched on its way to the Inter- lular changes infl uence bone cell sensitivity. national Space Station. On it was the authors’ entry into biological experiments in microgravity. Called the “Flow” Apart from the obvious disuse effects of mi- experiment, it was designed to test whether the osteocytes’ crogravity, the effect of spacefl ight on fl uid dis- production of NO and other signalling molecules changes tribution in the human body may be another when in microgravity versus when on Earth. Cultures in consideration. Without gravity to pull blood fl ight were subjected to both microgravity and simulated and other fl uids to the lower part of the body, Earth gravity using a centrifuge inside the Kubik incubator. these fl uids move more freely to the torso and Control cultures on the ground were subjected to identical head. There are data suggesting that, in humans, culture environment and stress stimulations. Unfortunately, bone mineral loss during spacefl ight is unevenly hardware malfunction resulted in a lack of electronic power distributed throughout the body, being most to fl ight containers. Despite this setback, the preparations pronounced in the legs and pelvis. In the head for the experiment indicate that this setup is viable for a fu- only a small but signifi cant increase in bone ture fl ight opportunity. The experiment has been consid- mineral density has been observed. Thus, chang- ered as a potential candidate by the European Space Agen- es in bone mineral density seem to correlate cy for one of the next Soyuz missions.

ESA - S. CORVAJA 2004 with changes in interstitial ﬂ uid pressure. In this

www.spaceﬂ ight.esa.int 47 An OHB-Technology Company

THE ADVENTURE IS EXPLORATION WE MAKE IT WORK!

TECHNOLOGY FOR SPACE www.ohb-system.de OHB and Space Research Towards the Future

From the early 1980’s OHB has been actively and determinedly involved in the development of hardware for Europe’s research endeavours in life and physical sciences in space. This reflects the company’s recognition of the key importance of this research not only for its application to our Earth-based concerns in developing new industrial processes and materials and the better understanding of human health issues, but also with its importance for future human exploration of the Solar System.

Recognising the importance of the huge opportunities for research offered by the International Space Station, OHB has been involved in the preparation of this research through its involvement in six Shuttle missions, four MIR missions and the Eureca mission. OHB is the second largest German company involved in the construction of the International Space Station (ISS) and the only European partner involved in all of the scientific payloads for the Columbus module of the ISS Biolab, the European Physiology Modules, the Fluid Science Laboratory, the Materials Science Laboratory and the European Modular Cultivation System.

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Timing Gravity High-precision experiments performed in space will put some of Einstein’s theories to the test. By Stefano Vitale, Christophe Salomon and Wolfgang Ertmer

ravity is everywhere. It is the reason an apple falls to the ground and why galaxies G gather in clusters stretching across millions of light-years. It pervades the universe, despite being the weakest of the four fundamental forces. Thanks to Einstein and his theory of general relativity, our understanding of gravity has progressed enormously. Since the publication of his theory in 1916, a number of experiments and observations have conﬁ rmed many of its basic tenets.

AT A GLANCE

■ Gravity is a universal force that acts consistently on all bodies; Galileo proved two bodies fall to Earth at the same rate. All experiments in physics yield in free fall the same results as in the absence of gravity.

■ Atomic clocks use atomic resonances to measure time and can accurately test Einstein’s predictions. Gravity limits their accuracy because atoms “fall” past the sensor. On the ISS, the atoms “ﬂ oat” longer near the sensor for more precise measurement.

■ Einstein said that when matter accelerates it emits gravitational waves. Satellites in development will allow the detection of these ripples as they pass by, a gravitational snapshot of the universe.

■ What happens if future testing shows that Einstein’s theories are slightly off? Many theorists are hoping for that to get a hint on how to reconcile gravity and the quantum world, an unsolved fundamental question in our understanding of the universe.

—The Editors SPACE CHANNEL / PHIL SAUNDERS

50 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research www.spacefl ight.esa.int 51 However in some key situations, gravity re- Timely Measurements mains poorly understood, such as within the vi- In the late 16th century Galileo Galilei recog- olent environment of a black hole. Here, condi- nized that in a vacuum, objects of different tions are so extreme that nothing escapes the weights, such as a cannonball and a feather, fall black hole’s grasp except gravitational waves! toward the centre of the Earth accelerating at In addition, how gravity operates within the exactly the same rate of 9.81 m/sec2. So, in limits of quantum mechanics, something that practical terms, if you fall off a cliff during a must have occurred during the very early stages mountain hike one thing you wouldn’t have to of the universe, is still a mystery. worry about is losing your rucksack or camera One of the most diffi cult tasks for physicists because they will fall with you at exactly your today is measuring gravity with suffi cient preci- rate of descent (ignoring any effects of air resis- sion to understand how it works at these ex- tance). They will appear to “fl oat” around you treme boundaries of our knowledge. An upcom- as you plunge toward the ground, almost as if ing experiment onboard the International Space gravity doesn’t exist. Station and others on a series of sophisticated Known as the principle of equivalence, this space probes may provide some answers. condition is a fact of life on the ISS. Astronauts,

ABOUT THE AUTHORS

Stefano Vitale is currently in the Department of Physics at the Uni- versity of Trento in Trento, Italy. Christophe Salomon is currently at the Laboratoire Kastler Brossel of the École Normale Supérieure in Paris, France. Top: In space under microgravity conditions, an atom can be kept in an observable volume for up to tens of seconds. On the ISS, atoms can be sent slowly across a two-microwave ﬁ eld zone set to the frequency of the Wolfgang Ertmer is currently at quantum jump between energy levels. Bottom: On Earth, gravity takes over and after just one-tenth of a sec- the Leibniz Universität Hannover in ond the atom is falling at 1 m/sec, and it keeps accelerating downward.

Hannover, Germany. SPACE CHANNEL / PHIL SAUNDERS

52 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research nuts, bolts and bits of equipment all appear to their low acceleration in microgravity will allow fl oat as if gravity has been switched off. In real- observation times signifi cantly longer than is ity, everything is in free fall, even the space sta- possible on Earth. tion itself. As it falls, however, it moves forward ACES is expected to reach an accuracy of 1 fast enough that it never actually plummets part in 1016 (one followed by 16 zeros). In terms Earthward. It keeps falling past Earth as it or- of time, this is equivalent to a clock being off by bits the planet. one second every 200,000 million years! For many experiments the ability to elimi- nate gravity is a major advantage, and for many The Relativity of Time researchers, the ISS provides an excellent labo- One of the predictions of general relativity is ratory to study natural phenomena without the that gravity affects clocks and the measurement impediment of gravity. Take clocks, for in- of time. Actually it is not just gravity. Relativity stance: they are a key instrument in the study of also dictates that the speed of light—the dis- gravity. tance light travels per unit of time—is the same for all observers: 299,792,458 m/sec. It may not Atomic Flip-out be obvious, but this also affects the measure- Atomic clocks are the most precise clocks that ment of time. exist. They use resonance frequencies of atoms Suppose a beam of light travels from the fi nal in certain elements, usually caesium or rubidium, car to the engine of a rapidly moving train just to keep extremely accurate time. Today our def- as it passes through a station. To an observer on inition of a unit of time is based on exposing a the train, the light will travel precisely the length free-fl ying, undisturbed atom of caesium-133 to of the train in X amount of time. But to an ob- microwaves. Quantum mechanics tells us that server standing on the platform while the train when the energy of the microwave signal is equal passes by, that same light beam will travel the to the energy separating two quantum states of length of the train plus the distance travelled by the caesium atom, the atom fl ips from one state the train during the experiment. As the velocity to the other. One second of time is defi ned as of light is the same for both observers, the clock 9,192,631,770 times the period of oscillation of of the person on the platform must measure a the microwave signal that makes a caesium atom time longer than X. The time between events, fl ip between two of its quantum states. the emission and collection of the light beam, is However, this applies only to undisturbed at- different for observers who see each other mov- oms, which is a fundamental limitation. The ing at velocities greater than zero. problem is that when an atom is released on Just as velocity affects clocks, so does grav- Earth, gravity takes over. The atom acquires a ity. Compare for instance a clock on the top of speed of one metre per second in just one-tenth a tower with an identical one placed at its bot- of a second, and it accelerates downward with tom. Each time the bottom clock ticks, it also every passing moment. So the best a physicist sends out a fl ash of light, so that while sitting at can do is to make a brief observation as the atom the tower top, you can compare the ticks of the zips past a detector. To obtain more accurate bottom clock, as marked by the fl ashes, with measurements, the atom needs to be observed for those of the clock at the top. To avoid being con- the longest possible time. fused by the effects of gravity, imagine the ob- In the microgravity environment of space, an server is free falling, which cancels out the ef- atom can be kept in an observable volume for fects of gravity. While the observer accelerates up to tens of seconds. On the ISS, atoms can be downward, he sees the tower and the clocks ac- slowly launched in free fl ight across a two-mi- celerating upward. With the tower top moving crowave-fi eld zone tuned to the “magic” quan- further away with constantly increasing speed, tum-fl ip frequency—the frequency of the quan- the fl ashing light that is travelling at constant tum jump between the energy levels of the at- velocity will have to cover a constantly increas- oms being used for the clock—and observed for ing distance. The fl ashes will thus be received at many seconds. longer time intervals than those with which they This is the feature that will be exploited by have been emitted. The bottom clock appears the Atomic Clock Ensemble in Space (ACES), to to run slower than the top one. be located on the external platform of the Co- This may seem like a theoretical exercise, but lumbus module on the ISS in 2012. Laser beams it is something that cannot be ignored even in will cool caesium atoms; the slow atoms and daily life. If you navigate using a global posi-

www.spacefl ight.esa.int 53 If a beam of light travels through a fast-moving train as it speeds through a station, the time for the light to travel from one end of the train to the other will be different for an observer on the train versus one standing on the platform. To the person on the train, the light travels the length of the train only in X time. But to the person on the platform, that same light travels the length of the train plus the distance travelled by the train. tioning system (GPS), and the software doesn’t cancellation is not a good thing. To make mat- correct for gravity and the satellite’s velocities, ters worse, it turns out that free fall is unavoid- your GPS will locate you many kilometres from able. This not a paradox. For example, we all your actual position. fall together, with the Earth, around the sun. The ticking of clocks in different gravitation- Thus on Earth, the gravity of the sun (and any al fi elds is so rooted in our understanding of the other celestial body) is turned off and so cannot principle of equivalence and the general theory be measured. of relativity that it has been tested repeatedly However, the cancellation of gravity in free with ever-increasing precision. The most accu- fall is not perfect. For instance the sun’s gravity rate measurements to date have been obtained pulls everything toward its centre. Thus falling by NASA’s Gravity Probe A. A hydrogen maser particles will converge toward that point and (microwave laser) atomic clock was launched on On our planet, these their paths will not be parallel. Also, objects a two-hour suborbital rocket fl ight to an altitude slight differences close to the sun feel a more intense gravitation- of 10,000 kilometres. The time of the onboard squeeze and pull al pull than do objects farther away, which clock was compared with two identical maser causes greater acceleration for the closer objects. clocks on the ground. Einstein’s theory was con- Earth’s oceans, The result is a compromise: most of the sun’s fi rmed to an accuracy of 70 parts per million. resulting in the gravity is cancelled by free fall, but small, local- ACES, with its more precise clock, is de- small relative ized differences remain. On our planet, these signed to improve Gravity Probe A’s test of the slight differences squeeze and pull Earth’s theory of relativity by as much as a factor of 30, accelerations that oceans, resulting in the small relative accelera- thereby providing a new challenge to Einstein’s we know as tides— tions that we know as tides—the only measur- theories. the only measurable able effect of the sun’s gravity on Earth. Thus, in free fall gravity causes relative accel- Relative Gravity effect of the sun’s eration of nearby free-falling particles, which, in Running experiments in microgravity can be gravity on Earth. its absence, would otherwise continue moving at useful. Free fall “turns off” gravity and actual- constant relative velocity. It was Einstein in his ly improves the operation of some instruments. general theory of relativity who pointed out that

But for scientists studying gravity itself, this this acceleration is the basic effect of gravity. SPACE CHANNEL / PHIL SAUNDERS

54 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Imagine two of these free-falling particles and is similar to what happens when a particle is plot their separation as a function of time. The moving on the curved surface of a sphere. The result is a graph of space–time: one axis is a space shortest path between two points is not a coordinate, the other is time. If the particles are straight line but an arc of a circle with same moving at a constant velocity, then the distance radius as the sphere. the particles travel is proportional to the time elapsed, and the plot is just a straight line. In Ripples in Space–Time ordinary, ﬂ at space, a straight line is the shortest While Einstein’s theory of relativity is our best path between two points and is called a geodesic. model for explaining gravity, some of its predic- In four-dimensional space –time (where there are tions are mind-boggling. For instance it pre- three directional coordinates plus time), things dicts that when matter accelerates, it causes rip- are a bit more complicated, but still, in the ples of curvature in space–time. These space– absence of gravity, a straight line is a geodesic. So time ripples move at the speed of light and are in the absence of gravity, particles follow geode- known as gravitational waves. sics in space–time. However, if gravity acceler- Intense gravitational waves were likely gen- ates one particle relative to the other, the rate of erated during the Big Bang, but there are other their separation change varies with time, and the candidate sources, for example, two black holes plot bends away from a straight line. orbiting each other. Such pairs are expected to In the theory of relativity, Einstein showed be quite common as there is strong evidence that that particles even in this case still follow geo- almost every galaxy carries, at its centre, a gi- desics. Because gravity curves space–time, how- gantic black hole up to billions of times the mass ever, geodesics are no longer straight lines. This of the sun. And galaxies often collide as they

Some evidence strongly suggests that every galaxy has a black hole at its centre that is up to billions of times the mass of the sun. Because galaxies often collide, their core black holes could form a binary system. If two black holes do merge, a powerful burst of gravitational waves would occur. SPACE CHANNEL / PHIL SAUNDERS

www.spaceﬂ ight.esa.int 55 In one upcoming mission, astronomers hope to detect gravitational waves created during the birth of the universe. Called LISA (Laser Interferometer Space Antenna), the NASA-ESA project will use three pairs of platinum and gold particles set in a triangle. These particle pairs can feel tiny oscillating accelerations relative to one another if they are hit with these waves.

wander through the universe, which gives their oscillation of the relative acceleration of these core black holes the opportunity to form a bi- particles in the range of one billionth of a bil- nary system. If two black holes eventually lionth of the acceleration caused by the Earth’s merge, the result will be a powerful burst of gravity. gravitational waves. When LISA is operational (perhaps as early Gravity waves are not just theoretical con- as 2019), it will open a new observational win- structs. In 1974 a binary pulsar was discovered, dow on the universe. Astronomers hope to de- in which the two stars are slowly spiralling in to- tect gravitational waves generated by the earli- ward each other. This will happen if the system est moments of the birth of the universe. Signals loses energy by emitting gravitational waves. It from the supermassive black-hole binaries scat- turns out that the stellar pair’s inward spiral is tered across the universe will help scientists occurring at exactly the rate predicted by gener- probe the large-scale structure of the cosmos. al relativity. So astronomers have detected a sig- The death of small objects spiralling into a black nature result of gravitational waves but not the hole will generate gravitational waves that waves themselves. That may soon change. should provide a map of the gravitational fi eld In the upcoming ESA–NASA mission called surrounding the hole. In short, astronomers LISA (Laser Interferometer Space Antenna), will fi nally get a close look at how gravity be- three pairs of gold–platinum particles will be haves in extreme conditions. set at the apexes of a triangle fi ve million kilometres on a side. These pairs of free-falling All Things Considered Equal particles, when hit by a gravitational wave, The principle of equivalence is such a central pil- will feel a tiny oscillating acceleration (relative lar of our understanding of how the universe to each other). A very precise motion sensor (a behaves that it is legitimate to ask the limits of

laser interferometer) will be able to detect an its validity. On Earth it has been demonstrated SPACE CHANNEL / PHIL SAUNDERS

56 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research that this principle is obeyed to at least the 13th temperature and standard pressure can be a few decimal point. In other words, the measured hundred metres per second). In microgravity, accelerations from all the physical systems that this speed limit can be pushed even lower, per- have been studied fall within this very small haps to less than 0.001 mm /sec. This is the speed margin of error. However, this is about the lim- at which atoms move when the temperature is it that can be reached in a standard laboratory less than one hundred billionth of a degree above on Earth because of the constraints of gravity. absolute zero (approximately -273º C). So the French Space Agency, in collaboration Various research groups are actively develop- with ESA, is developing a satellite called Micro- ing space-borne, cold-atom interferometers. scope to test how well the principle of equiva- ESA supports some of these studies through its lence is obeyed in space—with an accuracy 100 ELIPS research program. Prototypes have al- times greater than what can be accomplished on ready been tested in free fall, albeit for the short Earth. Two pairs of solid test-masses made from duration achievable on a drop tower in Bremen, different materials will free fall in Earth’s orbit, Germany (a facility where an experimental cap- and their acceleration will be measured with an sule is dropped from a great height in order to accuracy of one part in one thousand billion. achieve a few seconds of free-fall). When these Can the study be pushed even further? How experiments fi nally arrive at the ISS or some well might the principle of equivalence hold up other space-borne platform, the principle of for small quantum particles and not just for or- equivalence will meet a new challenge. dinary, solid matter? Can a test comparing the rate of fall of just a few isolated atoms be per- What a Drag formed? One complication is that the ISS does not actually provide a perfect, gravity-free environment. Chilled Out Mundane disturbances, like the residual drag of According to quantum mechanics, matter can Earth’s upper atmosphere on the ISS, will accel- behave like particles or waves. As a wave, light erate the station out of its trajectory. A perfect can pass through a grating or slit in an optical zero-gravity situation can be achieved only if no How well might the device and create interference patterns. Similar- other force acts on the laboratory itself. Admit- principle of ly, an atom behaving as a wave can create inter- tedly, it is not necessary for these external forc- ference patterns. In an atom interferometer the es to be exactly zero. If they can be measured, it equivalence hold up gratings and the slits are created by cleverly com- might be possible to compensate for them. for small quantum bined laser beams that guide the atoms along a This is the idea behind the so-called “drag- particles and not determined path. free” technique that forms the basis of the ideal For an atom-wave, its wavelength is directly gravitational lab. It consists of a small but heavy just for ordinary, related to its velocity. Any acceleration of the test mass fl oating freely inside a spacecraft. A solid matter? Can a atom will alter its wavelength and cause changes gravitationally quiet environment is created in test comparing the in the interference patterns, making an interfer- the neighbourhood of this test mass so that it ometer ideal to detect tiny acceleration differ- falls as freely as possible under the infl uence of rate of fall of just a ences between free-falling atoms. So in theory a gravity alone. A position sensor—a laser inter- few isolated atoms quantum-level test of the principle of equiva- ferometer or a proximity sensor—measures the be performed? lence is possible, but can it be carried out? position of the test mass relative to the space- Interferometers for atoms are not easy instru- craft without making physical contact. If non- ments to work with. Because the atom’s wave- gravitational forces such as air drag or solar-ra- length is inversely proportional to its velocity, diation pressure act on the spacecraft, the test and because the larger the wavelength the more mass will appear to accelerate. To compensate, precise the measurements can be, the slower the the spacecraft’s engines are activated to adjust atom moves through the interferometer, the bet- its fl ight path to follow the test mass. As a result ter. The challenge is to create an environment the spacecraft tracks the pure free-fall trajecto- conducive to very slow-moving atoms, and that ry of the test mass and counteracts any external turns out to be a cold environment. disturbances. This is achieved with laser cooling, a tech- Trying to gently adjust something as massive nique that uses laser light as a braking mecha- as the ISS is pretty much an impossible task. nism to slow the velocity of the atoms to as low Thus, any experiment, such as one testing the as a few millimetres per second. (In comparison, principle of equivalence or searching for gravita- the random velocities of atoms in a gas at room tional waves, that needs a perfect free-fall envi-

www.spacefl ight.esa.int 57 ronment will require a special drag-free satellite. Challenging Einstein In the case of Microscope, the spacecraft will use What happens if the results of any of these tests tiny ionic microthrusters, which adjust the craft disagree with principles of the theory of relativ- via the recoil from a jet of metal ions forced from ity or, at a more fundamental level, with the an onboard liquid reservoir. The force required principle of equivalence? If that is the case, phys- to compensate for the atmospheric drag present icists will have to consider other theories or new in low Earth orbit is miniscule—on the order of fundamental interactions. 10 millinewtons. This is less than the force need- Theorists are not waiting for experimental ed to lift an ordinary sheet of paper. results. Instead, they are continuing to try to LISA’s requirements are even more demand- model the behaviour of gravity in extreme con- ing: an environment for its free-falling particles ditions where quantum physics comes into play. that is almost one thousand times more strin- Some of these models predict extremely weak gent than for Microscope. A disturbance equal interactions between particles. If this is the case, to the force necessary to lift a small bacterium precise measurements, such as those about to be will disrupt LISA’s measurements! Although LI- attempted, may reveal tiny deviations from the SA’s interplanetary environment will be much principle of equivalence and the other basic quieter than Microscope’s (because LISA will principles of general relativity. Such deviations be situated farther from Earth), it is valid to ask would give physicists their fi rst real indication if LISA’s particle pairs can really be set in mo- of gravity’s behaviour at the quantum level, tion along pure space-time geodesic tracks with which dominated the early stages of the Big such minimal relative acceleration from outside Bang and a concept that Einstein was unable to forces. Such a question can only be answered by come to terms with. The general theory of rela- a test, and unfortunately the experiment can be tivity has stood the test of time remarkably well. performed only in the microgravity of space. But perhaps now we can fi nally begin to probe This diffi cult task belongs to LISA Pathfi nder. its limits. ■

Probing Gravity

Three European projects designed to probe gravity are already underway. LISA Pathﬁ nder will pave the way for a major ESA/ NASA mission planned for the near future: LISA (Laser Interfer- ometer Space Antenna), whose goal is detecting gravitational waves generated by massive objects such as black holes. It consists of two test-masses in a nearly perfect gravitational free-fall, and of controlling and measuring their motion with unprecedented accuracy. Such technologies are essential not only for LISA; they also lie at the heart of any future space-based test of Einstein's theory of general relativity. ACES (Atomic Clock Ensemble in Space) will bring a new generation of atomic clocks in the microgravity environment of the ISS. The ACES payload will distribute a stable and accurate time base that will be used for space-to-ground as well as ground-to-ground clock comparisons. The direct comparison of ultra-precise atomic clocks is crucial for the exploitation of ACES potential in many different areas of research. MICROSCOPE (MICRO Satellite à traînée Compensée pour l'Observation du Principe d'Equivalence) is a part of the microsatellite line of CNES (Centre National d'Études Spa- tiales, France). The objective of the project is to test the principle of equivalence with an accuracy of 10-15. ESA

58 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research

PHYSICS FUNDAMENTALS

Moving in Flow-Motion

60 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research A malfunctioning fuel pump and a harrowing near- disaster at Heathrow Airport illustrates the necessity of understanding ﬂ uid ﬂ ow dynamics. Removing gravity from this complex equation allows for new and exciting research in this area. May the capillary force be with you!

By Michael Dreyer, Ilia Roisman, Cameron Tropea and Bernhard Weingartner

ust after midday on the 17th January, 2008, a British Air- ways Boeing 777 made its fi nal approach to London’s AT A GLANCE Heathrow Airport. At 220 metres in elevation and 3.2 kilo- J ■ metres from touchdown, the autothrottle demanded increased Understanding how fl uids fl ow is vital to drainage, thrust from both engines. Nothing happened. The pilots took irrigation and other key over but could do nothing. After skimming across the roofs of systems used everyday; nearby houses, the aircraft thumped into the grass border some and science hopes to use 300 metres short of the runway. The 152 passengers and crew left weightlessness to unravel the plane via the emergency exits, terrifi ed and bewildered by physics’ complex secrets. what had happened. ■ Without gravity to inter- And what did happen? In May, the Air Accidents Investigation fere, surface tension and Branch concluded, “. . . that both engines had low fuel pressure at capillary forces can be the inlet to the HP [high-pressure] pump. Restrictions in the fuel better studied, controlled system between the aircraft fuel tanks and each of the engine HP and measured in small pumps, resulting in reduced fuel fl ows . . . work has commenced volumes. on developing a more complete understanding of the dynamics of ■ Fluid sprays can also be the fuel as it fl ows from the fuel tank to the engine.” better studied without Understanding how fl uids work really does matter. It touches gravity to affect their mo- our lives in ways that we do not realise: from the beauty of grav- tion; results can be applied ity-defying water droplets clinging to buds and twigs on a damp to cooling systems in elec- spring morning to fl uid fl ow choking in a high-pressure pump in- tronics and jet engines. side an aircraft engine. Although we have learned a great deal ■ Liquid columns or bridges about fl uid fl ows during many years of patient research, much of suspended in space help the science still remains mysterious. Venturing into space to dis- us to study capillary fl ows entangle gravity from the equation opens up exciting new ave- in varying temperatures, nues of research and provides greater insights into this fascinat- which is important to crys- ing subject. tal growth in electronics.

—The Editors SPACE CHANNEL / PHIL SAUNDERS

www.spacefl ight.esa.int 61 The Basics of a Strange Brew engineering technology. Spacecraft components Fluids are strange. A liquid fi lls a glass above its such as propellant tanks, heat-transfer units rim but does not overfl ow. An insect scuttles and life-support systems contain liquids, and across the surface of a pond as if walking on sol- these liquids must be moved around. In micro- id concrete. How is this possible? gravity the effects of natural convection, sedi- Every molecule in a liquid is pulled equally mentation, buoyancy and hydrostatic pressure in all directions, resulting in a net force of zero. are signifi cantly altered and cannot be used to At the liquid’s surface, however, a molecule is position, pump, separate, stratify or destratify attracted more by the interior fl uid than by the fl uids—with or without free surfaces. outside environment. But the force that pulls Fortunately, in the absence of gravity capil- the molecule toward the liquid’s interior is bal- lary forces can be much more dominant than anced by the resistance of the fl uid to compres- on Earth. These forces can be used to pump and sion, which causes the surface of a liquid to be- position liquids without the necessity of mov- have like a stretched elastic membrane. This ef- ing parts or hydrostatic pressure. For example, fect is called surface tension (attraction), which capillary channels (liquid-acquisition devices) in turn gives rise to capillary forces (adhesion). are widely used in propellant-management sys- These forces can be stronger than gravity; this tems to position and pump liquid toward exter- is interfacial fl uid fl ow in action. The impor- nal thrusters. tance of these forces increases dramatically in The advantage of this technique is that the fl ows over short scales (length or distance cov- inlet may be anywhere along the axis of the ered), but at smaller and smaller scales the infl ow channel. As long as the bulk of the liquid is vestigation of such fl ows becomes increasingly connected to the channel, liquid can be with- diffi cult to control or measure. drawn. Additional devices, such as refi llable res- This situation changes in microgravity, and ervoirs, ensure that bubble-free liquid is provided even in fl ows over long scales, the infl uence of to the spacecraft’s thrusters. An important pre- capillary forces can remain signifi cant. This is requisite to designing and using such devices is a why microgravity represents a powerful tool for fundamental understanding of how to model scientifi c research in this fi eld. And what better structures that contain liquids with partially free place to start than with spacecrafts themselves, surfaces for propellant management in space. since in the absence of gravity they are affl icted A simple containment structure is called a by numerous fl uid fl ow problems. parallel-plate capillary channel (see Figure 1). Two liquid menisci (the curved surface of a To Boldly Flow standing liquid) are formed between parallel All spacecrafts, whether simple meteorologic plates. External forces, such as a pressure drop satellites or complex space stations, are depen- created by a pump, drive the flow of liquid dent on and limited by fl uid fl ow physics and through the channel. Due to pressure gradients

Capillary forces allow this water droplet to defy gravity. SPACE CHANNEL / PHIL SAUNDERS

62 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Outlet

Inlet

Free surface

FIGURE 1. STABLE AND UNSTABLE FLOWS THROUGH PARALLEL-PLATE CHANNELS 1a. Above is an illustration of a stable fl ow through a parallel-plate capillary channel where the fl uid enters the system at the inlet and leaves at the outlet. The curvature of the free surfaces at the sides of the channel changes in the direction of the fl ow.

1b. On the left are two cases of ﬂ ow through a parallel-plate channel like that in 1a, but the view is from the top looking down on the channel. The liquid ﬂ ows from the bottom to the top and the free surfaces can be seen along the left and right sides of each image.

In the top row, the increasing ﬂ ow rate also increases the constriction of the free surface, but a steady ﬂ ow through the outlet is maintained.

In the bottom row, as the ﬂ ow rate increases, the pressure of the free surface along the outside can no longer be balanced by the internal pressure. The free surface collapses and gas ingestion occurs. This leads to unsteady two-phase ﬂ ow at the outlet.

along the channel’s axis, the curvature of the conducted in the low-gravity environment of a free surface changes in the direction of the fl ow. drop tower and non-orbiting, short-duration At a certain fl ow rate, surface pressure can no sounding rockets. A silicon fl uid was used with longer be balanced by internal pressure, and the a fl ow rate that varied between 4 and 8.4 ml/sec. surfaces collapse. For reasons not yet fully un- Videos were shot during the fl ight of the Tex- derstood, this stability limit depends on the ge- us-37 sounding rocket, which provided six min- ometry of the channel and the properties of the utes of microgravity time. liquid. So a key goal of ongoing research is to When fl ow rates are below a critical value, fi nd and explain these limits. the free surface remains stable and its curvature increases with increasing fl ow rate. If the steady- Choking on Fluid Flows fl ow velocity at the throat of the fl ow path is in- To understand the interaction of fl ow with a free creased above a critical value, ingestion of gas

SPACE CHANNEL / PHIL SAUNDERSgure 1A), gures ZARM (fi 1B) (fi surface, several parallel-plate experiments were occurs and the fl ow rate of the liquid is restrict-

www.spacefl ight.esa.int 63 ed. This phenomenon is called choking and sim- rigid wall, a thin, fl uctuating liquid fl ow is cre- ABOUT THE AUTHORS ilar effects can be seen elsewhere. For example, ated on its surface. This flow includes jets, when the velocity of a compressible gas fl ow sheets, crowns and capillary waves. In many Michael Dreyer received his doc- reaches the speed of sound in the throat of a cases the sheets are unstable and break up into torate in engineering in 1993 at the nozzle, the fl ow rate cannot be increased and secondary drops. Centre for Applied Space Technolo- gy and Microgravity at the Univer- choking occurs. Any study of these fl uid fl ows becomes even sity of Bremen and has led their During the fl ight of sounding rocket Texus- more diffi cult when spray strikes a very hot sur- multiphase fl ow group since then. 41, surface oscillations were observed that face. For example, what happens when fuel is He is chairman of a French/German helped to explain the effects of transient chang- sprayed onto a piston in an internal combustion project on the behaviour of propel- es on liquid fl ow rate. In an unsteady fl ow situ- engine? If the piston’s surface temperature lants in launcher tanks. His research is mainly funded by the Ger- ation, effects such as liquid acceleration and os- reaches a certain value higher than the boiling man Aerospace Centre DLR and cillation occur and result in changes in veloci- point of the liquid called the Leidenfrost point, ESA. He is the principal investigator ties, as well as a decrease in the maximum fl ow the liquid doesn’t actually touch the surface. In- on the NASA/DLR experiment CCF. rate compared with the steady fl ow rate. The stead, it evaporates beforehand, creating a thin Dr. Dreyer is the editor-in-chief of methods for predicting the free-surface shape vapour layer or wall between the piston and the Microgravity Science and Technolo- gy. He has published 2 books and and the critical fl ow rate were validated by these liquid. This process leads to the explosive break co-authored 53 journal articles. tests and can now be used to design liquid-ac- up of the liquid and the creation of secondary Ilia Roisman received his doctor- quisition devices for propellant management in drops (see Figure 2), a phenomenon quite com- ate in mechanical engineering in space or other applications. Based on these re- mon when cooking. If water is sprinkled onto a 1998 at the Technion-Israeli Insti- sults, the theory of choking has been modifi ed. skillet whose temperature is high enough, the tute of Technology in Haifa. He is On the other hand, choking does not neces- water breaks up into droplets that skitter across currently Privat Dozent at the chair sarily cause surface collapse. When the liquid the pan’s surface. of Fluid Mechanics and Aerody- namics at the Technische Univer- velocity reaches the limiting value, a supercriti- The thickness of the vapour wall created by sität Darmstadt and coordinator of cal fl ow regime is achieved. In other words, the spray impact is roughly 100 micrometers, and the research area for near-wall fl ow velocity is greater than the wave velocity the typical duration of the event is about 10 mi- multiphase fl ows at the Centre of and an unsteady, supercritical stable fl ow is croseconds, all of which makes the experimen- Smart Interfaces. He has coau- temporarily possible. Thus the theory defi nes tal examination of such fl ows rather demand- thored 1 book and about 20 journal articles. three fl ow regimes: subcritical fl ow, stable su- ing. However, in a microgravity environment percritical fl ow, and unstable fl ow. The super- these length and time scales can be expanded Cameron Tropea completed his critical stability limit is given by the dynamic in- without altering the underlying physics. This doctorate in engineering at the Technische Hochschule Karlsruhe dex, which is defi ned by the pressure interaction permits the investigation of gravity’s effect on in 1983. He is currently chair of Flu- and reaches a value of one for unstable fl ow. numerous outcomes including spray impact, id Mechanics and Aerodynamics at Capillary fl ows in different channels and the wall-fi lm thickness, the dynamics of the the Technische Universität Darm- with different fl ow modes will be studied on- splash and the heat transfer associated with the stadt, coordinator of a research board the International Space Station in the Mi- spray impact. training group on optical techniques in interfacial transport pro- crogravity Science Glovebox (MSG) with the A single drop hitting a wall creates a relative- cesses and is director of the Centre Capillary Channel Flow (CCF) experiment be- ly fast, radially expanding fl ow, which interacts of Smart Interfaces. Prof. Tropea is ginning in 2010. This experiment is a coopera- with the stationary liquid fi lm around the point editor-in-chief of Experiments in tive venture between NASA and the German of impact—think of raindrops hitting a puddle. Fluids and has served on several aerospace centre (the DLR). Upon impact, liquid in the puddle rises in a sheet ESA advisory boards. He has co-authored 6 books and about 100 jour- surrounding the drop’s entrance, forming what nal publications. A Not-So-Simple Shower looks like a raised crater. The motion around its Bernhard Weingartner received When a spray of water in your shower hits the edges is governed by the surface tension of the his degree in theoretical physics at wall, the result is something called an inertia- liquid. A liquid rim forms on top of this raised the Technical University of Vienna driven fl ow. It may look simple, but it’s not. For crater, almost like the lip of a cup. In fact the ve- in Austria where he is working on example, the spray impact also involves the locity of the rising liquid sheet is faster than that his PhD at the Institute of Fluid transfer of heat from the spray to the wall. This of the rim on its upper edge; the rim continues to Mechanics and Heat Transfer. With support from the Austrian Space exchange has a wide range of important appli- grow with the fl ow of liquid entering the rim Agency, future investigations are cations from cooling high-power electronics on from the sheet. If the rim lasts long enough be- planned on the sensitivity to the Earth to the design of spacecrafts, which also fore it collapses back onto the surface, then it be- ambient gas-phase conditions on have to work in high-temperature extremes. comes unstable and forms fi nger-like jets around thermocapillary fl ow. A corre- Because the hydrodynamics of inertia-driven its perimeter. When these become unstable, they sponding study for the Japanese module Kibo will be a collabora- fl ows on rigid walls is not completely under- pinch off into droplets. This is the physics be- tion among European and Japa- stood, the infl uence of gravity on the outcome hind what you see when pouring a glass of milk, nese scientists. of spray impact is not clear. When spray hits a and that last drop hits the liquid surface with a

64 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research splash resembling a crown. Those droplets studying these fl ows in a microgravity environ- pinched off the crown’s points are the secondary ment, it will be possible to glean a greater un- spray we know as splashing. A single drop hitting derstanding of the fundamental physics behind Now multiple drops hitting a puddle, as in a a wall creates a spray-fl ow patterns and droplet depositions on rainstorm, can complicate this ideal picture. In- a surface. dividual drop impacts may appear as described relatively fast, above, but if the drops land close to one anoth- radially expanding Bridges over Troubled Waters er or in quick succession, then the symmetry of In an experiment conducted on the ISS in 2003, the interaction is broken and a liquid sheet may fl ow, which interacts a thick fi lm of pure water was produced by sim- rise up between the two points of impact. In any with the stationary ply immersing a wire loop several centimetres case, these liquid sheets are less stable and may in diameter into a beaker of water. This liquid collapse earlier, often breaking up into jets and liquid fi lm around fi lm turned out to be remarkably resistant to later secondary drops—or a lot of splashing. the point of surface deformations. It seemed like a straight- Under microgravity conditions, the lifetime forward discovery, but then something unex- of the uprising liquid sheets may be much lon- impact—think pected occurred. ger because gravity no longer pulls them down of raindrops Injecting tiny fl akes of mica into the pure wa- into the fi lm’s surface. In such cases, the entire ter allows any hidden fl ows and swirls to be- liquid rim may detach from the surface fi lm hitting a puddle. come visible. The ISS astronaut conducting the and be suspended before it eventually breaks experiment observed that all motion within the up into individual drops around its circumfer- fi lm appeared to cease a few minutes after the ence. So, although there are an infi nite number experiment began. So he turned on a fl ashlight of scenarios of splash when a spray impacts a to have a better look. Suddenly the fl uid started wall, they are composed of a few number of to move again—almost as if someone was stir- basic interactions that can be well described ring it with an invisible spoon. and predicted. The explanation for this surprising effect, All this seems pretty esoteric, but inertia- known as thermocapillary fl ow, is not compli- driven fl ows have applications on Earth ranging cated. The focused beam of the fl ashlight heats from microelectronics to aircraft jet engines. By the water fi lm, thereby slightly lowering the sur-

FIGURE 2. TOO HOT TO HANDLE The black-and-white series shows a drop impact onto a hot surface. The impact leads to a splash and the creation of a thin evaporating liquid fi lm. When the surface temperature is high enough, a thin vapour layer forms between the surface and the liquid, which causes the drops to accelerate across the surface. The color series shows how grooves on a surface can increase the effectiveness of spray cooling. Here, an infrared camera captures a single drop as it hits a moist, grooved and heated surface in microgravity. The drop is colder than the liquid fi lm it creates on impact, and the cold water from the impacted drop collects in the grooves. The liquid fi lm is ruptured at the point where it meets the grooves due to the drop impact. ZARM

www.spacefl ight.esa.int 65 face tension on the spot where the light shines. a return fl ow in the interior of the liquid bridge. The resulting imbalance of surface tension Increasing the temperature difference above a drives a fl ow from the hot to the cold area and Experiments in certain threshold gives rise to fl ow patterns that back again, resulting in the appearance of a vary with time. Adding tracer particles to the “stirring” motion. Similar fl ows can also arise microgravity offer liquid reveals the fl ow. from variations in chemical concentrations or the opportunity to Understanding liquid bridges has a very the presence of an electric fi eld but experimen- practical application in the electronics industry tally, it is more revealing to generate a tempera- learn more about because at the heart of all electronics is a pure, ture variation. This ISS experiment could not thermocapillary single silicon crystal. It is produced by some- have been done on Earth as the fi lm would have thing called a fl oating-zone crystal-growth pro- immediately drained and collapsed. Under grav- effects, free from cess. The purity of the crystal depends on the ity only thin water fi lms are possible, and they the infl uence of quality of the fl ow of melted silicon, and liquid- have to be stabilized by adding soap. bridge science is the model for understanding A good test for the systematic investigation buoyancy. how this all works. of this thermocapillary fl ow involves the creation of a liquid bridge between the end walls of The Hidden Organisation two cylindrical rods. Setting the rods at differ- In Earth’s gravitational fi eld, water fl ows down- ent temperatures leads to a temperature varia- hill, masking deeper dimensions inherent in the tion in the liquid, which results in a difference fl uid itself. In a gravity-free environment, what in surface tension along the outer surface of the we see is the hidden organisation of fluid liquid cylinder. dynamics that is a basic part of fundamental When the temperature difference is small, a physics. steady vortex ring is established in which the ro- For example, under certain conditions in a tating fl uid takes on a toroid or doughnut shape. liquid-bridge experiment, tracer particles ac- This ring consists of surface fl ow moving from cumulate and form a three-dimensional spiral the hot to the cold side of the outer surface and string. Viewing it through the transparent top

FIGURE 3. Flow Dynamics

Particle accumulation seen in a liquid bridge experiment. The top view (a) shows particles forming a three-blade “windmill.” That this windmill rotates is an illusion. The particles follow the ﬂ ow, which is shaped like a doughnut, and the entire 3-D pattern rotates. The side view (b) illustrates the 3-D structure of the spiral particle string.

A C

(c) Top view of the fl uid-fl ow pattern on the free surface of an open circular container heated from below and cooled from above. In this case, the fl ow pattern resembles that seen in biological cells. The experiment was conducted in microgravity during the fl ight of MAXUS-2 sounding rocket. B ZARM

66 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research rod (a sapphire crystal), the particle accumula- serve small-scale thermocapillary effects on tion structure (PAS) looks like a “windmill,” Earth. For example, this is why the oil in a hot with three blades rotating at a constant speed. frying pan always moves toward the cold rim But the rotation is an illusion because the par- and why a thin layer of wine can climb the side ticles follow the fl ow, which is essentially toroi- of a wine glass, accumulate at a certain height, dal. In fact it is the three-dimensional pattern form droplets, and subsequently flow back that rotates, not the liquid, and the individual down the glass as “tears of wine.” particles organize themselves into what ap- Thermocapillary fl ows also arise in an as- pears to be a rotating wave (see Figure 3a). This sortment of important space applications. As separation from an initially homogeneous dis- an example, heat pipes are important for cool- tribution of particles can be traced back to par- ing electronic devices in microgravity. At the ticle-free surface collisions, but as yet it is not hot end of the pipe, liquid evaporates and turns fully understood. to vapour. The vapour then fl ows to the cold Another interesting example of fl uid fl ow end of the pipe, where it condenses. The re- self-organization appeared in the following plenishment of liquid at the hot end is accom- simple experiment. A liquid is poured into an plished by the thermocapillary effect, which open container that is heated from below and drives the condensed liquid towards the hot cooled from above. Since the temperature var- side of the pipe. ies perpendicular to the free surface, no sur- Another example is boiling liquids in weight- face-tension variation is initially present. How- less conditions. Intense thermocapillary fl ow ever, if the temperature difference across the arises near the point of contact where the hot layer rises above a critical threshold, the onset solid wall, the cold liquid, and the evaporated of patterns that resemble biological cells can be gas meet. The powerful fl ow induced by the lo- observed (see Figure 3c). This is known as Ma- cal surface-tension variation contributes signif- rangoni instability and is triggered by the ap- icantly to heat fl ow. pearance of weak hot spots on the surface. A similar process can be detected at the surface Complex but Essential of rapidly evaporating metallic paint, though Fluid physics is complicated but essential in our in this case the cooling (and resulting tempera- everyday lives. And as the 2008 incident at ture difference) is caused by evaporation. Heathrow Airport showed, it is of more than passing academic interest. Our knowledge of Tears of Wine this science is generally good but incomplete. As Under terrestrial conditions, the thermocapil- the technology derived from fl uid fl ows—high- lary effect described is usually masked by buoy- pressure fuel pumps, silicon chip manufactur- ant convection caused by the thermal expansion ing, and server cooling systems—becomes more of the liquid. Both effects are connected because exacting, filling the gaps in our knowledge both are caused by temperature variations. In becomes more important. order to properly analyse the effect, it is essen- While microgravity experiments provide in- tial to decouple the complex fl ow driven by the dispensable details and proof of the phenomena, two forces (buoyancy and thermocapillary) and it is essential that they be accompanied by theo- isolate the effect of each. retical analyses, numerical simulations, and Experiments in microgravity offer the op- ground-based testing. Together, all these efforts portunity to learn more about thermocapillary permit a direct experimental comparison, which effects, free from the infl uence of buoyancy. helps us understand the many processes in- Purely buoyant fl ows, on the other hand, can be volved in choking, capillary forces, thermocap- studied on Earth in closed containers where illary fl ow, and other fl uid motions. ■ there is no liquid/gas interface. Experiments performed in both environments will make it possible to understand the processes of combined buoyant-thermocapillary fl ow. Buoyancy forces are proportional to the vol- ACKNOWLEDGMENT ume of the liquid, while thermocapillary forces The authors would like to acknowledge the contribution of their act solely on the liquid’s surface. If the system American co-investigator, Prof. Mark Weislogel from Portland State involved is very small, the importance of buoy- University, Oregon. ancy diminishes. As a result, it is possible to ob-

www.spaceﬂ ight.esa.int 67 Breaking the Mould: Metallurgy in Microgravity

68 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research MATTER MATTERS

Factories ﬂ oating in space? Not quite, but the next giant step in metal processing technology may come from an unexpected source: the ISS. By Hans J. Fecht and Bernard Billia

e often think of breakthroughs such as artiﬁ cial hips and knees W and even dentures as modern day inventions. But these and millions of everyday products would not exist were it not for a process dating to the 1st millennium BC: casting. The processing of metallic alloys (a combination of two or more elemental metals) through melting and casting techniques, whereby the molten AT A GLANCE

material is poured or forced into a mould and ■ To produce increasingly allowed to harden, was invented several thou- complex metal products, sand years ago. Today, this processing is still an the physics behind casting important step in the industrial production processes must be better chain for a wide range of products from the very understood.

simple to the highly complex: turbine blades for ■ Gravity affects how a hot jet engines, low-emission engines for cars, so- metal cools by causing called supermetals used to produce wafer-thin parts to settle, which in sheets for electronic components, high-perfor- turn affects its structure mance magnets and ﬁ ne metallic powders to and function.

catalyze chemical reactions, to name just a few. ■ Industry needs better Clearly, while the metallurgy done today predictive models of how owes a debt to history, the process itself has un- structures form during dergone much—ahem—reﬁ ning. The end prod- cooling in order to keep ucts often need to perform well and retain their up with demands.

integrity under extreme circumstances, partic- ■ In space, advanced facili- ularly when used at high temperatures or when ties and methods to gather the product must be as light as possible in order data allow precise study of to conserve energy. To produce these high-per- processes in isolation from formance materials, the process must be closely gravity.

controlled for the sake of both optimal design —The Editors

SPACE CHANNEL / PHIL SAUNDERS and efﬁ ciency of production.

www.spaceﬂ ight.esa.int 69 1b ABOUT THE AUTHORS

Hans J. Fecht is director of the In- FIGURE 1. DENDRITE GROWTH stitute of Micro and Nanomaterials On the left, 1a shows the tree-like dendrite growth in the Faculty of Engineering and of a metal alloy in column-like formations. On the Computer Science at the University right, 1b is a computer-generated image of dendrite of Ulm in Germany. He is also director of the Institute for Dynamic microstructures. Dendrites are in blue, and they crys- Materials Testing at Ulm and a 1a tallise before the liquid portion of the melt, shown in group leader and senior scientist green and red, solidifi es. at the Institute of Nanotechnology of Research Centre Karlsruhe. He received his PhD in Materials Sci- Exactly how effectively this process is con- industry relies on the design and creation of ad- ence in 1984 from the University of Saarbrücken in Germany. After trolled means a great deal to the industry bot- vanced materials, which is accomplished by us- spending several years in the US, tom line. Profi ts must be balanced against costs. ing sophisticated computer codes to control the he returned to Germany where he The production and fabrication of alloys togeth- metallurgic processes. held several professorships since er with the casting and foundry industry gener- Although the industry does its job very well, 1990 and received the prestigious ate a considerable amount of wealth. For exam- competition is fi erce, and everyone is always G.W. Leibniz award of the German Research Society in 1998. Fecht ple, the 10 million tons of castings produced in looking for the next great breakthrough, that now directs a group of about 30 one year within the European Union is worth leap forward in technology that revolutionizes experts in the fi eld, has co-au- about 20 billion Euros. To continue generating the way business is done. The answer may lie in 2) Figure thored more than 300 technical pa- this kind of turnover, the casting and foundry a surprising place: space. A scientific pro- pers, has co-organized several In- ternational conferences and is a member of several international FIGURE 2. ENGINE COOLING advisory boards. Filling simulation and temperature distribution for a car engine block using Magmasoft computer simulations. In this image, the yellow areas are hot, the blue ones are cooler. Bernard Billia is research professor at the CNRS in the Institute of Materials, Microelectronics and Nanosciences of Provence in Mar- Temp C˚ seille, France where he heads research of the self-organized 1335 growth microstructures group. He 1321 has a background in physics and 1307 chemistry and obtained a Docteur 1293 des Sciences degree in 1982 at Aix- 1279 Marseille University in France after 1265 researching and modelling microstructure formation at the solid- 1251 liquid interface during solidifi ca- 1238 tion of alloys. Among his 1224 professional activities, he was a 1210 member of the Physical Sciences 1196 Working Group of the European 1182 Space Agency and of the European 1168 Space Sciences Committee of the European Science Foundation, and 1154 has served as project reviewer and 1140 materials science rapporteur at 1126

workshops. R. GLARDON AND KURZ, W. EPFL (Figure 1A); LEEDS UNIVERSITY, gure 1B); UK SPACE (ﬁ CHANNEL / PHIL SAUNDERS - DATA: MAGMASOFT (

70 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research gramme established by ESA using weightless- ity control as well as the engineering, tailoring, ness as an important research tool on parabolic and design of advanced materials for specifi c fl ights, on sounding rockets and, in the near fu- technological applications. ture, on the International Space Station. The idea of casting metal in space probably Cast in a Different Mould seems at best outlandish and at worst unneces- It seems an amazing contradiction that despite sary. Why should microgravity be an important the array of complex products currently manu- research tool when previous success has been factured, much of the basic physics behind these achieved using ground-based, gravity-condi- processes remains poorly understood. Clearly, tioned methods? To fi nd that answer we need the demands of industry call for the continued to look further into the melting and casting improvement of materials processing in the process. areas of composition, microstructure and performance by: Metal Works • Determining the thermophysical and relat- As previously stated, casting is a process by ed properties of metallic melts which a molten metal is solidifi ed. The liquid- • Mapping out basic relationships between to-solid transition is driven by the properties alloy and mould at the microstructure level of hot liquid metal and the way in which it • Developing predictive modelling of the for- cools over time, in the same way that water mation of grain structures during solidifi cation. freezes to ice at 0°C. From the standpoint of physics, castings belong to systems in which, rather than changing evenly in space and smoothly in time, the solid phase normally Dendrite Formation forms unevenly and with diverse microstructures (see Figure 1). Sophisticated computer models have been de- amed after the Greek word “dendron” for tree, the term dendrite applies to a veloped within the past 10 years that allow the N wide variety of structures with tree-like branches such as neurons, which are formation of such microstructures to be analysed composed of a cell body and dendrites that extend from that body. Snowfl akes, in great detail. For example, Figure 1b shows the which exhibit an infi nite number of elegantly shaped branches with hexagonal sym- evolution of the tree-like structures called den- metry, are the most familiar example of dendrites. After water vapour forms and so- drites (blue), which crystallise before the still-liq- lidifi es on a tiny dust particle (nucleation), the arms of the snowfl ake branch out directly from the newly formed ice crystal. uid portion fi nally solidifi es (green and red; see Their complex shape continues to evolve as a result of infi nite variations in condi- also “Dendrite Formation,” right). This interactions such as temperature, humidity and chemicals encountered by each ice crystal as tion between experiments and computer simula- it falls through the atmosphere. Most snowfl akes begin as relatively fl at, hexagonal tion has led to improved designs as well as im- shapes. The level of complexity of the dendritic arms results from the instability of proved performance undreamed of in the past. these smooth shapes and their reaction to the changing environment. In images (a) The fact that the transition from liquid to and (b) below, the six dendritic branches are basically identical to one another, which solid state is not a uniform process holds true indicates that the growth conditions were similar around whether one views the finished piece at the the growing crystal. If the arms were asymmetrical, how- smallest increment (nanometre) or on the larg- ever, this would indicate that the conditions surrounding est scale. In addition, the rate at which a cast- one branch were slightly different from those of its neigh- ing cools affects its microstructure, quality and bour. For comparison, image (c) shows snowfl ake-like properties. Generally, the areas that cool quick- grains growing in a melt. ly have a fi ne grain structure while the areas that cool slowly have a coarse grain structure. Figure 2 illustrates the temperature distribution within a car’s engine block immediately after the casting process was fi nished. Here we see that the temperature varies considerably between different locations within this large piece of metal since cooling occurs at a much higher rate close to the interface with the mould. It follows that for high-precision castings, the mas- tering of the grain structure during the liquid- AB C1 mm

K.G. LIBBRECHT, CALTECH (dendrite formation A, B); UNIVERSITY IM2NP, PAUL CEZANNE (dendrite formation C) to-solid phase transition is paramount for qual-

www.spacefl ight.esa.int 71 JET SET laboratory. Monitoring these experiments with Aluminum-based alloys are ideal sophisticated computer equipment has lead to for the blades in jet engine tur- improved validation of predictive models of this bines because of their strength, process—information that was much needed. reduced weight and resistance to damage from stress. Research by Levitation So the idea of taking a process thousands of years old and experimenting on it in space under weightless conditions is not so crazy after all. However, it is still necessary to consider what research can and should be carried out. ESA’s research programme focuses on a number of areas, including alloy solidifi cation when there are multiple phases and components, such as the multicomponent alloys used in many products, including the non-iron alloys used in jet engines and the growth of semiconductors such Using such advanced techniques to gather data on the as silicon for solar-powered cells. In order to perform the necessary experi- intricate processes of melting and casting ments, it is important to have access to extended periods of reduced gravity. Microgravity brings us closer to the design of new materials experiments in materials research conducted by ESA are to be carried out in a number of with better performance. multi-user facilities onboard the ISS, as well as on other microgravity platforms, such as para- Accomplishing these tasks requires bench- bolic flights and sounding rockets. One ISS mark data, and experiments performed in micro- facility important to the fi eld of materials re- gravity enable the study of the above properties search is the Materials Science Laboratory free of certain restrictions of a gravity-based en- (MSL). The MSL’s furnace inserts allow highly vironment. In space it is possible to suppress specialized solidifi cation experiments to be pergravity’s effects on the fl ow of molten metals and formed under microgravity conditions, with on sedimentation during solidifi cation. Without and without the application of fl ow-inducing gravity’s interference, it is possible to isolate oth- magnetic fi elds. er properties for investigation, such as diffusion Another crucial ISS facility is the Electro- and how it contributes to mass and heat trans- magnetic Levitator (EML). As fantastic as it port in the melt without the gravity-associated sounds, this equipment does precisely what the complications of certain solute ingredients being name implies: levitates molten metals. The EML more buoyant than others. Under normal cir- permits containerless melting and solidifi cation cumstances, the transfer of heat is modifi ed by of alloys and semiconductor samples. Further- convection in the melt, a process that makes por- more, the EML is equipped with highly ad- tions of the melt “fl oat” or “settle.” Solid grains vanced diagnostic tools that permit accurate most often settle under the opposing forces of measurements of thermophysical properties, as weight and buoyancy, which is called sedimenta- well as direct observation of the experiment tion. Obtaining evidence of this behaviour during fl ight by high-speed videography. through real-time x-ray studies on the ISS is one Using such advanced techniques to gather of the primary goals of experimentation. data on the intricate processes of melting and The resulting insights into alloy solidifi cation casting brings us closer to the design of new ma- and processing to be gained can potentially be terials with better performance. Such advanced used to produce new and unique microstructures. products can range from metre-sized objects to In addition, the space environment allows levi- micrometer-sized powders, for example: tated melts (see “Electromagnetic Levitator” on opposite page) to be controlled effectively at tem- • energy effi cient engines for the car peratures up to 2000ºC. This control enables the industry critical parameters of the liquid to be measured • metallic foams • medical implants much more accurately than in an Earth-bound SPACE CHANNEL / PHIL SAUNDERS

72 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research • powder production to improve catalytic implants and spacecraft, but society’s push for performance of modern fuel cells and continually stronger, lighter and more effi cient advanced combustion engines products requires that next great leap. • silicon-based materials for improved The most recent developments in technolo- solar cells gies have been seen in liquid processing, which • precision casting of detailed shapes and has the potential to improve the facilities respon- micro-thin sheets sible for the casting of complex parts and pro- • materials for high performance magnets duction of powders for the catalytic activation and of chemical reactions. To make this process • low-weight and high-strength materials more reliable, sophisticated computer models for modern space vehicles within the have been recently introduced in which all as- space exploration programmes. pects of industrial liquid metals processing are standardized to ensure long-term sustainability. To keep moving forward, ESA, together with the Intermetallics: European national space agencies, has set up a Not a Space-Age Rock Group scientifi c programme using the unique, gravity- A new international European Commission- free environment on parabolic fl ights, sounding Integrated Project (with ESA) has been estab- rockets and, in the near future, the ISS to devel- lished in the fi eld of intermetallic alloy solidifi - op new designs, structures and products. ■ cation. Christened IMPRESS—Intermetallic Materials Processing in Relation to Earth and Space Solidifi cation—this project represents an important step toward promoting scientifi c rec- iprocity between ground-based and micrograv- Metals that Float! ity experiments. The project comprises about 40 industrial and academic research groups t sounds like an oxymoron if ever there was one, but for more than twenty years, space from 15 European countries and will increase I agencies have been developing levitators that both lift – and heat – metal samples us- our understanding of the critical link between ing electromagnetic forces, acoustic waves or electrostatic forces. In these levitators, materials processing, structure, and fi nal prop- high-frequency alternating or static magnetic fi elds are generated by passing a current erties of new intermetallic alloys, such as TiAl through an assembly of coils shaped for positioning and heating. This electromagnetic (titantium aluminide) and NiAl (nickel alu- fi eld induces corresponding currents in the metal sample placed between the coils that minide). For additional information, see the heats (and sometimes melts) the sample. The machine can then use the combined cur- IMPRESS website at ESA http://www.space- rents and electromagnetic fi eld to generate a force that lifts the sample. fl ight.esa.int/impress/. Other than Houdini, who would want to levitate metal? The answer is quite possibly Called “aluminide intermetallics,” these alu- anyone involved in processing. Levitation of molten metals and other materials was de- minium-based alloys have a defi nite proportion veloped to avoid contact with container walls, which can lead to contamination of the of two or more elemental metals and have many sample and undesirable non-uniform crystallisation. When microgravity is applied, con- attractive mechanical, physical and chemical tainerless levitation processing becomes better and more effi cient. In the absence of properties due to their crystal structures and gravity, separation and sedimentation within the metal are eliminated, uniform mixing is achieved, and samples can be processed that are much larger than those on Earth. strong chemical bonds. For example, Ti-aluminides are well suited for blades in high performance aircraft engine turbines because of their reduced weight, improved mechanical strength and resistance to damage from stress whereas powders of Ni-aluminides have excellent catalytic activity for fuel cell applications. In both cases the use of these advanced materials improves the system performance tremendously.

Melting Away the Barriers Electromagnetic Levitator (EML), jointly As the products we make become more sophis- developed by ESA and the German Space ticated, it follows that their production process- Agency, the DLR, permits the accurate es must keep up. Advancements in liquid pro- property measurements of highly reactive cessing techniques have enabled industry to cre- liquid metal alloys.

DLR (GERMAN SPACE AGENCY) ate products such as jet engines, medical

www.spacefl ight.esa.int 73 74 74 Rattle and Using Vibrations Vibrations Using ■ ■ ■ GLANCE A AT By Daniel Beysens, Pierre Evesque and Yves Garrabos Yves and Evesque Pierre Beysens, Daniel By Champagne orsweep sand into apile to clean up. understood. must beproperties better the role of vibration on its and gas or liquid a solid, like act can Granular matter spacecraft. effi for trolled on use cient mustevaporation be con- fer, condensation and heat including trans- gases Behaviours of liquids and matter. inanimate on cial gravity like artifi act can - Vibrations MATTER LOOKING UP: Europe’s UP: LOOKING Quiet Revolution in Microgravity Research In space, you can’t toast your successes with So how can you make matter behave? — The Editors The Shake, Shake, as Gravity

MATTERS Roll: When weightless, it is not possible to pour yourself a glass aglass yourself topour not possible it is weightless, When W be have no attraction of sand grains The castle. asand build unrestricted by “up” or “down.” But what do we know know we do what “down.” But or “up” by unrestricted imate matter such as liquids, gases and granular mat mat granular and gases liquids, as such matter imate the liquid and wouldn’t rise to the top. Nor could you top. could Nor tothe wouldn’t rise and liquid the and thus spread like gas molecules in the air. the in molecules gas like spread thus and of the effects of weightlessness on the behaviour of inan- behaviour onthe of weightlessness effects of the and even if you succeeded, the bubbles would just sit in bubbles sit would just in the you if even succeeded, and won’t bottom— liquid tothe drop of Champagne—the piles. sand toform together stick naturally grains and gravity, they move weightlessly in a slow-motion ballet, aslow-motion ballet, in move weightlessly gravity, they sit at the bottom of the bottle, gas mostly rises to the top top tothe rises mostly gas bottle, of the bottom sit at the liquids that forgranted wetake Earth On sand? as such tween them, called cohesion, to keep them together; together; them tokeep cohesion, called them, tween national Space Station. Freed from Earth’s Earth’s from Freed Station. Space national nauts ﬂ e television have of seen footage astro- all oating around on Mir and the Inter- the and onMir around oating ter

SPACE CHANNEL / PHIL SAUNDERS www.spacefl ight.esa.int 75 Modelling Matter correspond to their “critical point,” which oc- The picture this presents is one of chaos. Clear- curs when pressure and temperature become ly human beings can direct their movements high enough so that liquid and gas can no lon- under these conditions, but are they left to swim ger coexist. Thus, the boundary between the through a sea of fl oating debris? More impor- liquid and gas phases disappears. The two states tantly, how does the spacecraft itself function? mix together as a dense gas (a gas with the den- Fortunately there are forces other than gravity sity of a liquid) and are called “supercritical fl u- at work. Although gas bubbles, liquid droplets ids.” For carbon dioxide, the critical point oc- and solid objects are no longer subject to the curs at a temperature of 31°C and pressure of 73 rules of gravity, they are, however, still bound bar, which is 73 times the atmospheric pressure. by inertia—that is they remain at rest until and For water, the critical point pressure is 220 bar ABOUT THE AUTHORS unless some force exerts pressure on them. and temperature is 374°C. When near their crit- That force can be quite minimal—in the form ical point, all fl uids behave in a similar manner; Daniel Beysens received his PhD of vibrations. Even a weak vibration in space so by studying one fl uid, the properties of all fl u- at Université Paris-6. He is now di- causes matter to feel the stress of that pressure ids can be deduced, a phenomenon called “crit- rector of research at the French and to respond with an equal reaction, such as a ical point universality.” (Kenneth G. Wilson Atomic Energy Commission (CEA), counter movement. As illustrated in the experi- won the Nobel Prize in 1982 for providing the and is also working at the Ecole Su- périeure de Physique et Chimie In- ments discussed below, these reactions often theoretical framework to estimate the universal dustrielle in Paris. He is interested mirror those like buoyancy that you would ex- features of critical point phenomena.) in phase transitions in fl uids and pect on Earth where gravity is present. Thus, the Supercritical fl uids possess other interesting has received several awards, in- vibration acts as an artifi cial form of gravity. properties, for instance supercritical carbon di- cluding the Ancel Prize from the Fluids provide a good model for investigation oxide is a very powerful solvent of organic mat- French Physical Society, the Gaz de France award from the French when they are under the specifi c conditions that ter and it is not harmful to living things or the Academy of Science with B. Zappo- li and co-author Y. Garrabos and the Prize for Innovative Technolo- gies for the Environment from the French institution ADEME. Condensation vs. Diffusion Pierre Evesque obtained his PhD at Université Paris-6, after which he entered CNRS, where he is currently director of research. He vis- Left: Condensation of a fl uid on Earth. Tiny liquid droplets form; then they fuse and run together to ited the Kavli Institue for form larger ones. The liquid is heavier, so gravity causes it to pool under the vapour. Right: Conden- Theoretical Physics in the US in sation in microgravity. The bubbles can only grow by haphazard collisions, known as diffusion. The 1997 and 2006 as an invited schol- result is this strange-looking mixture where vapour bubbles can exist inside liquid droplets. ar. He is currently working at Ecole Centrale in Paris where he studies the physics of granular matter and dissipative systems on Earth and in microgravity. Yves Garrabos received his PhD from the Université Paris-6. He joined CNRS at the Laboratoire des Hautes Pressions in Meudon-Belle- vue and at the Laboratoire d’Ingéniérie des Matériaux et des Hautes Pressions at the University Paris XIII-Villetaneuse. He joined the Institut de Chimie de la Mat- ière Condensée de Bordeaux in Pessac in 1993 as a director of research. He received the Gaz de France award from the French Academy of Sciences with B. Zap- poli and co-author D. Beysens. He participated as a principal or co- principal investigator for more than 10 experiments dedicated to fl uids and granular matter in mi-

crogravity. SPACE CHANNEL / PHIL SAUNDERS

76 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research SSg SS

AB CDE

FIGURE 1. SHAKEN, NOT STIRRED environment. It is also possible to burn danger- The pattern of heat distribu- allel—to the vibration, but this reaction can be ous wastes, like ammunitions, in supercritical tion in CO2 at that supercritical explained by the fact that other forces are trig- water effi ciently and safely. Fluids like oxygen point when the liquid/gas gered by the vibration force. distinction disappears. From and hydrogen are also used in space under super- left to right: critical conditions because they remain homoge- The Converts: neous regardless of the spacecraft’s accelera- (a) Heat source is labelled S; Condensation and Evaporation tions—or absence of accelerations—and the ori- (b) heat transfer under Earth’s We’ve all—quite innocently, of course—fogged entation of the gravity vector. gravity labelled g; up the car windows at some point. Under these Supercritical fl uids are very unstable and (c) double arrow shows heat normal conditions on Earth, when a fl uid starts particularly sensitive to the effects of gravity or transfer under high amplitude to condense, tiny liquid droplets form and these vibrations. This property and the fact they are (64 mm) and low frequency tiny droplets fuse and run together to form larg- universal models for fl uid behavior makes the (0.3 Hz) vibration in weight- er ones. Because liquid is the heavier substance, observation of their behaviour extremely help- less conditions on Mir; gravity causes it to pool together under the ful. Sand fi ts these criteria as well in that the (d) double arrow shows heat vapour, and the boundary between the liquid grains can act like molecules of a liquid or gas transfer under lower ampli- and gas phases is fl at (see Figure 2a). depending on the situation. tude (0.8 mm) and higher fre- Under weightless conditions, however, bub- quency (1.6 Hz) vibration in bles or droplets can only grow by the haphazard Being Hot is “Cool” weightless conditions on Mir; and slower process of collision, known as diffu- As anyone who has ever made pasta knows, (e) periodic convection near sion. What we are left with is a strange-looking when a liquid in a saucepan is heated from below the cell wall (fi ngering on the mixture: in the liquid there are a few large va- and before the liquid boils or the pasta is added, left and right cell walls) during pour bubbles that coexist with many tiny va- the layer closest to the heat becomes lighter than cool down as seen during Max- pour bubbles. In addition, the appearance of the its environment and rises. As it cools down at us 7 sounding rocket fl ight. mixture varies according to the vapour volume. the top, it sinks down and a rapid ordered When the vapour volume is greater than 30% of motion develops creating a circulatory pattern the fl uid, the bubbles are so close together that of rolls within the liquid, thus transferring the it forms an interconnected pattern (see Figure heat throughout, which is called convection. 2c). When the vapour volume is less than 30% This behaviour can also be seen with gases, in of the fl uid, we see that the boundary between which, for instance, cyclic clouds are quite often liquid and vapour is spherical (see Figure 2b), observed. unlike the fl at surface seen on Earth. Since microgravity removes any semblance In one experiment, we initiated the evapora- of up and down, you would expect to see an al- tion of a liquid under low amplitude, high fre- together different pattern of motion when a liq- quency vibrations (200 µm, 20 Hz). Under these uid is heated in space. However, we can repro- conditions, the vapour bubbles progressively duce the convection pattern using vibrations as align and stretch and fuse together. Then, rath- artifi cial gravity. When a container and its con- er surprisingly, stripes of gas form in space in a tents are heated and then vibrated, the pattern way that remind you of the fl at interface be- of motion resembles that seen here on Earth. tween vapour and liquid which we see on Earth This motion is detailed in Figure 1. Sometimes (see Figure 2d–i). when a substance reacts to vibrations in a Due to the lack of experimental time in space, weightless environment, the fl ow pattern ap- the stripes shown in Figure 2 are not the ulti- pears to defy common sense, such as when the mate equilibrium state, which we expect to be a

ESPCI / ECP / CNRS motion occurs perpendicular—instead of par- unique vapour stripe in the middle of the cell as

www.spaceﬂ ight.esa.int 77 On Earth

Vapour

FIGURE 2. PHASE SEPARATION a result of the symmetry between the vibration Separation between liquid g I A and the cell. and gas phases of CO2 and H2 samples. Is Sand a Fluid? Liquid Sand is a model for other granular matter. Its study in the weightless conditions of space rais- es numerous technical issues, such as the best Under weightlessness way to work with grains and powders in satellites and spacecraft. (Indeed, when travelling for L 20 years we will need to grow plants, to cut them V L in pieces, to harvest corns and seeds, to grind or B V C sow them to get food, and so on.) Such studies may also answer very fundamental questions concerning the formation of asteroids and the history of the Earth’s formation. At the same time we need to ask: “Should we consider granular matter as a gas or a liquid or a solid, since Under vibration on Earth sand can fl ow like liquid and form sand piles like a solid and, in weightless conditions, can fl oat and spread around like a gas?” In this context, grains act like fl uid molecules. D G Will they also exhibit properties of various states of matter: gas, liquid and solid? However, in order to be considered as either a fl uid or a solid, sand must exhibit a “temperature.” What does “temperature” look like? In a fl uid, temperature is achieved by the random motion of molecules: the higher their velocity, the higher the temperature. Thus, in terms of E H sand, the grains would acquire a temperature by reaching a random velocity, and that motion would generate energy known as kinetic energy. One way to create motion is by vibrating the walls of the sand container. so that the grains near the wall will collide, and then continue to collide with other grains. In this way grains behave like the balls struck on a snook- FIer table. Unfortunately, during the series of collisions, some energy is lost or dissipated, which is why vibrated granular matter is often called “dissi- Time pative matter.” Scientists are using weightlessness to address the different forms of energy (a) Separation of liquid and gas under Earth’s gravity. The demarcation line between found in this strange “dissipative” matter and phases is labelled I. to determine its characteristics. These studies (b) Under weightless conditions, with a small (<30%) vapour volume. Bubbles can are still in their infancy. However, in experi- only coalesce through collisions. A large vapour bubble forms, but many small bub- ments carried out on Earth, it turns out that the bles that did not coalesce remain isolated. (c) Also under weightlessness, but with a main physical parameter that determines sand’s large (>30%) vapour volume. The liquid/gas pattern is interconnected. L = liquid and V = vapour. behaviour is the number of layers of grain in the experiment container. This fi nding makes the (d-i) Phase transition under vibration of 300 µm amplitude, 20 Hz frequency. properties of granular matter very abnormal, as (d-f) Growth of interconnected gas and liquid domains. (g-i) Growth of bubbles. Note these properties change when the volume of the difference in how phases separate in microgravity in (a) and (b) without vibration matter is changed. Temperature is one example versus (d-i) with vibration. When vibrations are applied, the liquid–gas interface of such a property. As stated above, tempera- looks more like what is seen on Earth, with a fl at line between phases.

ture is determined by the velocity of the grains, ESPCI / ECP / CNRS

78 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research which is created through collisions. Because a it does not rotate. Think again of the snooker ta- small amount of velocity is lost by dissipation, ble. A player can strike the cue ball off-centre to the grains in the center of the container will impart “spin” and affect the movement of the have a velocity lower than the grains near the The behaviour ball using the rotation of that spin. But in this ex- vibrating wall. Thus, granular matter consisting periment, there is no spin. of many grains will be cooler at its centre in con- of granular The reason the grain does not rotate or “spin” trast to matter consisting of only a few grains matter in space is related to the shape of the container (a cube that exhibit nearly the same velocity. will be or a cylinder, chosen for sake of simplicity) and to the friction that results when a particle with In-Grained Behaviour understood an inclined trajectory rebounds with some rota- To illustrate this phenomenon, we ﬁ rst apply when the tion on one surface and with the counter-rota- vibrations to only one grain in a rectangular box. tion on a second surface. As energy dissipates in On Earth—where the properties such as gravity nature of the collision, the grain speed after the rebound and friction apply—our one grain can only col- temperature is less than before, a decrease that eventually lide with one wall. In space, however, not only “freezes” the grain rotation. What is also re- does the one grain bounce between two walls, it (velocity) and markable is that the grain moves at a much fast- also rebounds in a perfectly perpendicular the role of er speed than the vibrations of the wall—a par- straight line. This obviously does not happen on vibration are adox when one considers the energy lost during Earth where the grain does not reach the second each collision with the walls. wall but moves along by “jumping.” better The center image in Figure 3 is an illustration understood. Multi-Grain of two coherent grains moving at the same speed If one performs the experiment with two grains in a vibrated experiment container under weight- of the same size, both particles move at the same less conditions. The reason that the grain bounc- speed. Can this coherent motion be applied to es from wall to wall in a straight line is because more grains in such a way that they form a

FIGURE 3. On the Rebound

The behaviour of sand grains subjected to vibrations, indicated by the double red arrow.

Left: On Earth, the vibration is too weak for the grain to overcome gravity and reach the second wall. Thus it moves forward by “jumping.” Centre: Under weightless conditions two coherent grains move at the same speed in the vibrated container. Both particles collide only with the walls and make one round-trip per vibration period. Right: With many grains in the container, the grains collide with the walls and with each other. Higher grain density in the middle of the cell means that they cannot move as fast there, which also means the temperature is cooler there than along the sides where they move more freely. SPACE CHANNEL / PHIL SAUNDERS

www.spacefl ight.esa.int 79 However, the system is not completely random. The grains do follow certain behaviour patterns endemic to a liquid or gas. As discussed previously, the grains acquire temperature through movement, which creates kinetic energy. The distribution of speeds within the experiment container can vary considerably, and even the thickness of the container can make a difference. Grains no longer have the same velocity (or temperature) everywhere. Slower speeds in RESEARCH ON MIR the middle of the container mean cooler temper- The Russian space station, Mir, atures while faster speeds along the top and bot- was an extraordinary achieve- tom lead to hotter temperatures. ment. In 1976, in the days of the Soviet Union, Mir was intended to be the third generation of habit- Behaviour Matters able space systems that had start- Obviously, scientists’ chief concern in studying ed with Salyut. With the collapse these behaviours is not to make it possible to of the Soviet Union in 1991, an era build sand castles—or drink Champagne. Liq- of international space coopera- uids and gases are essential to the functioning of tion began and European re- spacecraft and to life support systems, but the searchers obtained access to Mir. weightless conditions in space make it quite dif- It was creaky, leaky, hot and fi cult to use them effi ciently. We have found that stuffy, but it worked and it was up vibrations can play the role of artifi cial gravity in space. As a research opportuni- for a number of key phenomena specifi c to the ty it was simply too good to miss! behaviour of liquids and gases, such as thermal Below, Astronaut Jean-Pierre convection, evaporation and condensation. Hot Haigneré on the Mir station in and cold fluid, bubbles and droplets find a 1999 setting up the CNES facility respective “up” and “down” when the vibration ”ALICE 2” where many of the ex- is turned on, thus simulating a reaction like that periments reported here have seen under normal gravitational conditions. As been performed. a matter of fact, physicians are already using vibration as artifi cial gravity to prevent muscle decay or bone decalcifi cation in space. Granular matter (sand), although often considered regular solid matter, exhibits unique behaviours and properties, some of which are still unknown. As we have seen, the individual grains act similarly to the molecules of a liquid “grain laser” in which a continuous stream of or gas. As such, vibration gives them a velocity sand grains is emitted like photons of light and a “temperature.” The behaviour of granu- beaming from within a laser? Unfortunately, lar matter in space will be understood when the when we increase the number of grains, they nature of this temperature and the role of vibra- start interacting strongly, colliding and repel- tion are better understood. Once the right con- ling each other as they cannot occupy the same ditions are determined and then applied, we place at the same time. The motion looses its could theoretically affect the behaviour of coherence and becomes erratic. grains so that they cohere as a solid, fl ow as a With many grains in an experiment contain- liquid, or expand as a gas depending on their in- er, the trajectories of those grains are then com- tended use. It is both a fundamental problem plicated by collisions (see Figure 3). If the grain that is related to the origin of the universe and concentration remains relatively small, the the formation of dust, asteroids and planets and, grains uniformly occupy most of the available more practically, it is quite essential for astro- space (the container length minus the zone nauts as it relates to the everyday necessities of where the wall vibrates). If we increase the con- growing food, managing waste and, in a word, centration of the grains past a certain point, they surviving during long-duration journey through ■ generate a cloud of gas. the universe. NASA; CNES / CADMOS

80 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research FASCINATED BY THE BRAIN AND MIND?

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The Complex Matter of Plasma

arely is a scientist able to announce the discovery of a major new state of matter; R in particular a state that common sense The 1993 discovery that complex plasma can exist as suggests should not exist. But this is precisely what happened at the International Conference soft matter changed the face of physics. Since then, on Phenomena in Ionised Gases in September research into its properties and behaviours has 1993 in Bochum, Germany. exploded, and experiments on the ISS have led to a A Different State of Matter Matter exists in four states: solid, liquid, gas- number of discoveries that could not have been made eous, and plasma. It might be argued that plas- in the presence of gravity. ma is nothing more than the ionized or charged state of gas. However, since the physics of interactions within plasma and gas is very dif- By Gregor Morﬁ ll and Hubertus Thomas ferent, the consensus is that plasma should be regarded as the fourth state of matter. This simple picture corresponds to the old elements of Earth, Water, Air, and Fire, but it is not the whole story. In the late 20th century, the Nobel laureate Pierre-Gilles de Gennes coined the term “soft matter” for a class of large (centimetre-sized) assemblies of substances that appear “soft” and consist of various supramolecular or “large” entities (large by atomic or molecular standards). He deﬁ ned soft matter as “supramolecular substances, which exhibit special properties such as softness or elasticity, which have an internal equilibrium structure that is sensitive to external forces, which may possess excited metastable states, and where the relevant physics occurs far above the quantum limit.”

82 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research AT A GLANCE

■ Traditionally, matter exists as a solid, liquid, gas or plasma, but complex plasmas or soft matter can be created by adding large or supramolecular particles to ordinary plasma.

■ Studying soft plasma matter on Earth is difﬁ cult because it must be levitated to be viewed, and gravity pulls it downward.

■ Experiments in space have resulted in supercoagulation in a neutral gas, which formed one giant conglomerate and a range of smaller ones.

■ Complex plasma research can be applied to many areas: waste removal, pollution control, solar cells, semiconductor chips and medicine.

—The Editors SPACE CHANNEL / PHIL SAUNDERS

www.spacefl ight.esa.int 83 Soft matter comes in two different states. ball of plasma; in fact plasma is quite common Granular solids, polymers (compounds made up in space. On Earth, plasma has multiple uses in of large molecules) and foams represent the sol- many industrial applications from lights to id state, while emulsions and colloids (mixtures plasma televisions. Ordinary plasma is a collec- containing particles of one compound suspend- tion of particles: positively charged ions (elec- ABOUT THE AUTHORS ed in another, milk is one example) represent trically charged atoms) and negatively charged liquids. The 1993 announcement in Bochum electrons. The system’s charge is neutral; in oth- Both Gregor Morfi ll and that supramolecular or complex plasmas could er words, there are as many positive as negative Hubertus Thomas are currently at the Max Planck Institute for Extra- be made to assume crystalline structures caused charges. The particles interact via their electric terrestrial Physics in Germany. quite a stir. Sir John Maddox, the editor of Na- forces instead of through atomic or molecular ture at the time, refl ected the opinion of the day interactions, as is the case in neutral gases. The when he stated: discharge that produces light in mercury- or sodium-vapour streetlamps is an example of “That physicists are the masters of artifi ce ordinary plasma in action. in the experiments they design is widely Complex plasma has an additional compo- recognised, but sometimes their tricks nent—charged supramolecular microparticles quite take the breath away. Most of what (see Figure 1). Although these microparticles can follows is about an experiment of outstand- be as small as one ten-millionth of a metre, they ing ingenuity, which, despite the electronic are extremely heavy—many billions of times equipment no doubt accompanying it, is heavier than atoms—and still carry a charge. In also simple. The word ‘elegant’ is over- fact if their number is suffi ciently large, they be- worked, but in this case it applies…” come the dominant component and as such will Maddox concluded his discussion of soft determine the structure of the system. This can matter by stating: “What the authors them- happen in an interstellar gas cloud when the ad- selves would like to do is to carry out their ex- dition of dust to the ordinary plasma present in periment in microgravity conditions. On the the cloud turns it into complex plasma. face of things, it is a more deserving candidate Because supramolecular microparticles have for room in some future Space Shuttle fl ight signifi cant mass, changes in their state or mo- than most of the others so far suggested.” tion are slowed by a factor of between one hundred thousand and one million, which means Complex Plasmas that individual particles can be seen. Hence the Plasma is everywhere, though it often goes physical processes taking place can be viewed unrecognized. The mammoth strike of a light- in super-slow motion and at the individual par- ning bolt and the tiny sparks of static electricity ticle level. It’s an experimenter’s dream or, per- are both examples of plasma. The Sun is a huge haps, a theoretician’s nightmare. Being able to observe individual microparticles and their super-slow motion may be ideal for experimental studies of the strong and weak coupling phenomena (the interaction between particles) in multi-particle physics. This, in turn, could lead to a better understanding of the fundamental principles that govern the stability and self-organisation of condensed matter and of critical phenomena such as phase transitions (regular changes in state), as well as the explo-

FIGURE 1. PLASMA PARTICLES An electron microscope image of typical particles used in complex plasma experiments. The particles are usually melamine formaldehyde and all are the Acc V same size, though occasionally metal-coated, nee- 10.0 kV 10 µm dle-shaped particles have been used. MAX-PLANCK-INSTITUT

84 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research CCD video camera FIGURE 2. PLASMA DISCHARGE DEVICE Macro lens The most common device used in complex plasma experiments is a Grounded radio frequency (rf) discharge de- electrode vice seen here. The plasma is usually a noble gas (argon, neon) at a Sheet of Particles laser light pressure of about 1 mbar. The Shunt Laser plasma is “ignited” using a radio circuit frequency signal on the two paral- 13.56 MGz Cylindrical lens lel electrodes. The microparticles generator Powered are illuminated by a laser and electrode Ring (copper) Matching viewed by recording the refl ected network light with a CCD camera. Using a thin (100 µm) sheet of laser light allows the simultaneous recording of a whole lattice plane. ration of the onset of cooperative phenomena— precise measurements needed to study three-di- phenomena that appear only when a large mensional systems and for stress-free processes, number of elements act together to produce ef- microgravity experiments are essential. fects not predictable from the properties of their individual components. Microparticles: To perform these observations on Earth, a Charging and Shielding device containing an upper and lower electrode Generally, a microparticle introduced into plas- creates an electrostatic force that levitates the ma will absorb electrons and ions and become microparticles and neutralizes gravity (see Fig- charged. In low-temperature plasma, the charge ure 2). But Earth’s gravity is so strong that the on the microparticles is negative because elec- electrostatic force can create only a thin, hori- trons are much faster than ions, and conse- zontal layer of free-fl oating microparticles. As quently electron impacts occur more frequently a result, Earth-bound experiments have natu- than ion impacts. The negative charge on the rally concentrated on fl at or monolayer systems microparticles repels the (negative) electrons (a single continuous layer that is only one micro- and attracts the (positive) ions. This continues particle, molecule, or atom thick). Still, these until, after a few microseconds, the collision are of great interest in many areas including the rates are the same, and the microparticle has study of surface physics, membranes and the reached Q, its equilibrium charge (typically a stability principles of condensed matter. few thousand electrons). In the absence of gravity, however, the micro- Around the microparticle the free electrons particles can occupy the whole plasma, and it’s and ions react to its electric fi eld and form a pos- possible to obtain experimental conditions when itive-charge cloud. The extent of this “shielding the system is isotropic (uniform), homogeneous cloud” (so named because it effectively screens and not stressed by the pull of gravity. For the or shields the local charge of the microparticle)

A B

Ion drift Negatively charged FIGURE 3. RAISE THE SHIELDS microparticle The shape of the plasma shielding cloud around a negatively charged o o microparticle in (a) a uniform plasma distribution and (b) ﬂ owing Negatively plasma. charged microparticle z

MAX-PLANCK-INSTITUT z

www.spaceﬂ ight.esa.int 85 FIGURE 4. AMAZING DISCOVERY Examples of supercoagulation as observed on the ISS. The “massive” coagulation of particles formed within seconds of the 1 mm start of the experiment.

is called its “Debye length,” named after the fa- Kristall Experiment (PKE-Nefedov, named for mous Dutch physicist Peter Debye. the Russian project scientist who died just before In isotropic plasma, where the plasma’s prop- the lab was installed on the ISS). The improved erties do not vary with direction, the shielding next-generation laboratory, PK-3 Plus, was cloud is spherical. If the plasma fl ows around the launched in 2005 and was built on the same prin- microparticle, the shielding cloud is elongated ciples as its predecessor but with a more advanced and anisotropic, with an excess positive charge design, better manipulation capabilities and downstream. This occurs in complex plasma ex- improved particle and plasma diagnostics. A periments conducted on Earth (see Figure 3). brief snapshot of experimental results from The force between the microparticles is elec- research performed on the ISS is summarized in tric and, due to the shielding, has a limited “Experimental Results” on the opposite page. range. If the particles are too far apart, they will While the decision to build the ISS was a po- be out of range and the system is said to be litical one, its ability to host complex research is “weakly interacting.” In nature—in star-form- a practical consequence. The presence of astro- ing gas clouds, zodiacal light (sunlight refl ected nauts is a bonus, especially for the type of exper- by a myriad of particles), planetary rings, light- iments that cannot be fully tested on Earth. By ning and so on—this is usually the case where designing the instruments so that astronauts can complex or dusty plasmas exist. perform the experiments themselves—exploring However, if the particles are close enough, parameters that may be ideal in microgravity but then the motion of each directly infl uences the impossible on Earth—the risks that always motion of its neighbour, and they become strong- plague purely robotic missions can be reduced. ly interacting. This presupposes that other forc- Such research, performed with dedicated, versa- es, such as particle interactions with any neutral tile, space-proven equipment—little “laborato- gas that happens to be present, are not more im- ries” that can be reconfi gured and adapted to portant. So for a description of strongly coupled study unexpected results and discoveries—can complex plasmas, two parameters are needed: yield a great deal of new knowledge at compara- one describing the coupling strength and the tively little extra cost. Astronaut operation of other the particle separation. the experiments has proven immensely valuable and has resulted in about 50 scientifi c publica- Human vs. Robot tions. Of those publications, two exemplify the As suggested by Sir John Maddox, experiments benefi t of a living, breathing and thinking hu- on complex plasmas have been conducted in man experimenter over a robotic one. space, though in the two complex-plasma laboratories on the International Space Station rath- Supercoagulation er than the Space Shuttle. From 2001 to 2005, The fi rst is the discovery of “supercoagulation.”

the ﬁ rst lab was the Russian-German Plasma Cosmonaut Yuri Baturin conducted a series of MAX-PLANCK-INSTITUT

86 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Experimental Results

Supramolecular substances. In this image the three states of complex plasma are revealed. Each white dot is a microparticle illuminated by laser light and recorded on camera. In the crystalline state, the microparticles occupy a well deﬁ ned, ordered lattice. In the liquid state, structural disorder and somewhat higher particle mobility is seen. In the gaseous state, the

particles move much faster, leaving long tracks captured during Crystalline Liquid Gaseous a single exposure.

Elastic deformation. This set of images, taken from a video recording, was acquired during an experiment conducted on the ISS. The complex plasma cloud is initially deformed and then reforms elastically; the time scale was approximately one second.

Excited states. This slice through a hcp 3-D plasma crystal All these experiments were carried out at temperatures was obtained in a (typically room temperature) when the thermal energies were laboratory on Earth. much higher than those of the quantum states themselves. The ground state (the These examples illustrate that the strong coupling possible fcc lowest level of crystal in complex plasmas leads to only one conclusion—the presence energy) exhibits a of the plasma state of soft matter. The fact that complex plasma 1 mm face-centred cubic can exist in solid, liquid and gaseous forms makes the complexi- (fcc) structure or a ty of these systems even more apparent. Hence the hierarchy of body-centred cubic. One can also see regions where the local system states shown below begins to resemble the classical structure is in an energetically excited, hexagonal, close-packed states of matter (solid, liquid, gaseous, plasma) —albeit with (hcp) state. Red indicates an fcc structure, green an hcp structure. much more diversity.

PHYSICALCAL STATES OF SOFT (granular) MATTERM

Complexomplex PlasmasPlasmmas LiquidLiquuid PPlasmaslasmas

AeAerosolrosool CloudsCCloouds Crystallineystalline PlPlasmasasmas

Response to external fi elds. These three images, ob- (Complexmplexx EElectrolytes)llectrrolytes) ColloidalCo Coulomb Crystals tained on the ISS, show the same liquid plasma under different CComplexompplel x FlFluidsluids ColloidalCo (neutral) Crystals external alternating current electric fi elds. The fi elds applied were (from left to right) 10, 12 and 19 volts. At the higher volt- GGranularranulara LLiquidsiiqquuids ages the particles align along the fi eld—a physical process simi- GGranularrannullar SSolidsoliids lar to electrorheology (electrically induced deformation and fl ow) in colloids, albeit at a much lower voltage and frequency. MAX-PLANCK-INSTITUT

www.spaceﬂ ight.esa.int 87 Adaptable Intelligence

Although robots are useful, a thinking human researcher is essential. Pictured are some of the astronauts/cosmonauts who carried out experiments in the PK-Laboratories on the ISS. Top left: Sergei Krikalev. Top right: Mikhael Tyurin, Konstantin Kozeev. Bottom left: Claudie Haigneré. Bottom right: Thomas Reiter.

experiments for which the methodology dif- made on Earth, because in a gravitational ﬁ eld fered from the way they had been designed and the particles would have simply fallen out of programmed. The original idea was to perform view in a fraction of a second. a plasma experiment, but the plasma was never This discovery may have some interesting ap- activated. So as an alternative, microparticles plications—in toxic waste removal, pollution were injected into neutral, argon gas. The result control and industrial production processes. So was unexpected: no plasma and no charge. understanding the origin of this behaviour is of Instead, within seconds the microparticles considerable interest beyond the simple intellec- formed one very large conglomerate and a tual puzzle of why and how nature can create whole spectrum of smaller ones (see Figure 4). something 100,000 times faster than predicted. This result had two surprising features. First, When a theory emerged based on the initial ex- the large conglomerate contained some 100,000 periments, the versatility of the PKE-laboratory microparticles, whereas the predicted coagula- and the human researchers made possible a se- tion was just two particles. Second, the size ries of subsequent experiments that conﬁ rmed spectrum of the smaller conglomerates was a the origin of this new physical process now power law. In physics, this is usually an indica- called supercoagulation. tion that a special “scale-free” process is operating, meaning that the size of the conglomerate Electrorheology is not dependent on the energy of the particles. Another example proving human superiority to

It’s unlikely such a discovery could have been robots involves “electrorheology,” an electrical- ESA / CNES

88 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research ly induced deformation and fl ow that is well Results validated be employed in everything from shock absorb- known in colloid physics. Electrorheology is the that adaptable ers to the protection of buildings against earth- process by which an external electric fi eld is quake damage. applied across a colloid fl uid and causes major human The question arose as to whether complex changes in the properties of the fl uid by rear- experimenters plasmas might also have electrorheological ranging the suspended particles—in other onboard the ISS are properties. In other words, if “electrorheologi- words altering its structure, or rheology. Viscos- cal plasmas” could be produced in a similar way ity is modifi ed, compressibility is changed and critical for as “electrorheological fl uids.” This seemed un- the shear modulus (the ratio of the shear stress following up on the likely as the conditions between the two systems to the shear strain) is altered. The fl uid may even basic experiments could not be more different. In colloidal or com- assume solid structure. This happens because plex fl uids the background fl uid is neutral and the electric fi eld induces a dipole in the (usually operated in “robotic the particles within are electrically active, while electro-active) colloidal particles. These induced mode” with in complex plasmas the background ion–elec- dipole fields lead to a rearrangement and available tron plasma is highly conductive and the mi- realignment of the particles, which in turn leads equipment. croparticles are insulators. In complex fl uids the to changes in the fl uid’s properties. The whole typical particle separation is approximately the process works because the suspended particles particle size (in other words they are densely are electro-active and the fl uid is neutral. Inter- packed); in complex plasmas the particle sepa- estingly, this property of colloidal fl uids could ration is roughly 50 to 100 times the particle

Supercoagulation

oagulation—the process of two or more particles joining tog- other cohesive forces, there was an electrostatic attraction as well as an ether—requires that the particles touch and the cohesive (bin- induced-dipole attraction (two equal but opposite charges). The result- Cding) forces overcome any elastic (repulsive) processes that would ing new theory predicted that below a critical level of particle density, make the particles “bounce off” each other. there should be no supercoagulation and no power law in the distribu- Numerous experiments have shown that the ability of particles to tion of the size of particles. Subsequent tests carried out at lower levels stick to each other depends on their surface material and collision dy- of particles confi rmed this. namics. For instance, it is much harder to get rubber balls to stick to- This is a perfect example of how the classical scientifi c method gether than snowfl akes. But when microparticles were injected into works. A rapid coagulation (supercoagulation) process was observed neutral gas and a coagulation rate 100,000 times faster than predicted in experiments carried out on the ISS, a theoretical explanation was (based on geometrical–collision estimates) was achieved, the results developed, and its predictions were confi rmed with further space- presented a challenge. based experiments. It turned out that the particles injected into the neutral gas were charged: some positive, some negative. Thus, in addition to all the

This sequence shows the process of coagulation by consecutive sticking of particles in slow (non-destructive) collisions. MAX-PLANCK-INSTITUT

www.spacefl ight.esa.int 89 wide economy depends in some manner on quantum mechanics, a theory that fi rst took hold about 100 years ago. Nevertheless some concrete technological possibilities, derived from complex plasma research, are already being investigated. One is based on fi ne-particle control in plasma. The concept is to continuously remove dust in plasma ABetching and plasma vapour-deposition devices where this unavoidable by-product of the manufacturing process causes signifi cant problems, including reduced effi ciency. These devices are used for manufacturing items such as fl at screens, solar cells, and semiconductor chips. Other new developments are likely to occur in totally unexpected areas—for instance, in “plasma medicine,” an emerging topic of consid- C D erable worldwide interest. The idea is to use room-temperature plasma in a “touch-free” process to treat bacterial and fungal skin diseas- FIGURE 5. ELECTRIC CHANGES size (they’re far apart). Nevertheless, theory es. The plasma penetrates the smallest pores An electric fi eld applied to com- predicted that an electrorheological process and openings, but all the patient feels is a plex plasmas causes its properties might exist for complex plasmas, too. “soothing, warm wind.” There are numerous to change by rearranging its particles, a phenomenon called elec- Testing the theory on the ISS required an ex- additional possibilities being discussed, includ- trorheology. In the upper panel periment that the equipment in the PK labora- ing its use on chronic wounds, in dentistry, sur- 6.8-micrometre-diameter parti- tory was not designed to accommodate. The gery and catheters. Even some cancer treat- cles were used, in the lower panel hope was to observe “something”—perhaps ments may be feasible. 14.9-micrometre particles. In both the ordering of a normal fl uid-like state into a The detection of a new state of matter— cases the particles aligned along string fl uid, where the molecules are arranged complex plasma and plasma’s soft matter “strings” at the higher fi eld inten- in loosely coupled chains of various lengths state—will permit new experimental approach- sities, labelled (b) and (d). along a given direction. A string fl uid induces es to the investigation of some major unsolved dramatic changes in the fl uid’s physical proper- physics issues. These include an understanding ties, a change that can be exploited in, among of the organisation of matter at the individual other things, shock absorbers. particle level during phase transitions; obtain- Of course, just in case the predictions were ing a full theoretical kinetic picture of the ori- at least close, the cosmonauts needed to gener- gin of turbulence; comprehending the scaling ate three-dimensional scans through the com- phenomena at the transition point between gas- plex plasma and provide the greatest amount of es and fl uids at the kinetic level in phase space; diagnostic results possible (see Figure 5). The and developing a comprehensive awareness of results ultimately confi rmed the new theory the fundamental stability principles of con- and, in the process, validated the notion that densed matter. adaptable human experimenters onboard the For scientists, any major discovery is always ISS are critical for following up basic experi- an exciting opportunity, and this one is no ex- ments operated in a predetermined “robotic ception. It opens up new areas of investigation, mode” with available equipment. creates opportunities for a better understanding of nature’s puzzles, and may ultimately lead Real Work for Soft Matter? to new knowledge and applications that will Are there practical applications for soft matter? benefi t humanity. The fact that a science lab on Very likely, but instant results should not be the ISS played a critical role in revealing the expected. German Chancellor Angela Merkel plasma status of soft matter cannot be over- put it best when she said: “People should not looked. The examples presented illustrate expect a basic science discovery made in the what can happen when experienced, well- morning to deliver an industrial utilisation in trained experimenters working in weightless the evening.” For instance, it’s generally not conditions are able to carry out unplanned, ■ well known that about 20% of today’s world- follow-up research. MAX-PLANCK-INSTITUT

90 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research HE_Space_Advert_ScientificAmerican_full_bleed_080711.ai 63.2566.6770.71 lpi 71.57°18.43°0.00°45.00° 11.07.2008 11.07.2008 13:29:20 13:29:20 Prozessfarbe CyanProzessfarbe MagentaProzessfarbe GelbProzessfarbe Schwarz

PLANT BIOLOGY

ZZLE PLANTS

nimal and plant life have been inter- twined for hundreds of millions of A years. Plants have oxygenated the atmosphere, and the fact that our planet can sustain Roots grow downward while such a large number of people today is almost entirely due to genetic mutations in wild wheat stems shoot upward. Clearly, AT A GLANCE that occurred at the end of the last Ice Age. plants “sense” gravity, but how? Charles Darwin not only alerted the world to ■ Darwin hypothesized that Years of research in gravitational plant motion is based on the origin of species, he investigated and de- both internal processes scribed the behaviour of plants in response to biology have yielded some and external forces such their environment, an environment that in- remarkable new insights—and as gravity. cludes gravity. A strange and interesting form of life, plants pose a conundrum: in the presence striking contradictions. ■ Research shows that plants grown in near- of gravity their roots grow downward and away weightless conditions are from sunlight, while their stems and shoots By Dieter Volkmann, more sensitive to gravity grow upward and toward sunlight. How, then, than those grown under do plants sense gravity? At a very basic level, ev- Anders Johnsson and Earth’s gravity. ery plant must have a mechanism that responds František Baluška ■ Plants will grow in micro- to gravitational force by transforming physical gravity, but they show dis- stimulus into a biological process. turbances in cell division, The study of these effects on living organ- photosynthesis and stor- isms is called gravitational biology. Not a new age reserves. area of research, numerous investigations have

■ Only when multiple gener- produced a variety of results—some expected, ations are grown in space some unexpected, but always fascinating. A his- will we get a clearer pic- torical review of these data reveals different ture of plant response to pieces of the larger puzzle—the remarkable microgravity. adaptability of plants to a condition never en-

—The Editors countered in evolution: the near-weightlessness

SPACE CHANNEL / PHIL SAUNDERS of microgravity.

www.spaceﬂ ight.esa.int 93 Results from the plants in the centrifuge were the most surprising, indicating that the initial effects can be seen at values between one thousandth and one hundredth of the Earth’s gravity.

Seeds of Research 1g µg Our story begins more than 25 years ago. When 86 79 48 [gs] 79 75 44 we began our research, access to the real envi- 0 ronment of microgravity in space was limited. So our research began in laboratories with equipment that simulates the absence of gravity: 10 clinostats and centrifuges, random positioning machines and magnetic levitation. Later we were able to use sounding rockets, providing up 20 to six minutes of microgravity. In addition, we also set out to study the other side of the equation—hypergravity, or forces of gravity greater 30 than those experienced on Earth. By 1983 we were able to get out of the labs and into space. For experiments in microgravity, 40 we used cress and lentil plants because both ger- minate quickly. In cress seedlings cultivated during fl ight on the centrifuge under Earth’s gravi- 50 tational conditions, the plant roots showed lower sensitivity to gravitational force compared with those cultivated in microgravity chambers. 60 These findings suggested that an organism [min] ABOUT THE AUTHORS grown in microgravity conditions adapts its biological processes in such a way that the organ- 11˚ 20˚ 21˚ angle 62˚ 47˚ 44˚ Dieter Volkmann is professor at ism’s sensitivity to gravity is affected. the Institute of Cellular and Molec- Further experiments performed by three Eu- FIGURE 1. ROOT SENSITIVITY Cress roots cultivated on the centrifuge in orbit and ular Botany, Rheinische Friedrich- ropean research groups demonstrated that roots Wilhelms-Universitaet Bonn in stimulated by various levels of gravity including from cress and lentil plants are in general high- Germany. Earth’s (1g) and observed in stimulus free environ- ly sensitive to gravitational force. Three plant ment for 60 minutes under reduced gravity. Values František Baluška is professor at samples were used: one on the ground, one in the Comenius University of Bratis- across the top indicate stimulation time in seconds. lava in Slovakia. orbit in a centrifuge that varied its speed (its ac- Values across the bottom indicate angle of root celeration force can mimic the effects of gravity) bending in relation to root axis. Seedlings grown in Drs. Volkmann and Baluška have between Earth’s gravity and one thousandth of microgravity show more prominent root bending, in- been publishing together in the dicating a greater sensitivity to gravity than seed- fi eld of plant signalling and behav- Earth’s gravity, and the third, also in orbit, un- iour for several years. Their team der microgravity conditions (see Figure 1). lings cultivated under Earth’s gravity. in Bonn has participated in experi- Results from the plants in the centrifuge were ments on ESA’s Spacelab, sounding the most surprising, indicating that the initial ef- rockets and parabolic fl ights. VOL. 1996. PAGES 19, 1195-1202; fects can be seen at values between one thou- such astonishingly low levels of gravity have been Anders Johnsson graduated from sandth and one hundredth of the Earth’s gravity well established in animals, which suggests that Lund University in Sweden. He is (10-2 and 10-3g, Earth = 1g). Their root growth plants and animals, including humans, share professor at the Department of

Physics at the Norwegian Universi- not only showed the weakest response to gravity some common response mechanisms to gravity. ENVIRON, CELL PLANT ty of Science and Technology, but it also mirrored the type of growth seen in The responses seen at various levels of grav- NTNU in Trondheim, Norway. He water. The results from the centrifuge were what ity may give some hints about another puzzling has published on biophysics of one would expect from a plant in the neutral question—plant memory. A Venus ﬂ y trap, for plants and other areas in biophys- buoyancy of water (where a body’s weight de- example, will close after consecutive stimula- ics and has participated in experiments in Spacelab and on the ISS. creases by the amount of water it displaces). At tion of just two sensory hairs, but not one. In E-mail address: anders.johnsson@ some point in distant history, this was probably our research, every two minutes we applied to

ntnu.no. exactly how plant evolution began. Reactions at cress plant roots a stimulus less than or equal to IN TEWINKEL. M. AND VOLKMANN D.

94 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research the minimum value necessary for a response. After two to three stimuli, the cress plants re- sponded clearly to repeated stimuli.

Actin’ Similar If plants and humans show similar sensitivity to microgravity, there is presumably a reason. One place to look is at the role of the molecule complex actomyosin. The actin family is a diverse and evolutionarily ancient group of proteins, shared by nearly all forms of organic life. Under normal gravitational conditions, actin’s main roles in both the plant and animal kingdoms are that of structural support as a so-called cytoskeleton within the cells and as an intracellular transport system. Actin combines with another protein, myosin, to form the complex actomyosin, whose Low speed centrifuge microscope (NIZEMI). role is movement. Given that these proteins share the important role of providing intracellular “scaffolding” that responds to gravity, they may tle like a sediment, moved in a direction opposite also share other common behaviours that evolved to the original gravity vector. To isolate gravity and adapted to the same state of Earth’s gravity. as the stimulating force, chemical compounds This hypothesis was tested for the ﬁ rst time were added to destroy the actomyosin system. on sounding rocket ﬂ ights using roots from cress Under these conditions, the statoliths did not plants and single cells called rhizoids from the move. These results indicate that under normal green algae Chara that function like a root. A re- conditions the position of statoliths is deter- action was seen within six minutes of exposure mined by two counteracting forces: an internal

ESA SP-1222; 1999 (Statolith positioning) to microgravity in both the rhizoids and in sta- force exerted by the actomyosin system and an tocytes, specialised cells in the cress root caps external gravitational force. Statoliths can reach that are involved in gravity perception. Within a stable position in microgravity, but to do so re- that short time, heavy (with respect to the cell cy- quires a functioning actomyosin system. toplasm) and sedimentous particles called stato- Longer and more sophisticated experiments liths, which exist in a web of actin and which set- became possible with the advent of 10-day

Seedlings cultivated Seedlings cultivated on centrifuge under reduced gravity

Control, Earth’s gravity 1g 1g cultivation followed by 1g cultivation followed by 1g cultivation followed by 1g cultivation followed by 13 min reduced gravity 29 min reduced gravity 46 min reduced gravity 122 min reduced gravity

FIGURE 2. STATOLITH POSITIONING Gravity perceiving cells from lentil roots showing the nucleus (N) and sedimentable particles (a = amyloplasts acting as statoliths). In normal gravity, the amyloplasts settle like sediment, but when subjected to more time in reduced gravity they are spread throughout the cell. W. BRIEGLEBW. (INSTITUTE OF AEROSPACE MEDICINE, DLR COLOGNE, GERMANY) (Nizemi); D. DRISS-ECOLE AND G. PERBAL. SPACEHAB, ON IN: BIORACK

www.spaceﬂ ight.esa.int 95 Spacelab missions. On one of these missions in ences like gravity and light, they also asserted 1996, one group used ESA’s Biorack facility to that plant motion has an endogenous or inner observe the positions of statoliths in lentil root “character.” They called a plant’s rotational, cells when exposed to different gravitational rhythmic movement circumnutation, and it is conditions, including the absence of gravity. affected both by gravity and by internal pro- Again the role of the actomyosin system in sta- cesses. In simple terms, circumnutation can be tolith positioning was evident (see Figure 2). described as a spiral movement around a central axis such as the plumb line. If you were to Feel the Rhythm observe a quick-growing plant for a block of While Darwin and his son had emphasised that time, you would see the tip nod successively plant organs move in relation to external inﬂ u- around the points of a compass (see Figure 3).

FIGURE 3. CIRCUMNUTATION This time-lapse sequence of Arabidopsis was recorded over a period of 75 minutes. As the plant grows, it revolves around a central gravitational plumb line. The direction of these revolutions is represented by the blue arrow. When Charles Darwin ﬁ rst developed his circumnutation hypothesis, he realised it was possible to measure these movements by using markers placed on the plant and recording the change in position over time. This principle is still used today; the yellow dots show the change in position at different time intervals.

13:57 13:02 12:42 13:32 SPACE CHANNEL / PHIL SAUNDERS, SOURCE: B. G. B. SOLHEIM AND A. JOHNSSON (NORWEGIAN UNIVERSITY OF SCIENCE AND TECHNOLOGY)

96 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research Can clear differences in the behaviour of cell groups when exposed to microgravity be explained by the differences in family or species, or are there other forces at work? And if so what?

The reason for this is the unequal growth rates exclusively found in land-based plants—have of plant cells on either side of the central axis. been characterized as the “backbone” of plants Cells on one side of the plant’s axis stretch, living on land. Because these plants evolved in an while those on the other side do not; then vice environment subject to gravity, long-term exper- versa. These rhythmic and synchronized chang- iments in microgravity were expected to prove es occur as cells periodically change their shape that they need those backbone-like cell wall around the plant stem in order to grow upward. components to function properly. To create unifi ed revolutions, plant cells must be Two teams were involved in this investigation, in tune with and respond to the movements of but came to different conclusions. An American their neighbours. The period of circumnutation group could not fi nd substantial differences in in plants fl uctuates widely, typically between 30 cellulose microfi bril (very fi ne fi bre-like strand) and 120 minutes and amplitudes vary. organization, cell wall thickening and lignin Darwin’s hypothesis that these vigorous composition in wheat seedlings after 10 days of movements of plant shoots around the plumb spacefl ight compared to ground controls. As a re- line had an inner origin could be questioned since sult all plants were essentially identical in size, gravity-oriented movements could be the deci- height and cell wall thickness, regardless of the sive mechanism. In fact, several experiments gravitational fi eld they experienced. point to the importance of gravity in rhythmic However, a Japanese group obtained different movement. Some plants stop their circumnuta- results under similar microgravity conditions. tions under weightless conditions simulated by a They saw increased elongation accompanied by clinostat; in others, the period of circumnuta- an increased mechanical extensibility of cell tions changes on a centrifuge; and mutant plants walls and decreased cellulose volume for differ- that do not grow according to the pull of gravity ent plant species and organs. While cell wall (agravitropic plants) have shown less movement. composition and the underlying metabolism do To test Darwin’s hypothesis, studies on the vary according to plant families, sometimes even infl uence of gravity on plant circumnutation to the species level, the question remains: can were carried out during the fi rst Spacelab mis- clear differences in the behaviour of cell groups sion in 1983 when an international consortium when exposed to microgravity be explained by from the US and Europe used the stems of sun- the differences in family or species, or are there fl owers for its investigations. Careful recordings other forces at work? And if so what? showed that circumnutations did not totally Space-grown plants also showed a reduction cease in weightlessness. Even if their amplitude in certain photosynthetic activities. Under light- diminished drastically, rotations were less fre- saturated photosynthetic conditions, the trans- quent and had a shorter duration. It would ap- port rate of electrons was reduced by 28%. This pear that Darwin’s hypothesis, put forward transport process begins when chlorophyll re- more than 100 years ago, is basically correct: acts to light and loses an electron, which is then plants do have an endogenous “character,” but passed through a chain of individual reactions gravity plays a role as well. as a key source of energy. There were also changes reported in chlorophyll content, structure Details, Details and number of chloroplasts within plant cells, More than a century later, what Darwin called and a decrease in the number and size of starch “character” is the study of plant biomechanics, a grains in the chloroplasts. These results have large fi eld concerned with the mechanical prin- been supported by other experiments. No dif- ciples that govern a living organism, right down ferences, however, were found in photosynthet- to the function of each living cell. In plants, an ic activity at the moderate light levels. important area of biomechanical research is the Again, results from these early experiments cell wall. Cell wall components, in particular the were fraught with contradictions. The most re- composite of cellulose and lignin—the latter is cent space experiments were conducted using

www.spacefl ight.esa.int 97 offered around 10 days. To really move forward in this fi eld, we need to observe what happens when plants reproduce in microgravity. Only when a new generation of seeds can be grown in microgravity will any long-term developmental effects between seedlings be observed. So far most reproductive plant experiments have been performed on the Russian space station Mir and more recently on the International Space Station. One study showed that cell and organ polarity of young seedlings and mature plants cultivated in microgravity did not devi- ate signifi cantly from control groups. Several corresponding reports agree that gravity is not required for germination, proper growth or development of seedlings, plants and even embry- os of the second generation. Nevertheless, the quality of seeds from generations grown in microgravity is different. Ara- bidopsis thaliana, a small fl owering plant widely used as a model organism in plant biology, has been cultivated on Mir. Seeds from these plants had 20% less storage tissue than found in seeds harvested from ground controls. Examination FIGURE 4. ROOT GROWTH more advanced equipment with careful controls, of the cell chemistry of storage reserves showed Cress roots in ground controls and no remarkable differences in plant mor- that starch was retained in the spacefl ight group, (above), and under reduced gravi- phology and cell structures have been reported. whereas protein and lipids were the primary ty (below). Roots in the control In reviewing this research, one of the things we storage reserves in ground control seeds. Protein group grow straight down while those in the experimental group have learned is the importance of controlling for bodies of plants produced in space were 44% bend at various angles. changes in the gas exchange process. In micro- smaller than those in the ground control seeds. gravity, convection—physically driven heat Developmental markers indicate that seeds and transfer—is prevented. When that movement is pollen produced in microgravity were physiolog- reduced, exchange of gases such as oxygen, car- ically younger than those produced under bon dioxide and ethylene is incomplete and has Earth’s gravity. From those results, researchers to be supported by sophisticated equipment. hypothesized that microgravity limits gas mix- During early phases of development, seed- ing even inside tissues and that the resulting gas lings—particularly their roots—consume oxy- composition surrounding the seeds and pollen gen. In the later stages, the concentration of car- retards their development. bon dioxide as well as ethylene dominates plant Other researchers in the US and Europe have growth and development. Thus, sophisticated observed differences in developmental process- gas control systems are critical to separate the es at the cellular level between in-fl ight speci- effects of microgravity, heat transfer and even mens and controls (see Figure 4). These differ- radiation. Early experiments in microgravity ences were not only related to storage of mate- did not provide adequate control for gas ex- rials such as starch, lipids and proteins but to change and created a stress situation for the cell division when anomalies were seen in chro-

samples. This might be one explanation for the mosomes, cytoskeletal elements and cell wall , VOL. 73, PAGES 438-441; 1986. divergent and sometimes contradictory results. components. In particular, one report illustrated the negative effects of microgravity on the Sex in Space cell division cycle. The initial phase of this pro- It is important to note that not only is micro- cess was prolonged; and then, when the chro- gravity an extremely new area of plant research, mosomes began to divide, breakage and frac- but this research is limited by the time available ture occurred. This suggests that microgravity in space to carry out the work. Simply put, might exert stress on fast-developing seedlings plants take time to grow. Sounding rockets pro- as well as on cell cultures. Although plants can

vide only 12 minutes at most; ESA’s Spacelab adapt to this environment, coping with multiple D. VOLKMANN ET AL. IN NATURWISSENSCHAFTEN

98 LOOKING UP: Europe’s Quiet Revolution in Microgravity Research effects from reduced gravitational force, de- to adapt to novel stress situations never encoun- creased convection as well as radiation are prob- tered during the evolutionary process. ably responsible for these outcomes. This is undoubtedly good news for astronauts on future missions as live, growing plants Gene Machines would be a potential resource for food and oxy- Given the complexity of a living organism’s gen on long journeys into the depths of the solar response to gravity, genetics must play a part in system. However, there is an even more funda- those regulatory processes. Experiments have mental aspect to the research. In almost all been performed in ground-based laboratories ﬁ elds of biology, the deeper you probe as a re- using microscopic array analysis with Arabidop- searcher, the more complicated things appear. sis thaliana. The results showed differences in So the more you can isolate particular factors, the expression pattern for up to 200 genes that such as gravity, the clearer the picture becomes. code for different classes of proteins including It took the discovery of the Rosetta Stone to re- transcription factors, cell wall modifying veal that Egyptian hieroglyphics were not in- enzymes, cytoskeletal elements and signaling comprehensible pictographs, but a complex and molecules. The relative increase or decrease in sophisticated language. several genetic components occurred quickly— The “character” of plants is ultimately what within two minutes after changing the plants we are exploring, and future experiments in from a vertical (normal) to a horizontal position space must include research into sensing and (see Figure 5). signaling of gravity, gravity as a driving factor Because there are few research opportunities of evolution and plant adaptation to various on long term space ﬂ ights, most of the currently stress situations. Obviously if plants are to be- available data were achieved with experiments come important in human exploration of the so- performed under hypergravity and under simu- lar system, then we need to know much more lated weightlessness. Results from two groups about the production of multiple generations, using young seedlings and cell cultures of Ara- the maintenance of seed quality and mass pro- bidopsis thaliana suggest that changes occur in duction, especially if we want to use them for gene expression under various gravitational food and oxygen production. Plants might also FIGURE 5. QUICK RESPONSE conditions and in different classes of genes; and contribute to the psychological wellbeing of as- Cress root under normal gravity, changes in the latter have been seen in plants tronauts traveling vast distances across the so- 24h after activation in standard under other forms of stress. By comparing the lar system. Meanwhile, back on Earth, in a time vertical position (left), followed expression patterns of selected genes from Ara- when food production has suddenly become a by 2h stimulation in horizontal bidopsis thaliana seedlings, data showed that major issue, the more we can learn about plants position (right). The root quickly genes coding for cell wall modifying enzymes, the better—and microgravity research is now a responds to a change in the gravi- cytoskeletal elements and signalling molecules part of that ‘toolbox’. ■ ty vector. are regulated in an opposite manner under hypergravity versus simulated microgravity on a clinostat.

Adaptation and Exploration Viewing these gravity-based experiments retro- spectively, we begin to form a more complete picture of the various molecular and cellular effects that range from disturbances in cell division to changes in photosynthetic and reserve storage products. Because these ﬁ ndings often appear contradictory, it is important to inter- pret them in the context of plant type, culture conditions and experimental procedures. Nevertheless, a common theme can be teased out from this wealth of data: plants will ﬁ nally develop under these stressful conditions more or less normally, and they are able to produce seeds over subsequent generations. These results are a 1 mm

H. BEHRENS PETERS AND P. (BOTANY INSTITUTE, UNIVERSITY OF BONN) testament to the remarkable capacity of plants

www.spaceﬂ ight.esa.int 99 BIBLIOGRAPHY

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Seibert et al., A world without in microgravity. Planta, 203, S57-S62. gravity, p. 186-210. Noordwijk, The Netherlands: European Volkmann, D., Baluska, F., Lichtscheidl, I., Driss-Ecole, D., & Space Agency. Perbal, G. (1999) Statoliths motions in gravity-perceiving Jarvis, D. J., & Minster, O. (2006). Metallurgy in Space. Materials plant cells: does actomyosin counteract gravity? FASEB J, 13, Science Forum, 508, 1-18. S143-147. Hibiya, T., & Egry, I., Eds., (2005). Thermophysical Properties of High Temperature Melts—The Use of Microgravity. Measure- ment Science and Technology, 16, number 2, Special Issue. ESSENTIAL TOOLS Sounding rockets play a crucial role in European microgravity research. The rockets provide up to 13 minutes of high quality microgravity levels at a relative low cost. The short turnaround time and flexible working procedures make it possible for the scientist to include the latest ideas in their experiments. The experiments performed on sounding rockets have produced high quality scientific results in many disciplines.

Swedish Space Corporation has been a provider of complete sounding rocket systems since 1987.

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