Preparing for the Impact

Although dramatic images of the impact may be sent to Earth in near-real time by the Deep Impact spacecraft and its impactor, the spacecraft themselves have limited remote sensing capability.
The parent spacecraft will observe the impact from 500 kilometres distance, and then turn to look at the other side of the nucleus, but most of the observations of the event will be carried out by other spacecraft and from Earth.

For this reason, a worldwide network of observers, both professional and amateur, is part of the Deep Impact project. Within the global network of space and Earth telescopes for this unprecedented astronomical event, Europe plays a significant role.

Two ESA spacecraft, ESA's Rosetta comet-chaser and its XMM-Newton space observatory, together with the NASA/ESA Hubble Space Telescope, will monitor the comet before impact, and then watch the impact and its aftermath.

ESO's Very Large Telescope (VLT) facilities in Chile will observe the event in a big observation campaign. ESA's optical ground station at Tenerife, Spain, will also look at the impact.

Rosetta is in the most privileged position in space to watch this unique event, and will be able to monitor the comet continuously over an extended period.

Rosetta is likely to be one of the key observatories of this event because of its set of powerful remote-sensing instruments.

The Deep Impact experiment will be the first opportunity in time to study the crust and the interior of a comet. As the material inside the comet's nucleus is pristine, it will reveal new information on the early phases of the Solar System.

Image Credit: ESO

This is a false-colour composite image of Comet 9P/Tempel 1 taken with EMMI on the NTT, during the night from 2005 May 4 to 5. North is up, East is left; the field of view is 2.5 arcmin. The exposure time is 30s in the V filter (associated to the Blue channel), 45s in R (green) and 30s in I (red). The images were obtained by M. Baes and M. Castillo, on behalf on the international team led by Olivier Hainaut (ESO, Chile). As the images in the various filters were obtained one after the other, and as the comet is moving in front of the background objects, the two stars visible in this image appear as sequences of coloured dots. The comet itself appears only very softly coloured, as its dust reflects almost uniformly the light from the Sun.

It will also provide scientists with new insight on the physics of craters formation, and thereby give a better understanding on the crater record on comets and other bodies in the Solar System.

The scientific outcome of the experiment depends crucially on pre-impact and follow-up observations. Before the impact, it is necessary to find out as much about the comet as possible, such as size, albedo (reflectivity) and rotation period. It is essential to have a good set of observations before the impact to unambiguously distinguish the effects of the impact from the natural activity of the comet.

Due to the currently limited understanding of the structure of these dirty 'snowballs,' it is not known what the effect of the impact will be. Some scientists predict the ejection of a plume and the creation of a football stadium sized crater. Others think that the comet could simply swallow the impactor with hardly any visible effect, or that it may eventually break up.

To prepare for the Deep Impact event, two teams of astronomers have already used ESO's telescopes over several months to do pre-impact monitoring, taking images and spectra of the comet both in the visible and mid-infrared wavebands. These teams make observations typically once per month, using either the 3.6m or the 3.5m New Technology Telescope (NTT) telescopes at La Silla.

ESO will also actively participate in the post-impact observations. As soon as Comet 9P/Tempel 1 is visible after the impact from Chile, and for a whole week thereafter, all major ESO telescopes - i.e. the four Unit Telescopes of the Very Large Telescope Array at Paranal, as well as the 3.6m, 3.5m NTT and the 2.2m ESO/MPG telescopes at La Silla - will be observing Comet 9P/Tempel 1, in a coordinated fashion and in very close collaboration with the space mission' scientific team. Among all observatories, the ESO La Silla Paranal Observatory will thus provide the best coverage of this one of its kind event.

The series of observations will provide unique clues to several questions related to comets. One will study in detail the chemical composition of the gas in the comet's coma, looking for fresh material from the nucleus' interior ejected during the impact. The careful study of this pristine material will provide important clues to trace the origins of comets, and so, on the formation of the solar system. The other series of observations will focus on the dust and boulders that should be released during the impact, thereby characterising the structure and composition of the nucleus. Astronomers should then finally know what these "dirty snowballs" are really made of. First images by ESO telescopes will be obtained shortly after midnight - European time - on the night of July 4 to 5

Based on ESA and ESO News Releases