This is an amazing discovery. Up until now, companion asteroids that more or less share an orbit with Earth have been theoretical. This class of asteroids is known as Trojans; Jupiter has them, and so do Mars and Neptune. Now Earth joins the club with 2010 TK7, an asteroid approximately 300 meters (1000 feet) across and 80 million km (50 million miles) from Earth.
|Asteroid 2010 TK7 is circled in green, in this single frame taken by NASA's Wide-field Infrared Survey Explorer, or WISE. Image credit: NASA/JPL-Caltech/UCLA|
NASA’s Wide-field Infrared Survey Explorer (WISE) managed to notice the small asteroid. WISE sees in the infrared, where warmer objects are easier to spot. 2010 TK7 is probably about the freezing point of water, which is pretty warm to astronomers. It also orbits such that it is mostly in the sky during daylight hours from Earth, so your typical amateur astronomer isn't going to be able to go out and spot this.
WISE was an asteroid hunting satellite that stopped operating back in February. Astronomers discovered 2010 TK7 by looking through the vast amount of data collected by WISE and confirmed it with Earth-based telescopes. This gives hope that other Earth Trojans might be buried in the data waiting to be found. Where there is one, there may be many.
What makes a Trojan, a companion asteroid, special, is that they orbit what is called a Lagrangian point. Five special spots between two astronomical bodies where gravity is essentially neutral. The points remain stable relative to where the one body is along its orbit around the other. The graph above maps out the locations of where these points would be.
2010 TK7 orbits Earth's L4 point, rather than remaining stable at the point or orbiting the Earth itself. It also has a kind of funky orbit that takes it closer and further away from the Earth, but it is still roughly 60 degrees in front of us and of no danger to the Earth. It just won't ever get close enough.
|This artist's concept (not to scale) illustrates the first known Earth Trojan asteroid, discovered by WISE. The asteroid is gray and its extreme orbit is shown in green. Image credit: Paul Wiegert, University of Western Ontario, Canada|
It is a tantalizing discovery because it opens the door to several new questions. We know little about this asteroid. It may be a candidate for future exploration, but because of it's odd orbit (it goes well above and below the orbital plane), it would be difficult to reach. But you can bet astronomers are going race to find more Earth Trojans. They have been looking for them for some time already, so it was only a matter of time.
And being an object of some importance, it will also need to be named. Coeus or Crius, the Titan sons of Gaia have been mentioned, but there are other sons of hers. Personally I wouldn't my the cyclopes sons of Gaia; Brontes ("thunderer"), Steropes ("lightning") and the "bright" Arges. But I guess we'll have to see.