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The Oddballs of the Solar System

The Oddballs of the Solar System

of the solartheof system oddballsThe PHYSICS & ASTRONOMY_Asteroids into the traditional categories. categories. traditional the into fit quite don’t that bodies system solar ”,small “active studying is Jessica Agarwal the At textbooks. in line official the was this years, many for – asteroids or either are the orbiting bodies Small 48 1.4 years, Phaethon approaches the Sun from theSunvariesconsiderably –every In thecourseofeachorbit,its distance the Sunonanelongatedelliptical path. identified in 1983 that the dust.ItcomesfromPhaethon, an omers havelongknowntheoriginof lar beliefwouldhaveit. make asecretwishcometrue,aspopu- the fleetingburstsoflightthatwill mosphere. Whenitdoes,produces vent forthisdusttoburnupinitsat- Ad- in preciselytherightpositionevery tiny grainsofcosmicdust,andEarthis These shootingstarsaremadeupof therefore knownastheGeminids. are the constellationGemini,andthey the meteorsappeartobecomingfrom extend thetrailsoflightbackwards, T TEXT MaxPlanckResearch 4 MaxPlanckResearch Taking amorerationalview, astron THORSTEN DAMBECK THORSTEN gift: shootingstars.Ifyou and 14 can hope fora special season betweenDecember4 the nightskyinAdvent hose who cast theirgaze to | 18 -

in Goettingen, in Max PlanckInstitutefor SolarSystem Research point amongthegeneralpublic. The the night sky even become a talking Sun. Then,thespectaculargoings-on in gas anddustwhentheyareclose tothe sometimes produce impressive tails of about sinceancienttimes andcan comets.” Thelatterhavebeenknown “Normally, meteor streams come from forSolarSystemResearch.Institute oftheMaxPlanck says JessicaAgarwal to bethesource ofameteorstream,” “It’s highlyunusualforanasteroid IN THE MAINBELT MOST ASTEROIDS surface andoutintospace. can catapulttinydustparticlesoffthe pressure fromtheintensivesunlight grees centigrade,andtheradiation kilometers wide,risestoover700de- surface ofthebody, whichisjustfive At thispoint,thetemperatureon (AU), oraround20millionkilometers. to adistanceof0.14astronomicalunits

that the asteroid asteroid the that the from images of sequence This illusion! optical An MarsandJupiter. belt. Thisliesbetweenthetwolarge Sun in what astronomers call the main sands, with most of them orbiting the cataloged intheirhundredsofthou 51). Theseobjectshavesincebeen page considered adwarfplanet(seeboxon resentative foralongtimebutisnow example, ,wastheirlargestrep- about foraround200years.Thefirst of ice. Asteroids have only been known too emitsdust,itdoessointheabsence taught intextbooks. ori swept awaybysublimatingiceandisthe dust that comets release into space is At any nuclei, and a dust tail. dust a and nuclei, two the envelops which , a – a of characteristics key the exhibits body tial celes the addition, In another. one orbiting ­gin oftheperiodicmeteorstreams. Phaethon isdifferent:althoughit rate, that is what has long been Hubble Space shows 288 P consists of two parts parts two of consists P

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Photos: NASA, ESA, and J. Agarwal (Max Planck Institute for Research) Photos: NASA, ESA, and J. Agarwal (Max Planck Institute for Solar System Research) September September September September August 22, 2016 22, 20, 29, 2016 9, 2016 1, 2016 2016 Cosmic odd and ends: the dwarf Ceres (left) is an asteroid, whereas Hale Bopp belongs to the family of comets. Until a few years ago, astronomers drew a clear distinction between the two classes because of their different features, but the boundaries are becoming increasingly blurred.

Asteroids and comets – Jessica Agarwal and which the researchers dated back ed the spot of light to be a large frag- knows her way around both fields, as to the year 2009. The following year, ment from an asteroid collision. An well as the boundary region between they observed the developing tail of X-shaped structure in the dust tail, di- them: “We conduct research at the in- P/2010 A2 for five months using the rectly behind the fragment, makes this terface between and geo- . It turned out asteroid a unique specimen. physics,” says the Goettingen-based that it was different from a normal Some of the active asteroids are also astronomer, whose work examines comet’s tail because, since comets emit referred to as main-belt comets. In their physical processes that can have a pro- gas and dust almost continuously while case, the astronomers suspect that the found effect on these small celestial they are near the Sun, they usually have dust emission is driven by water ice or bodies. One of the scientist’s work a fan-shaped tail. other volatile substances that subli- is an even more elusive subgroup of the mate and sweep near-surface dust par- minor planets: the so-called active - A UNIQUE X-SHAPED ticles away in the process, as happens oids. Phaethon belongs to this particu- STRUCTURE IN THE DUST TAIL in comets. One example of this group lar group of oddballs, whose common of objects is 288P, which follows an el- feature is that they can emit dust. A pa- In contrast, the linear tail of P/2010 A2 liptical path in the outer main belt and per published in 2015 already included suggested that it was formed by a sin- takes 5.3 years to complete a full orbit 18 such specimens, most of which are gle, brief event. On the first detailed of the Sun. only a few kilometers in size, with the Hubble photos, the researchers noticed This has only been notable examples of Scheila and Ceres, a spot of light and estimated the diam- known about since 2006, after the which have estimated diameters of 113 eter of the corresponding object to be telescope in Kitt Peak Na- and 975 kilometers respectively. around 120 meters. “The nucleus al- tional Observatory, Arizona, discovered Asteroids become smaller and smaller most seemed to be detached from the a small, weak spot of light; this tele- due to collisions with other small bodies dust cloud,” says Agarwal, who at that scope is specially designed for small so- in what Agarwal compares to a grind- time was still analyzing the images at lar system bodies. Just five years later, ing process. One example is the colli- the ESA’s research center in Noordwijk, the object was observed during several

sion that struck the object P/2010 A2 the Netherlands. The experts interpret- months of activity close the Sun. The Photos: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA (left), E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at)/CC-BY-SA 3.0

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The Main

Sun photos clearly showed a short dust tail ter of gravity; both have a diameter of pointing away from the Sun and an ap- around 1.8 kilometers. The striking proximately six-times-longer trail run- thing is their relatively large separation ning along its direction of motion. of approximately 100 kilometers. Even at that time, the long period of ac- The active is a good tivity pointed to the sublimation of ice example of how these bodies can turn as the driving force for dust emission. out to be fascinating objects. They al- September 2016 offered especially low researchers to study – live, so to Rich source: most asteroids orbit the Sun favorable conditions for observing speak – the processes of change they are on trajectories between the planets Mars and 288P, which came as close as 1.45 AU undergoing. 288P probably originates Jupiter, in what is known as the main belt. to Earth, providing a great opportunity from a collision that long ago destroyed for Jessica Agarwal and her colleagues a precursor body with a diameter of to unravel further secrets about their around 10 kilometers. That instant, ing, it is a very young family. A second object of study. High-resolution photos some 7.5 million years ago, marked the collision at a later stage might then taken by the Hubble Space Telescope birth of not only 288P but also a whole have split 288P into two fragments. Or now clarified what previous images had family of asteroids. At least eleven it might already have been a binary ob- only hinted at: 288P is binary! In other members of this group have already ject when it emerged from its violent words, it consists of two separate com- been identified, with their similar or- birth. “Neither option can be ruled ponents of roughly the same size, bits around the Sun betraying their out,” says Jessica Agarwal. “We don’t which orbit about their common cen- common origin. Astronomically speak- know exactly what happened.” >

AN ACTIVE GIANT

On the scale of the entire solar system, Ceres is only a dwarf, asymmetric hilltop rising from its center – this is the scene of with an average diameter of 975 kilometers and just 0.28% of past cryovolcanic activity. Scientists from the Max Planck the mass of the lightest planet, . However, it is a gi- Institute for Solar System Research have discovered that salt ant among the active asteroids, most of which are just a few solutions escaped from this location until well into recent kilometers in diameter. Since June 2018, the geological history. When the water evaporated, it left be- has been orbiting Ceres on a new, highly elliptical path. At hind light-colored deposits that spectroscopic measure- points, this NASA probe is only around ments have identified as 35 kilometers from the surface, bring- carbonate. Further bright spots in ing Dawn closer than ever to its object the eastern part of the crater are of study since its arrival in March 2015. probably also points at which a From this short distance, it has now water/salt mixture came to the been able to take detailed photos, surface. Several current studies some of them with ten times better suggest that the crust of Ceres resolution, of the 90-kilometer-wide contains large quantities of water Occator crater (see image), providing ice. It is likely that a steady stream the best view yet of the ominous of smaller impacts and bright spots that Dawn had previ­ously landslides uncover deposits of ice, discovered there. which then evaporate. This creates In the middle of this impact crater an extremely thin atmosphere of is a depression with a strikingly bright, water vapor, or “exosphere”. Graphic: designergold based on a draft from the MPG; photo: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

4 | 18 MaxPlanckResearch 51 disappeared from the scene entirely and has never been seen again. again. seen been never has and entirely scene the it end, from the In disappeared rock. of lumps large of agglomeration loose a once was object the Presumably, 52 the subliminggascouldproduceajet- near thesurface,then recoil from “For example,iftherearepatchesofice kind canbetriggeredbyvariouscauses. affect asteroids–andadramaofthis to occurincometsforsometime,also events, whichhavealreadybeenknown quickly aroundtheirownaxis.Such other words,whentheyrotatetoo be tornapartbycentrifugalforces, in scenario. Smallcelestialobjectscanalso There is, however, another plausible Total disintegration: the asteroid P/ asteroid the disintegration: Total MaxPlanckResearch 4 MaxPlanckResearch SHREDDED ASTEROIDS and Apollo. Evidently, the rotation of the P/ asteroid active the of rotation the Evidently, Apollo. and image). (see rotation asteroid’s the of acceleration or deceleration cause can effect the addition, In plane. orbital its to antiparallel or perpendicular, parallel, aligns then which rotation, of axis body’s the of position spatial the alter can This torque. continuous a producing directionally, body the from emitted is diation ra thermal when sunlight. It of occurs influence the under asteroids, as such ies, bod celestial small of state rotation the of alteration is agradual YORP effect The The effect was first demonstrated in 2007 by studying the asteroids YORP asteroids the studying by 2007 in demonstrated first was effect The Sun | 18 2013 R 3 practically crumbled before the astronomers’ eyes. hot of the mutual orbit, causing the two an importantroleinthedevelopment tation. Thiseffectmayalsohaveplayed precisely such a process of escalating ro- gine” effecttore288Papartthrough latest analysessuggestthatthis“jeten- up afterjustafewthousandyears.The stand,” saysAgarwal. level thatthebodycannolongerwith- tively quickly–untilitexceedsacritical like effectthatspeedsuprotationrela- cold Therefore, theasteroidcouldbreak zievskii, and and zievskii, ers research the the of surnames in letters first the of acronym an is effect the of name The forces. centrifugal resulting the to due frag ments ten least at into up it broke that extent an such to effect YORP the by ated Y arkovsky, arkovsky, 2013 R P 3 addack. O was acceler ’Keefe, R ad - - - - - ­ below). Anothervariant,knownasthe eventually tearthebodyapart(seebox of heat–untilthecentrifugalforces tion withsunlightandthere-radiation to speedup–simplydueillumina- small, irregularlyshapedcelestialbody YORP effectcancausetherotationofa over longperiodsoftime,theso-called cess that can alter a body’s rotation: ture of288Pinthefirstplace. allowed the largeseparationseentoday–which ther apartuntiltheyultimatelyreached components to movefurtherand fur- enly classifieditasacomet. Couldthe distinct tail, the researchers first mistak- quantities of dust,thereby producing a known asarubblepileasteroid. weak gravitationalpulltoform whatis loosely heldtogetherbythebody’s tion ofdustandlargelumpsrock, ably nothingmorethananagglomera- it wasdiscovered.P/2013R3prob- tember 2013–justafewmonthsbefore and Sep- eral stages between February the asteroidbeganbreakingupinsev- dividual fragments.Thisindicatesthat to extrapolatethe trajectories of thein Keck telescope, on Hawaii, can be used 100,000 tonnes. mass theresearchers haveestimatedat by acloudofdustanddebris,whose than 200 meters. This was accompanied largest ofwhichhasaradiusless fragments havebeenidentified,the caused by the YORPeffect:at least ten rapid rotation,whichwasprobably as theasteroidfellvictimtoitsown The researchers had an almost live view andothercolleaguesin2014. Agarwal fornia inLosAngelesalongwithJessica JewittoftheUniversityCali- David asteroid P/2013R3,aswitnessedby this ledtothefragmentationof and furtherapart. gether again–ormovethemfurther systembackto- ponents ofabinary YORPeffect,canbringthecom binary Experts alsoknowofanotherpro- Photos takenbyHubbleandthe As thecelestialbodyreleased large A particularlydramaticexampleof to reveal the binary na- Hubble torevealthebinary - -

Photos: NASA/ESA/D. Jewitt (UCLA) (top); graphic: designergold based on a draft from the MPG PHYSICS & ASTRONOMY_Asteroids

asteroid also have been destroyed by an impact? Agarwal thinks this is un- likely because, as she explains, the speed of the fragments is far too slow to have resulted from a collision. Like- wise, “jet effects” due to the sublima- tion of frozen substances – above all water ice – are highly unlikely, as there was no indication whatsoever of pro- longed sublimation-driven dust activity.

VANISHED NEVER TO BE SEEN AGAIN

When studying active asteroids, you al- ways have to be prepared for surprises. “P/2013 R3 disintegrated, disappeared, and has never reappeared since. The object P/2016 G1 was obviously just a dust cloud, or at any rate the nucleus was too small to be visible. P/2013 P5, on the other hand, exhibited nine highly individual dust tails,” says Jes­ sica Agarwal. This booming area of re- search has also drawn the attention of the space agencies. In January 2018, for example, the ESA considered launching a mission to several active small solar An eye for the unusual: Jessica Agarwal from the Max Planck Institute for Solar System Research in Goettingen dedicates her time to studying active asteroids. These celestial bodies have special system bodies, although initially this characteristics, such as being made up of multiple nuclei or outwardly resembling a comet. was merely a theoretical study. The Japanese space agency, JAXA, has made

more progress: with the Destiny+ space probe, it is planning to venture forth to SUMMARY the source of the Geminid stream – that l In recent years, researchers have discovered that there is no clear dividing line is, to Phaethon – with a launch planned between tail-forming comets and asteroids.

for 2022. l A group of transition objects in the are referred to as active Four years later, if everything goes asteroids because they can release dust and form tails.

to plan, a dust detector on board the l Asteroids have also been discovered that consist of a binary body. probe could be inspecting the raw ma- l Some asteroids seem to consist only of a dust cloud, while others break up before terial that brings shooting to researchers’ eyes. Earth’s sky each December; the instru- ment is being developed at the Univer- sity of Stuttgart. But Phaethon has a GLOSSARY volatile nature, with active phases also : One astronomical unit (AU) is the average distance between Earth followed by long periods of relative and the Sun and is equal to 149.57 million kilometers. tranquility. If Jessica Agarwal could Cryovolcanism: A form of volcanism in which volcanoes do not eject hot lava, as on wish for a target object for a space Earth, but rather methane, carbon dioxide, water, or . These substances are probe, her choice would be a different present in a frozen state inside a planet or . one, however: “288P! It shows comet- Sublimation: On Earth, substances such as water pass from the solid state to the liquid like activity, and the large separation state and then into the gaseous state. When one of these phases is skipped and a substance between its components is unique. That passes directly from the solid to the gaseous state, the process is known as sublimation.

Photos: NASA/ESA/D. Jewitt (UCLA) (top); graphic: designergold based on a draft from the MPG Photo: Ronald Schmidt/AFWK would be my favorite.”

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