The first spacecraft designed to blast into a comet鈥檚 icy heart is set to lift off in January 2005, mission managers announced on Tuesday.
NASA鈥檚 Deep Impact mission is scheduled to launch from Florida鈥檚 Cape Canaveral Air Force Station aboard a Delta II rocket sometime between 12 and 28 January. After a six-month journey, a pair of spacecraft will study the ice-and-rock heart 鈥� or nucleus 鈥� of Comet 9P/Tempel 1, which orbits the Sun between Mars and Jupiter.
The conjoined spacecraft will separate as they approach the comet鈥檚 nucleus. One craft will smash right into it while the other vehicle flies by the comet, observing.
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Comet nuclei are thought to reflect the original composition of the solar system. But only a handful have ever been seen as they approach the Sun 鈥� burning off obscuring clouds of dust and gas, called comas. And no spacecraft has yet touched a comet鈥檚 surface.
Peeling away layers
鈥淲e鈥檝e learned a great deal from studying imagery of the surface,鈥� says Tom Morgan, programme scientist for the mission at NASA鈥檚 headquarters in Washington, DC, US. 鈥淏ut what we don鈥檛 have is direct knowledge of the interior of a comet鈥檚 nucleus. Deep Impact will be the first mission to peel away these outer layers.鈥�
Getting under the surface is important because the outer layers are heated and altered every time the comet nears the Sun, says principal investigator Michael A鈥橦earn at the University of Maryland in College Park, US. The mission will help constrain models of how deeply the alteration penetrates.
And it will help answer why comets eventually stop releasing the gas that produces their bright comas. Astronomers do not know whether the phenomenon is due to the comets simply 鈥渞unning out of steam鈥� or to the growth of a crust that prevents ice from escaping the surface.
Plan of attack
The two spacecraft will separate near the comet鈥檚 six-kilometre-wide nucleus. The mother ship, called the flyby spacecraft, will then release a 370-kilogram copper-studded 鈥渋mpactor鈥� craft in front of the comet. On 4 July 2005, the comet will ram into the impactor at an estimated 37,000 kilometres per hour.
The impactor will be destroyed in the crash, but it will relay pictures of the comet until its very last electronic breath. These images will be the closest ever taken of a comet nucleus and could show details as small as 20 centimetres across.
The smash will blast away a crater that could be 14 stories deep and as wide as a football stadium. And it will eject debris that will cause the comet to brighten due to reflected sunlight. For about a day after the collision, the flyby spacecraft will snap visible and infrared images of both the debris and the crater to study their structure and composition.
All eyes watching
But many more observatories will be watching too, recording the event at a range of wavelengths and over many days as the ejected dust dissipates and fades. The Hubble, Spitzer, and Chandra telescopes will observe from space, and a host of instruments, including all of the powerful telescopes on Hawaii鈥檚 Mauna Kea, will observe from Earth.
Amateur observers between New Zealand and Arizona may be able to observe the actual impact with small telescopes 鈥� or even with the naked eye if the comet brightens enough.
Team members are quick to point out that the choreographed crash will hardly change the comet鈥檚 orbit. 鈥淚t鈥檚 a fairly small crater in comparison to the nucleus,鈥� says Karen Meech at the University of Hawaii in Hilo. She adds that objects hit comets all the time, as evidenced by the craters seen in images of nuclei.
The $330 million mission will effectively end about a month after the collision, when the flyby spacecraft finishes sending all of its stored data back to Earth. It will then move into a safe solar orbit or, if NASA provides more funding, scout out a second comet.
