Comet probe pays visit to Earth

Artist's impression of Deep Impact and comet Tempel 1

Artist's impression of the then Deep Impact spacecraft visiting Comet Tempel 1 in 2005. Now renamed EPOXI, the spacecraft will visit another comet in November 2010.

  • EPOXI mission bound for Comet Hartley 2
  • To make fly-by of Earth to pick up speed
  • Due to reach the comet in November 2010

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 kilometres (18,900 miles) above the South Atlantic.

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 fly-by, a close encounter with comet Hartley 2 in November.

Diagram showing EPOXI's orbit and fly-bys

EPOXI will make a fly-by of Earth on June 26, and reach Comet Hartley 2 in November 2010.

“The speed and orbital track of the spacecraft can be changed by changing aspects of its fly-by 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 fly-by and in some cases, like this one, it is the main reason for a fly-by,” said A’Hearn.

“The last Earth fly-by 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 fly-by to also change the shape of the orbit to get us to the comet.”

The Deep Impact mission made history and headlines worldwide when it smashed a probe into comet Tempel 1 on July 4, 2005.

“Earth is a great place to pick up orbital velocity,” said Tim Larson, the EPOXI project manager from NASA’s Jet Propulsion Laboratory. “This fly-by 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.”

A recycled mission

EPOXI is an extended mission of the Deep Impact fly-by 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).

Impact on Comet Tempel 1

In 2005, an impactor was collided with Comet Tempel 1, resulting in this huge flash.

On November 4, 2010, the mission will conduct an extended encounter with Hartley 2, studying the comet using all three of the spacecraft’s instruments (two telescopes with digital colour cameras and an infrared spectrometer).

On its original mission, the Deep Impact fly-by 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.

Although scientific objectives have never been a primary purpose of the Deep Impact/EPOXI spacecraft’s fly-bys 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.

Adapted from information issued by the University of Maryland / NASA / JPL-Caltech / UMD / Pat Rawlings.

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