FLUG REVUE April 2003: Rosetta seeks new comet

QUO VADIS, ROSETTA?

By Matthias Gründer

When almost 20 years ago the idea of the Rosetta mission was first formulated, astronomers and planetologists were fired with enthusiasm by the brave vision underlying the project. Implementation of this project would catapult European engineers and researchers to the foremost ranks of world space scientists, even if pessimists believed that such an ambitious goal could not be achieved: flight to a comet some distance from the sun where it has not yet developed a fully fledged dust tail, culminating in setting down a lander on the surface of the "dirty snowball” composed of ice and the primeval dust of the universe.

Rosetta

The goal which scientists had set themselves was the comet Wirtanen, whose trajectory would allow it to be "intercepted” from November 2011 at the peak of its orbit around Jupiter and then accompanied for closer study. For planetologists, such a mission constitutes an exciting journey back to the beginning of time as far as our solar system is concerned, as they are working on the assumption that the material in the nucleus of comets has not changed since its origin some 4.6 billion years ago. Such a deep-frozen archive of the universe would now be decoded, with enormous gains in knowledge for mankind.

The space probe itself consists of two parts, the main space probe which will circle around the comet and the lander, which is to set down on the celestial body. Both of these are to accompany the comet to its end, when it either falls apart in the distant future or collides with another planet.

Rosetta is to carry with it a message for all time: attached to the exterior of the space probe is a small nickel disk on which the first three chapters of Genesis are engraved in 1,000 languages. The San Francisco based Rosetta project created this to preserve the diversity of language. Linguists there fear that before the present century is over ninety-five percent of all human languages will have died out and many of them will no longer be readable. They therefore set themselves the goal of creating a "global Rosetta stone”, containing examples of text in every known language, which it would then be possible to decode at any time in the future.

The original Rosetta Stone was the model for both the language project and the space probe. On the 760kg basalt block which a soldier of the Napoleonic army discovered in Rosetta in 1799 is a text engraved in three different languages: Greek, Sumerian cuneiform script and Egyptian hieroglyphics. The first two were already known to academics, but up to that time hieroglyphics had been an unfathomable puzzle. With the stone it was suddenly possible to read this old pictographic writing system, opening the way to new discoveries about one of the advanced civilisations of mankind.

So much for the background, now for the 21st century mission. The thrust provided by the space launch vehicle would not be sufficient to send the space probe to the comet by a direct route, so to gain additional momentum, one Mars and two Earth gravity assists would be required. However, the planet configuration required for this would exist for only a brief time, so that the launch window was confined to the period between 13 and 31 January 2003. Moreover, within this period there were only six days on which the rocket could be launched, and on each of these days there were only two times, two minutes apart, in which the launch had to take place, accurate to the second. The laws of celestial mechanics are not easily outwitted.

The space probe was ready and so was its launcher rocket. But then came that fateful day of 11 December 2002, on which an Ariane 5 rocket exploded after launch. The cause of the catastrophe was narrowed down to a malfunction in the main engine of the first stage, and nobody could or was willing to make any guarantees for Rosetta. 700 million Euros of project costs and priceless scientific gains were at stake, so that the launch had to be called off. The rocket is still reserved, but it will probably have to be modified. Meanwhile Rosetta has been moved to a hangar where, surrounded by a protective nitrogen atmosphere, it awaits a new opportunity.

Was this the end of a research dream? "Not at all”, said Dr. Gabriele Arnold, head of the department of Optical Information Systems at the DLR Institute of Planetary Exploration in Berlin-Adlershof, in an interview with FLUG REVUE. Already during the development and construction phases of the space probe an ESA working party was in place whose job was to come up with alternative solutions in the event of disaster, and this party convened on 13 February 2003 to deliberate the matter. By May they are to present proposals for a new mission profile to ESA's Scientific Programme Committee.

The researchers' wish list contains several worthwhile target objects, including comets 88P Hover, 10P Temple 2 and 73P Strassmann/Wachmann. These and several others would be within reach of the space probe from the spring of 2004, but calculation of the appropriate trajectories is a relatively trivial problem compared with the fact that a "normal” Ariane 5 is not up to the job. A more powerful launch vehicle is needed. In the meantime, transferring the mission to the Russian Proton is not an option, as the high-gain antenna on the probe is too big for that vehicle. There is still a need for clarification here.

Far more difficult will be the task of modifying the lander to the new flight profile, for the landing weight and harpoons of the approx. 100kg high-tech box which was to be attached to the comet surface were designed precisely with the mass and hence the gravitation of Wirtanen in mind. A larger comet will require more stable lander legs and hence a certain amount of modification. Nevertheless, Dr Arnold is optimistic that all these issues can be resolved in time for the next comet.

From page 38 of FLUG REVUE 4/2003

Last updated 4 March 2003

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