It’s been a hectic 48 hours. On Wednesday, November 12th, after 10 years in space, travelling aboard its parent vehicle, Rosetta, the little lander Philae touched down on the surface of comet 67P/C-G/Churyumov–Gerasimenko (67P/C-G). It was the climax of an amazing space mission spanning two decades – and yet was to be just the beginning. Packed with instruments, it was hoped that Philae would immediately commence around 60 hours of intense scientific investigation, prior to its batteries discharging, causing it to switch to a solar-powered battery system.
Unfortunately, things haven’t quite worked out that way. As I’ve previously reported, the is very little in the way of gravity on the comet, so in order for Philae to avoid bouncing off of it when landing, several things had to happen the moment it touched the comet’s surface. As it turned out, two of these things didn’t happen, with the result that the lander did bounce – twice.
The first time it rose to around 1 kilometre above the comet before descending once more in a bounce lasting and hour and fifty minutes, the second time it bounced for just seven minutes. Even so, both of these bounces meant the lander eventually came to rest about a kilometre away from its intended landing zone. What’s worse, rather than touching down in an area where it would received around 6-7 hours of sunlight a “day” as the comet tumbles through space, it arrived in an area where it was only receiving around 80-90 minutes of sunlight – meaning that it would be almost impossible to charge the solar-powered battery system.
As noted above, the mission was designed so that most of the core science could be carried out in the first 60 hours of the mission, just in case something like this occurred. Even so, in order to prolong the life of the vehicle, it would have been nice to move it into a greater area of sunlight. A means of doing this had also been built-in to Philae: the three landing legs can be flexed, allowing it to “hop”. But as images were returned to Earth by the Lander, it became apparent that one of the legs is not in contact with the ground, making such a hop problematic. After discussion, it was decided not to attempt to move the lander, but focus on trying to achieve the planned science objectives.
As it turned out, the initial contact between the lander and the comet confused several of Philae’s instruments into “thinking” it had in fact landed, causing them to activate. These included the ROMAP magnetic field analyser, the MUPUS thermal mapper, the CONSERT radio sounding experiment and the SESAME sensors in the landing gear. Data received from these instruments, arriving on Earth some 30 minutes after initial contact with the comet, and the information which followed, help alert mission staff that something had gone wrong, and enabled them to subsequently piece together the events that occurred during the landing sequence, while the instruments continued to gather data and transmit it back to Earth via Rosetta.
On Friday, November 14th, the decision was taken to activate Philae’s sample-gathering drill, officially referred to as SD2. This had been postponed from the previous day, as the drill uses a lot of power. However, obtaining and analysing samples from inside the comet is a central part of the mission, the decision was made to push ahead with drilling operations.
At the same time, the MUPUS penetrator was also ordered into action. This can be deployed 1.2 metres from the lander and is hammered into the comet to measure the temperature of the subsurface and the strength of the material that Philae is resting on. Alongside these two tools, the Alpha X-ray Spectrometer (APXS) was ordered into action. This detects alpha particles and X-rays, which provide information on the elemental composition of the comet’s surface.
Another reason for delaying the use of the drill and the penetrator was that, as the lander is resting unsecured on the comet, there was a genuine risk either or both tools could end up moving it is unexpected ways, possibly resulting in damage. However, with the other instruments collecting data, and power reserves falling,the science team felt the risk was worth taking. “Any movement would likely be favourable,” mission scientist Jean-Pierre Bibring observed. In the event, both tools started operations around 12:30 UT on Friday November 14th, and appeared to function without adversely affecting the lander’s position.
The drill samples, if successfully gathered, would be delivered to COSAC, one of two evolved gas analysers carried aboard the lander, the other being PTOLEMY. The decision to analyse the samples using COSAC was taken on the basis that it has a lower power consumption than PTOLEMY, and that PTOLEMY itself was able to analyse the the dust in the coma around the comet by “sniffing” it.
However, as Rosetta passed out of communications range before the drilling operation had been completed, no-one would know if the operation had been successful until the next communications pass, which wouldn’t open until 21:00 on November 14th. What was worse, given the falling power levels, there was a reasonable chance that by the time Rosetta was back in range of the lander, Philae would have insufficient reserves too be able to establish contact.
As it turned out, contact was re-established at around 22:30 UT, and the Lander commenced transmitting data on its operations – including confirmation that the drill had successfully gathered samples from the comet and that they had been delivered to COSAC.
With data being received from Philae, a command was sent to the lander instructing it to rotate its body by 35 degrees in the hope that it would position one of the larger solar panels on the side of the lander to gather greater amounts of sunlight in the hopes of charging the solar batteries. Confirmation that this operation had been performed was received at around 23:00 UT, There had also been a plan to risk moving the lander by commanding it to “hop”, despite the fact one of the legs not touching the ground.
However, with power reserves dropping rapidly, the focus was on gathering as much additional data as possible. The saw Philae operating the PTOLEMY experiment, complete additional imaging operations and run its CONSERT experiment in parallel with the CONSERT instrument aboard Rosetta, allowing comparative data to be gathered.
By 00:30 UT on Saturday, November 15th, not long before communications with Rosetta were due to be lost as it travelled beyond Philae‘s horizon, ESA operation’s centre at Darmstadt reported that power levels were so low, the lander had powered-off all science instruments, leaving only the communications link. A further report at 00:46 UT from Darmstadt indicated the radio signal from Philae had been lost. A Tweet from the media team confirmed that transmissions had come to an end:
Even so, this is not necessarily the end of the story; as noted, it might be that one of the lander’s larger solar panels is now positioned to better charge the solar battery systems. If enough power can be obtained, it might be allow Philae to wake up. The chances are slim – the lander needs around 5 watts of power to boot itself back up – but they do exist, so this may yet be a “sleep well”, rather than a final “goodnight”. The earliest this might be known is during the next communications pass by Rosetta, scheduled to commence at around 10:00 UT on Saturday, 15th November – although with local “sunrise” on the comet taking place at around 06:00 UT, it’s not clear how much charge the solar batteries will have acquired by then.
But even if Philae doesn’t wake up, either in the next day or as the comet continues to “fall” towards – and around – the Sun, it has achieved nigh-on 100% of all its science goals, and successfully returned almost all of the data to Earth for analysis. In doing so, it has provided us with information that may further our understanding of the solar system and possibly of the origins and evolution of life itself. While understanding such things might sound meaningless or trivial in a world pressed with issues of its own, we have no idea what doors that knowledge may in turn unlock, or where it may lead us.
Nor is this the end of things. The Rosetta mission will continue through the months ahead, as the spacecraft travels alongside comet 67P/C-G on its journey to the Sun. Rosetta is the first spacecraft to rendezvous with and orbit a comet, retuning incredible incredible scientific data in the process – and the adventure is only just beginning.
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All images courtesy of ESA, unless otherwise indicated
Inara Pey: Living in a Modemworld 
Comets might not be so sexy as Mars, but, whew, I wouldn’t like to be part of the mission operations team at Darmstadt! So much that can go wrong! So much tension and expectation until the tiniest little bit of a clue is unraveled, the scientists have to do some quick decisions, and then lose all communications again for many hours (or days) to see if their decisions were the right ones…
Definitely space exploration is not easy! But nevertheless so exciting!
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No, space exploration isn’t easy! Even somewhere as “well known” as Mars isn’t a safe prospect – of the 49 mission sent there, almost 50% have either failed entirely or have failed to complete their primary science mission.
But Rosetta was and is truly a leap into the unknown. The fact is, prior to the spacecraft actually entering orbit around the comet on August 6th, 2014, we knew almost next to nothing about 67P/C-G, other than its size, the fact it was tumbling through space at the rate of about 1 rotation every 12 hours, and its likely chemical and mineral composition. But surface features, potential landing spots for Philae (if any), and so on, were entirely unknown.
And while orbital mechanics made the task of chasing and rendezvousing with the comet relatively straightforward, Rosetta had the odds further stacked against it even getting to the comet alive because it had to be solar-powered.
At distances beyond the orbit of Mars, solar power isn’t terribly efficient; that’s why all other deep space probes have relied on the decay of radioactive material (generally 238PU) to generate their electrical power in what’s called a radioisotope thermoelectric generator (RTG). However, the use of these nuclear “batteries” was never an option for the mission – instead, Rosetta had to (and does) rely on two incredibly fragile and vulnerable solar panel “wings” with a total span of 32 metres (that’s almost half the span of the space station’s solar panels) to draw sufficient energy from the Sun to power its electrical systems.
Even then, for a part of the mission, as the vehicle travelled out beyond the orbit of Jupiter, and 800 million kilometres from the Sun, it was simply too far way from the Sun for the panels to be able to even power Rosetta’s communications system. So in June 2011, the craft was put into a 31-month hibernation, with all systems switched off except for a small heating unit and a computer subsystem marking the passage of time. When the command was sent ordering the vehicle to do this, there was absolutely no guarantee that come January 2014, it would be able to wake itself up again as the timer expired.
But of course it did. And now Philae has completed its mission, and Rosetta has another 14 months of operations ahead of it which continue to present consider changes. For example, the comet’s coma is currently relatively quiescent. But as the comet continues to approach the Sun, this will change; it will become more and more active, with more and more dust and gas being thrown up into the coma, which extends for several thousand kilometres around the comet while Rosetta orbits as a few tens of kilometres. So the risk of the vehicle being struck by debris with sufficient enough size and force to damage it (especially the vulnerable solar panels) is going to become increasingly real.
… and I’ve not even mentioned the potential of the mission – quite outside of its scientific returns – to help in the planning of some quite major missions NASA is currently developing, including putting astronauts on the surface of an asteroid in the 2020s!
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