NASA Giotto Mission - Official NASA site gives an overview of the mission Giotto that was designed to study Comet P/Halley. Also gives details of its study of Comet P/Grigg-Skjellerup , which were completed during its extended mission.
NASA: Deep Space 1 (DS1) - NASA new technologies demonstration mission scheduled to fly by a near-Earth asteroid and encounter Comets Wilson-Harrington and Borrelly. From the official NASA site.
