Comet Halley as seen by the European Giotto spacecraft in 1986. Credit: ESA

Comets have been studied by several other spacecraft, not all of which were originally designed for that purpose.

Several new missions to comets are being developed for launch in coming years.

Past comet missions include:

• In 1985, NASA modified the orbit of the International Sun-Earth Explorer spacecraft to execute a flyby of Comet Giacobini-Zinner. At that point, the spacecraft was renamed International Comet Explorer. It successfully executed a flyby of comet Giacobini-Zinner in 1985 and comet Halley in 1986.
  
  
• An international armada of robotic spacecraft flew out to greet Halley's Comet during its return in 1986. The fleet included the European Space Agency's Giotto, the Soviet Union's Vega 1 and Vega 2, and Japan's Sakigake and Suisei spacecraft.
  
  
• Comet Shoemaker-Levy 9's spectacular collision with Jupiter in 1994 was observed by NASA's Hubble Space Telescope, the Jupiter-bound Galileo spacecraft and the Sunorbiting Ulysses spacecraft.
  
  
• Deep Space 1 launched from Cape Canaveral on October 24, 1998. During a highly successful primary mission, it tested 12 advanced, high-risk technologies in space. In an extremely successful extended mission, it encountered comet Borrelly and returned the best images and other science data taken from a comet to date.
  
  
• The Comet Nucleus Tour, or Contour, mission launched from Cape Canaveral on July 3, 2002. Six weeks later, on August 15, contact with the spacecraft was lost after a planned maneuver that was intended to propel it out of Earth orbit and into its cometchasing solar orbit.

• Deep Impact will purposely slam a 370-kilogram (approximately 820-pound) cylinder into a comet's nucleus so experts can study the interior of the comet. The launch of the Deep Impact spacecraft is planned for December 2004 and it is scheduled to reach out and touch its target, comet Tempel 1, on July 4, 2005.
  
  
• A European Space Agency mission, Rosetta will be launched in February 2004 to orbit comet 67P/Churyumov-Gerasimenko and deliver a science package to its surface via a lander. NASA is providing science instruments for the comet orbiter.