Mission navigators have tailored this trajectory to change the shape of the spacecraft's orbit and to boost it on its way to the mission's ultimate flyby, a close encounter with comet Hartley 2 in November.

"The speed and orbital track of the spacecraft can be changed by changing aspects of its flyby of Earth, such as how close it comes to the planet," explained University of Maryland astronomer Michael A'Hearn, principal investigator for both the EPOXI mission and its predecessor mission, Deep Impact.

"There is always some gravity boost at a flyby and in some cases, like this one, it is the main reason for a flyby. The last Earth flyby was used primarily to change the tilt of the spacecraft's orbit to match that of comet Hartley 2, and we are using Sunday's flyby to also change the shape of the orbit to get us to the comet," said A'Hearn, who won the 2008 American Institute of Aeronautics and Astronautics Space Science Award for his leadership of the Deep Impact mission - which made history and world-wide Deep Impact Spacecraft To Make Last Swing By Earth On Way To Second Comets when it smashed a probe into comet Tempel 1 on July 4, 2005.

A'Hearn also won the 2008 Kuiper astronomy prize for seminal contributions over his career to the study of comets, prominently including the Deep Impact mission.

"Earth is a great place to pick up orbital velocity," said Tim Larson, the EPOXI project manager from NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"This flyby will give our spacecraft a 1.5-kilometer-per-second [3,470 mph] boost, setting us up to get up close and personal with comet Hartley 2."

EPOXI is an extended mission of the Deep Impact flyby spacecraft. Its name is derived from this mission's two tasked science investigations - the Deep Impact Extended Investigation (DIXI) and the Extrasolar Planet Observation and Characterization (EPOCh). On Nov. 4, 2010, the mission will conduct an extended flyby of Hartley 2 using all three of the spacecraft's instruments (two telescopes with digital color cameras and an infrared spectrometer).

Note: On its original mission, the Deep Impact flyby spacecraft had a companion probe spacecraft that was smashed into comet Tempel 1 to reveal for the first time the inner material of a comet.

Flyby Science
Although scientific objectives have never been a primary purpose of the Deep Impact/EPOXI spacecraft's flybys of Earth, the mission team has used the spacecraft's instruments to find clear evidence of water on the moon and to study light reflected from Earth as a template that scientists eventually may be able be use to identify Earth-like planets around other stars.

The University of Maryland is the Principal Investigator institution. JPL manages EPOXI for NASA's Science Mission Directorate, Washington. The spacecraft was built for NASA by Ball Aerospace and Technologies Corp., Boulder, Colo.

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