The
Deep Impact mission was finished with the visit to comet
Tempel 1. But the spacecraft still had plenty of maneuvering fuel left, so NASA approved a second mission, called EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), which included a visit to a second comet (DIXI component) as well as observations of extrasolar planets (EPOCh component). The initial plan was for a December 5, 2008 flyby of
Comet Boethin, with the spacecraft coming within . The spacecraft did not carry a second impactor to collide with the comet and would observe the comet to compare it to various characteristics found on
9P/Tempel. A'Hearn, the
Deep Impact team leader reflected on the upcoming project at that time: "We propose to direct the spacecraft for a flyby of Comet Boethin to investigate whether the results found at Comet Tempel 1 are unique or are also found on other comets." He explained that the mission would provide only about half of the information collected during the collision with Tempel 1 but at a fraction of the cost. Mission controllers at the Jet Propulsion Laboratory began redirecting EPOXI on November 1, 2007. They commanded the spacecraft to perform a three-minute rocket burn that changed the spacecraft's velocity. EPOXI's new trajectory set the stage for three Earth flybys, the first on December 31, 2007. This placed the spacecraft into an orbital "holding pattern" so that it could encounter comet 103P/Hartley in 2010. "It's exciting that we can send the
Deep Impact spacecraft on a new mission that combines two totally independent science investigations, both of which can help us better understand how solar systems form and evolve," said in December 2007
Deep Impact leader and University of Maryland astronomer
Michael A'Hearn who is principal investigator for both the overall EPOXI mission and its DIXI component. EPOXI's spectrometer scanned the Moon on its way to Hartley, and discovered traces of "water or hydroxyl", confirming a
Moon Mineralogy Mapper observation — a discovery announced in late September, 2009.
EPOCh Before the 2008 flyby to re-orient for the comet 103P/Hartley encounter, the spacecraft used the High Resolution Instrument, the larger of its two telescopes, to perform photometric observations of previously discovered transiting
extrasolar planets from January to August 2008. The goal of
photometric observations is to measure the quantity of light, not necessarily resolve an image. An aberration in the primary mirror of the HRI allowed the HRI to spread the light from observations over more pixels without saturating the CCD, effectively obtaining better data. A total of 198,434 images were exposed. EPOCh's goals were to study the physical properties of giant planets and search for rings, moons and planets as small as three Earth masses. It also looked at Earth as though it were an extrasolar planet to provide data that could characterize Earth-type planets for future missions, and it imaged the Earth over 24 hours to capture the Moon passing in front on 2008-05-29. For a diagram of the EPOXI solar orbits see here. The mission's closest approach to 103P/Hartley occurred at 10 am EDT on 4 November 2010, passing to within of this small comet. The flyby speed was 12.3 km/s. The spacecraft employed the same suite of three science instruments—two telescopes and an infrared spectrometer—that the Deep Impact spacecraft used during its prime mission to guide an impactor into comet Tempel 1 in July 2005 and observe the results. Early results of the observations show that the comet is powered by
dry ice, not water vapor as was previously thought. The images were clear enough for scientists to link jets of dust and gas with specific surface features. == References ==