The Mission
The Deep Impact mission lasts six years from start to finish. Planning and design for the mission took place from November 1999 through May 2001. The mission team is proceeding with the building and testing of the two-part spacecraft. The larger "flyby" spacecraft carries a smaller "impactor" spacecraft to Tempel 1 and releases it into the comet's path for a planned collision.
In January 2005, a Delta II rocket launches the combined Deep Impact spacecraft which leaves Earth's orbit and is directed toward the comet. The combined spacecraft approaches Tempel 1 and collects images of the comet before the impact. In early July 2005, 24 hours before impact, the flyby spacecraft points high-precision tracking telescopes at the comet and releases the impactor on a course to hit the comet's sunlit side.
The impactor is a battery-powered spacecraft that operates independently of the flyby spacecraft for just one day. It is called a "smart" impactor because, after its release, it takes over its own navigation and maneuvers into the path of the comet. A camera on the impactor captures and relays images of the comet's nucleus just seconds before collision. The impact is not forceful enough to make an appreciable change in the comet's orbital path around the Sun.
After release of the impactor, the flyby spacecraft maneuvers to a new path that, at closest approach passes 500 km (300 miles) from the comet. The flyby spacecraft observes and records data about the impact, the ejected material blasted from the crater, and the structure and composition of the crater's interior. After its shields protect it from the comet's dust tail passing overhead, the flyby spacecraft turns to look at the comet again. The flyby spacecraft takes additional data from the other side of the nucleus and observes changes in the comet's activity. While the flyby spacecraft and impactor do their jobs, pro-fessional and amateur astronomers at large and small telescopes on Earth observe the impact and its aftermath, and results are broadcast over the Internet.
The Comet
Comet 9P/Tempel 1 was discovered on April 3, 1867 by Ernst Wilhelm Leberecht Tempel of Marseilles France while visually searching for comets. The comet was then 9th magnitude and described by Tempel as having an apparent diameter of 4 to 5 arcmin across. Later calculations revealed that the comet had been situated 0.71 AU from the Earth and 1.64 AU from the sun at that time.
Historical Highlights
The comet was very well placed for its 1867 discovery thanks to its closest approach to Earth (0.568 AU) and its perihelion (1.562 AU), which occurred on May 15 and May 24, respectively. Over the next five months after its initial detection, subsequent observations were frequently made. The comet was last detected on August 27, 1867 by Julius Schmidt, at which point the comet had become too faint for position measurements. At that time the comet was 1.30 AU from Earth and 1.81 AU from the sun.
The comet was first recognized as periodic in May of 1867 when C. Bruhns of Leipzig determined the orbital period to be 5.74 years. By the time the final observations had been made of the 1867 apparition, the orbital period had been re-calculated to be 5.68 years.
The comet was recovered on April 4, 1873 by E.J.M. Stephan of Marseilles, France. The comet remained under observation until July 1st of that year.
Predictions were made for an 1879 return, with the most ambitious being that of Raoul Gautier who computed definitive orbits from the two previous appearances before making his predictions for the upcoming return. Gautier's predictions enabled Tempel to recover the comet on April 25, 1879. The comet was observed until its last detection on July 8.
In 1881, comet Tempel 1 passed 0.55 AU from Jupiter. Due to gravitational interactions, the comet's orbital period was increased to 6.5 years and the perihelion distance was increased from 1.8 AU to 2.1 AU, making the comet an even fainter object. Subsequently, the comet was lost and it was not observed at its next expected return. Photographic attempts during 1898 and 1905 failed to recover the comet.
During 1963, B.G. Marsden conducted an investigation as to why comet Tempel 1 became lost. He found that further close approaches to Jupiter in 1941 (0.41 AU) and 1953 (0.77 AU) had decreased both the perihelion distance and the orbital period to values smaller than when the comet was initially discovered. These approaches moved Tempel 1 into its present libration around the 1:2 resonance with Jupiter. Subsequently, Marsden published predictions for the 1967 and 1972 returns in his paper On the Orbits of Some Long Lost Comets . (Courtesy of NASA Astrophysics Data System)
Despite an unfavorable 1967 return, Elizabeth Roemer of the Catalina Observatory took several photographs during 1967. Her initial inspections of these photographs revealed nothing. However, in late 1968 she re-examined the photographic plates and found that a single exposure taken on June 8, 1967 held the image of an 18th magnitude diffuse object very close to where Marsden had predicted the comet to be. Unfortunatly, the single image did not provide definitive proof of the comet's recovery.
During 1972, Marsden's predictions allowed Roemer and L.M. Vaughn to recover the comet on January 11 from Steward Observatory. The comet became widely observed and reached a maximum brightness of magnitude 11 during May of that year. The comet was last seen on July 10. This apparition proved that the single image taken by Roemer in 1967 was indeed comet 9P/Tempel 1. Since that time the comet has been seen at every apparition.
(posted: 4/19/05)
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