A small NASA space probe launched in 1998 October that flew past the asteroid (9969) Braille in 1999 July and comet 19P/Borrelly in 2001 September, imaging them both. It was the first space probe to use ion propulsion. Deep Space 1 ceased operation in 2001 December.

http://www.jpl.nasa.gov/missions/deep-space-1-ds1 Official mission website.