Thanks to Rosetta, scientists have selected five potential landing sites for Philae. (Image Credit: ESA)

In case you missed it, the European Space Agency's (ESA) Rosetta spacecraft has arrived at its destination, Comet 67P/Churyumov-Gerasimenko. The journey to comet 67P has taken nearly a decade and has seen the vehicle come to life after a record-setting 957 day hibernation period. Since its arrival, scientists have tasked themselves with collecting the data needed to select the best possible landing site. Later this year, around mid-November, Rosetta's Philae lander will make history by being the first craft to touch down on a comet's surface.

Since comet 67P had never been viewed close-up before, researchers could not select a proper landing site without Rosetta's help. Comets change very quickly as they journey closer to the Sun, and because comet 67P will only be 279 million miles (450 million kilometers) from the Sun at the time of landing, it's time to make a selection.

Rosetta's main objective is to study the comet's coma and analyze how it evolves as it orbits the Sun. Comets are cosmic time capsules in a manner of speaking, potentially carrying primordial building blocks of our solar system along for the ride. Scientists hope that through their analysis, they might determine what role (if any) comets play in spreading water — maybe even life — across the solar system. Rosetta, along with the help of Philae and its 10 science instruments, will also be on the look-out for changes in the comet's surface. Together, both the lander and the orbiter will work to map the comet's interior.

Artist rendition of the Philae lander touching down on the comet's surface. (Image Credit: ESA)

Landing on a comet is a complex process with many variables to take into account. According to both NASA and ESA, those variables include the following things:

  • Will the lander be able to maintain regular communications with Rosetta?
  • How common are surface hazards such as large boulders, deep crevasses or steep slopes?
  • Is there sufficient illumination for scientific operations?
  • Is there enough sunlight to recharge the lander’s batteries beyond its initial 64-hour lifetime without causing overheating?

"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," said Fred Jansen, Rosetta's mission manager from the European Space Agency's Science and Technology Centre in Noordwijk, The Netherlands. "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."

An initial selection process was conducted, yielding 10 potential landing sites. Each of those were subsequently analyzed and the field was narrowed down to five. Each candidate was assigned a letter. The order of which has no special significance. Over the next few weeks, each site will be carefully scrutinized, with the final selection, along with one backup, set to be announced around September 14. The next step will then be to develop a landing strategy.

"This is the first time landing sites on a comet have been considered," said Stephan Ulamec, Philae Lander Manager at the German Aerospace Center, Cologne, Germany. "The candidate sites that we want to follow up for further analysis are thought to be technically feasible on the basis of a preliminary analysis of flight dynamics and other key issues – for example, they all provide at least six hours of daylight per comet rotation and offer some flat terrain. Of course, every site has the potential for unique scientific discoveries."

Up-close view of potential landing sites as determined by Rosetta. Image Credit: NASA/ESA

One it successfully touches down, we're sure to acquire profound information about the object. Prior to touchdown, when Rosetta was still approaching the comet, scientists began to realize comet 67P was truly a unique specimen, containing two distinct lobes (which does nothing but further complicated the landing procedure). The five potential sites are spread over the lobes, with three options on the smaller lobe (B, I, and J) and two options on the larger lobe (A, C).

"Based on the particular shape and the global topography of Comet 67P/ Churyumov-Gerasimenko, it is probably no surprise that many locations had to be ruled out. The candidate sites that we want to follow up for further analysis are thought to be technically feasible on the basis of a preliminary analysis of flight dynamics and other key issues – for example they all provide at least six hours of daylight per comet rotation and offer some flat terrain. Of course, every site has the potential for unique scientific discoveries," stated Jean-Pierre Bibring, a lead lander scientist and principal investigator of the CIVA instrument.

Bibring continued, "The comet is very different to anything we’ve seen before, and exhibits spectacular features still to be understood. The five chosen sites offer us the best chance to land and study the composition, internal structure and activity of the comet with the ten lander experiments."

View of landing site A, as seen by Rosetta's OSIRIS camera (Image Credit: ESA)

Site A:

This interesting site is located on the larger of the two lobes and has a good view of smaller section. It would be ideal for observing the terrain between the two lobes, which is thought to be a possible source of out-gassing. Higher resolution images are needed to discern any potential surface hazards in advance (such as depressions and/or slopes). The illumination conditions need further analysis also, as they will play an instrumental role in making the final selection.

Site B:

This site is located in a crater-like structure on the small lobe. It has a flat terrain, but the illumination conditions could be an issue. Higher-resolution images would help in determining any potential boulder threats. This site in particular may not be as clear as other sites, as it could contain more recently-processed material.

Site C:

Location C is on the larger lobe and has varied surface features such as: depressions, cliffs, hills and even smooth plains. This site is well-illuminated and also note-worthy because of the number of interesting surface features it exhibits, but those same characteristics also increase the number of potential complications.

View of landing site I, as seen by Rosetta's OSIRIS camera (Image Credit: ESA)

Site I:

A relatively flat region on the smaller lobe, location I (pictured to the right) could be composed of fresh material, but high-resolution images will be able to give us a clearer picture of just how rough the terrain will be. Illumination issues should be non-existent at this locale.

Site J:

Very similar to site I, this location is also on the smaller lobe, possesses exciting surface features, and has excellent illumination levels. This particular location offers advantages for the CONSERT experiment, where Philae will attempt to map out the interior of the comet.

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