One year ago today, millions of people around the world watched as the Philae lander became the first spacecraft to touch down on a comet.
There were celebrations at the European Space Agency (ESA) as the first images were received – followed by confusion, as it appeared the probe had not quite landed as planned, and had bounced twice on Comet 67P Churyumov-Gerasimenko. Its ice harpoons failed to fire and attach to the ground, causing Philae to rebound twice, before coming to rest under a cliff.
Even now, scientists are still piecing together what happened during the landing, and we still aren’t even sure exactly where Philae is on the surface. But despite the bumpy landing, Philae still performed more than 90 percent of the science that was planned. Now, scientists are hoping to hear from it again as the orbiting Rosetta spacecraft swoops lower.
So, what have we learned from Philae, and what more can we get from the intrepid lander? Let’s take a look.
This awesome time-lapse shows Philae's journey to the surface of Comet 67P from Rosetta. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
We still don’t know where it is
Comet 67P is big, but it’s not that big. It’s about the size of London, if the city was molded into the shape of a rubber duck (seriously). Considering that Philae and the orbiting Rosetta spacecraft have spoken to each other, and Rosetta can take high-resolution images of the surface, surely we should have found it by now? Not so.
The problem is that, although we know the general area Philae is in, a location 20 by 30 meters (65 by 100 feet), it's really hard to pick out (see image below). Also, as the comet reached its closest point to the Sun on August 13, 2015, Rosetta had to move further away to avoid debris spewing from its surface, meaning it has been out of range. It is now dipping below 200 kilometers (125 miles), and will come as low as 10 kilometers (6 miles) over the next year, when it will almost definitely be able to get an image of Philae.
“One of the challenges has been we have not been able to get close, because the comet’s activity has been problematic in terms of visual identification,” Matt Taylor, Rosetta project scientist, told IFLScience. “We will get that before the end of the mission next year, and hopefully a bit more local information [from Philae].”
Philae is somewhere in this ellipse. ESA
But it was a good thing it bounced
Philae’s original landing zone, in a vast plain known as Agilkia, would have been interesting. But, exposed to the rays of the Sun, the probe would have burned out by March of this year.
However, by bouncing twice on the surface and becoming wedged under a cliff, it was sheltered enough to survive the comet’s close approach to the Sun. This has provided the scientists with a fascinating opportunity to get even more data from the comet than originally planned, if Philae wakes up again.
“It was a very welcome bonus to the lander,” Rosetta Science Operations Manager Laurence O’Rourke told IFLScience. “Especially because, one year later, we believe Philae is still alive. This would not have happened if it had stayed in the location it touched down first.”
The bounce also allowed the probe to get data from different locations on the surface, including using its magnetometer in more than one place. More on that later.
Shown is the first bounce, as seen by Rosetta. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Eight of its ten instruments worked
In its 60 to 70 hours of initial operation, Philae was able to use 80% of its instruments. This included the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT), which worked with the Rosetta spacecraft to measure the comet’s interior.
The only instruments not to work completely were the Alpha Proton X-ray Spectrometer (APXS), which would have studied the chemical composition of the surface, and a camera on the Comet Nucleus Infrared and Visible Analyzer (CIVA), a suite of six cameras intended to take a panorama of the surface.
Otherwise, the science returned from Philae was pretty impressive, and earlier this year the first papers of research from the lander were published.
Philae used its camers to take this, the first image from the surface of a comet. ESA
It discovered the comet has no magnetic field
As mentioned earlier, the bounce of Philae provided a boost for the science of the mission. In fact, it enabled scientists working on the mission to make an unexpected discovery.
“As with any planetary observations, you can make measurements from afar, but the best thing is to get and touch it,” explained Taylor. “And for Philae, I think we had a wonderful result. Our first result combing data from Philae and Rosetta together was measuring the magnetic field, and this was only possible because of the bouncing. There were multiple points of measurement, giving us a much better global feel for the local magnetic field. It indicated there was no remnant magnetic field in the comet.”
So there you go.
Rosetta (illustrated) and Philae have worked together on discoveries. ESA
It might come back to life again
Philae went into hibernation three days after landing when its main batteries ran out of power. But it has a secondary battery, powered by its solar panels, that has intermittently brought it back to life. Although the comet is moving away from its closest point, there could still be enough sunlight for Philae to wake up. Now that Rosetta is being lowered, it could be able to hear from Philae again.
“There’s a chance of Philae coming back to life for a third time in the coming months,” said Taylor. “It could come back this week, in fact, which would be a nice celebration, but I think the end of November is more likely, when we hope to be even closer.”
But the probe will be gone for good by January. This will be because the region it is in won’t have enough sunlight to keep it alive. So we’ve only got a few more months to make contact.
Rosetta, meanwhile, will remain in orbit until late in 2016, when it will be purposefully impacted with the surface.
Scientists at ESA broke into celebration when the probe landed a year ago. ESA
Philae isn’t done yet
If the lander wakes up, it will be instructed to begin a series of additional science experiments devised by the team on the ground. But it will also be told to take more images of its surroundings.
“There will be more pictures of the surface, around the lander, pictures underneath, and we’ll also run other instruments like the X-ray experiment,” said O’Rourke. “This is the plan, but it’s very much a case of when we can speak to the lander. We need good and steady communication.”
Comet 67P has now passed its closest point to the Sun. ESA/Rosetta/NAVCAM
But there’s no party planned
Both O’Rourke and Taylor said that there wasn’t any special celebration planned for the one year anniversary today, so unless they’re throwing a surprise party they didn’t want us to ruin, it appears that for most of the scientists at ESA it's business as usual.
“It’s just a day like any other day, just a year ago we did this thing that a lot of people are interested in,” Taylor said.
“I think, for me personally, it’s a day to celebrate the fact the lander is still alive, and to hope to hear from it,” added O’Rourke. “I’m more interested in celebrating when we hear from it again. It was a huge achievement for mankind in principle to land on the surface, but we are continuing to try and contact the lander, and the mission isn’t over.”
Oh well. We’ll celebrate for you. Happy birthday, Philae!
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