3753 Cruithne

Space is a vast, empty wasteland filled with debris. Take for instance the region of space in the vicinity of Earth's orbit. We are all familiar with the moon, but did you know about the Apollo Asteroids? The Apollo Asteroids are named after 1862 Apollo, a 1.5 km wide asteroid with a semimajor axis of 1.4 AU. Apollo Asteroids cross Earth's orbit and often Venus's as well.

Then there are the Aten Asteroids, named after 2062 Aten, a 1 km wide asteroid with a semi-major axis of less than 1 AU discovered in 1976. There are many Aten Asteroids (see list here) and even more possible candidates. One of the Aten Asteroids is 3753 Cruithne.

3753 Cruithne is a 5 km wide co-orbiting object, orbiting the sun in 1:1 orbital resonance with the Earth. This type of orbit is interesting because of how bizarre it appears from the Earth's perspective. Notice in the image on the right there are three orbits. The blue is Earth's orbit, the red is 3753 Cruithne's orbit about the Sun, and the yellow is how Cruithne appears to orbit from the perspective of Earth. I recommend going to this Wiki to better understand through a gif.

This orbit type, colloquially called a Horseshoe Orbit , is not that uncommon. It is important to note that the image above and the gif on wikipedia given earlier aren't precisely correct. A more correct gif of Cruithne's horseshoe orbit can be found here. If you viewed that last link, you saw that bean shaped orbit in the picture on the right precesses around the sun, creating the horseshoe shape. The total time it takes for one cycle of precessing to create the horseshoe shape is 770 years.

Cruithne's discovery in 1986 has since led to the discovery of other coorbiting objects with similiar resonances such as 5409 YORP, (85770) 1998 UP1, 2002 AA29 and 2009 BD.

So it turns out the solar system is filled with debris trapped in resonances. The two types of asteroids I mentioned today are just two of many different classes of similarly sized objects locked in orbits in resonance with the major planets such as Earth. It's easy to envision space as a vast empty nothing, and it is to a certain extent, but it is maybe a bit more chaotic than we envision in our simple solar system diagrams.