The Rosetta mission's Landing Site Selection Group has selected the primary and back up landing sites for the ESA's Philae probe ahead of an attempted touchdown on comet 67P/Churyumov–Gerasimenko on November 11. Selecting sites J and C as the primary and back up sites respectively was no easy task, with mission operators weighing up various factors and racing against time since the spacecraft entered orbit around the comet on August 6. Prior to this, the comet had simply been too far away to characterize.

As the distance closed between Rosetta and its target, and the instruments aboard the spacecraft got a better look at the comet 67P, the list of possible sites was reduced to five. Each of the sites had its own virtues and disadvantages, making the process a balancing act between the scientific, practical and realistic dangers.

Despite this, the final decision was arrived upon with unanimous consent. Site J represents an option with enough outgassing activity nearby to make the site extremely appealing scientifically, whilst also offering a relatively good chance of actually getting the lander down in one piece. Stephan Ulamec, Philae Lander Manager at the DLR German Aerospace Center highlighted the difficulties involved in making the decision, stating, “none of the candidate landing sites met all of the operational criteria at the 100 percent level, but Site J is clearly the best solution."

The main barrier to safely landing the probe is 67P's inherently irregular shape. The terrain at site J, which stretches over 1 sq km (0.38 sq mi) is relatively danger free compared to some of the other shortlisted sites. Whilst the ground is still broken and hazardous, there are relatively few boulders and most of the slopes characterizing site J are less than 30 degrees when compared to the local vertical. The site also offered sufficient illumination to recharge the lander's limited batteries whilst promising a decent frequency of communication passes with Rosetta, which will remain in orbit relaying data and commands for the remainder of the mission.

Furthermore, whilst there is activity nearby, site J is considerably quieter than it's rival landing zones, with less outgassing activity. However, this is not to say that the landing zone will not become more unpredictable as the comet sails closer to the Sun, with overall ougassing activity expected to increase substantially in the weeks leading up to the November landing attempt.

Site C, located on the body of the comet, was selected for the honor of the backup landing site due to it's higher levels of illumination paired with a relative lack of boulders.

Assuming that come November 11 Philae is setting its sites on site J, the descent should take roughly seven hours to complete from separation to touchdown. The unmanned explorer will, if all goes to plan, set down on the surface of 67P at the leisurely speed of just one meter per second (3.3 ft/s). The landing itself will be fully autonomous, with commands and contingencies uploaded to the probe prior to separation from Rosetta. Once on the surface the 100-kg 220-lb) probe will weigh only a single gram (0.03 oz) in the feeble gravity of 67P.

Once Philae is safely anchored to the surface of the comet she can begin to task her formidable array of scientific instruments to shedding light on some of the most fascinating questions regarding the birth of our solar system. If all goes to plan on the descent, Philae is expected to operate until roughly March 2015.

But even if the worst does come to pass and Philae fails to secure its place in the record books as the first unmanned lander to survive the descent to this most inhospitable celestial body, it would be difficult to consider the Rosetta comet chasing mission as anything but a success. It has paid for itself many times over, both as a scientific mission and as a source of inspiration.

Final confirmation of the landing date should take place no later than September 26, but in the meantime ESA has announced that it will be holding a public competition in order to rename landing site J.

Source: ESA

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