NASA mission 'E-Minus' one month to comet flyby

October 5, 2010, JPL/NASA
Logo of NASA's EPOXI mission, which is just one month away from its encounter with comet Hartley 2. Image credit: NASA/JPL-Caltech/UMD

Fans of space exploration are familiar with the term T-minus, which NASA uses as a countdown to a rocket launch. But what of those noteworthy mission events where you already have a spacecraft in space, as with the upcoming flyby of a comet?

"We use 'E-minus' to help with our mission planning," said Tim Larson, EPOXI mission project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The 'E' stands for encounter, and that is exactly what is going to happen one month from today, when our spacecraft has a close encounter with comet Hartley 2."

The EPOXI mission's Nov. 4 encounter with Hartley 2 will be only the fifth time in history that a comet has been imaged close-up. At point of closest approach, the spacecraft will be about 700 kilometers (435 miles) from the comet.

"Hartley 2 better not blink, because we'll be screaming by at 12.3 kilometers per second (7.6 miles per second), said Larson.

One month out, the spacecraft is closing the distance with the comet at a rate of 976,000 kilometers (607,000 miles) per day. As it gets closer, the rate of closure will increase to a little over 1,000,000 kilometers (620,000 miles) per day.

For those interested in what the "T-minus" stands for in a countdown to a – it translates to "Time-minus." For example, when a rocket is getting ready for liftoff, it will be lifting off at a specific time. If that time is 45 seconds away, it is said to be "T-minus 45 seconds and counting."

EPOXI is an extended mission that utilizes the already "in-flight" Deep Impact spacecraft to explore distinct celestial targets of opportunity. The name EPOXI itself is a combination of the names for the two extended mission components: the extrasolar planet observations, called Extrasolar Planet Observations and Characterization (EPOCh), and the of Hartley 2, called the Deep Impact Extended Investigation (DIXI). The will continue to be referred to as "."

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More information: For more information about EPOXI visit .

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1 / 5 (1) Oct 05, 2010
I'd love to know why the spacecraft can't do a closer flyby than 435 miles. At that distance, it is similar to trying to image Tallahassee, FL from Miami, FL. How about 50 miles away - wouldn't the amount of science obtained be far greater at such a closer distance? I've noticed the same problem with flybys of Jupiter's moons, for example. If the gravity is not a problem, why not get much closer?
5 / 5 (1) Oct 05, 2010
Why you keep a distance would be that it may be like flying into a sandstorm with too much loose stuff around the space craft would be destroyed. And too close an approach would lead to a very rapid rate of change of angle, and a blurred picture. The comet is only 570 meters across, so point the camera the wrong way and it will be missed. Also the comet may be visible to the naked eye on October 20, 2010.
not rated yet Oct 06, 2010
I've been following the comet for the past week as it travels through northern Perseus using 7x50 binoculars and a 4.25" f8 reflector. Not quite a naked eye object, but close, from my suburban location. On October 7 and 8 the comet passes within 1 degree of the Double Cluster in Perseus and should make it easier to locate in binoculars(and quite a sight). October 19-20 the comet will be slightly brighter(possibly a naked eye object) and near the bright star Capella in Auriga. Sky & Telescope has a page on observing Hartley 2: http://www.skyand...669.html

Finder charts: http://media.skya...2-bw.jpg

The comet's proximity to Earth will make it appear to move rather quickly across the sky over the next few weeks. Should be interesting to follow it from the ground and in space.

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