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Small Bodies of the Solar System Don Yeomans

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Small Bodies of the Solar System Don Yeomans news and views teakre Small bodies of the Solar System Don Yeomans ,-.......-..-.- "_.."..._..".."....".._..I........... ".........._ "_.""".." ".......... " "............_..._.." ....".."-..- ..-.... "....""... Discoveries of comets that behave like asteroids 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 Oort cloud at theedge of the Solar System. Asteroids were definedas bitsofrock anfimed mostly to a region between Mars and Jupiter 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 Sun and the ,ast few years, however,Mother Nature has four terreatrid planar:Mercury, Venu%Euth and Ma.fie lump of mdc that make up thenuin ticked over the boxes entirely, spilling the asteroid 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 comet 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 meteor shower (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 orbital period 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 astronomers should 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.
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