Today, NASA's spacecraft Deep Space 1 will encounter Comet Borrelly. While a comet fly-by has been done before, what makes this one interesting is the fact that Deep Space 1 is an experimental spacecraftnot designed for this sort of thing at all! In fact, many of the scientific instruments it will use to examine the comet's nucleus are experimental, and it has been navigating with an experimental camera designed for something else entirely, since its star-tracker camera (not an experimental device, by the way) failed about two years ago.
Deep Space 1 is perhaps most famous for its ion engine propulsion system (which presently holds the record for longest-operating propulsion system in the history of spaceflight) and its artificial-intelligence operations software (it's designed to make its own decisions on the spot, rather than relying on control from Earth with the long speed-of-light lag times). But it has a total of a dozen experimental technologies which it spent it's first year checking out (they all worked, surprisingly), and it's now running on "bonus time", doing more science with those instruments for only the ground systems' operational costs.
And it cost a total of less than $150 million.
In the past, it maneuvered itself on its AI autopilot to a 15-km pass by asteroid Braille, at the time the closest flyby of an asteroid (since then, of course, NEAR-Shoemaker actually landed on the asteroid Eros). It even successfully reoriented itself as it passed by, the better to image the asteroidwhile under autonomous control. For this comet approach, many of its experimental instruments have been reconfigured to collect scientific data, but it's worth remembering that they weren't designed for thisand the spacecraft itself was never intended for such a mission.
When the European spacecraft Giotto encountered Comet Halley in 1986, it was severely damaged despite the extra protection it carried. There's a good chance Deep Space 1 won't survive this encounter, since it has none of this protection; but then, it's survived far beyond its design expectations already. And if it does collect and return data, it will be immensely valuable information: even Giotto didn't carry some of the scientific probes that Deep Space 1 was launched with.
Like Clementine, Mars Pathfinder and Galileo in Jupiter orbit, it has proven to be more successful than even its designers and builders anticipated. My hat is off to the scientists and engineers on the project: this is a truly "heroic" spacecraft, and our return on investment is astounding. I'll be waiting with nervous anticipation for the outcomeand hoping the best for Deep Space 1.