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Over The Horizon

A Mini Moon for Earth

Researchers in Canada and have discovered that the near-earth asteroid (3753) 1986 TO is in a stable orbit with respect to the Earth, making the only known natural companion to the Earth other than the Moon.

York University postdoctoral researcher Dr Paul Wiegert and astronomy professor Dr Kim Innanen conducted computer simulations over the past eight months to support their research. Their Finnish colleague, Dr Seppo Mikkola of Turku University, independently verified their findings.

Although the five-kilometre asteroid was discovered 11 years ago, no one had studied its orbit thoroughly enough to detect its unique status.

The asteroid's orbital period is just a fraction less than Earth's, but not always. At intervals of 385 years, asteroid 3753 approaches close enough to Earth to have its orbit nudged into a slightly larger size. After that it takes slightly longer than the Earth to orbit the Sun. Then 385 years later another close approach occurs and the orbit is again made slightly smaller than Earth's.

If one plots 3753's orbit on a rotating frame, with the earth and sun fixed then the orbit sweeps out a kidney shape. Each orbit is slightly shifted in relation to the earth so, over 385 years, the kidney-shaped orbits sweep out a horseshoe shape. The asteroid then encounters the earth and reverses the sweep till the next encounter. Thus asteroids in this type of orbit are called "horseshoe trojans": "horseshoe" to denote the overall spread of the orbit;"'trojan"' after the two groups of Trojan asteroids that stay 60o ahead and behind Jupiter.

Saturn's moons Janus and Epimetheus show a similar sort of orbital relationship. One is moved into a smaller orbit by the gravity of the other. So it orbits Saturn faster and eventually catches up on its larger companion. It is then switched into a larger orbit so it lags behind. Thus it shuttles back and forth alternately lapping, and then being lapped by, it larger companion.

In the case of Earth and 3753, both are orbiting the Sun -- a unique pair. The asteroid's orbit is chaotic, but it remains a near-Earth object in the computer simulations for time-scales of a million years. However, not all of this time is spent as a horseshoe, as it switches between its current orbit and a non-horseshoe orbit on time-scales of a few hundred thousand years.

Asteroid 3753 passes from inside to outside the Earth's orbit, but its minimum yearly approach distance is usually quite large. During the closest approaches, which happen only every 385 years, the asteroid passes within roughly 40 times the Earth-Moon distance. The last such approach occurred about 100 years ago.

Asteroid 3753 crosses the orbits of Venus and Mars as well as the Earth's. Though its orbit does not currently intersect that of any planets, the axis of the orbit is slowly rotating. As a result, its orbit will intersect that of the Earth in 2,750 years, and (if it survives this crossing) that of Venus in about 8,000 years. Similarly, the asteroid's orbit intersected Mars' roughly 2,500 years ago.

These results suggest that the asteroid's current horseshoe orbit may not be stable for arbitrarily long times, unless there is some dynamical "safety mechanism" which preserves it against close planetary encounters. At this point, the existence of such a safety mechanism seems unlikely, and yet the very low a priori probability of an object being injected into such an orbit makes a recent origin seem equally unlikely.

Prediction of the asteroid's future is problematic owing to the chaotic nature of the orbit. A collision with the Earth seems very improbable. A strong gravitational interaction with Venus in 8,000 years seems quite likely, though the possibility of a collision with that planet at that time remains remote.

At a dim 15th magnitude, the asteroid will be visible only through telescopes as it crosses the southern sky in October-November.

Alan Gilmore & Pam Kilmartin
Mt John Observatory

Any observers interested in locating Asteroid 3753 can request location details by emailing

More information on the asteroid is available on the Web at: