Solar System Sentry

In 1990, for example, when Hubble was carried into orbit, scientists never dreamed it would provide them a front-row seat to a comet crashing into Jupiter.

"It never could have been imagined when we were designing the telescope," says astronomer Steve Maran, with NASA's Goddard Space Flight Center in Maryland.

"We can see a cosmic event happening before our eyes," says NASA's Astronomy and Physics programs director Anne Kinney, noting that the smallest particle of debris spotted by Hubble during the 1994 impact was the size of Earth itself.

Hubble also has been put to work scouting landing sites for Mars rovers, tracking the paths of comets streaking toward the sun, watching Saturn's beautiful aurora, spotting volcanic explosions on Jupiter's moon Io and following swirling clouds on Neptune.