The spacecraft, originally launched to test ion propulsion technology, executed a bonus flyby of comet 19P/Borrelly, returning detailed images of a dark, elongated nucleus venting jets of gas and dust. The encounter validated ion drive as a viable technology for deep space missions and provided planetary scientists with striking new data on cometary composition.
Deep Space 1 was never meant to be a comet hunter. Launched in 1998 primarily as a technology demonstrator for NASA's New Millennium Program, its main job was proving that xenon ion propulsion, a thruster emitting a barely visible blue thread of charged particles, could reliably power a spacecraft for years. It succeeded so well that mission planners decided to spend its remaining fuel on a daring bonus objective after its star tracker failed and was cleverly worked around.
On October 3, 2001, the probe threaded a narrow window past comet Borrelly at over 16 kilometers per second, close enough to capture images ten times sharper than any comet photographs before it. The nucleus proved bowling-pin shaped and remarkably dark, blacker than coal, with jets of gas erupting from isolated active regions rather than the whole surface evenly.
The ion engine that made the whole gambit possible ran for over 16,000 hours total, using a fraction of the propellant a conventional chemical rocket would have needed. That efficiency proof reshaped mission design for a generation of NASA and ESA deep-space probes, including Dawn's later visits to Vesta and Ceres.