Sniffing a Comet

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Illustration of Rosetta sitting on comet surface
Credit: ESA

ESA’s Rosetta mission launched in March of this year and will reach the comet Churyumov-Gerasimenko in November 2014. In the meantime though, Rosetta’s cameras can also be used to track automatically the two asteroids that Rosetta will be visiting during its long cruise, Steins and Lutetia.

One of the ingenious instruments on board Rosetta is designed to ‘smell’ the comet for different substances, analysing samples that have been ‘cooked’ in a set of miniature ovens.

ESA’s Rosetta will be the first space mission ever to land on a comet. After its lander reaches Comet 67P/Churyumov-Gerasimenko, the main spacecraft will follow the comet for many months as it heads towards the Sun. In 10 years time, it will have travelled distances of over one thousand million kilometers (600 million miles) from Earth, and about 800 million kilometers (480 million miles) from the Sun, to meet it cometary destination.

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Comet Wild 2 imaged by Stardust just after flyby. The image highlights the remarkably rugged surface of the comet, which in close-up stereo views shows hardened impact craters, cliffs, and mesas in the landscape.
Credit: NASA/JPL

Rosetta’s task is to study comets, which are considered the primitive building blocks of the Solar System. This will help us to understand if life on Earth began with the help of ‘comet seeding’.

The Ptolemy instrument is an ‘Evolved Gas Analyser’, the first example of a new concept in space instruments, devised to tackle the challenge of analysing substances ‘on location’ on bodies in our Solar System.

Weighing just 4.5 kilograms and about the size of a shoe box, it was produced by a collaboration of the UK’s Rutherford Appleton Laboratory and Open University.

The analysis of these samples from the surface of the comet will establish what the cometary nucleus is made from, providing valuable information about these most primitive objects.

After the lander touches down on the comet, the Ptolemy instrument will collect comet nucleus material, believed to be a frozen mixture of ices, dust and tar, using the Sampling, Drilling and Distribution system (SD2) supplied by Tecnospazio Milano of Italy. SD2 will drill for small cores of ice and dust from depths of down to 250 millimeters.

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Rosetta’s comet overn.
Credit:ESA

Samples collected in this way will be delivered to one of four tiny ‘ovens’ dedicated to Ptolemy, which are mounted on a circular, rotatable carousel. The German-supplied carousel has 32 of these ovens, with the remainder being used by other Rosetta instruments.

Of the four Ptolemy ovens, three are for solid samples collected and delivered by SD2 while the fourth will be used to collect volatile materials from the near-surface cometary atmosphere.

By heating the solid samples to 800 degrees C, the oven converts them into gases which then pass along a pipe into Ptolemy. The gas will then be separated into its constituent chemical species using a gas chromatograph.

Ptolemy can then determine which chemicals are present in the comet sample, and hence help to build up a detailed picture of what the comet is made from.

It does this using the world’s smallest ‘ion-trap mass spectrometer’, a small, low-power device built with the latest miniature technology. This device will find out what gases are present in any particular sample and measure stable isotope ratios.

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The approximately 4 kilometer cometary nucleus to be landed upon by the Rosetta and its harpooning landing hooks
Credit: Hubble

"Comets and asteroids are the building blocks of our Earth and the other planets in the Solar System. Rosetta will conduct the most thorough analysis so far of three of these objects," said Prof. David Southwood, Director of ESA’s Science Programme. "Rosetta will face lots of challenges during its 12-year journey, but the scientific insights that we will gain into the origin of the Solar System and, possibly, of life are more than rewarding."

In addition to its cometary destination, the two intermediate targets selected for Rosetta, Steins and Lutetia, have rather different properties. Steins is relatively small, with a diameter of a few kilometers, and will be visited by Rosetta on 5 September 2008 at a distance of just over 1700 kilometers. This encounter will take place at a relatively low speed of about 9 kilometers per second during Rosetta’s first excursion into the asteroid belt.

Lutetia is a much bigger object, about 100 kilometers in diameter. Rosetta will pass within about 3000 kilometers on 10 July 2010 at a speed of 15 kilometers per second. This will be during Rosetta’s second passage through the asteroid belt. Rosetta will obtain spectacular images as it flies by these primordial rocks. Its onboard instruments will provide information on the mass and density of the asteroids, thus telling us more about their composition, and will also measure their subsurface temperature and look for gas and dust around them.

Past missions that have flown by a comet have been: NASA’s ICE mission in 1985, the two Russian Vega spacecraft and the two Japanese spacecraft Suisei and Sakigake that were part of the armada that visited Comet Halley in 1986; NASA’s Deep Space 1 flew by Comet Borelly in 2001 and NASA’s Stardust, which flew by Comet Wild 2 earlier in January and has captured samples of the comet’s coma to be returned in 2006. On Valentine’s Day, 2001, the Near-Shoemaker spacecraft successfully landed on the asteroid, Eros. Its remarkable journey–to soft-land on a peanut shaped asteroid - about 176 million kilometers (109 million miles) from Earth, prompted Andrew Cheng, NEAR Project Scientist, to note: "On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life."


Related Web Pages

Harpooning a Comet
Comet Cruise Glimpses Earth
Extraterrestrial Capture
Reading Rosetta
Coma for Halley’s Comet