author: none
Jun 11, 2015
astro RosettaYou may remember that last year I reported on the European Space Agency (ESA) cometary probe Rosetta which was placed in orbit around a comet (nick-name: 67P), from where it dropped a lander, Philae, to the surface. Instead of anchoring itself to the surface when it touched down, Philae bounced, at least twice, and came to a stop in a shadowed location over a kilometer away. Before its batteries ran down, it photographed its surrounding (mostly its own feet and a wall of rock) and completed other experimentation. One of the experiments required that Philae and Rosetta stay in contact via transmissions that would pass through the lightweight material that 67P is made of .. the intent being to interpret variations in the signal as pointers to the comet’s internal structure.
Philae’s bounces took it into unknown territory but analysis of that radio signal allowed a recent refinement of Philae’s location. Also, Rosetta has been imaging 67P and, in addition to monitoring the comet’s increasing emission of dust and volatiles as it approaches the sun (now only 60 days till its closest approach), those images have been scoured for any signs of Philae. The problem is that the cometary surface is covered with objects about the size of Philae and those that are made of mostly of ice are bright, just like shiny Philae. Moreover, Philae is in a shadowed location, probably a trench or depression, maybe a even something like a cave, and is only lit by the sun briefly as 67P rotates.
The radio experiment has narrowed the probable location of Philae to a 160x16 meter ellipse. Superimposed over a Rosetta narrow-angle mosaic, it looks like this (the first touchdown site was at the far side of the flattish plane):
Close analysis of this area revealed six possible candidate bright spots for Philae’s resting place. Recent work has eliminated five of these locations; which is not saying that Philae is in the remaining location (because it may be at none of them), but it looks interesting. Two images taken in December show a brightness that was not there before Philae was dropped in November.
All the three frames have similar illumination .. the October frame was taken from about 10Km and the December ones from about 20Km. The region shown is a 20 meter square on the ground sightly above (but just outside) the center of the ellipse.
As 67P gets closer to the sun, and as the geometry changes, Philae will get more illumination. Assuming the onboard electronics have not been damaged, it is possible that Philae will get eventually enough sunlight to recharge its battery. If that happens, Philae will reboot and start transmitting a beacon signal; if Rosetta is in the right place it will hear the signal and report that fact. Transmitting the beacon signal will re-deplete Philae’s battery so this cycle could repeat several times. Once the ground knows Philae is waking, ESA controllers can try and get control of it to stop the “random” power recycling, get more power into the battery and resume some normal functions.
There are a lot of conditional words in that paragraph. An additional problem is with Rosetta; orbiting a chunk of ice, rock and dust that is approaching the sun is not a safe place! A few months ago, Rosetta was forced to abandon low (20-30Km) orbits around 67P because its navigation systems were unable to track stars among thousands of bright specks of dust being blown off the comet and, in some cases, Rosetta was probably bumped by drifting debris. It now maintains a respectable few hundred kilometers between itself and the comet. Rosetta will be able to resume close passes as comet activity fades after it passes its closest to the sun; so even if Philae is dead, if Rosetta is still functional and the comet still intact, we may, eventually, get an unambiguous image of Philae.
PS (little to do with the above): At the end of May, ESA released a large batch of Rosetta’s images from scientific embargo and, while browsing, I found this “beauty shot” (timed perfectly for the moonrise) from years ago, when Rosetta was doing a flyby of our planet to pick up speed: