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Newfound asteroid is companion to Earth, scientists say

April 7, 2011
Courtesy of Armagh Observatory
and World Science staff

A re­cently dis­cov­ered as­ter­oid has probably been fol­low­ing Earth around the Sun for over a quar­ter of a mil­lion years, and may be made of si­m­i­lar stuff to our plan­et, two re­search­ers say.

The space rock caught their eye two months af­ter it was found by the WISE in­fra­red sur­vey sat­el­lite, launched in 2009 by the Un­ited States. “Its av­er­age dis­tance from the Sun is iden­ti­cal to that of the Earth,” said Apos­to­los Chris­tou of North­ern Ire­land’s Ar­magh Ob­serv­a­to­ry of the aster­oid. The re­search, by Chris­tou and Da­vid Ash­er of the ob­serv­a­to­ry, ap­pears in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

“What really im­pressed me at the time was how Earth-like its or­bit was,” Chris­tou said. Most near-Earth as­ter­oids have very ec­cen­tric, or egg-shaped, or­bits that take the as­ter­oid right through the in­ner so­lar sys­tem. But the new ob­ject, des­ig­nat­ed 2010 SO16, was found to move on an al­most cir­cu­lar path so that it can’t come near any plan­et but ours.

Since as­tro­no­mers of­ten don’t in­i­tially know the pre­cise loca­t­ion of a new­found as­ter­oid, the two sci­en­tists es­ti­mat­ed it by cre­at­ing cre­at­ing computer-sim­u­lat­ed “clones” of the as­ter­oid for every or­bit it could con­ceivably oc­cu­py. They then sim­u­lat­ed the ev­o­lu­tion of these clones un­der the gra­vity of the Sun and the plan­ets for two mil­lion years in­to the past and in the fu­ture.

They found that all the clones closely mim­icked our or­bital mo­tion around the Sun, al­though as seen from Earth, they would seem to slowly trace out a horse­shoe shape in space. As­ter­oid 2010 SO16 probably takes 175 years to make the trip from one end of the horse­shoe to the oth­er, the in­ves­ti­ga­tors said.

So while on the one hand its or­bit is re­markably si­m­i­lar to Earth’s, “it keeps well away from the Earth,” To­lis said. “It has likely been in this or­bit for sev­er­al hun­dred thou­sand years, nev­er com­ing clos­er to our plan­et than 50 times the dis­tance to the Moon.” This is where it is now, near the end of the horse­shoe.

These “horse­shoe” or­bits are not un­com­mon: Earth has at least three oth­er “horse­shoe com­pan­ions,” ac­cord­ing to the as­tronom­ers. But, un­like the newly stud­ied one, the oth­ers lin­ger for a few thou­sand years at most be­fore mov­ing on to dif­fer­ent or­bits, Chris­tou and Ash­er said. Al­so, the new­found one is much larg­er, with an es­ti­mat­ed width of 200 to 400 me­tres or yards. 

Where the as­ter­oid came from is un­der in­ves­ti­ga­t­ion, the pair said. One possi­bility is that it could have “leaked” from a popula­t­ion of ob­jects near so-called tri­an­gu­lar equi­lib­ri­um points 60 de­grees ahead of and be­hind the Earth in its or­bit. Such a popula­t­ion has been pos­tu­lat­ed but nev­er ob­served, as such ob­jects would al­ways be near the Sun in the sky. If they do ex­ist, they may also rep­re­sent rel­ic ma­te­ri­al from the forma­t­ion of Earth, Moon and the oth­er in­ner plan­ets, sci­ent­ists say.


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A recently discovered asteroid has probably been following Earth around the Sun for over a quarter of a million years, and may be made of similar stuff to our planet, two researchers say. The space rock caught their eye two months after it was found by the WISE infrared survey satellite, launched in 2009 by the United States. “Its average distance from the Sun is identical to that of the Earth,” said Apostolos “Tolis” Christou of Northern Ireland’s Armagh Observatory. The research, by Christou and David Asher of the observatory, appears in the journal Monthly Notices of the Royal Astronomical Society. “What really impressed me at the time was how Earth-like its orbit was,” Christou said. Most near-Earth asteroids have very eccentric, or egg-shaped, orbits that take the asteroid right through the inner solar system. But the new object, designated 2010 SO16, was found to move on an almost circular path so that it can’t come near any planet but ours. Since astronomers often don’t initially know the precise location of a newfound asteroid, the two scientists estimated it by creating creating computer-simulated “clones” of the asteroid for every orbit it could conceivably occupy. They then simulated the evolution of these clones under the gravity of the Sun and the planets for two million years into the past and in the future. They found that all the clones closely mimicked our orbital motion around the Sun, although as seen from Earth, they would seem to slowly trace out a horseshoe shape in space. Asteroid 2010 SO16 probably takes 175 years to make the trip from one end of the horseshoe to the other, the investigators said. So while on the one hand its orbit is remarkably similar to Earth’s, “it keeps well away from the Earth,” Tolis said. “It has likely been in this orbit for several hundred thousand years, never coming closer to our planet than 50 times the distance to the Moon.” This is where it is now, near the end of the horseshoe. These “horseshoe” orbits are not uncommon: Earth has at least three other “horseshoe companions,” according to the astronomoers. But, unlike the newly studied one, the others linger for a few thousand years at most before moving on to different orbits, Christou and Asher said. Also, the newfound one is much larger, with an estimated width of 200 to 400 metres or yards. Where the asteroid came from in under investigation, the pair said. One is that it could have “leaked” from a population of objects near a so-called triangular equilibrium points 60 degrees ahead of and behind the Earth in its orbit. Such a population has been postulated but never observed as such objects are always near the Sun in the sky. If they do exist, they may represent relic material from the formation of Earth, Moon and the other inner planets.