It was early on Sunday morning. I woke up and played games on my iPad then, on an impulse, checked Twitter before I got out of bed. The first thing I saw was euphoric chatter: Philae is back!
I stayed glued to my iPad through breakfast, enjoying the long, slow tide of more and more people in American time zones waking up on their lazy Sunday morning to the happy news.
Quickly, Randall Munroe updated his xkcd cartoon chronicling Philae's life.
Fellow science writer Ed Yong and I had a moment on Twitter, finding the perfect soundtrack for the happy event. (Okay, it's artery-cloggingly cheesy, but for nerds of a certain age it's perfect.)
Now that we've all had a few days to recover from our euphoria, it's time to ask: what does this mean? The last time we heard from Philae, I was there in mission control, watching through the glass as a graph of its bus voltage trended to zero. I'm not ashamed of the tears I had in my eyes at the end. Philae had worked very hard in its final hours, taking and returning new data until the very last moment. But there was a lot of science left undone; Philae's awkward landing position and various mechanical problems had caused experiments that depended upon reaching and penetrating the comet's surface to be incomplete.
They would be incomplete forever if Philae never woke up. But project manager Stephan Ulamec always expressed confidence the probe would eventually return to action. It wouldn't be soon -- their understanding of the geometry suggested that the comet must get close to perihelion before the lander would receive enough power to run its transmitter. Maybe as late as August, he said, but possibly as early as May.
Here we are: it's June, and the lander is awake and talking again. How did that happen, what is the lander's status, and what's next for the mission? Let's take these in order.
The last signal was heard from Philae at around 00:30 UT on November 15, 2014. Philae was in too much shadow for its solar panels to generate sufficient power, but the changing season and approaching perihelion were expected to improve the situation. Rosetta began listening for Philae in March. That Rosetta update described the conditions necessary for Philae to make contact:
- During its low power state, all available solar energy would be used to heat the lander.
- Once the temperature exceeded -45ºC and the panels generated 5.5 watts, it would power up and attempt to charge its battery, while listening for a signal from Rosetta once every 30 minutes.
- Once the available power exceeded 19 watts, it could turn on its transmitter.
- But the transmissions can only be received by Rosetta when Rosetta is above the horizon.
- Since the lander likely only has power to transmit while it is in sunlight, Rosetta must be above Philae's horizon while Philae is in sunlight.
All of these conditions were finally met, on June 13, 2015, during Rosetta's fourth listening attempt. Rosetta received signals at 21:28 UT, which were immediately relayed to Earth and received at 21:45. Contact lasted 85 seconds. Three more contacts happened over a period of four minutes on June 14 (two comet rotations after the previous signal), but each contact lasted only 10 seconds due to poor viewing geometry.
(If you like that chart, you can find a live-updated version here.)
Philae's status: Warm with plenty of power
The initial report showed Philae with plenty of power (24 watts) and, if not warm by Earth definitions, at least not too cold (-35ºC). A later, briefer burst of telemetry showed the lander even warmer, -5ºC. The initial packets of data from the lander indicated that it had been awake for at least a few days, possibly as long as a few weeks, before successfully making contact. A Rosetta blog entry from Monday tells us that the lander began generating about 13 watts at comet sunrise, reaching a high of more than 24 watts at noon, and that the solar panels are receiving more than 135 minutes of sunlight each comet day. (The comet rotates once in 12.4 hours, so that's still a short day and a long night, but it's enough for Philae to be active.)
This is more than enough power for science, especially for instruments that require less power (like ROMAP, the magnetometer). Early work "would also likely include measurements that did not previously generate science in November," the blog entry states (more on that below). But before they can do science, they need to improve the stability and quality of communications between orbiter and lander. Rosetta's orbit is not currently a good one for long periods of high-quality lander communications. So the next order of business is to shift Rosetta's orbit to one that can support robust communications. They will shrink Rosetta's orbit to a 180-kilometer one, and keep the orbiter aimed downward toward the comet's surface.
The ESA blog notes that the unfortunate events of Philae's landing, which were so frustrating in November, are now beneficial for the lander; shadowing keeps its temperature from rising too high now. "Any operation of Philae’s instruments up to or through perihelion on 13 August – the comet’s closest point to the Sun along its orbit – will allow in-situ study of a comet during its peak activity. Had Philae landed at the planned site, at Agilkia in November 2014, its mission would likely have ended in March because of the higher temperatures of that location as solar illumination increased."
Philae's activities: what's next?
Once Rosetta can communicate more reliably with Philae, the mission will be able to get back to work. They held a press briefing at the Paris Air Show today to explain the current status and future plans for the mission. You can watch the whole briefing below:
Here are some key points from the briefing.
From Elsa Montagnon, deputy flight director for Rosetta: The comet is very active at the moment. There have been numerous "star tracker events" in which the spacecraft's star trackers confuse dust particles for stars, and lose their ability to determine the spacecraft's orientation. The number of star tracker events is continuing to increase as the comet approaches perihelion. Perihelion is on August 13. Based on current activity levels, the mission has determined that the closest safe distance for Rosetta to the comet is 180 kilometers. But if comet activity increases, they will have to retreat. At present, the Rosetta mission is planned to end on December 31. However "we know we have enough resources until September 2016." It was not clear whether she meant financial resources or spacecraft resources -- I would assume she meant the latter, but don't know for sure. By that time, comet activity will have decreased markedly.
From Barbara Cozzoni, a Lander Control Centre engineer: Analysis is ongoing to understand and calculate exactly when the lander first switched on; it was "probably days before" June 13. Both of the computers (DPUs, Data Processing Units) are working. The computer's software is able to detect the day/night cycle on the comet. The lander internal compartment is warm, already at -36C at sunrise. Its minimum operating temperature is -45C. Four solar panels (1, 2, 3, and 6) are collecting energy. Moreover, they are performing in line with predictions, meaning that no comet dust has settled on them to obscure them. "So it looks like Philae is ready for operation now." But to be really sure they need more data, and for that they need Rosetta to be in a better orbit.
They are now getting ready to do science. "In the last seven months we were not only praying for the lander to come back; we were also planning." They plan to begin long-term science in the next few days. Science plans will take the form of science "slots," each lasting a single comet day. Some slots will be used for charging the batteries, because some science operations require battery power. Science observations will be performed autonomously. They don't need daily Rosetta communications to make this work, but they do need to know how many slots will pass in between opportunities to communicate with the orbiter, so they need predicted contact times from Rosetta.
From Philippe Goudon, Philae project manager at CNES: in November, Philae accomplished about 80% of the goals of its first science sequence. The two things it did not do were to measure the surface with APXS and to collect a sample with SD2 and deliver it to COSAC and Ptolemy. They had intended to land in a spot in the northern hemisphere, which would have given them 9 hours of sunlight out of the 12.4 hour day. It would also have killed the lander after March. Instead, they landed in the southern hemisphere during winter. The main reason that the power situation has improved is that the comet is now much closer to the sun, but the change in seasons has also helped. The best time for lander power will be in August because the comet will be close to the sun and because there will be the longest days.
The first planned activities will be least power-demanding and least risky, requiring no movement of the lander. These include SESAME PP, which measures the permittivity of the surface through the lander feet; ROMAP, which measures magnetic fields; and MUPUS' Thermal Mapper. Next will come slightly more power-hungry activities, which still do not require moving the lander: CONSERT, which acts together with an instrument on the orbiter to perform radio ranging and radar sounding; ÇIVA-P, which takes panoramic images around the lander; ROLIS, which images below the lander; and SESAME DIM and CASSE, which count dust impacts on top of the lander and measure acoustic properties of the ground beneath the lander feet, respectively. They can also use COSAC and Ptolemy to "sniff" the atmosphere.
Cozzoni said that further operations are being considered but have not yet been planned, pending a better understanding of the current condition of the lander. They are discussing with the science team the priority of riskier activities, including operating SD2 and lifting and rotating the lander. Goudon discussed lander movements more freely. They will not move the lander in the next few weeks, but will try it in the next month. For instance, they would like to rotate the lander to place the APXS instrument into a better position from which it may succeed in making a measurement. (When APXS did not work in November, there was discussion to the effect that its lens cap had not come off; at the time it was my impression that it wasn't recoverable, but it now sounds like the problem was that the awkward position of the lander left the APXS instrument too far from the surface after deployment.)
Goudon did not discuss using the drill, but the next presenter, Philae chief scientist Jean-Pierre Bibring, did. In November, he said, they were unlucky; the SD2 drill could not reach the surface, and then they ran out of time. Now, though, they know how much they need to rotate the lander (about 30 degrees) in order to place the surface within reach of the drill. He is clearly eager to attempt this, and it's obvious why: what they have learned about the composition of the comet from the orbiter and from the sniffed "atmosphere" samples has shown that the granular material from which the comet is built is unexpectedly rich in organics. They have instruments on the lander to gather comet material and taste it; that's the main unfinished business that Philae has left to do.
It's going to be a very exciting summer for comet science, as Philae returns to work on the surface of a comet near perihelion. I'm sure it will not all be smooth sailing. It will be very challenging for the Rosetta navigators to keep the spacecraft in an orbit close enough for good communications and yet not risk too many safe modes and lost communication sessions due to star tracker events. The intense comet activity will, unfortunately, prevent Rosetta from getting close enough to the comet to directly image Philae for some time. Last week ESA posted a list of the current best guesses at landing site locations, but which one (if any) is the correct site is unknown. Continued radio communications, coupled with work by the CONSERT radar sounding experiment, could both contribute to localizing the lander. But for 100% certainty they need a photo, and for that they'll have to wait for comet activity to die down enough for Rosetta to approach closer again.
Stay tuned for the continuation of Philae's dramatic story!