As of 1995, 878 comets have been cataloged and their orbits at least roughly calculated.

1
Stardust and Deep Impact Cosmic History
How many comets are there?
As of 1995, 878 comets have been cataloged and
their orbits at least roughly calculated. Of
these 184 are periodic comets (orbital periods
less than 200 years) some of the remainder are
no doubt periodic as well, but their orbits have
not been determined with sufficient accuracy to
tell for sure.
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Stardust and Deep Impact Cosmic History

• Comets are small, fragile, irregularly shaped
  bodies composed of a mixture of non-volatile
  grains and frozen gases.
• They have highly elliptical orbits that bring
  them very close to the Sun and swing them deeply
  into space, often beyond the orbit of Pluto.
• Comet structures are diverse and very dynamic,
  but they all develop a surrounding cloud of
  diffuse material, called a coma, that usually
  grows in size and brightness as the comet
  approaches the Sun.
• A small, bright nucleus (less than 10 km in
  diameter) is sometimes visible in the middle of
  the coma. The coma and the nucleus together
  constitute the head of the comet.

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Stardust and Deep Impact Cosmic History
Background/Philosophy
The Stardust and Deep Impact missions are part of
a new generation spacecraft which requires a
careful trade-off between science and cost in
order to produce a mission with the highest
possible science value per unit cost. Each
mission is a partnership of industry, university,
and government organizations including national
education organizations and technology
alliances. STARDUST WILL BE THE FIRST SAMPLE
RETURN MISSION PAST THE MARS/MOON ORBIT DEEP
IMPACT WILL BE ANOTHER FIRST - LOOKING DEEP
INSIDE A COMET
http//discovery.jpl.nasa.gov/
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Stardust Bringing Back Cosmic History
Launch February 7, 1999 Mass 385 kilograms
(848 pounds) total, consisting of 254-kilogram
(560-pound) spacecraft and 46-kilogram
(101-pound) sample return capsule, plus 85
kilograms (187 pounds) fuel Science-related
subsystems Aerogel dust collectors, sample
return capsule, comet and interstellar
dust analyzer, dust flux monitor, navigation
camera GOAL Collect interstellar and cometary
particles for the Eventual return to Earth in
January 2006.
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Stardust Bringing Back Cosmic History

• Important Fact
• Stardusts total encounter time is 2 hours?
• The signal will be transmitted live!
• First, image will be made available January 2 at
  approximately 6pm to the world!

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Deep Impact First Look Deep Inside a Comet
Launch December 2004 Mass Total - 1020
kilogram (2,244 pounds), Flyby Spacecraft -
650-kilogram (1,424-pound) Impactor -
370-kilogram (820-pound) Science-related
subsystems Flyby spacecraft High resolution
camera visible w/filters Medium resolution
camera infrared/visible Impactor Medium
resolution visible/no filters
ITS Instrument
Star Tracker
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Deep Impact First Look Deep Inside a Comet

• Important Facts
• Deep Impacts total encounter time after impact
  is 14 minutes.
• The crater made at impact will be up to the
  length of a football stadium and several stories
  deep.
• Near real time images from both the impactor and
  flyby spacecraft will be sent to Earth.