In my last post, I wrote about the asteroid 2009 BD, which has an orbit very nearly that of Earth's. But, it is not Earth's only orbital companion. I know of more than a half dozen asteroids whose orbits would be considered co-orbital with Earth. The two that I want to talk about today have even stranger orbital properties. These are the asteroids 2002 AA29 and 2003 YN107.

As with 2009 BD, these asteroids have an orbit almost the same as that of Earth, but not quite. Both have orbits that are a bit inclined with respect to Earth's orbit, as you can see in the accompanying orbital diagrams. These diagrams, courtesy JPL's Small Body Database, show an oblique view of the Solar System, so you can see how the orbits drift above and below Earth's orbit. They also have orbits nearly as elliptical as Earth's, but the long axes of their orbits is not aligned with that of Earth's orbit. Thus, they drift closer and farther from the Sun that Earth. This drifting closer and farther while going up and down (compared with the plane of Earth's orbit) makes their orbit appear almost helical if you were to look at the orbit from Earth's perspective instead of from the Sun's point of view.  Paul Wiegert has a nice explanation of this sort of orbit, and I'm using a couple of his images here, including this one:

This is a typical orbit for this sort of asteroid, as seen from Earth.  Furthermore, it is also what we call a horseshoe orbit.  The name comes from the fact that typically, if the asteroid is moving a little faster than Earth, when it approaches from behind in Earth's orbit, Earth's gravity pulls on it a bit, it speeds up, slides to an orbit just a shade farther out than Earth's, slows down, and Earth pulls away from it.  Effectively, it would look like the asteroid got close to Earth in its helical orbit and then started going back the other way.  Eventually, Earth catches up with it again, and the reverse happens.  So, the asteroid simply goes back and forth along paths that loop in and out of Earth's orbit, and whenever it gets close to Earth, the orbit of the asteroid shifts ever so slightly so that it continues to loop in and out of Earth's orbit for years, decades, or centuries until it comes close to Earth again.  One of the first asteroids that I heard about that does this sort of thing was 3753 Cruithne, discovered in 1986.   Clearly, these orbits are of bodies about the Sun, but they are greatly influenced by the presence of Earth and Earth's gravity.  But, there are plenty of other bodies in the Solar System influencing these bodies, and in order for them to behave as they do, they have to have an orbit that is unaltered by these other bodies.  That isn't going to happen.  Thus, these orbits, while pretty stable for a long time, are ultimately unstable.  For most unstable orbits, that means that they can be shifted into an orbit that either comes too close to Earth at some point, resulting in their being thrown out of this neat orbital relationship with Earth.  They can even wind up on a path that would ultimate collide with our planet, though that is far less likely than their being tossed into another orbit.

But, one really interesting thing that can happen is if one of these asteroids approaches Earth at just the right spot in its orbit.  Then, their orbit becomes even more tightly entwined with Earth's.  In this case, they orbit the Sun always in the vicinity of Earth.  If you were looking at the asteroid from Earth, it would look almost as if the asteroid were spiraling around Earth.  This would be a case of an asteroid whose orbit, instead of going back and forth in the helical horseshoe orbit above, would be trapped in the gap in the horseshoe shape shown.  Such an orbit would look like the following (also from Wiegert):

For a while, Earth would be orbiting the Sun with a companion.  It is important to note that, while the asteroid looks like it is looping around Earth, it is in fact still orbiting the Sun, not Earth.  That means that it is not really a satellite, or moon, of Earth.  It is still a sun-orbiting body, but its orbit looks almost like that of an Earth satellite body, so we sometimes call it a quasi-satellite or a quasi-moon.  The asteroid 2003 YN107 got trapped in just such a situation a few years back.  For about ten years, from 1996 to 2006, it made loops around Earth, taking one year to make one complete circuit.  It is small and wasn't discovered until 2003, having already been a temporary companion of Earth for ten years.  In 2006, though, it managed to get a slight nudge to get back into its normal horseshoe orbit.

2002 AA29 does something similar.  It orbits the Sun.  But, its orbit looks like the horseshoe seen above.  About once per century it approaches Earth, where it is turned back.  Eventually, it approaches from the other side (one time it is gaining on Earth, and the next Earth gains on it).  Again, it is turned away.  It bounces back and forth, gaining on Earth, then losing ground until Earth catches it.  It, then, pulls away once again.  But, once in a while it gets caught into a temporary holding pattern, spiraling around Earth.  That last happened perhaps nearly 1500 years ago, and it may happen again in about 600 years.  There are other asteroids with this behavior, too.

So, Earth does not orbit the Sun alone.  Besides our Moon, there are a variety of other bodies out there keeping us company.

-Astroprof

Orbit diagrams credit:  JPL Solar System Dynamics, Paul Wiegert's 2002 AA29 page