Deep Impact

At approximately 1:52AM (EDT) on July 4th, NASA's Deep Impact spacecraft will collect data as its impactor probe carves a crater a size somewhere between a house and a stadium, in the nucleus of comet Tempel 1. Launched on January 12th from Cape Canaveral Air Force Station, Deep Impact has traveled 431 million kilometres in 173 days to encounter Tempel 1, which has been traveling through space at 37,100 km/h. It's a mission described as "the astronomical equivalent of a 767 airliner running into a mosquito," by mission scientist Don Yeomans.

About 22 hours before the encounter, Deep Impact will release the 1 metre wide, 372 kg impactor and settle back to observe. Two hours before the impact, as the 1 billion ton Tempel 1 comes hurtling at 10.3 km/s, the impactor will switch to autonomous navigation mode and move itself into a collision trajectory. On impact, a crater up to 14 stories deep will be created, revealing what is below the surface of Tempel 1's nucleus. From 500 km away, Deep Impact will have about 13 minutes to observe before being bombarded by comet guts. Observations will be radioed back to NASA. The encounter will not cause an detectable alteration in Tempel 1's speed or trajectory as it continues on it's way in orbit around the Sun.

Studying comets is key to understanding our early solar system, as comets have remained relatively unchanged since the solar system was formed about 4.6 billion years ago. The tale, as we're taught in school, tells us that comets condensed in the outer solar system during its formation. Initially, comets resided in what is now known as the Kuiper Belt -- a flat belt of comets located beyond the orbit of Pluto. In the early solar system, the gravitational influence of the outer giants propelled some comets into a sphere around the solar system, called the Oort Cloud. Comets from the Kuiper Belt are short period, while those from the Oort Cloud are long period -- falling into orbit around the Sun due to gravitational perturbations.

Tempel 1 is from the Kuiper Belt. Since comets have resided beyond the reach of the Sun's heat most of their lives, they have remained relatively unchanged since the formation of the solar system. They represent a record of the material that went into the creation of the solar system. Equally important is the composition of comets -- they are 50% frozen water and 10-20% carbon. The terrestrial planets are actually quite poor in water and carbon, and it is believed that comets delivered water to Earth and seeded the planet with life. Comets may also have been responsible for some of the mass extinctions that have engulfed the planet in the past.

Deep Impact is part of NASA's ongoing fast, cheap and high impact missions under the Discovery Program. Other missions from the program have included:

Near Earth Asteroid Rendezvous (NEAR) -- the first spacecraft to orbit and land on an asteroid, Eros, in February 2000.

Mars Pathfinder -- delivered a robotic lander on Mars on July 4, 1997.

Lunar Prospector -- entered lunar orbit on January 12, 1998.

Stardust -- the spacecraft flew through the coma of Comet Wild 2 on January 2, 2004, collecting samples, and will return to Earth on January 2006.

Genesis -- the spacecraft collected solar wind samples beyond the Moon and returned to Earth on September 8, 2004.

Comet Nucleus Tour (Contour) -- was lost six weeks after launch, on August 15, 2002.

Mercury Surface, Space Environment, Geochemistry and Ranging (Messenger) -- the spacecraft will arrive at Mercury in September 2009 to begin detail mapping of the planet.

Dawn -- the spacecraft will be launched in May 2006, and will travel to two of the most massive asteroids in our solar system: Vesta in 2010, and Ceres in 2014.

Kepler -- to be launched in the fall of 2007, the mission of Kepler is to find Earth-like extrasolar planets.

Other cometary missions that have increased or will be increasing our knowledge of comets, are:

International Comet Explorer -- executed a flyby of Giacobini-Zinner in 1985, and Halley in 1986.

Halley was also visited in 1986 by ESA's Giotto, the Soviet Union's Vega 1 and Vega 2, and Japan's Sakigake and Suisei spacecrafts.

Comet Shoemaker-Levy 9's collision with Jupiter in 1994 was observed by the Hubble Space Telescope, and the Galileo and Ulysses spacecrafts.

Deep Space 1 -- made a successful encounter with Comet Borrelly.

Stardust -- flew within 236 km of Comet Wild 2 on January 2, 2004, and is scheduled to return to Earth with samples in January 2006.

Rosetta -- was launched by ESA on March 2, 2004 on a mission to orbit and deliver a lander on Comet Churyumov-Gerasimenko in 2014.