EPOXI

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EPOXI
Illustration of the Deep Impact space probe used on EPOXI mission.
Organization	NASA
Major contractors	Ball Aerospace & Technologies Corp., JPL
Mission type	Flyby
Launch date	January 12, 2005
Launch vehicle	Delta II
NSSDC ID	2005-001A
Webpage	EPOXI website
Mass	650 kg (1,430 lbs)

NASA's EPOXI is a University of Maryland-led unmanned space mission that uses the existing Deep Impact vehicle to begin a new series of observations. First it will investigate extrasolar planets and, in 2010, it may visit and study Comet Hartley 2. The new mission was originally announced in NASA Press Release 07-147 on 3 July 2007 as including flyby of Comet Boethin, but Boethin was too small and faint for its orbit to be calculated accurately, so the mission was subsequently retargeted to Hartley 2. Scientists theorize Comet Boethin may have broken up into pieces too small for detection. NASA and the University of Maryland confirmed funding for the Hartley 2 flyby in news releases issued on December 13, 2007.^[1] ^[2]

EPOXI combines two targets. These are the Deep Impact Extended Investigation (DIXI) and the Extrasolar Planet Observation and Characterization (EPOCh). Deep Impact will conduct both, first during the cruise phase to Hartley 2 (EPOCh) and secondly at flyby (DIXI).

"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 Deep Impact leader and University of Maryland astronomer Michael A'Hearn who is principal investigator (PI) for both the overall EPOXI mission and its DIXI component.^[3]

[edit] Mission

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 allows the spacecraft to use Earth's gravity to begin a new mission in a path towards another comet. The extended mission is called EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) and in January 2008 will have Deep Impact begin studying the stars around several known extrasolar planets to attempt to find other nearby extrasolar planets. The larger of the spacecraft's two telescopes will attempt to find the planets using the astrometry and transit methods.^[4]

The initial plan was for a December 5, 2008 flyby of Comet Boethin, with the spacecraft coming within 435 miles (700 kilometers). Instead of using another impactor to collide with the comet (which the spacecraft does not have), the spacecraft will 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."^[5] He continued on to reveal that the mission would provide about half of the information as the collision of Tempel 1 but at a fraction of the cost.^[5] Deep Impact will use its spectrometer to study the comet's surface composition and its telescopes for viewing the surface features.^[4]

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 will target Deep Impact toward Comet Hartley 2. However, this will require an extra two years of travel for Deep Impact. NASA retargeted the spacecraft toward Hartley 2 and has approved the additional funding required.^[6]

The flyby of comet Hartley 2 is targeted for October 11, 2010.

In addition to investigating comet Hartley 2, the spacecraft will point the larger of its two telescopes at nearby previously discovered extrasolar planetary systems in late January 2008. It will study the physical properties of giant planets and search for rings, moons and planets as small as three Earth masses. It also will look at Earth as though it were an extrasolar planet to provide data that could become the standard for characterizing these types of planets.

"The search for exosolar planetary systems is one of the most intriguing explorations of our time," said Drake Deming, Epoxi deputy principal investigator at NASA's Goddard Space Flight Center, Greenbelt, Md. "With Epoxi we have the potential to discover new worlds and even analyze the light they emit to perhaps discover what atmospheres they possess."

The mission's closest approach to the small half-mile-wide comet will be about nearly a thousand kilometers (620 miles). The spacecraft will employ the same suite of two science instruments the Deep Impact spacecraft used during its prime mission to guide an impactor into comet Tempel 1 in July 2005.

If Epoxi’s observations of Hartley 2 show it is similar to one of the other comets that have been observed, this new class of comets will be defined for the first time. If the comet displays different characteristics, it would deepen the mystery of cometary diversity.

"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, Calif.

Mission controllers at JPL began directing Epoxi towards Hartley 2 on November 1. They commanded the spacecraft to perform a three-minute rocket burn that changed the spacecraft's velocity. Epoxi’s new trajectory sets the stage for three Earth flybys, the first on December 31, 2007. This places the spacecraft into an orbital "holding pattern" until it’s time for the optimal encounter of comet Hartley 2 in 2010.

"Hartley 2 is scientifically just as interesting as comet Boethin because both have relatively small, active nuclei," said Michael A'Hearn, principal investigator for Epoxi at the University of Maryland, College Park.

Epoxi’s low mission cost of $40 million is achieved by taking advantage of the existing Deep Impact spacecraft.