Asteroids caught in the Lagrangian points of the Sun-Jupiter system.
Adam Rifkin stashed this in The Universe
We are just seeing the orbits of two asteroid groups (Hilda and Trojan). All of the groups can be seen here:
For more of this guy's work, check out the animation page of his website: http://sajri.astronomy.cz/asteroidgroups/groups.htm
Lots of cool stuff about asteroids.
Reddit comment on Lagrange points:
If you look at any orbital system, each orbiting object will have 5 points relative to their orbit that has a neutral gravity. That means that if you put a space station (or apparently an asteroid) near one of those points with the correct velocity, you don't need to expend any further fuel keeping it there.
They're commonly proposed as the ideal spot to put space stations. If you don't use one of the 5 Lagrange points of the earth/moon system, the station would need constant adjustments to stay stable. Modern satellites have tiny jets they use to counter the gravity of the Moon / other planets / the sun, but a large space station would use fuel far faster than small satellites.
Another Redditor adds:
If something were at a Lagrange point of the Earth/Moon system, it wouldn't move compared to the Earth and the Moon. Since the Moon's orbiting around the Earth, the satellite would orbit in a (mostly) circle as well. No fuel would be required to hold it at that point (assuming it was a stable Lagrange point, but we're not going there).
It's kinda like a geosynchronous orbit (one orbit per day), but compared to a system, as opposed to a single body.
The Lagrange points come from the orbiting satellite being proportionally far away from the larger body and closer to the smaller body. In our above case, the larger body is the Earth and the smaller body is the moon. The gravitational attraction to the two bodies is equal, so the satellite doesn't want to move compared to them.
Lagrange points are kinda like the balance point between a basketball and a baseball on a yardstick (except backwards because you need to be closer to the smaller object to have an equal gravitational force... on second thought, maybe that wasn't the best analogy).