The majority of asteroids are just silly little rocky bits of junk between the orbits of Mars and Jupiter, the crumbs that fell from the plate as those planets were served up on the solar dinner table. This crumby region is known as the Asteroid Belt.
[insert suspender joke]
There are other tribes of asteroids gathered here and there. For instance there is a gaggle of them both 60 degrees ahead of, and 60 degrees behind Jupiter in its orbit. These are called Trojan asteroids because they are always horsing around. Another two groups live ahead of and behind Mars. They are called Martian Trojans. There are plenty of other gangs elsewhere in the solar system, including inside Earth's orbit of the Sun. Some of the creepy little critters, like Cruithne, have very odd orbits indeed. I think you'll like Cruithne. More about him in a moment.
Most asteroids are the size of gravel, mere cosmic specks, but at least 16 asteroids have a diameter of 240 km or more. You don't want to get hit by something like that. Ceres is the big boss. He has a diameter of about about 914 km which is roughly the size of Texas. The surface of Ceres reminds me of parts of Texas. If Ceres hits Earth, I hope it hits Texas.
It might seem reasonable to speculate about the material in the asteroid belt in terms of a "failed planet." One problem with this quick-spun hypothesis is that all the asteroids added together wouldn't even make a respectable moon. It would take 343 of Ceres to equal one Mars and like I said, most asteroids, unlike Ceres, are stupid little pebbles. On the other hand you could argue that in days of old there were a lot more asteroids but Jupiter ate them. Jupiter is well known for such mischievous and self-indulgent shenanigans. If Jupiter wasn't such a pig there might have been another planet out there instead of a big bumpy belt.
Ceres, the largest of all the asteroids, was accidentally discovered by a careless Italian astronomer named Giuseppe Piazzi in 1801. He was looking and whoops, there it was, the very first asteroid ever discovered. Everyone called it a planet and assumed that it would always be a planet. Eventually they realized it was only one of a whole shit-load of a new type of objects so it got demoted from smallest planet to biggest asteroid. I don't know if that is really a demotion or a promotion though. I mean, it went from being a crappy planet to Boss of the Belt. Anyway everyone soon got use to the new arbitrary compartmentalization created by the International Astronomy Union's solar system nomenclature, until Pluto came along. Then they suddenly realized that astronomers are just a bunch of jerks.
Now for the good stuff. Cruithne (CREW-een-ya) is an asteroid (#3753 if you're taking notes) with a crazy-ass orbit. When viewed from an Earth frame of reference (stationary Earth), it describes a series of bean shapes. Over the course of 770 years the series of beans completes a big horseshoe-shaped movement around the Sun with the Earth in the gap of the horseshoe. It approaches the Earth from one direction, then it moves away and makes more bean shapes across the sky as it moves around the Sun the other way, until it approaches Earth from the other side, and then moves away again. Studying the animation will show you that Cruithne never really goes around Earth. At times the orbit brings Cruithne underneath the south pole of the Earth (40 times farther away than the Moon), and at other times it can be seen on the far side of the Sun.
Weird fact: The orbital period of Cruithne is one Earth year! Cruithne is at a balance point where its average orbital period around the Sun is precisely the same as Earth, 365 and 1/4 Earth-days. So they have leap year there too! Sort of. Um, no not really.
Don't be fooled, Cruithne's orbit isn't really horseshoe beans. That's just the shape of the path as seen from planet Earth. Looking down from above you would see that it makes a nice ellipse that takes it inwards towards the orbit of Mercury, and then way outside the orbit of Mars. Cruinthe's fast motion when it is close to Mercury's orbit is compensated by its slow motion when it lies beyond the orbit of Mars. Meanwhile the Earth helps maintain the stability of the orbit of Cruithne by "shepherding" it. At a slow point, the Earth's gravity gives it a pull and speeds it up. At a fast point, the Earth's gravity slows it down. That's why its average period matches ours, we MAKE it so.
Because the weird little bugger and the Earth do not go around the Sun in exactly the same amount of time each and every year, (the asteroid is currently going around slightly faster than the Earth), the position of the bean-shaped loop relative to the Earth varies over time. If the asteroid and the Earth didn't have a special arrangement, the Earth would face potential danger as the asteroid would eventually drift towards our planet. However, in their current relationship (the friend zone), the direction of drift is reversed every time it approaches the Earth. It is as if the Earth uses gravity in a clever way to regulate the asteroid and keep it at a safe and appropriate distance. They can never get together even if they are both drunk and desperate at the same time.
Cruithne is about three miles in diameter (5 km) and was discovered by D. Waldron in 1986. It cannot be seen by the naked eye. The oddness of the orbit was discovered in 1997 when nerds working at Queen Mary and Westfield College in London came up with the mathematical models to describe its path. Since then other asteroids have been discovered which travel in a similar weird Earth-shepherded way as Cruithne; 1998 UP1 and 2000 PH5.
The name Cruithne refers to one of the earliest Celtic tribes known to have inhabited Britain and Ireland. They may have been (probably were) the very first Celtic tribe to migrate from the mainland. Or they may have been descended from prehistoric tribes indigenous to the British Isles since the stone age. Or they may have descended from ancient aliens from another planet. Or maybe an inappropriately behaved asteroid.