Rosetta is a mission of firsts. It is the first mission to successfully chase and rendezvous with a comet, and hopefully in November it will be the first spacecraft to make a soft landing on a celestial body that's so unlike anything we have explored before. Unfortunately, as is so often the case with firsts, there are untold risks and complications that are difficult to foresee ahead of time, but here are some of the dangers that we know about.

Currently, 67P/Churyumov-Gerasimenko (67P) is relatively calm, with conditions around the comet actually exceeding the ESA's predictive models. However, appearances can be deceiving. Rosetta is essentially locked in a battle against the clock, requiring mission operators to choose an optimum landing site for the Philae lander, and bring Rosetta in range to drop the tiny laboratory onto the surface of the comet as quickly and as safely as possible.

Any delays to this process could make life much more difficult, as the comet's ever-decreasing distance from the Sun causes it to become ever more unpredictable. The heat from our star creates a phenomenon known as outgassing, a term given to the expulsion of trapped gas pockets. Whilst this occurrence does not pose a serious threat to the safety of the spacecraft, it may be enough to alter Rosetta's trajectory, creating yet another headache for mission operators. Furthermore there is a concern that dust thrown out as a by-product of the outgassing could impair Rosetta's optical sensors, diminishing the quality of pictures to be snapped by the mothership as it remains in orbit.

Philae's landing process is almost fully automated, as the time lag between command and execution created by the distance between the robotic explorer and Earth does not allow for direct control. Therefore the lander is forced to operate on pre-uploaded commands supplemented by contingency procedures should the unexpected occur. The team must give Philae the greatest chance of success they can by selecting a site with the optimal surface composition. This is vital, as Philae's descent procedure involves thrusters pushing the 100 kg (220 lb) lander to the surface of the comet. If all goes to plan, she will anchor herself to the surface with the use of hooks mounted on her feet and two anchor-like appendages that fire into 67P's surface, further securing the explorer.

Five landing sites have been shortlisted for the November landing (Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

If the site is too hard, the hooks on Philae's feet and the anchors would be all but useless. In this scenario, even drilling the comet for samples could force the lander off its surface. Conversely if the chosen area is too dusty, there is a chance that this dust could envelope the lander, compromising the solar panels required to recharge Philae's limited onboard batteries (that are expected to run out after the first few days on the comet). In essence, the team is looking for a Goldilocks site: not too dusty, not too solid, just right.

Prior to attempting a soft landing on 67P, there are a number of observations and procedures that the team can undertake to try and increase the chances of success for both the landing and the scientific mission as a whole. For example, in an attempt to avoid any surprises in equipment malfunction once Philae is deployed, mission controllers are carrying out a series of comprehensive checks on the lander's vital systems. Most importantly, the team will asses the readiness of Philae's batteries and camera, and will test the communication system between the lander and its mothership, a vital component for the relay of orders and data between Earth and the lander.

Furthermore the team is in the process of learning everything it can about the surface composition in order to pick the Goldilocks site. One of the ways this is being accomplished is via an analysis of the images sent back from Rosetta detailing the landing sites. For example, a smoother surface indicates the presence of large amounts of dust settling in the ordinarily broken surface of the comet, whilst an excess of boulders could indicate a predominantly rocky surface.

Recently, Rosetta initiated another descent phase that aims to bring the mothership to within 29 km (18 miles) of the comet by the 10th of this month. Undoubtedly as the comet closes on its quarry, mission controllers will get a better idea of what to expect in the lead-up to the historic landing date.

Source: ESA Google+

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