EPOXI

TheDeep Impact spacecraft atBall Aerospace & Technologies in July 2004.
Mission type	Flyby (85D/Boethin · 103P/Hartley · (163249) 2002 GT) Space observatory
Operator	NASA
COSPAR ID	2005-001A
Website	epoxi.umd.edu
Mission duration	8 years, 18 days
Spacecraft properties
Spacecraft	Deep Impact
Manufacturer	JPL · Ball Aerospace · University of Maryland
Launch mass	650 kg[1]
Power	620.0W
Start of mission
Entered service	July 21, 2005 (2005-07-21) (20 years, 3 months and 22 days ago)
End of mission
Last contact	August 8, 2013 (2013-08-08) (12 years, 3 months and 4 days ago)
Flyby ofHartley 2
Closest approach	November 4, 2010 (2010-11-04) ~03:00UTC (15 years and 8 days ago)
Distance	694 kilometres (431 mi)
Official insignia for the re-targetedDIXI mission to Hartley 2

EPOXI was a compilation ofNASADiscovery program missions led by theUniversity of Maryland andprincipal investigatorMichael A'Hearn, with co-operation from theJet Propulsion Laboratory andBall Aerospace.EPOXI uses theDeep Impact spacecraft in a campaign consisting of two missions: theDeep Impact Extended Investigation (DIXI) andExtrasolar Planet Observation and Characterization (EPOCh).DIXI aimed to send theDeep Impact spacecraft on a flyby of another comet, after its primary mission was completed in July 2005, whileEPOCh saw the spacecraft's photographic instruments as aspace observatory, studyingextrasolar planets.^[2][3][4]

DIXI successfully sent theDeep Impact spacecraft on aflyby of cometHartley 2 on November 4, 2010, revealing a "hyperactive, small and feisty" comet, after threegravity assists fromEarth in December 2007, December 2008 and June 2010. TheDIXI mission was not without problems, however; the spacecraft had initially been targeted for a December 5, 2008 flyby of cometBoethin, though, the comet could not be located, and was later declared alost comet, prompting mission planners to reorganize a flyby of an alternative target, Hartley 2. After its flyby of Hartley 2, the spacecraft was also set to make a close flyby of theApollo asteroid(163249) 2002 GT in 2020. The mission ended after contact with the spacecraft was suddenly lost in August 2013 and attempts to re-establish contact in the following month had failed.^[5] Mission scientists theorized that aY2K-like problem had plagued the spacecraft's software.

TheDeep Impact mission was finished with the visit to cometTempel 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).^[6]

On July 21, 2005,Deep Impact executed a trajectory correction maneuver that placed the spacecraft on course to fly past Earth on December 31, 2007. The maneuver allowed the spacecraft to use Earth's gravity to begin a new mission in a path towards another comet. In January 2008Deep Impact began studying the stars with several knownextrasolar planets in an attempt to find other such stars nearby. The larger of the spacecraft's two telescopes attempts to find the planets using thetransit method.^[6]

The initial plan was for a December 5, 2008 flyby ofComet Boethin, with the spacecraft coming within 435 miles (700 km). 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 on9P/Tempel. A'Hearn, theDeep 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."^[7] 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.^[7] (EPOXI's low mission cost of $40 million is achieved by reusing the existing Deep Impact spacecraft.)Deep Impact would use itsspectrometer to study the comet's surface composition and its telescopes for viewing the surface features.^[6]

However, as the Earth gravity assist approached, astronomers were unable to locate Comet Boethin, which is too faint to be observed. Consequently, its orbit could not be calculated with sufficient precision to permit a flyby. Instead, the team decided to sendDeep Impact to comet103P/Hartley requiring an extra two years. NASA approved the additional funding required and retargeted the spacecraft.^[8] 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 theDeep 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 2007Deep Impact leader and University of Maryland astronomerMichael A'Hearn who is principal investigator for both the overall EPOXI mission and its DIXI component.^[4]

In June 2009,^[9] EPOXI's spectrometer scanned the Moon on its way to Hartley, and discovered traces of "water or hydroxyl", confirming aMoon Mineralogy Mapper observation — a discovery announced in late September, 2009.^[10]

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 transitingextrasolar planets from January to August 2008.^[11] The goal ofphotometric observations is to measure the quantity of light, not necessarily resolve an image. An aberration in the primary mirror of the HRI^[12] 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.^[13] EPOCh's goals were to study the physical properties of giant planets and search for rings, moons and planets^[14] as small as three Earth masses.^[15] 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.^[11]

The spacecraft used Earth's gravity for the second gravity assist in December 2008 and made two distant flybys of Earth in June and December 2009. On May 30, 2010 it successfully fired its engines for an 11.3 second trajectory correction maneuver, for a velocity change (Δv) of 0.1 meters per second (0.22 mph), in preparation for the third Earth flyby on June 27. Observations of 103P/Hartley began on September 5 and ended November 25, 2010.^[16] For a diagram of the EPOXI solar orbits seehere.

The mission's closest approach to 103P/Hartley occurred at 10 am EDT on 4 November 2010, passing to within 694 kilometers (431 mi) 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.^[17]

Early results of the observations show that the comet is powered bydry 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.^[17]

"When comet Boethin could not be located, we went to our backup, which is every bit as interesting but about two years farther down the road," said Tom Duxbury, EPOXI project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "Hartley 2 is scientifically just as interesting as comet Boethin because both have relatively small, active nuclei," saidMichael A'Hearn, principal investigator for EPOXI at the University of Maryland, College Park.^[3]

In November 2010, EPOXI was used to make some test-training deep sky observations, using the MRI camera that is optimised for cometary imagery. Images were made of theDumbbell Nebula (M27), theVeil Nebula (NGC6960) and theWhirlpool Galaxy (M51a).^[18]

Wikimedia Commons has media related toEPOXI.

External image
Deep Impact trajectory

• EPOXI home page
• NASA's EPOXIArchived 2016-02-13 at theWayback Machine page
• NASA's Deep Impact Begins Hunt For Alien Worlds - 8 Feb 2008
• Movie of the Moon transiting the Earth
• EPOXI Mission Archive at the NASA Planetary Data System, Small Bodies Node

• Missions to comets
• Discovery Program
• Exoplanet search projects

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