Rendezvous with a chunk of primeval rock

Rendezvous with a chunk of primeval rock

Although the comparison with the manned Moon landing may appear somewhat exaggerated, Rosetta is undoubtedly one of space travel’s most daring enterprises: For the first time in history, a probe is accompanying a comet on its orbit around the sun, and in the middle of November it will set down the Philae lander on its surface. Scientists from the Max Planck Institute for Solar System Research in Göttingen have front row seats for the evaluation of the images and data from the comet named 67P/Churyumov-Gerasimenko.

Text: Helmut Hornung

MARCH 2, 2004; 8:17 a.m. CET

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Comet experts discuss the latest images from Rosetta: Martin Hilchenbach, Holger Sierks, Paul Hartogh and Hermann Böhnhardt (from left) at the Max Planck Institute for Solar System Research.

Darkness still shrouds the Kourou spaceport in French Guiana as a type Ariane 5G+ rocket shoots up into the cloudy sky leaving a fiery trail in its wake. On board a freight that is intended to shed more light upon the origins of the solar system: Rosetta. The probe itself is slightly larger than a Smart car. Its two long wings with solar panels for the energy supply give it the appearance of a strange insect. Aboard Rosetta, in addition to the SREM detector for detecting high-energy particle radiation, are eleven instruments and a box as large as a refrigerator: Philae. It is to perform the trick of landing on the nucleus of a comet and conduct measurements there over several months with the aid of ten scientific devices.

Rosetta’s history goes back to 1984. At that time, the European Space Agency ESA decided to undertake a mission to the nucleus of a comet, initially in partnership with NASA. After budget cuts forced the Americans to withdraw, the Europeans continued the plan alone. In Germany, the German Aerospace Center and the Max Planck Institute for Aeronomics (since 2004: Solar System Research) were initially the major players in the project; the latter made a significant contribution to the development and construction of the lander, among other things.

On January 13, 2003 Rosetta and Philae were due to lift off to the Wirtanen comet. But a rocket from the Ariane 5 series, like the one which was to transport the ambitious mission, had managed to perform a spectacularly unsuccessful lift off. The European space program was halted temporarily, the start of the comet scout postponed by more than a year. Moreover, a new destination had to be found. The choice was ultimately 67P/Churyumov-Gerasimenko. Two scientists, Churyumov and Gerasimenko, at the Institute for Astrophysics in Alma-Ata/Kazakhstan, had discovered the celestial body as a tiny star on a photographic plate in fall 1969.

In the past, the gravitational field of the planet Jupiter had influenced the trajectory of “Chury.” Before 1840 it orbited the sun at a considerable distance, and the lack of heat meant it had not been able to develop any cometary activities up to this point. Meaning: The nucleus should still be relatively fresh and pristine – which researchers see as a benefit. In its present orbit the comet approaches the sun to within a distance of around 193 million kilometres (Earth’s distance to the sun: 150 million kilometres) every six years and 203 days.

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Zoom on "Chury": Holger Sierks, who manages the  camera systems OSIRIS, shows an excerpt of the nucleus; the boulder next to Sierks's index finger was christened 'Cheops'.

SEPTEMBER 7, 2008; 10:14 p.m. CEST

Signals from the depth of space appear on the screens in the ESA Control Centre in Darmstadt. The flight engineers immediately forward them on to the Max Planck Institute for Solar System Research. Although it is the middle of the night, 14 scientists here filter the first images from the raw data. These show a rock with an elongated shape which tapers to a point at one end; countless craters cover its surface, a particularly large crater two kilometres in diameter is located at the North Pole. The subject is around 360 million kilometres from Earth – and shows the Šteins asteroid. Rosetta passed by it on September 5, 2008 at a distance of 800 kilometres.

Although the OSIRIS telecamera switches into safety mode nine minutes before the rendezvous and only the wide-angle camera is working, the researchers are satisfied with their instruments. OSIRIS is the abbreviation for Optical, Spectroscopic and Infrared Remote Imaging System. Two cameras constructed as mirror systems take images in the ultraviolet, visible and infrared spectral ranges. The light-sensitive CCD detectors are made up of 2048 by 2048 pixels; each of these pixels measures 13.5 micrometres (one-thousandth of a millimetre).

Before its rendezvous with Šteins, Rosetta had flown past Earth on March 4, 2005 at a distance of only 1955 kilometres. It was even possible back then to see the probe with binoculars. On February 25, 2007 Rosetta passed Mars; on November 13 of the same year it again gained momentum with the aid of our planet. After the rendezvous with Šteins, it executed another Earth fly-by on November 13, 2009.

Why this extremely complicated trajectory? A direct flight from Earth to Churyumov-Gerasimenko would have required enormous quantities of fuel. During the so-called swing-by manoeuvres on tortuous paths, Rosetta gained the energy it needed from the gravitational fields of the planets free of charge, as it were. During each of the three Earth swing-bys, the speed of the vehicle increased by around 20 000 kilometres per hour.

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