On 8 June 2011, the Rosetta spacecraft will be put into hibernation after having travelled through space for more than seven years. To reduce energy consumption, the European probe will be flying in 'economy mode' as it heads towards its destination, the comet 67P/Churyumov-Gerasimenko.

But this will be no break for researchers at the German Aerospace Center; they will continue to train in preparation for Rosetta's arrival in May 2014. Six months after reaching the comet, Philae, the Rosetta lander, will become the first spacecraft to land on a comet.

Rosetta has travelled a long way since 2 March 2004, but it still has a long journey ahead. On its way to comet 67P/Churyumov-Gerasimenko, the spacecraft has captured images of the asteroids Steins and Lutetia during high-speed flybys, and has also passed close to Earth and Mars on several occasions. Rosetta has now been placed in a dormant state to enable it to reach its destination safely.

"At the moment, the probe is too far from the Sun to maintain full operations using solar power," explains Stephan Ulamec of the DLR Microgravity User Support Center (MUSC) in Cologne.

This is where the lander control centre, from where the Philae lander will be operated when it is on 67P/Churyumov-Gerasimenko, is located. "The spacecraft will remain pointed towards the Sun over the next two-and-a-half years, but only a clock, some heaters that are part of the thermal control system and a radio receiver will remain in operation."

Training for landing
Until the spacecraft and its lander reach their destination, the researchers will continue to train for what is the most difficult manoeuvre of the mission - setting the lander down on the comet's surface. With a diameter of about four kilometres, the comet is rather small; its gravity is just one ten-thousandth that of Earth's. Weighing about 100 kilograms on Earth, Philae becomes a ten-gram lightweight on 67P/Churyumov-Gerasimenko. "We still know nothing about the surface of the comet. Is it hard? Is it rough? What is the comet's axis of rotation?"

The landing manoeuvre has been tested using what is known as a ground reference model - a replica of the lander constructed from spare parts. Using this model and their software, researchers are able to simulate, not only the sequence of operations that will be carried out during the landing in November 2014, but the operations on the surface of the comet as well. "Until arrival, we will be going through various descent scenarios and the use of the instruments on board the lander," explains Ulamec, the project manager for Philae.

Mysterious comet
The planetary researchers take the long time before arrival in their stride: "comets still hold many secrets," says Ekkehard Kuhrt of the DLR Institute of Planetary Research in Berlin, project manager for the various DLR instruments on the spacecraft and lander.

"What we have learnt about comets so far has often generated more questions than answers." Among other things, the scientists want to use the Rosetta mission to gain a better understanding of the comet's chemical composition and the physical processes that take place to make a comet active and form a tail of dust and gas.

The planetary researchers are expecting the first spectacular images as soon as the spacecraft enters orbit around the comet in May 2014. "To date we have only explored comets during high-speed flybys.

With Rosetta, we have the first opportunity to study a comet over an extended period of time from a distance of just a few kilometres." In the years leading up to the arrival, the researchers will mainly be focussing on one thing - optimising the spacecraft and lander software for 2014. Philae will be the first craft to touch down on a comet. Scientists are developing software to analyse the data acquired after arrival.

Flying in hibernation
A total of eleven instruments on the Rosetta spacecraft and ten experiments on the Philae lander, several of which DLR is contributing to, will collect data on this first close encounter with the comet. The OSIRIS dual camera system on the spacecraft will use its narrow-angle channel to observe the nucleus and its wide-angle imager to examine the environment surrounding the comet.

The scientists will use these images to select the landing site on 67P/Churyumov-Gerasimenko best suited for Philae. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) has been designed to analyse the surface of the comet and its temperature from the orbiter.

As Philae touches the comet for the first time, a harpoon will be fired to penetrate the surface and prevent the lander from 'bouncing' back into space. The following experiments developed at DLR will be on the Philae lander: the ROLIS camera will image the comet's surface during and after landing; the SESAME experiment will perform seismic investigations on the comet; and the MUPUS instrument will measure a wide range of physical parameters.

Until then, Rosetta will be travelling to its destination in hibernation mode. "The clock on board the spacecraft will re-establish contact with Earth in January 2014," explains Stephan Ulamec. All the systems will be checked after their awakening from hibernation, which will be followed by a slow approach to the comet and the start of the first investigations.

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