Taking Inventory 
The census of objects in our own solar system is never done. Astronomers discover thousands of new objects every year -- most of them rocky asteroids beyond the orbit of Mars. But a few are closer in -- some of them on orbits that bring them close to Earth.
The number of known objects in the asteroid belt is more than half a million and still climbing. All but a few thousand were discovered in the last 25 years, thanks to dedicated searches and better technology.
The technology includes spacecraft. In fact, a single craft discovered more than 33,000 asteroids in just one year.
The craft was WISE -- the Wide-field Infrared Survey Explorer. It was launched at the end of 2009 to map the entire infrared sky. It operated until the end of January, when its instruments became too warm.
Most of the asteroids it found are in the asteroid belt -- a broad region between the orbits of Mars and Jupiter. But more than a hundred follow orbits that bring them close to Earth's orbit.
These Near-Earth Objects are of particular interest for a couple of reasons. First, they could be targets for future exploration by both automated probes and astronauts. Most important, though, they could someday hit Earth, causing planet-wide destruction. Tracking these objects can warn us of possible collisions far in advance -- making the census of the solar system a matter of life or death.
More about the census tomorrow.
Script by Damond Benningfield, Copyright 2011