By now you may have heard about this interesting video showing how many asteroids we've discovered since 1980. It's pretty cool!
I have no idea how accurate it is, but the numbers seem about right; I know there are several hundred thousand known asteroids in the main belt between Mars and Jupiter [Note: the creator of the video talks about this in the comments below]. Note that a lot of the ones you see toward the end get close to Earth; according to the JPL Near-Earth Object site, almost 7200 near-Earth asteroids have been cataloged as of August 20, 2010! Of these, 815 are larger than about 1 kilometer in diameter, and 1137 are considered to be potentially hazardous; that is, have a chance (however small) of hitting the Earth.
It's interesting to see where the asteroids are when discovered; usually in the opposite direction of the Sun, because that's where surveys tend to look. Right at the end you'll see two white patches at 90° from the direction of the Sun on either side. If I were a betting man -- and I am -- I'd wager those were from WISE, an infrared survey satellite. It scans the sky constantly, looking at right angles to the Sun, and I know it's designed to find asteroids.
More interesting, to me, is how crowded the asteroid belt looks! But don't be deceived. The distance between Mars and Jupiter is a bit roomier than depicted in the video. Remember, Mars is about 220 million km (130 million miles) from the Sun, and Jupiter is about 800 million km (480 million miles). That's a whole lot of real estate: almost 2 quintillion square kilometers (670 quadrillion square miles)! Written out, that's 2,000,000,000,000,000,000 square kilometers.
Yeah, a whole lot of real estate.
And that assumes those asteroids all lie in the same plane. In fact, many of their orbits are tilted, so we're really dealing with volume. Even allowing that they may move above or below the plane of the solar system the paltry amount of a million kilometers, that means there's really 2 septillion cubic kilometers: 2,000,000,000,000,000,000,000,000 cubic km! The volume of the Earth is only about a trillion cubic kilometers, so we're talking a volume of space that could fit a trillion Earths in it!
So there's plenty of room for these asteroids. The biggest, Ceres, is less than a thousand km across, so it's small compared to Earth. Most of those asteroids identified in the video are roughly a few kilometers across, so they are positively tiny on this scale. The only reason the video looks crowded is because each asteroid is represented by a pixel that's much, much larger than the actual asteroid size on that scale -- even the Sun would be smaller than a single pixel at that scale! Displaying the asteroids actual size would be difficult; on my monitor a 10-km wide rock would only be about a nanometer across. A human hair is 100,000 times wider!
My friend Dan Durda -- you'll see lots of him on my TV show premiere Sunday night -- studies asteroids. He once told me there are a billion asteroids in the main belt bigger than about 100 meters across. Even so, on average, there are millions of kilometers between them!
Space is big. Really, really big.
Despite what you see in this video and in movies, the asteroid belt is not a swarming mess of rocks going every which way. It's actually mostly empty space. In fact, when we send probes to the outer solar system, the odds of even getting anywhere near one are incredibly small. Scientists have to physically aim the probe on purpose to get near enough one to get good images of it. The fact that we have, many times, is a testament to how good we're getting at this sort of thing.
So anyway, the lesson here is twofold: we are getting better at finding asteroids, but there's still plenty of room for them to exist in.