What was your first reaction when Philae woke up?
Philippe Gaudon : “Well, I was the lucky one who was the first to see it wake up and I couldn’t believe my eyes! We’d been waiting for this moment at the SONC for months. We’d been keeping a close eye on things and we’d set up an e-mail alert system combined with permanent watches in the evening and at weekends. On the evening of Saturday 13 June it was my turn and I was watching the 24 Hours of Le Mans on the TV with my laptop next to me. When I’d checked the parameters in the afternoon, it was ‘business as usual’ and the curve was flat. I glanced at the laptop and saw a spike! From that moment, I lost all interest in the car race. I zoomed in on the spike and there were several steps, so I called my 2 colleagues at the SONC to ask them to check and they got right back to me saying the signal contained data from Philae. We then contacted our colleagues at the LCC in Cologne and at 3:05 a.m. they confirmed contact with the lander. We knew Philae could wake up at any moment and we’d been waiting for it to happen, but our jaws just dropped and we were really thrilled to hear from it again at last!”
Contact was regained between Rosetta and Philae at the fourth attempt, is that right?
Philippe Gaudon : “We’d planned to start listening for a signal on 30 May, but because of a slightly altered trajectory of the orbiter, the likelihood was that we would have a real chance of making contact from 10 June onwards. So the window of opportunity had only just opened.”
And contact was made again on the evening of 14 June?
Philippe Gaudon : “Yes, but it wasn’t as good, much more unstable. We kept losing the signal and we didn’t retrieve much data. It’s also very difficult to date the packets in Philae’s memory because its clock switches off at each stop/restart cycle, but it would appear the data we retrieved are the most recent, in other words those that had just been collected in situ during Philae’s current operating cycle, but we still need to confirm that.”
Are all the data just housekeeping data from Philae or have you retrieved something from the instruments?
Philippe Gaudon : “There’s nothing from the instruments, which have all been shut down since the end of the initial science mission in November last year.”
So, what happens now?
Philippe Gaudon : “First, we have to try to extend the communications windows. There are two likely limitations: either Philae still isn’t generating quite enough power or the orbiter’s alignment with its antenna might not be optimal. So, we’re going to attempt to manoeuvre Rosetta into a better position with respect to Philae, which means we’ll have to change its trajectory and move it in closer while retaining enough safety margin to shield it from the effects of increased activity of the comet’s nucleus. That’s going to take a few days or maybe a week to make these adjustments and all the while we’ll keep listening to Philae but we don’t plan to send it any commands yet. For that, we’ll need a longer and more stable communications link.”
Once you’ve established a better link, how are you going to restart the instruments?
Philippe Gaudon : “We’ve already devised the logic for switching on the instruments, giving initial priority to the least power-hungry ones and those that don’t need to move anything on board the lander and therefore won’t put it at risk. Then, we’ll start firing up those that draw a little more power but are stationary, like the cameras and chemical analysers. Only after that will we be able to start using the instruments with moving parts that might affect Philae’s stability.”
Do the data received last Saturday [13 June] contain any information about the battery’s health?
Philippe Gaudon : “Yes, and the good news is that the battery’s compartment is at a temperature of around 0°C and it would appear to be charging. Over the past few months, since we were facing the risk that Philae might not be able to generate enough power at its location next to a steep cliff with lots of rocks that could be casting shadows and in its tilted position, we’d devised an operating mode that would draw power directly from the solar panels without having to first charge the battery. But analysis of the 300 packets of data received Saturday shows that Philae must have been powering up and down for some time, and that it has warmed its battery compartment and is probably charging. So, if we leave it to charge over the next few days we might be able to consider resuming science activities. That’s how the lander was designed to operate and the LCC has no intention of changing that.”
That would presumably make it possible to conduct longer science sequences?
Philippe Gaudon : “It would be great news for the drilling tool in particular, since it would’ve been very hard to drill with just the direct power from the solar panels. So, yes, if it’s confirmed that extra power is going to be very welcome indeed!”
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, the German Aerospace Centre, the Max Planck Institute for Solar System Research (MPS), CNES and ASI, the Italian space agency. Rosetta is the first mission in history to orbit a comet, escort it on its course around the Sun and deploy a lander on its surface.