3753 Cruithne is an asteroid that is a quasi-satellite of Earth, having a horseshoe orbit in regards to Earth. That means it's a big rock that orbits the sun in about the same amount of time as it takes the Earth to orbit the sun. It appears (from earth) to move in a big kidney-bean-shaped pattern in the sky, it's always on the same side of the sun as we are, and never passes behind it.
The earth comes close to 3753 Cruithne every November, and will continue to do so until 2015. After that, it will slowly drift away for a while, till another period of near passes happens in 2292. This orbital relationship is not entirely stable, but is expected to maintain itself in similar cycles for the next several million years, anyway.
3753 Cruithne is about 5km in diameter.
Game and Story Use
- For a nice disaster movie feel, 3753 Cruithne gets sped up, slowed down, or knocked out of orbit by some passing comet. The comet doesn't hit us, but Cruithne does - possibly decades later.
- Of course, a far more believable scenario would be one where we get hit by 99942 Apophis. It's not nearly as big, but it'll still mess us up plenty.
- In a setting with a Counter-Earth, Cruithne could actually change orbits or get snatched up by the gravity of the other earth.
- Such a setting may instead have other asteroids that occupy some weird quasi-satellite position balanced between the gravitational pulls of two earths and the sun
- or a "Counter-Cruithne" asteroid that stays on the same side of the sun as the Counter-Earth does.