NASA's Stardust mission launched in Jan. 1999 on a seven-year round-trip journey to collect pristine particles from a comet and return them back to Earth for analyses—helping scientists around the world unravel the mysteries surrounding the birth and evolution of the solar system.
+ Stardust Archived site

Comets are time capsules that hold clues about the formation and evolution of the solar system. NASA's 8th Discovery mission, Deep Impact, is the first to ever attempt to probe beneath the surface of a comet and reveal the secrets of its interior.
+ Deep Impact Archived site

The Discovery and New Frontiers missions provide the diversity of vantage points – flybys, orbiters, landers, and sample returns – to not only dramatically advance our understanding of the solar system, but also to allow NASA to further refine its exploration strategy.
+ Discovery & New Frontiers Program