Where oh where is the comet?

William Atkins
Monday, 17 December 2007 18:52

Science - Space

That’s what they said when NASA scientists decided to send their Deep Impact spacecraft to Comet 85P/Boethin. When they looked again for the comet, it was no where to be found. So, Deep Impact was re-direced to Comet Hartley 2.     

On July 4, 2005, Deep Impact, launched on January 12, 2005, successfully completed its primary mission.

It ejected an 820-pound (370-kilogram) metallic projectile (“Smart Impactor”) into the nucleus of comet 9P/Tempel and recorded the results as debris flew in every direction.

The explosion was equivalent to 4.7 tons of TNT.

See Deep Impact at two NASA websites: http://solarsystem.nasa.gov/deepimpact/index.cfm and http://www.nasa.gov/mission_pages/deepimpact/main/index.html.

It then put the spacecraft to sleep, waking it up every six months as a safety check.

With an idle spacecraft on their hands, NASA mission managers decided to carry out another mission to a comet. Comet 85P/Boethin was in the neighborhood because it had been detected on two earlier occasions in 1975 and 1986. However, when Deep Impact was directed to intercept its course, the comet didn’t show up at the point where and when it was expected.

Astronomers think the comet could have been demolished on its last close encounter with the Sun in 1997. But, another more plausible theory is that during its approach to the Sun, it broke up into several pieces and, then, these pieces took on trajectories different from the original course—which would explain why Deep Impact didn’t find it.

Mission managers knew, when they originally planned to send the spacecraft to Comet 85P/Boethin, that they might not get very close to it because of a lack of precise tracking on the comet. But, they thought it was a risk they would take considering its relative closeness to Comet 9P/Tempel. However, early on in the mission planning stage, astronomers at several observatories could not find the comet. So, the mission managers abandoned their plans to go to Comet 85P/Boethin.

So, on November 1, 2007, mission managers, instead, had Deep Impact use its rocket engine to setup a flyby (the first of three) of Earth on November 31 so that it could use Earth’s gravity to propel it onward to Comet Hartley 2.

On its way to Comet Hartley 2, Deep Impact will also study five stars that have known planets (called extrasolar planets, or exoplanets) circling them. This mission is called Extrasolar Planet Observations and Characterization (EPOCh).

When Deep Impact reaches Comet Hartley 2 around October 11,  2010, it will approach to about 550 miles (1,000 kilometers) of it. Comet Hartley 2 is about 1.0 mile (1.6 kilometers) in diameter. It will use two cameras and one spectrometer to study the comet. This part of the mission is called Deep Impact eXtended Investigation (DIXI).

The combined mission is called EPOXI, a combination of parts of EPOCh and DIXI.

On December 13, 2007, University of Maryland astronomer Michael A’Hern, the principal investigator for the EPOXI mission, said, "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." [University of Maryland article “Deep Impact Extended Mission Heads for Comet Hartley 2”.

[Note: Author corrected mistake of units, from kilometers to kilograms....oops, as the saying goes.]

[Note: It must have been a bad day to write. Yes, as a reader points out, the spacecraft did not go to Comet 85P/Boethin, but early on in the planning stages of the mission, when the managers found out that the comet was missing, they recalibrated the probe's course to go to Comet Hartley 2, instead. I've made the necessary changes to the article, and I think it looks better now.]