Howdy, all! Starting tomorrow, I will be off for a few days as I travel to Kitt Peak National Observatory for two nights of asteroid hunting. You might recall a couple of recent news stories about Earth's close encounters with these bodies. It's the job of the Minor Planet Center (located at Harvard) and a team of (mostly volunteer) astronomers around the world to find and track these rocks to determine if they pose a threat to our planet.
Asteroids are pieces of rock left over from the formation of the solar system. About 4.5 billion years ago, the solar system consisted nearly entirely of these sorts of bodies, which at this stage are called planetesimals. These planetesimals combined together to form the rocky planets and what was left formed much of the asteroid belt. Today, most of these asteroids are confined to the region between Mars and Jupiter. A few, however, have more eccentric orbits which can take them closer to our planet. These bodies are called Near-Earth Objects.
Once an asteroid is detected outside of the asteroid belt, the process of determining its orbit begins. Initially, this orbit is extremely uncertain; we've only seen it once, so it could be travelling in pretty much any direction. Using our best guess, astronomers look for the asteroid again in the first few months after its detection (after that, the uncertainties often become too large). If we can find it again, we now have twice as much data with which to compute its trajectory. If there is no possibility of this rock hitting Earth, the process stops and the asteroid is placed in a database. If there is still a chance, the asteroid is looked for again and the process repeats. Eventually, basically all asteroids are determined to be safe and placed in the ignorable column.
Astronomers use two scales to discuss the danger posed by an asteroid. The Palermo scale is similar to the Richter scale used for earthquakes and is based on a statistical definition of how likely an impact of a given size is to occur. More user-friendly is the Torino scale, which assigns each asteroid a value from zero to ten. Zero means exactly that: virtually no chance of impact. The scale then continues up through ten, the "we're all going to die" level. You can view the complete scale here. The good news? Of all the known asteroids, there is exactly ONE level one asteroid; all the rest are zeros. So, we're pretty safe right now.
What would happen if we did discover an incoming asteroid? First off, it's important to remember that the Earth is being constantly hit by smallish rocks. As these burn up in the atmosphere, they produce shooting stars. If, however, we found a bigger one, it's not clear that there really is anything we could do. Researchers have suggested attaching rockets to it, trying to blow it up, etc. Perhaps the most intriguing idea is to simply paint it black on one side. Black things absorb and radiate heat more effectively; given enough time, extra radiation from one side could push the asteroid off track.
For now, though, we're just spending a few sleepless nights searching for the darn things.