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Emily LakdawallaJune 29, 2005

News: All of Earth's Eyes Are on Tempel 1 as Deep Impact Zeroes In on Comet...

With four days remaining until Deep Impact crashes into comet Tempel 1, the comet is looming larger and larger in the public view. Dozens of observatories around the world, both on the ground and in space, have been participating in an unprecedented worldwide campaign to study Tempel 1 in the months and years leading up to the encounter. The campaign has already paid off for comet scientists: multiple observatories were watching on June 14 and June 22 when explosive bursts of gas and dust erupted off of the comet.

Deep Impact animation on approach to Tempel 1

NASA / JPL-Caltech / UMD

Deep Impact animation on approach to Tempel 1
This movie is made up of many frames captured by Deep Impact's Medium Resolution Imager between June 22 and June 24, 2004. A brightening by a factor of about 5 and a rapid decay to baseline brightness were observed on June 22. The same outburst was also observed by the Hubble Space Telescope and several ground observatories.

"Normally, you observe a small comet for 4 or 5 days," says Luisa Lara, the Principal Investigator for the Tempel 1 observing campaign being conducted at the Calar Alto and Sierra Nevada observatories of the Instituto de Astrofisica de Andalucia in Spain. "Then you don't see any [outbursts] like this. It was common for Hale-Bopp, which was an enormous, very active comet -- but [Tempel 1] is a small comet. It was really something that we were there at the right time, and the comet decided to explode a little bit."

Such outbursts are typical for comets as they approach the Sun, although astronomers do not understand why they occur -- whether they are jets from new fissures opening at the surface of the comet, or if they represent a chunk of material wafting free from the surface and disintegrating into dust. That is among the many mysteries that the Deep Impact mission hopes to answer by creating a new crater on the comet to see what's inside. The Earth-based observational campaign is designed to establish a baseline of observations of Tempel 1's activity, so that the effects of the Deep Impact crash can be put into proper perspective.

In fact, says Hermann Boehnhardt, Principal Investigator for one of two observational campaigns being conducted for Deep Impact at the European Southern Observatory (ESO) facilities at La Silla and Parañal, it is the Earth-based observatories that will likely provide most of the science output for the Deep Impact mission. "The DI mission is unique in the respect that it relies on Earth-based observations for the science return much more than other cometary missions. The mission focuses on producing the crater, and has only 3 scientific instruments on-board. So, Earth-based observations are meant to contribute a lot."

Boehnhardt is now in Chile, monitoring the observational campaign being conducted there. "ESO gave the two Deep Impact proposals all four 8.2-meter telescopes at the Very Large Telescope Observatory and the three 2.4-meter telescopes at La Silla, for 8 nights, plus some extra time for pre-impact observations). This is something that has never happened in ESO history before. And given the multiplexing capabilities of the telescopes and instruments and the tight lines within our observing team, we hope to contribute a good science part for this mission."

The worldwide focus on one small comet is highly unusual, Lara says. "It is a normal, typical small Jupiter family comet. I would not say it is a boring comet, but I can tell you I wouldn't have been monitoring the comet if not for Deep Impact support." Still, participating in the worldwide campaign is "very interesting and exciting," Lara said. "There is going to be coverage in every longitude where you can find a telescope."

Estimates vary as to the number of observatories that are already focusing on Tempel 1, in part because one observatory may consist of many telescopes, each of which may have many instruments. Following is a partial list of the observatories that are participating in the campaign, according to University of Hawaii astronomer Karen Meech, who is coordinating the international effort:

Spaceborne observatories:

Ground-based optical observatories:

In addition, over 50 radio telescopes are watching from a dozen different sites, and hundreds of amateurs around the world are watching Tempel 1 in participation with the Deep Impact mission's Small Telescope Science Program.

Why so many telescopes? "The post-impact phenomena have different timescales for their evolution, from seconds to days," explains Boehnhardt. "So, one needs to follow them over a longer time interval and all around the clock. This alone requires good coverage by telescopes around the globe. The comet cannot be observed all night long from the ground. The other reason is that we expect to see phenomena in different wavelength regions, different spatial and spectral resolution, and no single telescope has the proper instrumentation to do all of this. So, the observational tasks need to be distributed among the various facilities to make best use of the existing instrumentation for the best science return."

Spaceborne observatories have the additional advantage that they can look at Tempel 1 without interference from the Earth's atmosphere. Among these, ESA's Rosetta spacecraft is in a unique position, because it is not in orbit around the Earth and can consequently watch Tempel 1 for 24 hours a day. In addition, Rosetta's instruments are designed specifically for comet observation, since its mission is to rendezvous with a comet. Rosetta's observations begin today and will continue around the clock until July 14.

The unprecedented worldwide campaign will generate vast quantities of data, Lara says. "Since January, I haven’t done anything but [observe Tempel 1]. At La Silla, they are going to observe the event with every instrument in every telescope. So they are going to gather I don’t know how much data. And the data from the spacecraft adds even more. At the time of the impact we will do a very quick analysis and do our press release. But if you really want to make the most of this information, you will need a couple of years to get a full picture, before you really understand what happened at the time of the impact."

Read more: pretty pictures, comets, comet Tempel 1, Deep Impact

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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