Comet Encounter 
The last time a spacecraft visited Comet Tempel-1, it gave the comet a big whack. It fired a probe at the comet, blasting out tons of ice and rock in a spectacular explosion.
Another spacecraft is scheduled to fly past Tempel-1 this evening. It won't batter the comet, though. Instead, it'll look at the scar left by the earlier encounter.
This craft is called Stardust. And although it wasn't the craft that visited Tempel-1 before, it is making its second visit to a comet. It flew through the tail of Comet Wild-2, gathering grains of comet dust in a small capsule. It later dropped the capsule into Earth's atmosphere, where it parachuted safely to the ground.
The spacecraft itself was still working, so NASA gave it a new target: Tempel-1.
A craft called Deep Impact had fired the probe at Tempel-1. The goal was to excavate a crater, allowing the craft to peer deep into the comet.
Comets are icy leftovers from the formation of the solar system, so they contain clues to the materials and conditions that gave birth to Earth and the other planets. But their surfaces have been changed by impacts and radiation. So the only way to get pristine material is to dig below the surface.
Deep Impact did just that. But the outburst from the impact was so bright and long lasting that the craft couldn't see into the crater it gouged. So Stardust may pick up where Deep Impact left off -- looking into the birth of the solar system by looking deep into a comet.
Script by Damond Benningfield, Copyright 2010