Observations

The Deep Impact experiment is a unique opportunity 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. It will also provide scientists with new insight on crater physics, and thereby give a better understanding on the crater record on comets and other bodies in the Solar System.

By forming a deep crater in Comet Tempel 1, scientists want to find clues to the formation of the solar system. The Deep Impact objectives are:

• Observe how the crater forms
• Measure the crater's depth and diameter
• Measure the composition of the interior of the crater and its ejecta
• Determine the changes in natural outgassing produced by the impact

 The scientific outcome of the experiment depends crucially on pre-impact and follow-up observations. Before the impact, it is necessary to accumulate a significant amount of data to fully characterise the comet, in terms of size, albedo (reflectivity), rotation period, etc. It is also 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 a football stadium sized crater, others think that the comet will simply swallow the impactor with no visible effect, or 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. An example of the images obtained can be seen as eso0518 or on the Photos page.

ESO's telescopes will also be used in the post-impact observations. As soon as the comet is visible after the impact from Chile, all major ESO telescopes - 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

Two observing programes will perform imaging and sepctroscopic studies for one week follolwing the impact: one is dedicated to the study of the dusty component while the other will look into the gas. The "Exploring the Dust Component" programme will use the Wide-Field Imager on the 2.2-m MPEG/ESO telescope, the EMMI and SOFI instruments on the 3.5-m NTT, the infrared imager TIMMI2 on the 3.6-m, as well as NACO, FORS 1 and VISIR on the Very Large Telescope. NACO is an adpative optics instruments capable of obtaining images not blurred by the atmosphere's turbulence, while FORS 1 is a multi-mode instrument able to take images and spectra. VISIR provides diffraction-limited imaging at high sensitivity in the two mid infrared (MIR) atmospheric windows: the N band between 8 to 13μm and the Q band between 16.5 and 24.5μm, respectively. In addition, it features a long-slit spectrometer with a range of spectral resolutions between 150 and 30000. This observing programme aims at observing the non-volatile components - dust and boulders - released immediately around impact time in order to characterise the structure and composition of the nucleus. The leader of the team is Hermann Boehnhard from the Max-Planck Institute in Lindau (Germany).

The second observing programme, "Exploring the Gas Component", makes uses of the UVES, FORS 2 and ISAAC instruments on the VLT to analyse the gas of the coma of the comet. UVES is a ultraviolet and visible spectrograph, while ISAAC performs observations in the near-infrared. The aim is to study the chemical composition and isotopic ratios of the gas around the nucleus. By searching for differences between the normal cometary coma before the impact and after the impact, unvaluable information can be obtained on the pristine material ejected by the impact. Knowing the isotopic ratio of this more pristine material will provide important clues to trace the origins of comets. The Principal Investigator is Heike Rauer from the German Space Agency (DLR).

All seven major telescopes of ESO will thus be used in what is most likely the most ambitious ground-based study of the Comet 9P/Tempel 1. For each telescopes, two astronomers - in addition to the ESO support astronomers - will carry the work. A few days before the event, all the astronomers who collaborate on this will meet in ESO's Vitacura Offices in Santiago (Chile) to establish the final most optimum observing strategy, before heading towards the two observing site, the 2600m high Cerro Paranal and the 2400m La Silla. Once the observations have been carried out, eight days following impact, the astronomers will meet again in Vitacura to perform the final data reduction and discuss the scientific results. These will then be presented at an international scientific conference.

Just after the impact on July 4, the telescopes will start observing as soon as possible. With TIMMI2 on the 3.6-m ESO telescope at La Silla, it will possible to observe in daylight, as TIMMI2 performs infrared observations. Images will thus be available from around 16:00 Chilean Time, or 22:00 in Garching. The images taken with the other telescopes - at La Silla and Paranal - will start as soon as it is dark enough, i.e. when twilight ends in Chile, i.e. around 19h30 Chilean Time, i.e. 01:30 am in Garching on July 5. s on. The images will be processed in almost real time in Chile by the astronomers and then made available through the Web site.