Hurtling toward Hartley 2

Hartley 2 as imaged by NASA’s Wide-field Infrared Survey Explorer, or WISE. The comet, known officially as 103P/Hartley was discovered fairly recently, in 1986, by Malcolm Hartley in Siding Spring, Australia. It probably originated from an icy orbit close to that of Jupiter’s, before something knocked it on a path toward the sun. Credit: NASA/JPL-Caltech/UCLA

On November 4, NASA’s Deep Impact/EPOXI spacecraft will fly within 435 miles of comet Hartley 2. Various instruments closer to Earth are helping prepare for the encounter.

Hubble Space Telescope observations taken on September 25, 2010 show that the comet’s nucleus has a diameter of approximately 0.93 mile (1.5 kilometers), which is consistent with previous estimates that indicated the nucleus was less than a mile in diameter.

Hubble’s spectrographs — the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS) — are expected to provide unique information about the comet’s chemical composition. The Hubble team is specifically searching for emissions from carbon monoxide (CO) and diatomic sulfur (S2). These molecules have been seen in other comets but have not yet been detected in Hartley 2.

The infrared vision of NASA’s Wide-field Infrared Survey Explorer, or WISE, complements what EPOXI will see with its visible-light and near-infrared instruments. A WISE image of Hartley 2, taken on May 10, 2010, shows an extensive view of Hartley 2’s trail, the dusty path left by the comet on its repeated 6.46-year journey around the Sun. WISE will be analyzing the comet’s nucleus, as well as the sizes of dust particles in the coma that surrounds the nucleus. This information will help reveal how the comet has changed over time.

The comet is in a highly active state as it approaches the Sun, warming up after its cold journey in deep space. The comet started to show signs of activity in the spring, spitting out gas and dust, and by July there were clear jets of gas. Although Hartley 2’s nucleus is small, its surface off-gasses at a higher rate than seen in other cometary nuclei.

“Comparing the dust early on to what we see later with EPOXI helps us understand how the activity started on Hartley 2,” said Michael A’Hearn, the principal investigator of EPOXI at the University of Maryland in College Park.

This image of comet Hartley 2 was captured by amateur astronomer Byron Bergert on Oct. 6 in Gainesville, Florida using a 106 mm Takahashi astrograph. Image credit: Byron Berger

The Hubble data show that the dusty coma is remarkably uniform. There is no evidence for the types of out-gassing jets seen from most “Jupiter Family” comets, of which Hartley 2 is a member. Jets can be produced when the dust emanates from a few specific icy regions, while most of the surface is covered with relatively inert, meteoritic-like material. In stark contrast, the activity from Hartley 2’s nucleus appears to be more uniformly distributed over its entire surface, perhaps indicating a relatively “young” surface that hasn’t yet been crusted over.

On November 4, when EPOXI swoops down into the comet’s coma, the spacecraft’s cameras will view the nucleus up-close and take high-resolution photos (7 meters per pixel at closest approach).

"We hope to see features of the comet’s scarred face: craters, fractures, vents," says Sebastien Besse of the science team.

The aim of the mission is to gather details about what the nucleus is made of and compare it to other comets. Because comets spend much of their time far from the Sun, the cold preserves their composition – and that composition tells a great story.

"Comets are left-overs from the ‘construction’ of our solar system," explains Besse. "When the planets formed out of the ‘stuff’ in the solar nebula spinning around the Sun, comets weren’t drawn in."

Researchers study these pristine specimens of the primal solar system to learn something about how it formed, and how it birthed a life-bearing planet like Earth.

Deep Impact finished its prime mission in 2005, when it successfully delivered an impactor to comet Tempel 1. The spacecraft, renamed EPOXI, is now heading for comet Hartley 2. Credit: NASA

"These flybys help us figure out what happened 4.5 billion years ago," says Lori Feaga of the EPOXI science team. "So far we’ve only seen four nuclei. We need to study more comets to learn how they differ and how they are the same. This visit will help, especially since Hartley 2 is in many ways unlike the others we’ve seen."

EPOXI will provide not only a birds-eye view of a new world but the best extended view of a comet in history.

"This spacecraft is built for close encounters. Its instruments and our planned observations are optimized for this kind of mission," says Feaga. "When, as Deep Impact, it flew by Tempel 1, it turned its instruments away from the nucleus to protect them from debris blasted up by the impactor. This time we won’t turn away."

The comet should be viewable in the night sky this evening (Wednesday, October 20), as it reaches its closest approach to Earth since it was discovered in 1986. The comet will be approximately 17.7 million kilometers (11 million miles) away, or about 45 times the distance to the Moon. The comet may be visible to the naked eye as a 5th magnitude “fuzzy star” in the constellation Auriga, near the constellation Perseus, if viewed in dark skies. Observers will need binoculars or telescopes from urban areas in the Northern Hemisphere. Southern Hemisphere stargazers will be able to see the comet later in the month.