[meteorite-list] Earth Gets a New Companion for Trip Around Sun (Asteroid 2014 OL339)

[Ron Baalke]

http://www.newscientist.com/article/dn26291-earth-gets-a-new-companion-for-trip-around-sun.html

Earth gets a new companion for trip around sun
by Rebecca Boyle
New Scientist
30 September 2014

Add one to the entourage. A newly discovered asteroid called 2014 OL339 
is the latest quasi-satellite of Earth - a space rock that orbits the 
sun but is close enough to Earth to look like a companion.

The asteroid has been hanging out near Earth for about 775 years and it 
will move on about 165 years from now, say Carlos and Raul de la Fuente 
Marcos of Complutense University of Madrid in Spain, who have just described 
it.

Quasi-satellites orbit in resonance with Earth, allowing the planet's 
gravity to shift the rock's position much like an adult pushing a child 
on a swing, says Martin Connors, an astronomer at Athabasca University 
in Canada. The asteroid orbits the sun every 365 days, as Earth does, 
but Earth's gravity guides it into an eccentric wobble, which causes the 
rock to appear to circle backward around the planet.

Earth's retinue

The asteroid, which is between 90 and 200 metres in diameter, is among 
several different categories of space rock in Earth's retinue besides 
our one satellite, the moon. Rocks that hang out at a gravitational middle 
ground known as a Lagrange point, where they follow or lead Earth in its 
orbit, are called Trojans.

Mini-moons, meanwhile, are small asteroids that get sucked into Earth's 
gravitational pull and orbit the planet, but only for a few months or 
a year, says Paul Chodas at NASA's Near Earth Object Program. He spotted 
what appeared to be a mini-moon back in 2002, but it turned out to be 
the third rocket stage of the Apollo 12 lunar mission.

Most planets and even some large asteroids are accompanied by hangers-on. 
With four quasi-satellites catalogued so far, Earth comes in second only 
to Jupiter's six, though the gas giant probably has many more that we 
can't see. The same is probably true of Earth, as small space rocks are 
difficult to find - astronomers didn't spot the first till 2004.

"If you go into your kitchen and you see some big cockroaches, you know 
there are a lot of little ones there, too," Connors says.

Journal reference: arxiv.org/abs/1409.5588v1