The press room was nearly empty this morning -- only a few stalwarts are left. I, Eric Hand (from Science), and Steven Young (Astronomy Now) were hanging around to follow what could be Philae's very last day of work on the comet before falling silent, and Chris Lintott and Alok Jha were wandering around getting last bits of video for BBC.
Because Philae is in such a shadowed position, it is not receiving enough sunlight to recharge its batteries, so has only the battery power it left Rosetta with. That power will definitely take it through today, hopefully through tonight, and possibly but not likely into the next day, but no further.
So, since we're coming to the end of things, they've started taking more risks. They've completed all the science they can do without moving any of the mechanical devices on the lander, getting about 80% of the data they would have expected from the first science sequence. Last night, almost immediately after I posted my last update, they made the decision to command the lander to deploy the MUPUS soil penetrator. The first uplink attempt failed, but a second attempt got the sequence through, and around midnight European time the instrument was on and at work. Philae fans around the world were able to follow all of these events in detail through the active MUPUS Twitter account.
This device can measure the temperature of the subsurface, how fast heat is flowing out of the comet, and how rapidly the comet's uppermost surface conducts heat. The MUPUS team shared this video of a test, showing how the self-hammering probe operates. (Caution: audio levels are high.)
The next morning, ESA confirmed the successful deployment of MUPUS and made the decision to perform the riskiest remaining operation: to try to drill into the comet and retrieve samples for the lander's gas analyzers, which will try to measure directly the chemical and isotopic composition of the comet. The drill had protruded 25 centimeters from the lander before Rosetta set below Philae's confined horizon. We will have no further contact with Philae until after 9:00 p.m. local time. Did the drill go to completion? Did it deliver a sample to COSAC? We don't know.
It's possible we'll never find out. Philae is very, very close to draining its battery. During a Google+ Hangout today, Valerie Lommatsch, an engineer at the Lander Control Center at DLR in Germany (and also, incidentally, the first woman scientist or engineer I've seen on a panel at this event), gave a detailed explanation of the lander's power situation. Here are my notes on her remarks (which are as close to verbatim as I could take them, but which are still my impression of her remarks):
CNES battery team has been doing simulations continuously to find out how long batteries will last; last night we calculated we needed 80 watt-hours to complete the SD2 and COSAC block. [SD2 is the drill, COSAC the science instrument to which the drill is delivering a sample.] We thought we might have 100 watt-hours left, if primary battery temperature stays up. One thing that is unfortunate is that the one command line that didn't reach the lander was one that would've put it into a low power mode which uses about 2 watts less energy than standby mode. And that is the mode that we have to be in as we wait for link to come up. It's going to be really, really close whether we make it to the link or not.
We're thinking about two ideas right now during the next block with the landing gear. One is we might rotate panel 1. We think there is shadow cast on panel 2, so rotating a larger panel [into the sun] could result in same situation as now. One other possibility is running the landing gear up without any controls, just running up and then hoping we can maybe bounce our way out, but it's very unlikely. We're just throwing around ideas right now.
So that will be it, tonight: the last-ditch effort to cause Philae to move into a position in which its solar panels will be able to recharge the batteries. Mechanical movements will drain power quickly, so if they do manage to move things, it seems almost certain that that will be Philae's last act unless the unlikely happens and these last-ditch efforts really do effect a major change in the power situation.
A lot of people have been asking me whether the tiny amount of sunlight reaching the solar panels now would be enough to recharge the battery, given enough time. Lommatsch put that idea to rest.
It is very unlikely right now. We have 1.5 hours [of sunlight] at less than 1 watt, and 20 minutes of 3 or 4 watts. The lander needs 5 watts to boot....In order to charge the secondary battery, we have to heat it to 0 degrees Celsius. We need about 50-60 watt-hours a day in order to reach 0 degrees and still have daylight left to charge the battery. So it doesn't look that great. What we could hope for is if we are closer to perihelion, near 1 AU, maybe we could have enough energy on our one solar panel, maybe every once in a. Having [a communication] link requires additional power again.
You can watch the whole Hangout here:
After the Hangout ended, Matt Taylor invited me to visit the Main Control Centre here, where I was able to shake the hands of just a few of the dedicated team that has kept watch over Philae 24 hours a day since Tuesday.
I had thought that this was going to be my last act here at ESOC, and that I would be monitoring the situation from my hotel room this evening. But it looks like I may get a chance to return to ESOC to be here while they wait for possibly the last contact they ever get from Philae. If I don't collapse first! Stay tuned for more.