Philae photographed by Rosetta

Philae photographed by Rosetta

As it came within 2.7km of the surface of comet 67P, the European Space Agency’s (ESA) Rosetta spacecraft was finally able to photograph, without a shadow of a doubt, the Philae lander in the location where engineers believed it had come to rest on 12 November 2014.

“The Raider of the Lost Philae” could be the title of an adventure film with, this time, the Rosetta spacecraft playing the role of a space and robotic Indiana Jones searching, not for the Ark of the Covenant, but the 100kg lander which marked the first landing on a comet on 12 November 2014. And behind Rosetta, the entire mission team (engineers, scientists, ground controllers, etc.) who have never given up on discovering where Philae had indeed come to rest after its unexpected double bounce.

Rosetta - Philae

Philae on comet 67P uncovered by Rosetta on 2 September 2016. We can see the robot’s central body and its legs shining due to light from the Sun. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

In the video below from the French space agency (CNES), Phillip Gaudon explains Philae’s discovery by Rosetta.

The search for Philae

Let’s back up a little. Launched on 2 March 2004 by Ariane 5 from Kourou, Rosetta completed a 10-year journey before successfully reaching the comet 67P/Churyumov-Gerasimenko on 6 August 2014. Instead of flying past the nucleus of the fiery-tailed star, Rosetta became the first spacecraft to be placed in orbit around it. In addition to examining 67P with its instruments, on 12 November Rosetta released the Philae robot lander, which marked the first ever landing on a comet. On that day, the Cité de l’Espace in Toulouse played host to more than 6,000 people who came to follow the event as shown in the video clip below.

However, the arrival did not go as planned. On reaching the ground, Philae bounced and then travelled over a kilometre before coming to rest. Unfortunately, the lander was in shadow and unable to charge its batteries. At the Toulouse Space Centre belonging to CNES (the French space agency), the SONC (Space Operations and Navigation Centre) teams, led by Philippe Gaudon, reprogrammed the robot so that it would make maximum use of its instruments before it stopped working.
In the video below, Philippe Gaudon talks about this race against time in front of a full scale model of Philae, which was then on display at the Cité de l’Espace.

By analysing various data, engineers and scientists calculated the most likely places where Philae might be located. Rosetta took photos of the zones in question, but since it was around twenty kilometres from the nucleus, the resolution prevented definitive identification.

Rosetta - Philae

This comparison of images taken by Rosetta before (left, 22 October 2014) and after (12 and 13 December 2014) Philae’s landing show a shiny “object” which seems to have “arrived” between October and December. This was the most serious candidate for Philae’s location but the resolution meant that it was uncertain at the time. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Finding Philae: a scientific challenge

Things recently changed with Rosetta set to end its mission with a controlled crash on 30 September. In fact, the spacecraft is currently edging ever closer to 67P in preparation for this event. And on 2 September, when it was passing just 2.7 km from the surface of the cometary nucleus, Rosetta’s high-resolution OSIRIS camera captured the region that would seem to be most likely. Success!  With a resolution of 5 cm/pixel, the shape of Philae, which is about the size of a washing machine, can clearly be seen and this time, there is no doubt.
Philae‘s position in relation to the comet’s surface also explains why it was almost impossible to re-establish communications despite two brief radio signals received by Rosetta in June and July 2015.

Rosetta - Philae

The photo from Rosetta’s OSIRIS camera on 2 September 2016 vividly shows Philae on the small lobe of 67P, at the location where engineers and scientists thought it most likely that it had landed. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

However, finding Philae is not just satisfying a curiosity, it is also scientifically useful. Indeed, after coming to rest, Philae took many measurements. But since it bounced, it was no longer in the targeted location. Consequently, without information about its location, scientists could not place where the instrument had been working with any certitude. Since the nucleus of 67P is a complex and varied environment, this precision was however essential. It has now been validated thanks to Rosetta. As Matt Taylor, ESA’s Rosetta project scientist says, we can now “put Philae’s three days of science into proper context, now that we know where that ground actually is.”