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EPOXI: Deep Impact, Part 2

by Louise Good

Comet Hartey 2

The EPOXI spacecraft took this image of Comet Hartley 2 as it flew by the comet around 3:59 a.m. HST on November 4 at a distance of about 435 miles (700 km). Jets can be seen streaming out of the nucleus. Photo courtesy NASA.

Remember the Deep Impact mission? In July 2005, the Deep Impact spacecraft left an impactor in the path of Comet Tempel 1 so that the two would collide, enabling scientists to see the inside of a comet. The impactor was, of course, destroyed, but the main part of the spacecraft continued to function well, and NASA, not wishing to waste a perfectly good spacecraft, gave it a new mission.

They named this mission EPOXI because it actually was two missions stuck together: EPOCh, the Extrasolar Planet Observation and Characterization investigation, which observed stars with known transiting giant planets from January to August 2008, and DIXI, the Deep Impact eXtended Investigation of comets. The goals of DIXI are to understand how features of a cometary nucleus (the solid part of the comet) relate to those of the coma (the envelope of gas and dust around the nucleus), to study the diversity of cometary nuclei and the heterogeneity within a cometary nucleus, and to learn more about the origin of the solar system by studying the structure, composition, and formation of cometary nuclei.

In the early morning hours of November 4 HST, the spacecraft made its closest approach (435 miles/700 km) to Comet Hartley 2. It is the smallest comet ever seen close-up, a mere 0.37 miles (0.6 km) in average radius, but the nucleus itself is highly elongated, that is, it is long and narrow.

University of Maryland astronomer Michael A'Hearn, one of the originators of and principal investigator for both Deep Impact and EPOXI, said, "The images are full of great cometary data, and that's what we hoped for."

IfA astronomer Karen Meech is a member of the DIXI science team. She has been in charge of the ground-based observations for the mission. She said, "We have now seen five comets up close. Since comets tell us about the chemistry and physical conditions of the early solar system, the more we learn about comets, which delivered some of Earth's early water and organic compounds, the more we may learn about how life began on Earth. The images of this nucleus were absolutely amazing!"

All together, an international consortium of more than one hundred observers from 10 countries have been allocated over 500 whole or partial nights with 58 telescopes to observe the comet from Earth from summer 2008 through winter 2011. This includes using seven space and airborne observing platforms. IfA researchers using the Hubble Space Telescope and the Gemini North (on Mauna Kea) and South (in Chile) telescopes played a key role in understanding how fast the nucleus was spinning, and gave us our first early hint that the nucleus would be elongated.

Comparisons with data from Tempel 1, taken with the exact same instruments, will be particularly useful for determining which cometary features represent primordial differences and which result from subsequent evolutionary processes. Hartley 2 is smaller and more active than Tempel 1, and there was more sunlight during the encounter with the former because the encounter occurred slightly closer to the Sun, so all measurements should be significantly improved.

Comet 103P/Hartley 2, discovered in 1986 by Malcolm Hartley, makes one orbit of the Sun every 6.4 years. The comet made a close approach to Earth on October 15, 2010, at a distance of 11 million miles (18 million km), which is only 47 times the average distance to the Moon. This gave astronomers on Mauna Kea and at other observatories to get an excellent opportunity to take high-resolution images of the comet, including radar images from the nucleus.

Comet Hartley 2 became the target of the DIXI mission when Comet Boethin could not be located before the December 2007 Earth flyby needed to redirect the spacecraft to the comet (see Deep Impact and the Case of the Missing Comet).