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A Companion to the Earth?

Week of June 16, 1997
Orbits are funny things. Our common sense conclusions about the way objects orbit one another will almost always lead to trouble. Take the strange case of an asteroid discovered back in 1986. Dubbed simply "3753", it orbits the Sun at almost the same distance as the Earth does. Now, objects closer to the Sun orbit faster, and objects farther orbit more slowly. So imagine 3753 is in an orbit just smaller than the Earth. It orbits just a tad faster, slowly increasing its orbital lead. Eventually (after 385 years!) it has almost lapped the Earth. Approaching the Earth from behind, it starts to feel a tug from the Earth's gravity, yanking it forward in its orbit.

You might expect that since it will feel an acceleration from the Earth, it will speed up and move in closer to the Sun. But this is where our experience fails us: it is not the speed of an object around the Sun that determines its distance, it is the energy in the orbit. Since the Earth has given the tiny asteroid extra energy, the asteroid actually moves farther out from the Sun. It's like rolling a ball up a hill. Give it more energy and it will roll higher up.

Since asteroid 3753 is in a larger orbit, it slows down, and is now moving slower than the Earth. Mind you, it never actually passed the Earth; the extra energy was given to it while it was trying to pass the Earth.

So now the asteroid is lagging behind Earth. 385 years later, the Earth catches up with it. The Earth pulls backwards on the rock, robbing it of energy. It falls back a bit, towards the Sun, and speeds up. Note that the Earth never passes the asteroid, nor does it ever pass the Earth! The asteroid follows what is called (simplistically, in this case) a "horseshoe orbit". If you were to carefully map out this kind of orbit, it would look like a giant horseshoe, with the tips of the shoe very close together. These types of orbits were known to be possible theoretically for a long time, but it wasn't until Voyager went to Saturn that actual objects were found that orbited this way. Some of Saturn's moons perform a similar complicated dance that the Earth does with asteroid 3753. The actual orbit of 3753 is much more complicated, however.

Although 3753 was discovered over ten years ago, its orbit wasn't fully understood until Paul Widget (York University) and his colleagues used computer simulations to map out its behavior. Their findings were recently released, and even got some news coverage, although the news coverage I saw did not accurately describe the situation (for regular readers, this won't come as a surprise! ;-).

©2008 Phil Plait. All Rights Reserved.

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