Deep Impact Spacecraft Closing in on Comet Hartley 2

After five years as a lonely sentinel in space, an American spacecraft that has already crossed paths with one small comet is about to visit another icy wanderer: Comet Hartley 2.

The spacecraft in question is Deep Impact, a NASA probe launched in 2005 that visited the Comet Tempel 1 on July 4 of that year and smacked the comet with an impactor vehicle to kick up material and see what it was made of.

Astronomers used telescopes in space and on Earth to watch the Deep Impact's comet collision, and now they're ramping up again for the spacecraft's flyby of Comet Hartley 2. The comet flyby occurs Thursday (Nov. 4) at about 10:01 a.m. EDT (1401 GMT).

But Deep Impact's new comet rendezvous ? which NASA calls EPOXI ? won't be noticeable to Earth-based observers when the spacecraft makes its closest approach to Hartley 2.

Unlike in 2005, when Deep Impact's impactor raised a cloud of dust from the nucleus of Tempel 1, no such audacious maneuver is planned during its flyby of Comet Hartley 2. As a result, ground based observers will see no changes in the appearance of the comet and besides, the spacecraft itself is far too small to be seen using ground based telescopes.

But the images that will be transmitted back to Earth are expected to give astronomers new clues about the nature of comets. [Photos of Comet Hartley 2]

Finding another target

Comet Hartley 2 was not NASA's first choice for Deep Impact's second comet rendezvous.

Mission scientists originally planned for a December 2008 flyby of Comet Boethin and on July 21, 2005 the Deep Impact probe fired its thrusters in a maneuver that allowed it to use Earth's gravity to begin a new mission in a path toward Comet Boethin.

But soon scientists was realized that there was a problem.

Comet Boethin was missing.

Astronomers could not find Comet Boethin in surveys and suspect that it apparently fragmented into myriad tiny particles. This meant rewriting a new game plan for Deep Impact.

A new target was selected: Comet 103P/Hartley 2.

To reach this new comet, however, would require an extra two years of travel for the spacecraft. On May 28 of this year, the spacecraft fired its thrusters in a correction burn sent it whipping past the Earth on June 27 to use our planet?s gravity to aim itself on a course for a flyby of Comet Hartley 2. ??

Comet Hartley 2 up close

Comet Hartley 2 is a relatively small comet with orbital period of about 6.46 years. It was discovered by Malcolm Hartley in 1986 at the Schmidt Telescope Unit in Siding Spring, Australia.

Unlike Comet Boethin, Hartley 2 has been well observed by both amateur and professional astronomers in the weeks and months leading up to its encounter by the Deep Impact spacecraft.

The comet passed within 11.2 million miles of Earth (about 45 times the distance to the moon) on Oct. 20.? Last week, the comet arrived at perihelion ? its closest approach to the sun ? at a distance of 97.6 million miles (157 million km) from the star.? It is now moving away from both the sun and Earth.

?This spacecraft, and mission team, have logged 3.2 billion miles over the past five years, and we are confident that we have a successful plan in place to give Hartley 2 a thorough look-see," said Tim Larson, project manager for NASA's EPOXI mission at Jet Propulsion Laboratory in Pasadena, Calif.

Tracking the comet

Amateur astronomers have been tracking Comet Hartley 2 in recent weeks with binoculars and small telescopes. It has appeared mainly as a dim, circular patch of light.

Observations by the Hubble Space Telescope have also helped plan Thursday?s flyby of the comet.

The observation campaign has been a tantalizing tease of things to come and provided EPOXI?s science team the best extended view of a comet in history during its pass through the inner solar system. The Hubble observations will continue through the encounter phase of the mission.

The latest observations of Hartley 2 shows that the comet's nucleus, its core of ice and rock, has a diameter of approximately just under a mile (1.5 km), which is consistent with previous estimates.

Barring an unforeseen breakup (like what happened to Comet Boethin), Hartley 2 should be able to survive up to another 100 circuits through the solar system (about 700 years) at its current rate of mass loss.

Having passed closest to the sun on Oct. 28, Comet Hartley 2 is now is in its most active state.

Deep Impact is closing in on Comet Hartley 2 at a rate of 7.8 miles ?per second (12.5 kps).

On Nov. 4 at about 10:01 a.m. EDT (1401 GMT), the spacecraft will make its closest approach to the comet at a distance of about 434 miles (700 km).

?It will be only the fifth time that a comet has been photographed up close and the first time in history that one spacecraft has observed two different comets with the same set of instruments.

The mission?s encounter phase begins late Wednesday (Nov. 3), when the spacecraft is about 18 hours from the time of closest approach to the comet?s nucleus.

At that time the spacecraft will stop transmitting through its large high-gain antenna and reorient itself so its two visible-light and one infrared imager maintain lock on the comet for the next 24 hours or so.?

At that point, the spacecraft will begin beaming down its cache of comet close-ups while continuing to take new images. It is expected to take several hours for all the images aboard Deep Impact to be beamed back to Earth.

Why comets?

Comets seem to resemble frozen froth left over from the tempestuous birth of the solar system some 4.5 billion years ago.

Astronomers think the interiors of comets have changed little since then and contain pristine ice, gases, dust and other materials from the solar system's formation. But their true nature is probably more perplexing than that of any other object within the gravitational grip of the sun.?

So scientists believe that understanding comets is a way of understanding how the solar system was born.?

Another reason to study comets is that they can pose a threat of colliding with Earth, just like asteroids, and have the potential to cause cataclysmic damage.?

Defending against such possibilities requires knowing more about these objects in hopes of possibly deflecting or even destroying objects those that might potentially cross paths with our planet.

• The Best Comet Photos of All Time
• Deep Impact's Top 10 Comet Crash Images
• Comet Hartley 2 Could Deliver 'Bonus' Meteor Shower

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.