NASA's Deep Impact probe strikes comet successfully
Monday, July 4, 2005
NASA's Deep Impact probe collided with the comet Tempel 1 on Monday, as intended by scientists. The collision took place at 5:45 UTC and NASA held a press briefing shortly after at the Jet Propulsion Laboratory (JPL) in Pasadena, California, NASA's non-manned space flight control center. A full-fledged press conference by NASA is scheduled to be at JPL on Monday afternoon at 2 p.m. PDT (9:00 UTC).
The experiment, intending to provide more information about the make up of comets, consisted of a 370 kg projectile being fired at the comet in order to observe and analyse the resultant impact. With information gleaned from the impact, the comet's general make-up can be established and more accurate hypotheses regarding how the Solar system and the universe came to be.
Deep Impact is a NASA space probe designed to study the composition of the interior of a comet. Previous space missions to comets, such as Giotto and Stardust, were merely fly-by missions, only able to photograph and examine the surfaces of cometary nuclei. The Deep Impact mission will be the first to examine a cometary interior, and thus, scientists hope, reveal new secrets about these small frozen bodies.
Following its launch on January 12, 2005, the Deep Impact spacecraft took 174 days to reach Comet Tempel 1 at a cruising speed of 103,000 kilometers per hour (64,000 miles per hour). Once the spacecraft reached the vicinity of the comet on July 3, 2005, it separated into two portions, an impactor and a flyby probe. The impactor used its thrusters to move into the path of the comet, impacting 24 hours later at a relative speed of 37,000 kilometers per hour. The impactor has a mass of 370 kilograms and approached Tempel 1 with a relative speed of 10.30 kilometers per second (6.3 miles per second), thus delivering 1.96 × 1010 joules of kinetic energy, the equivalent of 4.5 tons of TNT. Scientists believe that the energy of this high-velocity collision will be sufficient to excavate a crater up to 100 meters wide (larger than the bowl of the Roman Colosseum), although the crater has not yet been spotted in post-impact images because the cloud of debris is obscuring the view.
Just minutes after the impact, the flyby probe passed by the nucleus at a close distance of 500 km, taking pictures of the crater position, the ejecta plume, and the entire cometary nucleus. The entire event was photographed by Earth-based telescopes and orbital observatories, such as the Hubble, Chandra, Spitzer and XMM-Newton. In addition, the impact was observed by cameras and spectroscopes on board Europe's Rosetta spacecraft, which was about 80 million km from the comet at the time of impact. Rosetta should determine the composition of the gas and dust cloud kicked up by the impact. 
The total Deep Impact mission cost is US$330 million.
The Deep Impact mission will help answer fundamental questions about comets, such as:
- Is the composition of a cometary nucleus the same throughout, or has some physical process caused the interior to become differentiated from the surface? In other words, is the nucleus layered?
- Are cometary nuclei highly cohesive and tightly-packed, or porous conglomerates?
- Do any parts of a cometary nucleus contain pristine material that have been untouched since the creation of the comet during the Solar System's early history?
Scientists hope that these questions will be answered, at least in part, by data from the Deep Impact mission. For example, the size and shape of the crater produced by the impact will tell scientists how well-packed the cometary material is.
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