Rendezvous with a Primordial Rock

Although the comparison with the manned moon landing may appear somewhat exaggerated, Rosetta is undoubtedly one of space travel’s most daring enterprises: For the first time in history, a probe is accompanying a comet on its orbit around the Sun – and in mid-November, it set down the Philae lander on its surface. Scientists from the Max Planck Institute for Solar System Research in Göttingen have front row seats for the evaluation of the images and data from the comet named 67P/Churyumov-Gerasimenko.

TEXT HELMUT HORNUNG

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This is the very heart of the comet! 67P/Churyumov-Gerasimenko as seen by the onboard camera of the Rosetta probe on August 3, 2014.

MARCH 2, 2004, 8:17 a.m. CET

arkness still shrouds the Gerasimenko. Two scientists, Churyu- Optical, Spectroscopic and Infrared Re- Kourou spaceport in French mov and Gerasimenko, at the Institute mote Imaging System. Two cameras Guiana as an Ariane 5G+ for Astrophysics in Alma-Ata/Kazakh- constructed as mirror systems take im- rocket shoots up into the stan had discovered the celestial body as ages in the ultraviolet, visible and infra- cloudy sky, leaving a fiery a tiny star on a photographic plate in red spectral ranges. The light-sensitive D trail in its wake. On board is a freight the fall of 1969. CCD detectors comprise 2048 by 2048 that is intended to shed more light on In the past, the gravitational field of pixels, with each of these pixels mea- the origins of the solar system: Roset- the planet Jupiter had influenced the suring 13.5 micrometers (one-thou- ta. The probe itself is slightly larger trajectory of “Chury.” Before 1840, it sandth of a millimeter). than a Smart car. Its two long wings orbited the Sun at a considerable dis- Before its rendezvous with Šteins, with solar panels for the energy supply tance, and the lack of heat meant it had Rosetta had flown past Earth on March give it the appearance of a strange in- not been able to develop any cometary 4, 2005 at a distance of just 1,955 kilo- sect. Aboard Rosetta, in addition to the activity up to this point. In other words, meters. At that time, it was even possi- SREM detector for detecting high-en- the nucleus should still be relatively ble to see the probe with binoculars. On ergy particle radiation, are eleven in- fresh and pristine – which researchers February 25, 2007, Rosetta passed Mars; struments and a box as large as a refrig- see as a benefit. In its present orbit, the on November 13 of the same year, it erator: Philae. It is to perform the trick comet approaches the Sun to within a again gained momentum with the aid of landing on the nucleus of a comet distance of around 193 million kilome- of our planet. After the rendezvous and conduct measurements there over ters (Earth’s distance to the Sun: 150 with Šteins, it executed another Earth several months with the aid of ten sci- million kilometers) every 6 years and fly-by on November 13, 2009. entific devices. 203 days. Why is this extremely complicated Rosetta’s history goes back to 1984. trajectory necessary? A direct flight At that time, the European Space Agen- SEPTEMBER 7, 2008, 10:14 p.m. CEST from Earth to Churyumov-Gerasimen- cy ESA decided to undertake a mission ko would have required enormous to the nucleus of a comet, initially in Signals from the depths of space appear quantities of fuel. During the so-called partnership with NASA. After budget on the screens at ESA’s European Space swing-by maneuvers on tortuous paths, cuts forced the Americans to withdraw, Operations Center (ESOC) in Darmstadt. Rosetta gained the energy it needed the Europeans continued the plan alone. The flight engineers immediately for- from the gravitational fields of the In Germany, the German Aerospace ward them to the Max Planck Institute planets free of charge, as it were. Dur- Center and the Max Planck Institute for for Solar System Research. Although it is ing each of the three Earth swing-bys, Aeronomics (since 2004: Solar System the middle of the night, 14 scientists the speed of the vehicle increased by Research) were initially the major play- here filter the first images from the raw around 20,000 kilometers per hour. ers in the project, with the latter making data. These show a rock with an elongat- a significant contribution to the devel- ed shape that tapers to a point at one JULY 10, 2010, 6:00 p.m. CEST opment and construction of the lander, end; countless craters cover its surface, among other things. with a particularly large crater measur- The pace is infernal: Rosetta is travelling On January 13, 2003, Rosetta and ing two kilometers in diameter located at no less than 54,000 kilometers per Philae were due to lift off to the Wir- at the North Pole. The subject is around hour as it points its camera eyes at the tanen comet. But a rocket from the Ari- 360 million kilometers from Earth – and asteroid Lutetia. This cosmic potato – ane 5 series, like the one that was to shows the Šteins asteroid. Rosetta passed its longitudinal dimension of around transport the ambitious mission, had by it on September 5, 2008 at a distance 126 kilometers making it significantly performed a spectacularly unsuccessful of 800 kilometers. larger than Šteins – has a varied land- lift-off a few weeks prior. The European Although the OSIRIS telecamera scape with mountains, large numbers space program was halted temporarily, switches into safety mode nine minutes of large and small craters, scattered and the start of the comet scout post- before the rendezvous and only the boulders and parallel grooves. The sur- poned by more than a year. Moreover, a wide-angle camera is working, the re- face of the celestial object seems to be new destination had to be found. The searchers are satisfied with their instru- covered by a thick layer of fine-grained,

Photo: ESA/Rosetta/MPG for OSIRIS Team MPG/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA choice was ultimately 67P/Churyumov- ments. OSIRIS is the abbreviation for loose material (regolith). >

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In October 2012, the probe reaches the point of its trajectory furthest from the Sun; around 795 million kilometers separate it from the warming rays. At this enormous distance, even the 64 square meters of the solar sail generate only small amounts of power. Never- theless, it is sufficient to keep the vehicle reasonably warm and a clock on board ticking. JANUARY 20, 2014, 10:59 a.m. CET

An alarm clock rings in space. Its job is to bring Rosetta out of hibernation after 957 days. The probe is whizzing Ragged space potato: Rosetta photographed the two asteroids Šteins (left) and Lutetia during around in the depths of space, nearly the fly-bys in September 2008 and July 2010. Despite the fleeting encounters, it succeeded in obtaining extremely detailed views, particularly of Lutetia. Craters, mountains and scattered 815 million kilometers from Earth. It’s rocks can be seen on its surface. the beginning of anxious hours of wait- ing for the technicians, engineers and scientists in the European Space Oper- With 3.5 grams per cubic centimeter, vides images with a resolution of 60 ations Centre (ESOC) in Darmstadt. Lutetia has an unusually high density. meters per pixel. Some images even Will Rosetta really wake up? If yes, From the spectra, specialists conclude provide indications of a landslide the plan is as follows: The star sensors that it has a composition comparable within a crater. All in all, it’s a success- will slowly heat up to operating temper- to those of meteorites belonging to the ful dress rehearsal for the meeting ature, open their eyes and determine Ro- carbonaceous chondrites, which have with Churyumov-Gerasimenko. setta’s position from the sky. The steady a high carbon content. However, there rotation of the probe about its axis will are also similarities with the so-called JUNE 8, 2011 gradually come to a halt, and the para- enstatite chondrites, which contain bolic dish measuring 2.2 meters in diam- the mineral enstatite. After the engineers and technicians had eter will align itself toward Earth and Rosetta can’t solve the mystery of installed upgraded software for the cen- send the first sign of life. which family Lutetia belongs to, but tral computer in December 2010, they the scientists are certain of one thing: switched it off. Some months later, and JANUARY 20, 2014, 7:18 p.m. CET “This is a completely new world that slightly earlier than planned, on June no one has ever seen before,” says Hol- 8, 2011, they then put the complete Owing to the huge distance, it takes a ger Sierks from the Max Planck Insti- spacecraft into hibernation. Rosetta bit more than 45 minutes until the sig- tute for Solar System Research, head of now slowly rotates about its own axis nal from Rosetta reaches Earth, is re- the OSIRIS team. The instrument pro- for stabilization. ceived by a dish in Goldstone, Califor-

THE ROSETTA STONE AND THE ISLAND IN THE NILE

In 1822, Egyptologist Jean-François Champollion (1790 to 1832) suc- Before the start, ESA announced a competition to find a ceeded in deciphering hieroglyphics. He did this by analyzing the name for the lander. Philae came out on top – named after Rosetta Stone, which bears the same text in three languages: hiero- an island in the Nile on which a temple complex once glyphics, demotic and Ancient Greek. Taking its inspiration from stood. One of the remaining obelisks bears an inscription this stone, the Rosetta mission aims to help solve the mystery of in Greek and Egyptian, and was used to help decipher hi- comets and the early evolution of the solar system. eroglyphics. Photos: ESA/Rosetta/MPG for OSIRIS Team MPG/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

48 MaxPlanckResearch 4 | 14 UMWELT & KLIMA_Biochemie

Comet experts gather: Martin Hilchenbach, Holger Sierks, Paul Hartogh and Hermann Böhnhardt (from left) discuss the latest images of the Rosetta space probe in the Max Planck Institute for Solar System Research.

nia, and appears as a small line in an in sight after a ten-year trip through edge of our planetary system beyond irregularly jagged green curve on the space is an indescribable feeling,” says Neptune’s orbit is more than 30 times monitor in Darmstadt. Still, some sci- the Max Planck researcher. The unas- as far from the Sun as the Earth is, and entists had expected that this would suming little star is none other than is populated with thousands of cosmic take place around 6:30 p.m. on January 67P/Churyumov-Gerasimenko, as seen rocks. Under the influence of Neptune, 20, 2014. It is now already after 7:00 through OSIRIS’ eyes. the orbits of individual bodies gradu- p.m. and Rosetta has been overdue for The camera system on board Roset- ally shift toward the other gas giants three-quarters of an hour. ta must operate to its performance lim- in the planetary system – right down The nervousness is increasing at its. After all, more than five million ki- to Jupiter. ESOC. Everyone present is looking up to- lometers separate the space probe and “Despite this migration – and the ward a monitor, as if spellbound. The the comet from one another. This ex- collisions that may have preceded it – clock in the control room shows 7:18 plains why the comet on the photos comets in the Jupiter family are some p.m. when a line finally appears on the covers only a fraction of a pixel. What’s of the purest material that has been pre- screen – first short, then slowly becom- more, the celestial body glimmers like served from the birth of the solar sys- ing longer. Two technicians at the con- a very dim light; a series of exposures tem more than 4.6 billion years ago,” soles are the first to throw up their arms. from 60 to 300 seconds and additional says Ulrich Christensen, Director at the The cheering starts. People hug each oth- image processing are required to make Max Planck Institute for Solar System er. A glass breaks somewhere. Rosetta is it visible at all. Research. While particularly the inner awake! It’s now setting off on the final While Rosetta and its instruments are planets, such as Mercury and Venus, stage of a more than ten-year journey. slowly awakening, Churyumov-Gerasi- have undergone great changes as a re- menko is still in a kind of slumber. This sult of the heat and the particle bom- MARCH 21, 2014 is typical behavior for a comet. As with bardment from the Sun and have lost all members of the Jupiter family, it volatile components, this material has The picture looks as if it was taken by spends most of its life in the icy depths been stored in an unaltered state in the a well-equipped amateur astronomer. of the planetary system at a similar dis- ice of the comets for billions of years. The magnificent globular cluster known tance from the Sun as the gas giant Jupi- When the cosmic vagabond moves as Messier 107 in the Serpent Bearer ter. This is where countless bodies mea- nearer to the Sun on its orbit, its surface constellation shimmers slightly left of suring several kilometers in size whiz begins to warm up, and water and fro- center. Diagonally above M 107 there about as dead lumps of rock and frozen zen gases evaporate and entrain tiny appears a weak spot of light, which a gases, such as carbon dioxide and ice. dust particles. The comet becomes ac- layperson could easily overlook. But In all probability, “Chury” and its tive: an atmosphere (coma) forms for Holger Sierks, it’s something very peers have their origins in the Kuiper around its nucleus, and it finally devel-

Photo: Ronald Schmidt special: “To finally have our destination Belt. This ring-like region at the very ops the characteristic tail. >

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This material is what Rosetta will inves- croscope before being bombarded with molecules. Water and other substanc- tigate in greater detail than ever before. indium ions. Ions are then released es leave characteristic fingerprints on “The space probe is a kind of laborato- from the surface of the dust particles in the light in this wavelength range. ry that is operated on-site at the com- this process and can be analyzed fur- “The signals that water molecules et,” says Max Planck researcher Martin ther. “We can identify not only individ- leave in our measurement data can be Hilchenbach, head of the COSIMA ual elements, but also, above all, organ- detected particularly well,” says Paul team. COSIMA is one of the instru- ic molecules,” says Hilchenbach. Hartogh from the Max Planck Institute ments that are intended to specifically for Solar System Research, who super- coax some of the secrets out of the APRIL 30, 2014 vised the development and construc- cometary dust. In the microscopic, cau- tion of a subsystem of MIRO. The re- liflower-shaped pores of carriers mea- “Chury” really is a comet! At least the searcher is pleased with its sensitivity: suring just a few millimeters in size, the images taken by OSIRIS show a real it’s as if one were to discover the evap- dust catcher collects individual parti- coma. It stretches around 1,300 kilo- oration of a cup of hot tea on the cles that are first localized under a mi- meters into space and envelops the nu- moon from Earth. cleus with gas and dust. The research- Five at one go: The circled regions were ers are astonished by this cloud, JULY 14, 2014 selected as landing sites and designated A, because the comet is still more than B, C, I and J. J – located on the “head” of the comet’s nucleus – ultimately won. 600 million kilometers from the Sun. A rubber duck is orbiting in space! At However, the splendor doesn’t last very least this is what the images that show long: photos from the beginning of Churyumov-Gerasimenko from a dis- June show 67P as a tiny star again with tance of less than 12,000 kilometers no activity whatsoever. The comet ap- suggest; on Earth, this would corre- parently woke up too early and imme- spond roughly to the distance between diately nodded off again. Germany and Hawaii. The images from OSIRIS further reveals that the nu- the middle of July prove that the nucle- cleus, which measures five by three ki- us of the comet consists of two clearly lometers, rotates about its axis once separated parts: a larger “body” on every 12.4 hours – 20 minutes shorter which a smaller “head” sits. than determined previously from Earth. If this isn’t due to a measure- JULY 21, 2014 ment error, something must have re- duced the rotational time. This proves The rubber duck has a collar. Rosetta is once again that comets are always now just 5,500 kilometers away and good for a surprise. providing images with a resolution of 100 meters per pixel. The images clear- JUNE 6, 2014 ly show that the “neck region” located between the “head” and the “body” ap- The comet is sweating. It’s releasing a pears much brighter. This is where an large glass of water every second, or abyss some 1,000 meters deep opens more precisely, around 300 grams of up, and where the camera discovers so- water vapor. This was determined by called jets – fountains of dust. The an instrument called MIRO. The small brightness of the band could be caused radio telescope receives signals from a by differences in material, different respectable 350,000 kilometers; this is grain sizes or topographic effects. It is now the separation between Rosetta still unclear how 67P got its duck-like and 67P. shape. A few researchers speculate as to MIRO analyzes the microwave ra- whether the “head” and “body” were

diation that originates from the gas originally two separate objects. Photos: ESA/Rosetta/MPG for OSIRIS Team MPG/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

50 MaxPlanckResearch 4 | 14 Photo: Roland Keller/www.rkech.blogspot.com ity inApril–isfinallyfullyawake. asimenko –afterabriefphaseofactiv- And soitseemsthatChuryumov-Ger- a distanceofmere2,500kilometers. but itcan’t beimaged by structure shouldbesignificantlylarger, particle densityishighest.Thetotal ner regionofthecoma,inwhich lometers. Itappearstobeonlythein- view, anareameasuring150byki- completely fillsthecamera’s fieldof around thenucleus.Thediffusecloud ages showanextensivecloudofdust The comahasappearedagain.im- JULY 25,2014 The spaceprobeinitiallyapproached “triangular” orbitaboutthe nucleus. midal orbitsaround67P, following a signal. Darmstadt hadreceivedtheredemptive ment, theEuropeanControl Centrein Sierks. Justbeforehemadethisstate- says MaxPlanckresearcher Holger almost unrealtohaveactuallyarrived,” approach fornearlyadecade,itseems billion kilometers.“Afterflyingonthe Arrived atlast! AUGUST 6,2014,11:30a.m.CEST to the right of Sierks’ index finger Cheops. Cheops. finger index christened Sierks’ of right the to researchers nucleus; the of asection to points system, camera OSIRIS the of head Sierks, Holger “Chury”: into Zoom Rosetta hadswiveledintopyra- Rosetta hastravelled6.4 Rosetta from partially formedbythecomet’s activi- also beseen. smooth surfacesandroundhillscan escarpments nowappear. Expansesof sharp-angled cliffs,mountains differences inbrightnessonthesurface, nuclei.Inadditiontostark cometary pixel, surpassingallpreviousimagesof tion oftheimageswas5.5metersper neuver onAugust6,thespatialresolu- been 50kilometersandless. since thebeginningofSeptember, ithas the comettowithin100kilometers; cleus of 67P/Churyumov-Gerasimenko cleus of67P/Churyumov-Gerasimenko conditions thattheregionon thenu- interesting:thesearethe scientifically shade, nottoomuchsunlight −and As levelaspossible,not too much AUGUST 25,2014 create differentgeologicalformations. where theyaccumulateinplacesand the comet,theyfallbacktosurface, slow toleavethegravitationalfieldof these dustparticlesaretooheavyor vaporized andentrainedjetsofdust.If Sun, volatilesubstancesfromitssurface ty. Duringearlierapproachestothe It isalandscapeofstrangebeauty, Even shortlybeforethethrustma- and deep lar batteries.However, the Sunmust least sixmonthstocharge dayforat every light arenecessary landing. Moreover, sixhours ofsun- level aspossibleshouldensure asafe mands anddata.Asurface thatisas order toexchangeoperationalcom- dio contactwiththespaceprobein landing site,theremustberegularra- still hasno-goareas. the landeritself,mapofcomet different possibledetachingspeedsof ment entations ofthespaceprobeatmo- ceivable velocities,trajectoriesandori- if onetakesintoaccountallthecon- This doesnotapplytoallregions.Even Lead Scientistofthelandermission. Institute forSolarSystemResearch, mann BöhnhardtfromtheMaxPlanck the areasinfirstplace,”saysHer- course, wasthat nated A,B,C,IandJ. er “body”ofthecomet.Theyaredesig- “head,” andtheothertwoonlarg- of thepotentiallandingsitesareon nounce fivepossiblecandidates:three day, researchers andengineersan- where Equally importantisthat,fromthe “The mostimportantcriterion,of Philae Philae undocks,aswellthe willlandmustfulfill.To- 4 | 14 Philae MaxPlanckResearch MaxPlanckResearch the rock isabletoreach Philae ’s so- 51 THE ONBOARD INSTRUMENTS...

…of Rosetta …of Philae OSIRIS* camera system with telecamera and wide-angle camera SD2 drills and sampler ALICE UV spectrometer COSAC* gas chromatograph, mass spectrometer VIRTIS spectrometer for the visible and infrared range Ptolemy gas chromatograph, mass spectrometer MIRO* microwave spectrometer APXS** alpha X-ray spectrometer RSI radiowave experiment ROLIS camera system CONSERT* radar tomograph CIVA camera system, infrared spectrometer, microscope ROSINA* neutral gas and ion mass spectrometer MUPUS penetrator, thermal probe, radiometer COSIMA* dust mass spectrometer SESAME* acoustic seismometer, dust monitor MIDAS dust microscope CONSERT* radio wave experiment GIADA dust analyzer ROMAP*** magnetometer, plasma monitor RPC plasma analyzer

* Participation of the Max Planck Institute for Solar System Research ** Participation of the Max Planck Institute for Chemistry *** Participation of the Max Planck Institutes for Solar System Research and for Extraterrestrial Physics

not shine for too long, either, other- wise the lander could overheat. ROSETTA SEPTEMBER 10, 2014

Mapping a comet. The images transmitted by OSIRIS achieve a resolution of 75 centimeters per pixel. With areas char- acterized by steep cliffs, depressions, craters, scattered boulders and parallel grooves, 67P exhibits many different landscapes. Some of these regions appear quiet; others were apparently formed by the activity of the comet. The scientists combine the various expansive landscapes into a map – and puzzle over it: “So far, nobody really un- derstands how the morphological differences that we see have formed,” says Max Planck researcher Holger Sierks. SEPTEMBER 16, 2014

J wins. Although the images show a very rugged terrain there, almost in the center of the “head” of the comet, com- putations show that the chances for a landing are good. The researchers and PHILAE technicians take into account the to- pography of the area and the mechan- ical characteristics of Philae’s landing gear. The spot on which Philae touches down can be determined to within only 500 meters or so. “This means we need, not one per- fect spot, but a complete region where as many landing scenarios as possible have a happy end,” says Hermann Böhnhardt. This area is dotted with relatively few large rocks that could pres-

ent a danger to Philae, for example. In Graphics: ESA/ATG medialab PHYSICS & ASTRONOMY_Solar System Research

addition, measurements indicate that organic material is present here. Final- ly, CONSERT should find very accept- able conditions for its radio observa- tions at the landing site selected. CONSERT is the only experiment of the Rosetta mission that is part of both the orbiter and the lander. It is to use radio waves to investigate the inner structure of the comet’s nucleus. A radio signal will be transmitted from the space probe through the nucleus to the lander and back. The shape of the nucleus and the flight path of the orbiter mean that not every spot on the surface of 67P is equally suited to penetrate the whole of the comet’s nucleus. Landing site J is one of the best sites for this task. On October 14, 2014, J is confirmed. It is named “Agilkia.” One of the greatest On firm ground: This image of the surface of comet 67P/Churyumov-Gerasimenko was adventures of space travel can begin. snapped by the CIVA camera on board the Philae lander. At the front left, one of the three landing legs can be seen. NOVEMBER 12, 2014, 9:35 a.m. CET cold gas thruster should now push the legs. The first images show rocky mate- Already on November 8, the ground lander gently onto the comet’s surface, rial and a cliff. Sunlight reaches the land- crew had sent Philae the computer se- harpoons should be launched, and the ing site for only one and a half hours a quence that controls the landing via its screw-like tips of the three landing legs day, so the solar-powered batteries aren’t orbiter. On the morning of November should also bore into the surface. But taking in enough energy. Thanks to the 12, the Rosetta mother ship is still hov- the cold gas thruster and the harpoons primary battery, however, the board in- ering more than 22 kilometers above fail to work! Philae bounces off and struments start to record data and con- the surface of the comet. After the con- floats for approximately 110 minutes duct measurements as planned. trol center gives the final “Go!” three around one kilometer further away. This “critical phase” of the scientif- preloaded springs maneuver Philae The probe then touches down again, ic work ends after just under 60 hours. away and push it off into space at 9:35 lifts off slightly and, after a second lit- On November 15, 1:36 a.m., Philae falls a.m. The refrigerator-sized box drifts tle “hop,” finally lands for a third time. into an automatic sleep mode; contact away from Rosetta. Around two hours with it is interrupted. Whether the after separation, data transmission NOVEMBER 15, 1:36 a.m. CET lander will make contact again remains starts. The lander sends signals to the uncertain. One thing, however, is cer- orbiter, which transmits them to Earth The exact landing site is initially un- tain: the mission has been successful. at the speed of light. Since the comet is known. It’s possible that Philae touches Rosetta will revolutionize our knowl- flying through space at a distance of the ground with only two of its three edge of comets! more than 500 million kilometers from

our planet, these signals take 28 minutes and 20 seconds to arrive. Around two hours after separation, TO THE POINT the OSIRIS camera aboard Rosetta turns ● Rosetta has been travelling through space for more than a decade already. its gaze toward the landing probe as it In August 2014, the probe reached its destination, the comet 67P/Churyumov- retreats. “Philae is making good prog- Gerasimenko.

ress,” says Hermann Böhnhardt. The ● Rosetta is the first mission in the history of space travel planned to accompany current image proves that the landing a comet on its orbit around the Sun for several months, and put down the Philae gear has unfolded as it was intended to. lander on its nucleus in mid-November 2014. ● Churyumov-Gerasimenko has an elongated, two-part shape. It reminds some NOVEMBER 12, 2014, 4:34 p.m. CET researchers of a rubber duck, with the “body” and “head” connected by a bright “collar.” The nucleus measures approximately five by three kilometers. Touchdown, Philae has landed! The ● With steep cliffs, depressions, craters, scattered boulders and parallel grooves, 67P probe touches down at a speed of ap- exhibits many different relief features.

Photo: ESA/Rosetta/Philae/CIVA proximately one kilometer per hour. A

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