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USA in Space, 3rd Edition Deep Impact Date: Beginning January, 2005 Type of Mission: Planetary exploration Deep Impact is the first space mission designed to study the interior of a primitive celestial body directly. This was accomplished by slamming a heavy impactor into Comet Tempel 1, creating a fresh crater. Never before has any space mission tried to make an impact crater of this size in any object. Summary of the Mission In July, 1999, the National Aeronautics and Space Administration (NASA) approved funding under its Discovery Program for a space mission to slam an object into a comet and to study the ejecta from that impact. This proposed mission became the Deep Impact project. By November, 1999, serious work had commenced in design of the project. Except for minor variations, the plans were finalized by May, 2001. These plans called for the development of a flyby spacecraft to study the impact ejecta and for a smart impactor that would guide itself to the target. The comet selected as target was Tempel 1, discovered in 1867 by Ernst Tempel. Comet Tempel 1 orbits the Sun every 5.5 years. It approached perihelion, the closest point to the Sun in its orbit, in early summer, 2005. A target date for comet rendezvous was selected as July 4, 2005. Earth's orbit placed it in position to launch a mission to arrive at Tempel 1 on this date during a period of only a few weeks in January, 2004, and again for several weeks a year later. These two time periods during which a launch is possible are known as launch windows. Initially, plans called for the mission to be launched during the first launch window, in January of 2004. However, the spacecraft was not ready for launch at that time, and so launch was postponed until December 30, 2004, the first day of the second launch window, initially reserved as a backup in case the launch could not occur in January, 2004. The launch window was only about three weeks long, and if the spacecraft was not launched in this time, then no rendezvous mission would have been possible. In November, 2004, the launch was further postponed until January 8, 2005, at 19:40 Coordinated Universal Time (UTC). This date was well into the last launch window, and thus it gave very little leeway if problems developed with weather or with the launch vehicle. The launch vehicle for Deep Impact was a Delta II 2925, a variant of the Delta rocket with nine strap-on solid rockets. As with most launch vehicles, the components were assembled at the launch pad. The first stage of Deep Impact's launch vehicle was lifted into place on November 22, 2004, at Space Launch Complex 17B of the Cape Canaveral Air Force Station, located adjacent to the Kennedy Space Center, in Florida. The solid rocket boosters were attached three at a time over the following eight days. Meanwhile, the Deep Impact spacecraft was being prepared nearby at the Astrotech Space Operations facility near Titusville, Florida. On October 23, 2004, the spacecraft had arrived from the Ball Aerospace & Technologies factory at Boulder, Colorado, where it had been constructed. NASA's Deep Impact mission finally launched at 18:47:08 UTC on January 12, 2005, from Cape Canaveral's Complex 17. Six solid rocket boosters fired at liftoff and were jettisoned in flight at a point before the final three solid rocket boosters ignited. Three minutes into ascent, the final solid rocket boosters had burned out and dropped away, leaving the main engine to continue to burn. After main engine cutoff and second-stage ignition, the protective fairing covering the spacecraft separated safely, exposing Deep Impact to space conditions. Nine minutes after liftoff, the second stage shut down and left the remaining booster/spacecraft combination in a parking orbit. At 19:11 Coordinated Universal Time (UTC), the second-stage engine reignited for 95 seconds. Third-stage ignition followed second-stage shutdown and separation, and the third-stage engine continued to fire until 19:16 UTC. Spinning motions of the spent third stage were damped out, and at 19:22 UTC the Deep Impact spacecraft separated from the spent booster. Shortly after beginning its journey to the comet, the spacecraft's onboard computer detected an error and put the spacecraft into a safe mode, a shutdown designed to prevent damage to the spacecraft until the problem is corrected. Only minor course corrections were required over the course of a six-month cruise phase. On July 3 the flyby spacecraft released the impactor so that it could intercept Comet Tempel 1. To prevent collision between the flyby and Tempel 1, the flyby performed an avoidance maneuver, firing thrusters that slowed it down by 120 meters per second (270 miles per hour). Moving down and below the comet, the flyby spacecraft had to turn its cameras back toward Tempel 1 to record the impactor's crash. The impactor's on-board guidance system executed four maneuvers as people all over the world watched on cable news, NASA television, and the Internet. Deep Impact provided tremendous celestial fireworks on the Fourth of July, striking very close to its target at a relative speed of 10.2 kilometers per second (22,800 miles per hour). The time of impact (5:52 UTC) was selected in order for the impact to happen above the horizon simultaneously for two of NASA's Deep Space Network radio telescopes, allowing both receivers to record real-time data transmitted from the spacecraft. The impact excavated a crater in the comet nucleus, allowing the flyby spacecraft to study the material from inside the comet. The flyby spacecraft began imaging one minute before impact, and it was 10,000 kilometers (6,200 miles) away at the moment of impact. The flyby spacecraft continued collecting data as it closes with the comet nucleus, eventually passing within 500 kilometers (310 miles) of it. Selected images were sent back to Earth in near-real time; however, data were collected far more quickly than the communication system could send the information to Earth. Thus, recorded data began to be sent back to Earth starting about fifty minutes after impact, with most of the data returned within a day with provisions to extend data playback for nearly a month if necessary. Shortly after this highly successful encounter, Deep Impact program officials sought funds to continue the mission and redirect the flyby spacecraft toward another comet encounte; preliminary permission was given, but budget consideration held the potential for cessation of the extended mission. The flyby spacecraft was somewhat irregular in shape, measuring about 1.7 meters wide, 2.3 meters high, and 3.3 meters long and having a mass of 650 kilograms. Power was provided by a 7.5-square-meter solar panel located on one side of the spacecraft bus. The two main science instruments are located on the other side of the main bus. The spacecraft bus housed the propulsion, communication, and computer systems. Mounted to the top of the bus, a 1-meter-diameter dish provided primary communication with Earth. A separate antenna communicated with the impactor. Debris shields, called Whipple shields, protected the spacecraft from damage from impact with high velocity dust particles shed by the comet. The principal science instruments on the flyby spacecraft were the High Resolution Instrument (HRI) and the Medium Resolution Instrument (MRI). Both instruments were Cassegrain-type telescopes, and CCD (charge-coupled device) cameras with filter wheels to permit observations in different colors of light. The HRI was one of the largest instruments dedicated to solar system astronomy ever put aboard a spacecraft. With its telescope having a diameter of 30 centimeters, the HRI has five times the resolution of the MRI and is capable of resolving features smaller than 2 meters when nearest the comet nucleus. The HRI also is able to make measurements in infrared light. The MRI, with its 12-centimeter-diameter telescope, also had a filter wheel to allow observations in different spectral ranges, and it acts as a backup to the HRI. Additionally, at close approach its larger field of view provided images of the entire nucleus, rather than merely a portion like the HRI, which saw only a small portion of the nucleus at a time. Additionally, in the last ten days before the encounter, the MRI acted as a navigation camera for the spacecraft. The impactor was a 350-kilogram cylindrical device about 1 meter in diameter and 1 meter in length. The impactor was attached to the flyby spacecraft bus until twenty-four hours before impact, and it received all electrical power from the flyby spacecraft's solar panel. After separation, it operated entirely on battery power. The impactor was destroyed in the impact with the comet nucleus, and so it was made of mainly copper, because copper provided the least interference with spectral measurements of the impact ejecta. The impactor contained a small propulsion system to make corrections to its trajectory, in order to impact at the proper point on the comet nucleus, as well as a communication system to send data back to the flyby spacecraft. The main instrument on board the impactor was the Impactor Target Sensor (ITS), which was virtually a duplicate of the MRI, except without the MRI's filter wheel. Thus, in addition to guiding the impactor to the correct target zone, the ITS was capable of taking useful science images on its way to impact with the comet nucleus. Upon impact with the comet, the impactor delivered an energy of 19 gigajoules (equivalent to about 4.8 tons of dynamite) and excavated a sizable crater on the surface of the comet nucleus. Contributions Comets form in the outer solar system of mostly ice, with some dust and rock mixed in, and may be virtually unchanged since the formation of the solar system. For most of the time since their formation, comets have remained either in a zone beyond the planet Pluto called the Kuiper Belt or in a cloud around the solar system called the Oort Cloud. Occasionally the orbits of comets are altered, sending the icy bodies into the inner solar system, where heat from the Sun begins to vaporize the ices that make up a comet's nucleus, shedding dust debris and forming a cloud around the nucleus and the tail of the comet. This cloud prevents astronomers from observing the nucleus of a comet with great detail. The ITS optics aboard Deep Impact were expected to be destroyed prior to impact by collisions with dust particles from the comet, but they still provided the most detailed images of a comet nucleus to date. Some concerns over the imaging capability of the flyby spacecraft's HRI were reported on March 25, 2005. After a bakeout cycle had been completed to remove residual gases from the optics, mission controllers were unable fully to achieve optimal focus with the HRI because of a slight imperfection in the instrument's mirror. By June, 2005, however, mission scientists were confident that they would be able to produce high-quality images using the HRI, despite its inability to achieve perfect focus. After the images were sent back to Earth, a mathematical process called deconvolution was applied to the images to correct for the focusing problem. This technique should produce images nearly as good as could have been achieved had the HRI mirror been able to provide perfect focus. By blasting into the comet, Deep Impact permitted the inner layers of a comet to be studied for the first time. The outer portions of a comet in the inner solar system have been altered by the heat from the Sun, but the inner parts may remain as they were when the comet formed. Also, in striking the comet, Deep Impact helped astronomers determine the consistency of the comet nucleus. It is not known if the nucleus is rock-solid, like an iceberg, or is soft and fluffy, like a snowflake. Preliminary analysis of images of the comet and data collected by examination of the plume created during crater excavation indicated a surface with much more dust and dirty water ice than had been expected. Scientists would continue to study Deep Impact data for years to come. Context Knowing more about comets permits astronomers to know more about the formation of the solar system. Comet studies also help scientists understand Earth, because some of the water and organic materials needed for life are believed to have been brought to Earth by comets. However, understanding comets better has a more practical application as well, because comets can impact with planets. If a comet were on a collision course with Earth, knowing the structure and consistency of the nucleus of the comet might permit a strategy to be developed to deflect the comet, given sufficient time. Deep Impact and its sister comet mission, Stardust, were both funded under NASA's very successful Discovery Program. Discovery missions are relatively inexpensive space missions with a narrow focus. An earlier Discovery mission, Deep Space 1 (1998), passed by a comet, but that mission's primary goal was development of a new ion drive for space probes. Other than the Comet Nucleus Tour mission (CONTOUR) of 2002, which lost contact with its spacecraft, Stardust and Deep Impact were the first U.S. missions designed primarily to explore comets. They form part of an international effort to study comets, including Rosetta, launched in 2004 by the European Space Agency (ESA) toward Comet Churyumov-Gerasimenko. Deep Impact also marks a first in space exploration, in that it attempts to alter the body being studied, in this case by blasting a crater into it. All other space missions have been more passive in their approach, with only minimal impact on the body being studied. See Also Asteroid and Comet Exploration; Cape Canaveral and the Kennedy Space Center; Dawn Mission; Stardust Project. Further Reading Brandt, John C., and Robert D. Chapman. Introduction to Comets. Boston: Cambridge University Press, 2004. Provides a detailed examination of virtually every cometary phenomenon. Crovisier, Jacques, and Thérèse Encrenaz. Comet Science. Translated by Stephen Lyle. New York: Cambridge University Press, 2000. An excellent treatise on comet structure and the study of comets, though somewhat technical in places. Davis, John. Beyond Pluto. New York: Cambridge University Press, 2001. A popular book about the Kuiper Belt, a region beyond Pluto from which many comets are believed to originate. Levy, David. Comets: Creators and Destroyers. New York: Touchstone, 1998. Discusses some of the theories of comet impacts bringing life-giving materials to Earth, and perhaps resulting in some of the past mass extinctions. Sagan, Carl. Comet. Rev. ed. New York: Ballantine Books, 1997. Written by a noted popularizer of science, this work describes the pristine remnants left over from the origin of the solar system called comets. Liberally illustrated with photographs, Sagan's book humanizes the study of science and teaches a history of astronomy along the way. Verschur, Gerrift L. Impact: The Threat of Comets and Asteroids. London: Oxford University Press, 1997. Describes the nature of near-Earth asteroids and comets, and their threat of devastating impacts on Earth. Whipple, Fred L. The Mystery of Comets. Washington, D.C.: Smithsonian Institution Press, 1985. An older book, but one that gives an excellent history of comet studies, as well as an easy-to-understand, basic presentation of the standard model for comets as explained by Fred Whipple himself. Raymond D. Benge, Jr. |
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