Bombing the Comet

Image of Borrelly taken by Deep Space 1.
Ground based image of comet 9P/Tempel 1 set for July 4th encounter and impactor.
Credit: NASA/JPL

On July 4, 2005, the NASA Deep Impact spacecraft will visit Comet 9P/Tempel 1. It will launch a 360 kilogram (kg) impactor that should produce a crater on the surface of the comet and a plume of gas and dust.

This experiment will be the first 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.

The scientific outcome of the experiment depends crucially on pre-impact and follow-up observations. Before the impact, it is indeed necessary to accumulate a significant amount of data so as to fully characterise the comet, in terms of size, albedo (reflectivity), rotation period, etc. It is also essential to have a good baseline of observations before the impact to unambiguously discriminate 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 – which is a rather precise definition of a comet – it is indeed far from clear what the effect of the impact will be. Although the most likely model predicts the ejection of a plume and a football stadium sized crater, other model predictions vary between the comet simply swallowing the impactor (with barely any visible effect) to the eventual break-up of the nucleus.

dust_particle
Comet Halley imaged by European flyby.
Credit: ESA

As part of a very large international collaboration, two teams of astronomers have 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 NTT telescopes at La Silla.

New images shows the latest of these monitoring pictures. Obtained during the night of May 4 to 5 with the EMMI instrument on the New Technology Telescope (NTT), it shows the comet, 100 million kilometres away from Earth. The coma extends more than 30 thousand kilometres from the comet nucleus, which is a 5 km diameter snowball hidden in the central bright core of the coma.

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.

Eros_surface
The rocks inside a crater on the Asteroid Eros, as imaged before impact with the NEAR spacecraft. Numerous small impacts on the asteroid show brown boulders visible interior to the less exposed (white) lip of the crater. False-color for emphasis.
Image Credit: NASA/Eros

The other series of observations will focus on the dust and boulders that should be released during the impact, thereby characterizing 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.

 

 

 

 

 

 

 

 

 


Related Web Pages

European Southern Observatory
Deep Space 1
Dr Marc Rayman’s DS1 Mission Log
Cometary Closeup
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Winter Boon From Deep Space