After you become the first spacecraft to orbit an asteroid, what do you do for an encore? For ESA’s Rosetta comet probe, the answer is to get ready for the first soft landing on a comet nucleus. Only weeks after going into orbit around comet 67P/Churyumov-Gerasimenko, the unmanned Rosetta explorer is engaged in a fast-paced reconnaissance of the comet and has identified five candidate sites for putting down the Philae lander in November.

On August 6, Rosetta made history as it went into a triangular orbit around 67P/Churyumov-Gerasimenko, which circles the Sun once every 6.5 years and is about 522 million km (324 million mi) away from it at present. Simply achieving the remarkable feat of chasing a comet over many years in a trajectory that involved several flybys of Earth and a detour out beyond the orbit of Jupiter is cause enough for celebration, and going into orbit around 67P/Churyumov-Gerasimenko is one for the books, but the ESA is planning to cap that all off by making a soft touchdown on the comet’s nucleus with the 100 kg (220 lb) Philae lander in November.


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It wasn’t possible to study 67P/Churyumov-Gerasimenko in detail from Earth, and there haven’t been any previous reconnaissance missions to provide maps. To make up for this, Rosetta is taking high-resolution images from a distance of 100 km (62 mi), as well as detailed measurements of the the comet’s rotation, mass, temperature, gas pressure and density, and surface gravity.

Philae candidate landing sites (Image: ESA)

The problem is that landing on a comet is very different from touching down on other planets, moons, and even asteroids. Rosetta has only a very limited window of opportunity for the landing before 67P/Churyumov-Gerasimenko comes so close to the Sun that it blows out too much gas from evaporating ice for a landing to be possible. Therefore, the Rosetta team is faced with a rush job before time runs out.

It’s a balancing act between the engineering requirements for safely landing Philae on the comet and the scientific goals of the mission. The site selected needs to be able to provide a clear line of sight for radio communications between Philae and Rosetta, as well as being free of steep slopes, crevices, and boulders. It also needs enough sunlight to keep the lander’s batteries charged, but not so much that it overheats.

In addition, Philae needs a one-kilometer square landing site because of the difficulty of piloting in the vicinity of the comet. This means that Rosetta must make a detailed map of the comet in only a few weeks to give mission control time to select a landing site. Another complicating factor is that a comet nucleus is very different from a planet or even an asteroid, with many more rough and even craggy areas caused by the evaporation of ice over the centuries.

So far, the ESA has narrowed the number of candidate sites down to five locations spread across the two lobes that make up the nucleus. Each is an example of the trade offs that need to be considered during selection. Site A is likely to see outgassing, but it may have surface hazards and light problems; site B is inside a crater and seems a safe landing site, but there may be boulders present and the area may have been recently disturbed, which reduces its scientific value; site C has good lighting, but is very rugged; site I is flat, well lit, but is covered with rough terrain; and site J is well suited for experiments to study the nucleus’ interior, but is marked by boulders and terracing. Which of these will be chosen depends on how all these factors can be squared.

The findings from Rosetta now go on to the Landing Site Selection Group for further consideration as to the recommended sites’ respective technical and scientific feasibilities. Meanwhile, the spacecraft will move in to a distance of 50 km (31 mi) for a closer look.

Landing Site B (Image: ESA)

The ESA says that by September 14 the five sites will be assessed and ranked. After the final site and back up are selected, a detailed strategy for the landing maneuver will be drawn up. This will be aided by Rosetta moving to within 20 to 30 km (12 to 18 mi) from 67P/Churyumov-Gerasimenko. The landing is scheduled for November 11, though this could change depending on what the survey finds.

"The process of selecting a landing site is extremely complex and dynamic; as we get closer to the comet, we will see more and more details, which will influence the final decision on where and when we can land,” says Fred Jansen, ESA Rosetta mission manager. “We had to complete our preliminary analysis on candidate sites very quickly after arriving at the comet, and now we have just a few more weeks to determine the primary site. The clock is ticking and we now have to meet the challenge to pick the best possible landing site.”

Source: ESA

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