Rosetta's CONSERT heads for a real cool venue
What is a comet really like? What is its interior like? Could it be a fluffy agglomeration of snow and dirt? Or perhaps it is solid all the way through like an iceberg encrusted with black organic material? Some have even likened it to a chocolate cake with a dark surface overlying a mixture of porous and solid material! Identifying the nature of a comet is just one of the key questions that ESA's Rosetta mission is intended to answer, and the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) instrument on the Rosetta Orbiter and Lander will play a major role in revealing the true nature of these cosmic wanderers.
CONSERT has already made its mark by becoming the first of the scientific experiments that will fly on the Rosetta Orbiter to be delivered to Alenia Spazio in Turin. The remainder of the comet chaser's state-of-the-art payload will follow in the coming weeks, paving the way for the start of the Orbiter's payload integration phase.
Sounding a comet with radar
CONSERT's mass is limited to only 3 kg, but there is nothing lightweight about its mission to gently probe the interior of a comet -- something that has never before been attempted.
The experiment, built in France and in Germany, will reveal the internal structure of Comet Wirtanen's nucleus by using an ingenious radar sounding technique. As the Rosetta Orbiter swings around the tiny ice world at a distance of less than 30 km, a transmitter on board the spacecraft sends a radio 'pulse' towards the comet nucleus at a frequency close to 90 MHz.
The Lander, which is sitting on the far side of the nucleus, behaves rather like a mirror. It receives the signal after it has travelled through the icy nucleus and transmits a new 'pulse' back towards the Orbiter. This re-transmitted signal eventually arrives back at the Orbiter, where it is compressed and stored for off-line scientific analysis.
Some 3000 of these measurements will be taken during each orbit of Comet Wirtanen. By studying the time delays as the signals pass through the nucleus from different directions, the scientists will be able to estimate the dielectric constant of the materials inside the comet (a measure of its ability to reflect the radio signals and their velocity). They will then be able to determine the internal structure (if any) of the nucleus -- the denser the material is, the slower the pulse passes through it.
So will it work?
The experiment's principal investigator, Professor Wlodek Kofman of the Laboratoire de Planetologie (CNRS-UJF-OSUG) in Grenoble and also affiliated with the Service d'Aeronomie (CNRS), France, has been studying this problem for many years.
"Based on our current understanding of the composition of comets, we believe that electromagnetic waves of the right frequency will pass right through the nucleus," said Professor Kofman.
"Obviously, we have to try out the technique on Earth in order to see if it works before we launch it towards a comet or planet," he said. "In 1993 we actually went to Antarctica to carry out a radar experiment and we found that we could successfully deduce the structure inside the ice."
"We were intending to fly a radar on the Russian Mars-98 mission in order to measure the thickness of the Martian permafrost," he continued, "Unfortunately, the mission was cancelled, so Rosetta will be our first opportunity to fly an experiment in space."
"We have recently completed a successful test of the CONSERT Electrical Qualification Models on the roofs of the University of Bochum," he said.
"The two antennae and mock-ups of the Lander and Orbiter were placed on two different roofs so that we could characterise the radiation pattern of the antennae with a minimum of interference from the ground and perform the end-to-end test of the equipment," he explained.
"The 'Lander' was placed on one roof, about 80 metres away from the 'Orbiter', and we picked up the return signal loud and clear," said Kofman.
"When we are in orbit around Wirtanen, we should be able to detect large structures or layers within the comet, and even recognise small-scale irregularities," he added.
One of the great unknowns is the lifetime of the experiment, since its success depends on the continued operation of the Rosetta Lander.
"The life of the Lander is expected to be quite short -- possibly only a few days or weeks," explained Professor Kofman. "If the Lander survives for a long time, we will carry out the experiment many times. Obviously, I would like it to operate for many orbits!"
The CONSERT instrument was developed by three European institutes: the Service d'Aéronomie in Paris; the Laboratoire de Planétologie in Grenoble; and the Max-Planck-Institut für Aeronomie in Lindau. Other contributions have been made by the European Space Technology Centre (ESTEC) in the Netherlands and the University of Bochum.
For further information contact:
Professor Wlodek Kofman
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The Rosetta Orbiter swoops over the Lander soon after touchdown on the nucleus of Comet 46P/Wirtanen. (Photo courtesy Astrium).