NASA Comet Missions Pulling Double Duty; EPOXI Encounter on Thursday

NASA’s Deep Impact spacecraft will have a second chance at fame this Thursday, November 4. Rechristened EPOXI, it will give scientists a close-up view of the nucleus of comet Hartley 2. The closest approach will occur at 9:50 am EDT (13:50 UT, 6:50 am PDT). It is the first of two comet encounters by NASA spacecraft in the next three months, both by spacecraft pulling extra duty after completing their original missions.

As Deep Impact, the spacecraft now known as EPOXI successfully encountered – literally – comet Tempel 1 in 2005. One part of the spacecraft separated from the flyby spacecraft and impacted the comet – or to be precise, it was placed in front of the comet so the comet would run into it. Instruments on the flyby spacecraft studied the material ejected into space, imaged the comet’s surface and relayed images transmitted by the impactor. The flyby spacecraft remained in good condition after the encounter and was given new life as EPOXI.

EPOXI is only one of NASA’s comet explorers. On September 10, 2010, NASA celebrated 25 years of comet research with a symposium at the Newseum in Washington, D.C. Scientists talked about EPOXI and two other NASA interplanetary missions that received multiple assignments associated with comet research: ISEE-3 and Stardust.

The sheer number of comets in our solar system may come as a bit of a surprise. Dr. Anita Cochran of the McDonald Observatory revealed not only that there are 10¹⁴ (10,000,000,000,000) comets today, but that in the last several years NASA’s STEREO and Europe’s SOHO spacecraft have observed about 1,000 of them making a final death plunge into the Sun. With that many meeting their doom in a short span of time, one can imagine how many existed when the solar system formed 4.5 billion years ago. Dr. Cochran estimates that only about 10 percent of comets remain today.

Dr. James Green, Director of NASA’s Planetary Sciences Division, entertained the gathering with stories of the “bad rap” comets once had as “harbingers of gloom and doom.” Today they are the subject of intense scientific interest because they hold clues to what happened early in the formation of the solar system. “Yes, they are leftovers,” said Dr. Cochran, “but they are fundamental leftovers” that can answer the question of “where did we come from.”

Halley’s Comet holds special fascination with a cycle that brings it close to Earth every 76 years, appearing the year that legendary author Mark Twain was born and again the year that he died. In 1986, the last time it was in our neighborhood, Europe, Russia, and Japan sent probes to study it. Europe’s Giotto mission sent back fascinating images of its nucleus as it made the closest approach of all the spacecraft. (ESA’s Rosetta mission is currently on its way to a long-term rendezvous with another comet and will emplace a lander on its surface.)

NASA could not afford a Halley’s Comet mission. However, Dr. Robert Farquhar, an orbital dynamics wizard, calculated a way to redirect a NASA spacecraft already in space – the International Sun-Earth Explorer-3 (ISEE-3) – to intercept a different comet, Giacobini-Zinner. Dr. Farquhar humorously recounted for the audience the challenges he faced in convincing NASA to reposition ISEE-3, which was part of a three-spacecraft ensemble studying solar-terrestrial physics. Ultimately he succeeded and ISEE-3, renamed the International Cometary Explorer (ICE), flew past comet Giacobini-Zinner on September 11, 1985, months prior to the Halley’s Comet encounters by the other spacecraft. Thus, NASA went into the record books for sending the first spacecraft to a comet.

That was just the first NASA interplanetary spacecraft to get double duty. Deep Impact/EPOXI was the second, and it was a merger of two ideas on how to continue using the Deep Impact flyby spacecraft. As outlined by Dr. Michael A’Hearn of the University of Maryland, EPOXI is the merger of his idea to send it to a second comet – the Deep Impact Extended Investigation (DIXI) proposal, and NASA’s Drake Deming’s proposal to use it to search for extrasolar planets – the Extrasolar Planet Observations and Characterization (EPOCh) concept. The two ideas and their acronyms were combined into the cleverly designated EPOXI. The extrasolar planet detection phase of EPOXI’s mission has been completed.

A third opportunity to reuse a spacecraft already in space came with Stardust, which in January 2006 brought back to Earth a sample of material from the tail of comet Wild 2. The sample canister was recovered on Earth, while the mother spacecraft remained in space and given a new job. Now named Stardust-NExT, it will revisit Tempel 1 – the comet that collided with Deep Impact – to allow scientists to try and locate the crater caused by the collision and study other geological features. Cornell University’s Dr. Joseph Veverka explained that Tempel 1 is a planetary geologist’s dream, with “tremendous geological diversity on its surface.” Deep Impact saw only about one-third of the comet’s surface, and “we want to have a better look at the layered terrains,” he said. Stardust-NExT will reach Tempel 1 on Valentine’s Day (February 14) 2011.

Meanwhile, stay tuned for the EPOXI closest approach to Hartley 2 on Thursday morning. Information on how to view events live are available on NASA’s website.