NASA’s Wide-field Infrared Survey Explorer (WISE) has found the very first asteroid that (more or less) shares an orbit with Earth! Called 2010 TK7, this asteroid is about 300 meters (roughly 1000 feet) across, and is the first in an up-to-now theoretical class of objects called Earth Trojans.
In the July 28 2011 issue of the journal Nature , astronomers announce that the as-yet-unnamed near-Earth asteroid with the temporary designation 2010 TK7, shares the Earth’s orbit with it. As the asteroid and the Earth both orbit the Sun, 2010 TK7 remains in step with our planet, remaining ahead of us as seen from Earth.
Like one of a pair of dancers performing a complicated tango, the asteroid moves in an elaborate path that brings it sometimes closer and sometimes farther from our planet. The Earth and the asteroid remain in sync however, with the asteroid always preceeding the Earth as they both move around the Sun.
That asteroids could perform such complicated motions while staying in step with their companion planets was first proposed by Joseph Louis Lagrange in 1772. The first asteroid discovered to have this property was asteroid 588 Achilles, spotted by Max Wolf in 1906, and which accompanies the giant planet Jupiter. As the largest planet in our Solar System weighing in at over 300 times the mass of the entire Earth, Jupiter is now known to have thousands of such asteroids. Frequently called “Trojan” asteroids because of the tradition of naming these bodies after the heroes of the Greek and Trojan War as immortalized by Homer in the Iliad and Odyssey, only the planets Jupiter, Neptune and Mars were known to harbour such objects until now.
The vast majority of asteroids orbit the Sun within the main asteroid belt between the planets Mars and Jupiter, and thus remain far from the Earth. Near-Earth asteroids (NEAs) represent a small population of asteroids which have been kicked out of the asteroid belt by various complex processes, and which may pass much closer to the Earth at times.
Asteroid 2010 TK7 is a near-Earth asteroid in the sense that its orbit takes it much closer to Earth than main belt asteroids ever do, but it does not constitute a threat to our planet. Quite the opposite in fact. Other NEAs move around the Sun in ways which are uncoordinated with our planet, and this allows for some degree of danger from collision. Because the motion of 2010 TK7 is in sync with that our planet the asteroid, like a good dancer, always neatly avoids blundering into its partner.
At its closest point to Earth, 2010 TK7 is still many times the distance to the Moon from us, much much further than other asteroids that occasionally pass near the Earth, like 2011 MD. In the next ten thousand years, 2010 TK7 will not approach us any closer than 20 million km, which is over 50 times the 384,000 km distance to the Moon.