Now Philae down to sleep We pray a sunbeam soon to sweep And if the hibernation break We have more science yet to make— Exclamation Snark (@BadPhysics) November 14, 2014
As I was wrapping up the final Philae press briefing yesterday (summarized here), Daniel Scuka, ESA’s Senior Editor for Spacecraft Operations came down to where I was writing, and asked me if I’d be interested in returning to ESOC in the evening to watch with the ESA web team during what might be Philae’s final contact with Earth. “Really?” I asked. I made eye contact with Steven Young across the table, and asked Daniel, “Can he come too?”
In the end it was three of us serving as social media eyewitnesses to the end of Philae, with Chris Lintott joining too, unaccompanied by his usual camera crew. We were there only to watch and tweet, in the same room as the ESA web team including Daniel as well as stalwart ESA bloggers Emily Baldwin and Claudia Mignone. That room was right behind the main mission control center. The control center was in the hands of mission manager Stephan Ulamec and flight director Elsa Montagnon, with another eight or so spacecraft operations engineers involved.
We arrived at around 9:30 in the evening, prepared for what might be a long wait for Philae to regain contact. Philae has only a relatively weak radio antenna, and depends upon Rosetta for relay. The comet rotates once in twelve hours; Rosetta, on a 30-kilometer orbit, passes around it once in about two weeks. So there is roughly one opportunity for contact between Rosetta and Philae in each comet day, drifting a little later each day as Rosetta moves slowly in its orbit.
Had Philae been in the expected landing site, Rosetta contact times would have been certain, and the radio contacts would have lasted nearly half a comet day — six hours. As its location is not known, the contact times are uncertain. Contacts routinely happening earlier or later might give a hint to lander longitude on the comet, but what was actually happening on radio contacts, Stephan Ulamec explained to us, was that the lander made contact nearly an hour later than expected, and lost contact nearly an hour earlier than expected. Furthermore, the connection, when initially made, was wobbly, coming in and out of signal lock for as much as an hour before becoming strong (Montagnon joked that “there is a tree in the way”). All of these indicate the lander’s view of the horizon is occluded by objects nearby.
While we waited, Daniel showed us some NavCam images he’d gotten from the flight dynamics team, showing what they thought was a bounce mark on the comet from the initial landing. But the images were a little confusing as they weren’t co-registered. I offered to help register them a little better and annotate them, and this was the result:
The second image should actually contain Philae, post-bounce, but the pixels are 1.3 meters across, so it’s hard to say. For what it’s worth (probably not much), here’s a guess at its location.
Rosetta regained contact with Philae at about 11:23 in the evening, local time (10:23 UTC, Earth Received Time). At first, the signal was wobbly but it stabillized in only a few minutes. Science data, stored onboard Philae from activities performed while it had been out of contact with Rosetta, streamed down to Earth via the orbiter. Housekeeping data came first, followed by COSAC science data. On Twitter, the SD2 drill team confirmed that their drill extended its entire distance out and then in again — a hopeful sign — though there is no way of knowing until results are analyzed whether it actually managed to reach the surface and grab a sample.
With all the science data off the lander, Rosetta autonomously sent Philae a command sequence to move. After several days of agonizing, the team had finally decided upon the movement action with the highest probability of success. They commanded the lander legs to lift the body by 4 centimeters, hopefully raising it a bit farther above the horizon, and then rotating the lander body 35 degrees to place the largest solar panel in a position where it could hopefully receive the most sunlight.
Telemetry confirmed that the lander moved as it had been commanded to do. There was no way of knowing whether the motion of the solar panel would increase available power, because by this time it was local night, with sunrise expected at around 6:00 UTC. But following the two commanded motions, Stefan Ulamec proclaimed the battery to be “good” — I was not quite sure what this meant, only that he seemed pleased by the amount of power remaining.
Still, there wasn’t much left. From our position behind the glass, Steven, Chris and I watched the engineers as they, in turn, watched a set of graphs on their screens — graphs that were declining steadily. Shortly after the motion was commanded, the main bus voltage plunged. We were told that it needed a minimum 21.7 volts to function, and after that, the end would be fast. My eyes misted, but I kept thinking of Huygens, 10 years ago. Against all odds, Huygens had survived for hours after the landing, long after Cassini had passed over its horizon, and after Saturn set beyond the Earth radio antennae that had been watching, just in case it survived landing. It had always troubled me that we hadn’t watched Huygens to the end. For Philae, we could stand watch.
And yet, Philae was untroubled by its distressingly low power state. It carried on executing a series of science operations from its new position. The downlink signal remained strong. It acquired and returned ROLIS images from its new orientation. It began performing a third CONSERT ranging experiment. CONSERT is intended for ranging through the comet, so doesn’t ordinarily get used when the orbiter is overhead, but its high-precision radar ranging would be useful for pinpointing Philae’s uncertain landing location; a third complete session would complete a triangulation.
The link stayed strong for a while, and instruments continued to acquire and return data. But, inevitably, the end came. Around 1:30 in the morning local time (00:30 UT), Daniel told us that the lander had switched to standby mode due to low power. All instruments were shut down. Ulamec came in and told us that the lander had gone into a mode where its voltage dipped and it shut down and then climbed briefly and it booted and sent one data packet every few minutes. It wouldn’t be long. But he was smiling, and in fact during this time there was frequent laughter inside the mission control center — Chris and I figured it was either gallows humor or just plain surprise that the lander was still going.
Ulamec came in again and said it had been a few minutes since the last packet — there might be one or two more, but that was probably it. But he was smiling. “We are happy. We even can watch it falling asleep, which is a little bit sad, but it can give us data that we want to have.” And he was thrilled that the CONSERT ranging pass had run to completion.
At 1:44 local (00:44 UT), Elsa Montagnon came in and told us “the link has broken. It is not far from the time we expected it to break. Rosetta’s radio receiver for Philae is, at the moment, always on. So it will be listening.” It was theoretically possible that the new solar panel position could even allow Philae to wake tomorrow after sunrise, though nobody really seemed to expect that to happen. If the solar power situation has improved, it will probably take several days of charging to achieve wakeup. So Rosetta will watch. But watching won’t prevent Rosetta from doing its science mission.
"We are privileged," Montagnon said a little bit later, "to have seen Philae going to sleep ‘live.’"
I and Steven and Chris were tweeting all this, and Tweetdeck on my computer looked like a slot machine, the way notifications and retweets were spinning by. The entire world was watching through us, to Philae, to keep it company as it passed into slumber. I showed this to Elsa Montagnon, and she was surprised and touched. The other engineers in mission control center also seemed surprised to hear how many people were following along.
As I tweeted my last picture, the engineers began shutting down the screens. It seemed a fitting way to end the evening. Lights out.
It’s been quite a week. Philae had a rough landing, but a successful one, and managed to operate all of its science instruments. Although there were problems, I am confident that the data returned from this tiny mission will effect a major change in our understanding of comets. But we’ll have to be patient. Good science takes time. Meanwhile, Rosetta is still fully functional and has more than a year ahead of it at least, to watch as the comet goes through perihelion next summer. And who knows? As the comet approaches the sun and its seasons shift, we may yet hear from Philae again.
And now for some acknowledgments. Thanks to Daniel Scuka for the invitation to ESA as social media, and to the other folks on the ESA web team including Emily Baldwin and Claudia Mignone, for their help, hard work, support, and at the end, amazing access to the final hours of our watch over Philae. Thanks, too, to ESA more generally, for stepping up their public outreach efforts on Rosetta and Philae; I don’t know who deserves all the credit for that, but some at least goes to Mark McCaughrean, Fred Jansen, and Matt Taylor on Rosetta in Europe, and Claudia Alexander in the U.S. They can tell me who else I should thank! Thanks to all the Philae instrument teams who are keeping us informed with science updates on Twitter, in particular MUPUS, who is tweeting out science results from their experiment even as I write this blog entry!
I also need to thank my peers who were with me in the ESA press room. Space journalists are, first and foremost, space enthusiasts. Sure, there is competition among us to get the best stories. But during confusing and unfolding events like this one, I find space journalists to be open and cooperative, working together to make sure we all have the right, best stories. The writers and reporters huddled around the table in Darmstadt freely shared juicy tidbits of information with each other, collaborating to get more, better, more accurate information out to as wide an audience as possible. Jonathan Amos, Stuart Clark, Eric Hand, Chris Lintott, Ivan Semeniuk, Paul Sutherland, and Steven Young — without their generosity, I wouldn’t have had nearly as much information to share with you, and more of the stuff that I’d told you would’ve been wrong.
And a HUGE thank you to Planetary Society members, whose generous financial support made the trip possible.
And that’s it for me. Fittingly, my battery is dying on my laptop as I descend at the end of my flight to Los Angeles. Thanks for sticking with me for this adventure, and stay tuned for further news from Churyumov-Gerasimenko, which I’ll be posting on planetary.org.