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. “ How many comets are there? ” Stardust and Deep Impact “Cosmic History”

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.

Stardust and Deep Impact “Cosmic History” 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 Background/Philosophy:

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.

Stardust “Bringing Back Cosmic History” Important Fact: Stardust’s 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!

Deep Impact “First Look Deep Inside a Comet” ITS Instrument Star Tracker Launch: December 2004 Mass: Total kilogram (2,244 pounds), Flyby Spacecraft kilogram (1,424-pound) Impactor 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

Deep Impact “First Look Deep Inside a Comet” Important Facts: Deep Impact’s 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.