news A newfound asteroid orbits the Sun on a track similar to Earths, making it behave at times almost like a second moon of our planet.
It is not a second Moon, astronomers stress, but they are calling it the first quasi-satellite of Earth.
"As a quasi-satellite, it is going around the Sun very close to Earth," explains Martin Conners of Athabasca University in Canada. "Both are in orbit around the Sun but near each other for a long period of time."
The asteroid is under the influence of Earths gravity, however. It does a horseshoe-shaped jig as it circles the Sun, and this aberration brings it periodically close enough to Earth that it becomes visible to large telescopes, even though it is no bigger than a football field.
The object was discovered in images taken Jan. 9 of this year by the Lincoln Laboratory Near-Earth Asteroid Research (LINEAR) survey.
"Among asteroids, the orbit of 2002 AA29 is the most strikingly Earth-like, since it is very nearly at the same distance from the Sun and also very circular," Conners and his colleagues write in the journal Meteoritics and Planetary Science.
The newfound space rock is not currently in a quasi-moon state. It was, however, in the year 572 and will be again in the year 2575. Each session can last decades.
Other space rocks have been found in similar orbits, including one called Cruithne, discovered a few years ago.
"They both go around the Sun in one year, which is why Earth can have a big effect on their orbits," Conners told SPACE.com. "However, AA29 follows Earth's orbit quite closely, while Cruithne has a very elliptical orbit that takes it quite far outside and inside our orbit."
Most asteroids orbit the Sun between Mars and Jupiter. A few, however, travel the same general space traversed by Earth. Most of these do not have orbits as circular as 2002 AA29. But like most asteroids, AA29 travels at a strong inclination to the Earths orbit about 10 degrees in this case which means it moves above and below the plane that Earth moves in.
It is due to make a closer approach 3.7 million miles (5.9 million kilometers) on Jan. 8, 2003. Thereafter, it begins a 95-year journey away from the planet, before coming back again even later. It will not be visible to the naked eye at any time. Nor is there any danger of it hitting the planet for at least thousands of years, if ever.
The Moon, Earths only true and natural satellite, is about 238,900 miles (384,402 kilometers) away.Objects that move in a sort of lock step with a planet are called Trojans. Jupiter is known to have several Trojan asteroids. But 2002 AA29 represents what Conners and his colleagues say is a new class of object near Earth.
"Finding the first Earth co-orbital object, 2002 AA29, provides incentive to search for other horseshoe-orbiting objects or Earth Trojans," the researches write, suggesting that studying them would teach astronomers more about how asteroids orbits evolve. They also suggest the object could be a target for a space mission.
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