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article imageESA's Rosetta probe enters orbit at Churyumov-Gerasimenko comet

By Robert Myles     Aug 6, 2014 in Science
Paris - The European Space Agency’s (ESA) comet-chaser mission, already 10 years in transit, has rendezvoused with its target the 67P/Churyumov–Gerasimenko comet. The ESA Rosetta probe enters orbit around the comet today, Wednesday.
Starting at 09:00 GMT (11:00 CEST), orbit entry will be triggered by a thruster firing lasting just under six and a half minutes. The thruster burn will tip Rosetta into the first leg of a series of three-legged triangular paths about the comet. Each leg is about 100 kilometers long. Rosetta will take between three and four days to complete each one.
The rendezvous with comet Churyumov–Gerasimenko, named after two Ukrainian astronomers who discovered it in 1969, is just the curtain-raiser for an ambitious mission that aims to put a robot lander, named Philae, on the comet’s surface. Philae will piggy-back aboard the comet as it approaches the Sun.
Rosetta was launched in March 2004 atop an Ariane 5 rocket. The spacecraft comprises two main components: the Rosetta orbiter itself carrying 12 scientific instruments and the Philae robotic lander with its separate payload of nine instruments.
Rosetta is about to embark on a 17 month long tryst with comet 67P during which time it will carry out the most in-depth study of a comet yet attempted. Rosetta will carry out a comprehensive survey of the, presently unknown, comet’s surface so that a suitable landing site for the Philae lander can be selected.
When the decision is made to deploy Philae, provisionally fixed for sometime in November 2014, the lander will approach Churyumov–Gerasimenko at relative speed around 1 metre per second (about 2.2 mph or 3.6kph). When Philae makes contact with the comet’s surface, two harpoons will be fired into the comet to secure the lander, limpet-like, to the comet’s surface and stop Philae from bouncing back off into space. Additional drills will be used to further bind Philae to its target.
Once on the comet’s surface, Philae’s scientific research will start. The principal aims of Philae’s mission are threefold:
• Characterisation of the nucleus of the comet
• Determination of the chemical compounds present
• Study of comet activities and developments over time
This image of comet 67P/Churyumov-Gerasimenko that was obtained on August 3rd  2014 shows very diffe...
This image of comet 67P/Churyumov-Gerasimenko that was obtained on August 3rd, 2014 shows very different surface structures such as steep slopes and wide planes.
ESA/Rosetta/MPS for OSIRIS Team
The other side of the comet imaged on August 3rd  2014 from a distance of 285 kilometers.
The other side of the comet imaged on August 3rd, 2014 from a distance of 285 kilometers.
ESA/Rosetta/MPS for OSIRIS Team
The Rosetta probe is named after the Rosetta Stone, an ancient basalt slab of Egyptian origin dating from the reign of Ptolemy V (around 196 BC). The Rosetta Stone featured a decree in three scripts, one in ancient Egyptian hieroglyphs, the middle portion Demotic script, and the lower script composed in ancient Greek. Due to the Rosetta Stone presenting essentially the same text in all three scripts, (apart from minor differences), it unlocked modern understanding of Egyptian hieroglyphs.
Just as the Rosetta Stone led to a greater understanding of the Egyptian system of recording information, so the aim of the ESA’s Rosetta probe is to glean more information about comets and the early stages of formation of our solar system.
Maintaining the Egyptian connection, the Philae lander is named after Philae Island in the river Nile. It was on Philae Island that an obelisk was found which, along with the Rosetta Stone, helped in deciphering Egyptian hieroglyphics.
The Rosetta probe has a 32 meter (105 feet) wingspan, measured from one end to the other of its two huge solar panels each 14 meters (46 feet) in length. Even so, Rosetta is still dwarfed by comet 67P, which is estimated to be 4 kilometers (2.5 miles) wide. The Philae lander is about the size of a refrigerator and weighs just over 45 kilograms (about 100 pounds).
The Rosetta mission has been a long time reaching fruition. First approved by the ESA in 1993, Rosetta was initially projected to rendezvous with another comet, 46P/Wirtanen. The original launch date was postponed, however, with the result that comet 67P/Churyumov–Gerasimenko was chosen as the alternative destination.
After launch, Rosetta flew three times around the Earth to achieve a “gravity assist” boost aimed at saving fuel for its 10 year journey to comet 67P. Clocking up 6 billion miles (9.6 billion kilometers) en route to 67P, Rosetta also sent back some amazing images of two asteroids, Lutetia and Steins, as it passed close to each.
The trickiest part of the mission will be landing Philae on the comet. Once on the comet’s surface, Philae’s analytical instrument, appropriately named “Ptolemy” will be provided with just a few “grains” of solid sample for analysis. The comet samples will be heated in miniature ovens.
Once gaseous, the samples will be fed into a sophisticated chemical analysis system and from there into an ion trap mass spectrometer. The gas is ionized by an electron-source and then a controlling high-voltage field is used to selectively eject ions of differing mass into a counter, enabling isotope ratios to be measured to very high precision.
From that process scientists should be able to gain a better understanding of the comet’s chemical makeup.
Putting the technology used in perspective, Professor Richard Holdaway, Director of STFC RAL Space, part of the design team for “Ptolemy,” said, “We took a chemistry set the size of an average kitchen and shrank it down to fit into a shoebox. It had to be robust enough to survive the rigors of the launch and 10 years in space -- and now, when it begins to take the measurements of the comet material, it’s going to do that in incredible detail.”
Remarkably, all this will be accomplished by mission controllers and scientists presently 405 million kilometers away with commands to Rosetta, even travelling at the speed of light, taking about 23 minutes to reach it.
The ESA is providing updates as today’s rendezvous with the comet unfolds on a dedicated webpage and via @ESA_Rosetta on Twitter.
Update: In the few hours since the Rosetta spacecraft arrived at comet 67P, it's beamed back close-up images of the comet (see below). Never before has a comet been seen in such minute detail.
Rosetta spacecraft s OSIRIS narrow-angle camera obtained this close-up detail of a smooth region on ...
Rosetta spacecraft's OSIRIS narrow-angle camera obtained this close-up detail of a smooth region on the "base" of the "body" section of comet 67P/Churyumov-Gerasimenko on August 6, 2014.
Credit: ESA/Rosetta/MPS for OSIRIS Team
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