news and views teakre Small bodies of the Solar System Don Yeomans

,-...... -..-.- "_.."..._..".."....".._..I...... "...... _ "_.""".." "...... " "...... _..._.." ....".."-..- ..-.... "....""... Discoveries of that behave like and asteroids that behave like comets are making us reassess our view of Earth's smallest neighbours.

cientists have a strong urge to place Mother Nature's objects into neat Sboxes. For most of the past halfcentury, the comets and asteroids of the Solar System did seem to belong in two separate popula- tions -each within their own box. The rules werethatcomets,withawiderangeoforbits, were solid, dirty iceballs originating in the so-called at theedge of the Solar System. Asteroids were definedas bitsofrock anfimed mostly to a region between Mars and and travelling roughly in the same plane and inthe same direction as the planets about theSun (Fig. 1).Fromtjme to Lime over the past 50 years, objects were bund that did not really belong in either )OX, but they were onlyconsidered as occa- iional exceptions to the rules. Within the Figure 1 The usual view of corneta and utcroida. The inna Soh System cuntlinr the and the ,ast few years, however,Mother Nature has four terreatrid planar:, Venu%Euth and Ma.fie lump of mdc that make up thenuin ticked over the boxes entirely, spilling the bdt between him and Jupiterorbit the Sun in the meplane andthe medirection PJ the :ontents and demanding thatscientists rec- planets. Most tometa have highly dlipticnl orbits, whichmeans they spend most of their timein the )gnizE crossover objects - asteroids that outer reache of the Solar System with only briefpasages dose to the Sun. )ehave like comets, and comets that behave ike asteroids. As a result, the line between accepted that this asteroid, and a handfulof throughold Palomar Sky Survey plates Dmets and asteroids is no longer clearly others that have associated meteor streams, showed thatthe orbit predicted for this Irawn. are defunctcomets that have lost the ability asteroid was identical to thatof a dis- to emit gas and dust. covered by Wilson and Harrington in1949 :ros.over oqects A few years earlier,in 1977, the asteroid (ref. 6). So asteroid 1979 VA had been a The modern model for the nucleus of a Chiron hadbeen discoveredin an orbitthat comet 30 years earlier and is now known as :omet began with Fred Whipple in 1950-5 1. takes it from just inside the orbit of Saturn 107P/Wilson-Harrington = (4015) Wilson- Vhipple's 'dirty iceball' model for a to just inside the orbit of Uranus (Fig. 2, Harrington. 'ometary nucleus proposed a solid body, a overleaf). There are now a few dozen ofthese The third object to receive a dual desig- ew kilometres across, that is made up of so-called Centaurs, asteroids whose orbits nation is 133PIElst-Pizarro = (7968) Elst- ,arious ices (frozen water, methane, ammo- lie in the outer planetary region. Although Pizarro. Its orbit is very similar to thatof a lia, carbon dioxide and hydrogen cyanide) initiallylabelledasanasteroid,byearly1988, main-belt asteroid circling the Sun between n which dust is embedded12.This model can when it was approaching its minimum dis- the orbits of Mars and Jupiter, but it dis- xplain the impressive dust tails we associate tance from the Sun (its perihelion), Chiron played a temporary, comet-like dust tail in vith comets passing the Sun. Dust particles began to act in a decidedly non-asteroidal 1996(ref.7). re liberated when the ices vaporize as the and more comet-like way. First, it became omet approachesthe Sun, and they get abnormally bright; then in 1989itdeveloped Crossed paths llownaway by solar radiation pressure, a dust atmosphere; and by January 1990 In the 1950s Jan Oort argued that comets ften forming impressive, gently curved cyanogen gas emission was detected spec- with long orbital periods (millionsof years) lust tails. tros~opicallf~.Chiron was the first object must spend most of their time in a vast As the comet ages, dust becomes strewn to receive a double designation as both an spherical cloud surrounding the planetary I1 the way around its orbit. So when the asteroid and a comet. It is now known as system. There is no direct observational evi- arth intersects this streamof cometary the ninety-fifth periodic comet (9SP) and dence for this so-called Oort cloud, but it is ebris, a (or storm) is the the two-thousand-and-sixtieth numbered thought to extend outto about 100,000 tsult. Almost all well observed meteor asteroid (2060). So we have 95P/Chiron= times the Earth's distance from the Sun, or bowers are associated with known comets. (2060) Chiron. ~OO,OOOastronomical units (AU). Later work 'his was a neat, easily understood picture To date, three objects havebeen officially on the origin of the Long-period comets f the cometary ageing process. But it was recognized as having splitpersonalities and established that they formed in the colder fihipple himself who pointed out in 1983 have receiveda dual designation. The second region between the orbits of Jupiter and lat the orbitof the Geminid meteor stream is asteroid 1979 VA, which was discovertd Neptune as the leftover bits and pieces from 'as verysimilar to that of a recently discov- in 1979 in an eccentric, comet-like orbit. It the formation of the Solar System. Once red asteroid (3200 Phaethon) rather than a comes as closeas the Earth to thesun. so one formed,manyofthesecometssuffereddox met. Asteroid 3200 Phaethon is in a rather would expect it to prnduce gas near its peri- gravitational encounters with themajor :centric,comet-likeorbit,anditisgenerally helion if it werean active comet. A look back planets and were thrown either out of the 839 news and views feature

formation, they offer clues to the primitiv‘e composition and conditions of the earlY Solar System.Spacecraft missions are key tl 0 understanding what is inside these object!I, and future generationsmay actually benefiit from these explorations. Interplanetar Y colonists will need to exploit the minera I, metal and water supplies of cornets an1d asteroids, and mission planners will need tl 0 know which objects are the richest in naturaI1 resources and the easiest to mine. It is possible that theultimate answers tc D all these questions will not be found until th e smallbodiesoftheSolarSystemareexplomd moredoselybyspacecraft (Box I). Nonethe lessthelossofourstandardpictureofcometS and asteroids is already providing a morse diverse spectrum of possible structures -- from porous balls of ice to solid rocks ant slabsofiron.

Fire2 cmamver objeaa in the outer %luSyetam There PIL dasteroids (ruch u (3iron) Comets In transition whose orbitr tokc them outride the main utcroid ut, into the out- Solu System. (%iron has an If all comets were solid,.dirty balls of water of lo yeusand has been secnto emit gn, MJ dust like a comet. Other ostaoidr, such ice, then theirbulk densities would be about as 1996 PW (orbital period, 5,900ycus) luia 1997 MDlO (orbital period, 140 yeam), have very 1 g cm”. But it seems that some comets havc e eccentric orbits, more like tharc of comets than asteroids. rather crumbly, low-density structures that are made from several bits held together b! Y Solar System altogether or into the distant the first objects in the Solar System to bedes- little more than their own self-gravity. Thi:S Oort cloud. ignated as asteroids with retrograde orbits. conclusion arose because some comets werc Upon reaching the Oort cloud, some As mentioned earlier, object Elst-Pizarro observed to breakupas a result of tidal force:E bodies are thrown back into the planetary has been given a dual comet and asteroid from either the Sun or Jupiter, and mort system by the gravitational perturbations designation because it has shown cometary than two dozen othershave split apart for nc ) of individual passing stars, the Galactic disk activity even though its nearly circular orbit obvious reason at all. of stars, or giant molecular clouds of gas and is very similar to those asteroids in the main The most dramatic example of a tidall]I dust. Coming from the roughly spherical belt between the orbitsofMars andJupiter. split comet was the disintegrationof come1t Oort cloud, these long-period comets enter We now have comets in asteroid-like Shoemaker-Levy 9 intomore than tM the inner Solar System with random pro- orbits and asteroids in comet-lie orbits. dozen fragments when the comet passerI grade (sa,me direction as planets) or retro- Both comets and asteroids can evolve from close to Jupiterin July 1992 (Fig. 3). This Wac 5 igrade (opposite direction to planets) orbits. the Oort cloud into highly inclined, even before it crashed into thesurface ofthe gian!t This is unlike the short-period comets (peri- retrograde, orbits about the Sun, so orbital planet two years later. Some contemporar)1 ,ods less than 200 years), which have come behaviour is no better than physical behav- press accounts reported that “the might)r Iunder the gravitational influence of Jupiter, iour for telling them apart. Our attempts to tidal forces of Jupitertore the comet tc ) and usually orbit closer to the main planeof sortcometsandasteroidsintoseparateboxes pieces”,but the reality was rather less impres- 1the Solar System in a prograde direction. havefailed and astronomersshould now sive. The tidal acceleration on thecomet wa8i Most of the objects in the Oort cloud are considertheseobjectsasmembersofahighly nomorethanawimpy3mms-*or0.0003g.11f 1probably comets that formed in the outer diverse family - the small bodies of the you could have held a piece of comet Shoe SIolar System. But up to 3% of the current Solar System. maker-Levy 9 in your hand, it would have Fmpulation could be asteroids that formed easily broken apart with only modest pres- jl ust inside Jupiter’s orbit and then were New family vduem sure. Ifwe assume roughlyequalsizes for the Flushed out, by way of gravitational inter-. The blurring of the boundaries between six comets that have split as a result of tidal a dons with Jupiter, to the very edge of the comets and asteroids forces us to reassess our forces from Jupiter or the Sun, then comet Siolar System.The peculiar asteroid 1996 PW knowledge of the nature and origin of the Shoemaker-Levy 9 suffered the greatest Could be one of these objects (Fig. 2). it small bodies of the Solar System. From time tidal disruptive force. It seems likely that Shows no comet-like activity and yet it has to time, some ofthese objects come too close the other five comets were at least as fragile a very eccentric orbit and an orbital period to Earth for comfort, and the skies are being and probably more so. atf about 5,900 years, indicating that it is anxiously scanned for asteroids on a colli- A comet made up of discrete chunks and li,kly to have evolved back into the inner sioncoursewithEarth.1fwedofindathreat- held together by little more than self-gravity S‘olar System from the Oort cloud’. ening object then we need to understand its is best described by the ‘rubble pile’ model9. In addition, there areseveral other aster- structure andcomposition, because plans to Arubblepilehasalmostnointernalstrength, 0tidal objects in highly eccentric orbits - deflect a loose, fragmented structure out of very high porosity and correspondingly low 0 nce considered the hallmark of comets. our way would be entirely different from bulk density. This model could explain how 7’hese include (3200)Phaethon with an those to deflect a solid rock. a comet like Shoemaker-Levy 9 can break 0 rbital period of 1.4 years, 1997 MDlO with Astronomers are also interested in the up under very modest external forces. It a n orbital period of 140 years, and the structure of comets and asteroids because could also explain why some comets,suchas t-1 xently discovered 1999 LE31 and 1999 they represent some of the least processed comet Biela in the 185Os, can break apart L D31 with orbital periods of 23 and 120 material in the Solar System. As the left- before disintegrating completely into a Years, respectively.The latter two objects are over bits and pieces from early planetary stream ofmeteoric particles with no remain- 830 news and views feature

iing nucleus. Possibly the most fragilecomets are created by millions of years of mutual I:ollisions in theouter planetary system, 1Nhere the nuclei are first broken apart and t:hen re-accrete as loosely bound rubble Ides". Or, near the end of their active life- t ,imes, comets may lose the ices that cement t ogethertheseparatepieces. Comets that have already gonefrom adve to quiescent (for example, Wilson- 1farrington) suggest that some bodies do t mme defunct and join the ranks of the Figure 3 Frapcnta of Cornet Shocnuker-Lcvy 9. Thii ahort-periodcomet was in orbit around asteroids. CometEncke, with its stable orbit Jupitulndsplitinto pie- Wfien it got too dose to the giant planetin July 1992.This image ww Vvithin the orbitof Jupiter, is generally con- taken by the Hubble Space Telescopein March 1994, four months before the cometdived intothe Sidered to be an active comet in transition to atmosphereof Jupiter. a n asteroidal object. Comet lSP/Finlay may aIs0 be a low-activity transition object. It is two groups, the C-type andS-type. C-type to be the most primitive because they haw tl he only active comet with an orbit suitably asteroids have low reflectance () and not been chemically differentiated, whm Close to that of the Earth's that does not may contain mixtures of hydrated silicates, S-typeasteroids may have experienced inter- generate meteor showers, suggesting that carbon and organic compounds.S-type nal or external heating that has separated it : produces very little dust". Low-density asteroids have higher and can con- themintodifferentlayersofmaterial(similax extinct comets can probably explain a sig- tain pyroxene (silicates containing mag- to the Earth's separation into a core, mantle n,ificant fraction of the near-Earth asteroid nesium, iron and calcium), olivine (mag- and crust). The metals found in some S- Population, so we cannot assume that all nesium and iron silicates), and nickel-iron and "type asteroids can be explained by 0bjectsthatthreatenEarthwiuhavethtspe metal. melting processes similar to those seen in CI Dmpositionorstructure. The darker, C-type asteroids are most volcanic rockson Earth. common in the outer partof the mainaster- Meteorites areasteroid collision frag- U)own to Earth oid belt, whereasS-type asteroids are mostly ments that have fallento Earth, and as such Asteroids have been classified according to found in the inner asteroid belt. The less are thought to hold clues regarding the early tk le light reflected from theirsurfaces- their commonM-typeasteroids containmixtures history of asteroids. Because most asteroid 0:ptical spectra. Although no two spectra are of nickel-iron metal and magnesium or fragments are rocky, they can survive the 0cactly alike, most asteroids fall into oneof iron silicates. C-type asteroids are thought passage throughthe Earth's atmosphere, - Box 1 Love at first sight

I3n Valentine's day (14 February) ThernassoftheasmMhasbeen 2000, the Near-Earth As$rold detennii by tracking the spacecraft I?endezms (NEAR) spacecraft was as it slowly abits Ems. By dividing !successfully placed in orbit about the the asternid's mass by L volume. a i Isternidbas-namedfwtheGreek preliminary estimateof 2.6 g cm" god of he.This successful has been determined for the rendem took place after the NEAR asteroid's bulk density -abcut the spacecraft had already madean en densityoftheEarth'scrustAlthOugh mute fly-by of the C-lype astemid it S too early to draw definite MaMide in June 1997 and a @-byof conclusions. the asteroid's densty bas itself in December 1998. This coUPledWithsOmelayered~reS Imosaicoffwrimageswastakenon Seen in tha Eros images imply Uut 14!+iuary2000atadlstanceof Emsisnot~porousastheasterdd about 320 km frm Em. The view Mathilde; obsanred by NEAR in 1997. 1- i1- down on the nath polar ragion As well as the Imaging camera t1 Eras. Currently, the orbiting NEAR usedtotakathispiciure,lheNEAR 3pacecraftcan cnly image me sunlit spacecraft carries a full suite of Iwrthem hemisphere: images of the sclentmc instruments, hcludlng an physical make-up and sbuclure of occurred on 22 and 23 March 2000. Ismahhg Ems landscapa will not be infrared speammeter b identii th~sotyect and to aentny what l)p Intense sdzv X-rays from these flare IXLSsiiMe until the SOumem various minerals. a lidar altimeter tor of meteorite might be produced by an events caused certah chemical tlemisphere becomes sunlit In June detennming the details of (he S-type asteroid like Eras. elements on me sum d Eros to mcQ. asteroid's shape. a magnetometer for Althcugh only designed to operate fluoresce with characteristic energ& The large crater in this image is chaacterizing the &emid's at an dtitude d 50 km or less. the allowing the detection of magnesium, f i km acrcss, and cl-ddles a boulder magnetic field, and an X-ray and lidar began working successfully at aluminum, silicon, caklum and imn. 5lbout100minsire.EmsismS-type gamma-ray spectrometer to its first opportun~Qm29 February The NEAA spacecraft will remain in aIstemid, which is heaviiy cratered determlne its chemical composition. 2000 at a distance of 290 km from orbt abcut Eros fw a year, making Cwer most of its surface, suggestmg AstheNEARspaceaaftmovesto Eros. The X-ray spectrometer has detailed studies of its surface 11hat it is relatively old. Images like lower and lower about Bm. also been able to deliver useful morphology, shape and magnetic Uhis one are used to determine the these measurements will be used to informatlon earlier than anticipated fieid, as well as its minerabgicd and 1: 1: hape of Ems and hence ItE volume. Improve w knowledge of the Banks to three so(ar flares that chemlcd composlbon. IY. news and views feature

important in the unlikely went that one is found on an Earth-threatening trajectory. The best technology to deflect an object out of harm’sway would dependcritically on the nature of the object itself. More than one deflection strategy is needed to deal with fluffball ex-comets and slabs ofsolid iron. Those small bodies that most closely approach Earth are also the most accessible forthe future mining of theirnatural resources. In terms of landing a spacecraft on their surface, there are several asteroids that are more accessiblethan theMoon itself If the inner Solar System is to be colonize( within thecomingyears, the necessarymate rials for interplanetary structures, such a: denrity(-2.6 gan”). habitats, are likely to come from the weaIth o minerals and metals provided by asteroidsU whereas debris from comet streams nearly have been even less than it is now. Because it costs several thousand dollars pel always burns up in the atmosphere, some- Mathdde and Eugenia must have very pound to launch materials into Earth‘s orbi 1timesproducingspectacular meteorshowers porousstructures (greater than 50%) ifthey and beyond, it would be far more cost effec. in the sky but leaving little evidence on the have the same composition as meteorites tive to build these structures from nearb) surface of the Earth. The most common found on Earth. Meteorites that are thought natural resources found in space. The warel Imeteorite is theordinarychondrite, whiJlis to be collision fragments from C-type aster- supplies necessary for sustaining life and Immposed mostly of rocky silicates, and SO oids have bulk densities about twice that of for providing hydrogen and oxygen fol 1has not experienced the chemical ditTerenti- their parent asteroids. There is growing evi- rocket fuels are also likely to come from ation associated with melting. These are dence that at least some asteroids have interi- comets. Asteroids and comets may one day 1:hought to be some of the most primitive orstrucruresdoselyresemblingrubblepiles. become the workshops, or fuelling stations 1rocks in the Solar System, although their The NEAR spacecraft orbited an S-type and watering holes, of future planetary 1,arent asteroid type is not clear. asteroid (433 Eros) earlier this year, and the exploration. I On 22 March 1998, an ordinary chon- asteroid‘s bulk density was found to be Don Romans is at thelet Propulsion Laboratory, < kite was seen to fall to Earth by seven boys in 2.6gcm”(Box 1). In 1993 theGalileospace- California Instituteof Echnolop, Pasadena, IMonahans, Texas. W~thin48 hours,this craft imaged the S-type asteroid 243 Ida and Califinria 91 209, USA. Ineteorite was being examined at the John- its moon (Fig. 4b). The bulk densities calcu- e-ma& donaLikymmans@!iplna.gov Sion Space Center in Houston, Texast2.Labo- lated for both S-type asteroids -Eros and 1. whipple. F. 1. Mmphyx I. I 11.375394 ( 1950). I’atory analysis of the Monahans meteorite Ida - are about twice that of the C-type 2. Whipplc,ELAYmphlr/.113,16U74(1%1). 3. Thdm. D. 1. Harmann, W. K. & CruWank. U. P. IAU Ck. (letected crystals’embedded with water salt asteroids Mathilde and Eugeniat7-I9,so S- NR 4554 (1980). I n the formofbrine, andthe salt crystals were type asteroids may be more solid than their 4. Bur, 5. I.. BMl,E Hams, A W. & Hewin. A. V. fcunu Z’.

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