Separation, Descent, and Landing

Separation, Descent, and Landing (SDL) events are described in detail on the Rosetta blog.

Separation

The lander will be pushed from the orbiter at a commanded, precise speed. If the commanded deploy does not succeed, a backup spring can push the lander off at a speed of 18 centimeters per second. CIVA will take a "farewell" image of the orbiter, which should be returned during the descent. Hopefully Rosetta's NavCam will capture the return image, of Philae departing.

Descent

It will take Philae 7 hours to free-fall the 22.5 kilometers to Churyumov-Gerasimenko's surface. Within an hour of separation, Philae will establish radio contact with Rosetta, unfold its three-legged landing gear, deploy the CONSERT radio antenna, deploy the boom holding the ROLIS camera, and begin taking photos with ROLIS.

Forty minutes after separation, Rosetta will perform a divert maneuver. Two hours after separation, Rosetta will turn to watch Philae drift toward the comet.

Throughout its descent, Philae will be actively gathering data. ROLIS will shoot descent images. COSAC, Ptolemy, and SESAME will attempt to sample gas and dust around the comet. ROMAP and SESAME will measure magnetic fields and plasma. CONSERT will measure the separation between lander and orbiter, and also try to actively sense the surface of the comet.

A little of the descent data will be transmitted to Rosetta during the descent, so regardless of what happens to Philae when it contacts the comet, there will be some science returned. Descent data may include the CIVA farewell image and CONSERT data on the range from orbiter to lander with respect to time, among other information. (Ref)

Landing

Philae's legs are designed to damp out the forces of a hard landing to reduce the lander's chance of bouncing. When Philae touches down, it will fire two harpoons to attach it firmly to the comet's surface. A thruster on top of the lander fires at the same time as the harpoons, keeping the lander on the ground. Ice screws also deploy from the three lander feet.

Upon landing, the first science activities will be CIVA panoramic imaging, MUPUS acceleration measurements, and SESAME elastic properties measurements. The descent data and these first images will be returned as soon as possible so that mission engineers can determine the orientation of the lander, and then command it to rotate it to best illuminate its solar panels and optimize the positions of the lander instruments. Mission engineers will also use these early data to calculate tables describing when the orbiter will be visible from the lander (for communication) and when the ground will be illuminated. (Ref)

If the comet turns out to be very soft (with a compressive strength less than 2 kPa, similar to lightly compacted snow), the lander could sink in up to its baseplate, which would prevent it from rotating. The lander would only sink farther if the compressive strength were less than 100 Pa (similar to powder snow). (Ref 1, ref 2)

The first science sequence lasts five days and can be performed entirely under battery power; with solar power, the lander could survive and continue doing science for much longer. For more post-landing plans, read the Rosetta blog.

Whatever happens on landing day, I'll be there to report it! Stay tuned to this blog, to my Twitter feed, and to my list of official Rosetta and Philae tweeters for news.