Hanging On 
The old adage "what goes up, must come down" works fine here on Earth. But there are tens of thousands of worlds in the solar system where that's not the case. Throw a baseball into the sky, and it'll just keep going.
These worlds are the asteroids -- chunks of rock and metal left over from the birth of the planets. Even though thousands of them are the size of mountains, their surface gravity is incredibly weak. Toss a ball gently into the sky and it could escape the asteroid and float away.
That sounds like fun, but it'll be hard for asteroid explorers to deal with -- even a normal-sized step could propel them into space.
That's not just an academic problem. Earlier this year, President Barack Obama scrapped NASA's plans to send astronauts back to the Moon, and instead instructed the agency to plan an expedition to a Near-Earth Asteroid -- one that passes within a few million miles of Earth's orbit around the Sun. Astronauts will need a way to anchor themselves to the surface to keep from drifting off into space.
Once they reach an asteroid, the explorers will drill into the surface for samples that haven't been changed by impacts and solar radiation -- samples that might not have changed since the birth of the solar system. The samples could help scientists fill in some of the gaps in our knowledge of how Earth and the other planets were born.
More about asteroids tomorrow.
Script by Damond Benningfield, Copyright 2010