Philae landing site selection process under way as Rosetta closes to within 60 kilometers of the comet

When Rosetta arrived at comet Churyumov-Gerasimenko on August 6, it began its survey of the comet from a standoff distance of roughly 100 kilometers, though that number varied within a small range as the spacecraft executed its odd triangular-shaped path in front of the comet. The initial survey has provided the mission with the data they need to make a preliminary selection of about five candidate landing sites for Philae (more on that below). On August 17, Rosetta spacecraft changed course to one that would approach the comet more closely. This evening, Rosetta will halt the approach, arriving at its new standoff distance of approximately 60 kilometers. As it did previously, the spacecraft will circulate around a point in space sunward of the comet, completing three triangular arcs over the next 10 days.

The comet has grown increasingly large in the NavCam field of view as the spacecraft approached it.

The last three images in this series, taken from closer distances than ever before, are especially nice:

Rosetta fans have been very focused on the images, but there are other kinds of scientific observations being performed at the comet. One measurement that's important both for science and for mission operations is to determine the mass of the comet. According to the ESA blog, radio tracking information has yielded the first reasonably precise estimate of the comet's mass at 1 × 10^13 kilograms, plus or minus 10%, a number that English-speaking Europeans would call "10 trillion" "10 billion" and Americans call "10 quadrillion" "10 trillion" kilograms. And hey, since India is an interplanetary nation now, I might as well add for fun that English-speaking Indians call this "10 lakh crore" kilograms. With all this international English number-word confusion it's probably best to stick with scientific notation for large numbers!

It's interesting to compare this number with an estimate made before Rosetta's arrival: 3.14 × 10^12 kg. The new estimate is larger by a factor of three. But they didn't report an error on the earlier estimate, and the uncertainty would've been quite large; it's quite possible that the new estimate is consistent, within error, with the earlier estimate.

Once you have the mass, you can determine the density, if you know the volume. I have not yet seen a specific estimate for the dimensions or volume of the comet based on OSIRIS imaging. But we've got great NavCam images, so this suggests a weekend homework problem. Are you ready? Use Rosetta's NavCam images to estimate the volume of the comet. Then estimate its density. Here are some useful facts: NavCam images have a field of view of 5 degrees and are 1024 pixels square. The Rosetta team always tells you the distance separating the spacecraft and the comet for every NavCam image. So you can get the field of view (that is, the width or height) of any Navcam image by doing: [2 × Rosetta's distance × tan(2.5°)]. You can get the pixel resolution by dividing that number by 1024. I might try approximating the comet as two ellipsoids, one for the "head" and one for the "body." You need three measurements through the ellipsoid, so you'll have to look at multiple NavCam images from different points of view to get those measurements. Most image processing software (even Microsoft Paint) will allow you to measure pixel distances on images. The volume of an ellipsoid is (4/3) times pi times the radii of the ellipsoid's three axes. If you do the math, share your estimate in the comments! After modeling it as two ellipsoids, I might also try modeling it as two elliptical cylinders to see how much my model for the shape of the comet affected my estimate of the volume. Then divide mass by volume to get density. What do you get? Anything under 1000 kg per cubic meter is less dense than water ice, so would imply a porous interior.

The survey of the comet at 100 kilometers has provided the Rosetta team with the information they need to perform an initial selection of landing sites. They are under considerable time pressure. To balance between the availability of solar power (which requires proximity to the Sun) and the hazards of increased comet activity (which goes up as the comet approaches the Sun), the landing must happen in mid-November. They'll finish the process of selecting candidate landing sites this weekend, and have stated that they'll announce the candidate sites on Monday.

In this initial round of landing site selection, they are primarily concerned with engineering constraints. They're looking for places that they can deliver the Philae lander at the right speed, with the right illumination conditions along the way, while remaining in contact with the lander all the way down. The site will need sufficient sunlight to generate solar power but not so much that Philae will overheat, so the landing sites will necessarily be out of permanently sunlit or shadowed regions. You can read more about landing site selection constraints on the ESA website. Here is a timeline of the landing site selection process, interleaved with the Rosetta operational timeline I posted earlier. The operational timeline is in italics.

• This weekend, the Landing Site Selection Group is analyzing about 10 possible landing sites.
• August 24: Landing Site Selection Group downselects to 5 sites.
• August 24-September 3: Meanwhile, Rosetta will perform its second triangular "orbit" at a standoff distance of 60 kilometers, with turns on August 27 and 31.
• August 25: OSIRIS team delivers digital terrain models of candidate landing sites to Lander Control Centre
• August 25: trajectory analysts at Rosetta Mission Operations Centre begin studying feasibility of delivering Philae to specific sites
• September 3-10: Meanwhile, Rosetta will transition to a global mapping orbit at 30 kilometers.
• September 5: Rosetta Mission Operations Centre report on their analysis. Some landing sites may be rejected as unfeasible
• September 10-24: Rosetta will perform global mapping from a real orbit with the comet at the center of the orbit and the spacecraft at an average distance of 30 kilometers
• September 13-14: Landing Site Selection Group considers flight dynamic analyses as well as new Rosetta science data. Group will select primary and possibly backup landing sites.
• September 16 to October 11: Rosetta gathers high-resolution data on proposed landing sites. The spacecraft will be close enough for OSIRIS to map boulder distributions.
• September 23: OSIRIS team will deliver high-resolution images and maps of boulder distributions to Rosetta Mission Operations Centre.
• September 24-29: Rosetta performs "night excursion" and transfer to 20-kilometer orbit
• September 26: Rosetta Mission Operations Centre delivers operational scenario for primary landing site.
• September 29-October 10: Rosetta performs close observations at 20-kilometer altitude
• Beginning October 10, Rosetta performs close observations at 10-kilometer altitude
• October 12: Landing Site Selection Group meets to make "Go/No Go" decision about Philae landing at primary site.
• October 14: Lander Operations Readiness Review will (hopefully) culminate in official permission for landing Philae at the selected site
• October 13 to November 3: Philae science planners work to adapt Philae's First Scientific Sequence to the choice of specific landing site
• November 9: Final landing sequences will be uploaded to the lander.
• November 11: Landing!