On Sunday, NASA's historic Deep Impact spacecraft will fly past Earth for the fifth and last time on its current University of Maryland-led EPOXI mission. At time of closest approach to Earth, the spacecraft will be about 30,400 kilometers (18,900 miles) above the South Atlantic. Mission navigators have tailored（调整使适应） this trajectory（轨道，弹道） 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（飞越） 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（前任，前辈） mission, Deep Impact.

"There is always some gravity（重力） 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（倾斜） 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（航空学） and Astronautics（航天学） Space Science Award for his leadership of the Deep Impact mission -- which made history and world-wide headlines when it smashed a probe into comet Tempel 1 on July 4, 2005. A'Hearn also won the 2008 Kuiper astronomy prize for seminal（种子的，精液的） 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（轨道速度） ," 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（来自） 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红外光谱仪). 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.