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Is this Philae? Rosetta team find candidate for lost lander

Paul Sutherland, Feature writer
Jun 11, 2015, 23:20 UTC

Sen—Rosetta scientists say they may have located their lost lander Philae on the rugged surface of Comet 67P/Churyumov-Gerasimenko.

Since the washing-machine sized probe bounced onto the surface of the comet in November, its exact wherabouts have been a mystery. The team suspected it had ended up in the shadow of a cliff where lack of sunlight caused its batteries to die.

For the last three months, Rosetta has been listening for signals from Philae, in case improved lighting conditions closer to the Sun have allowed it to wake from hibernation. At the same time, the ground team have been scouring images of the surface to see if they can spot the lander.

A number of bright spots have been ruled out as Philae on close inspection. But tantalisingly, one very close to the area where it is calculated to be remains as a possible candidate for the lander. It is not visible in images taken on Oct. 22th, 2014, before the landing, but shows up in photos taken on both Dec. 12th and 13th.

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Images taken before and after Philae's landing show the bright spot that could be the missing craft. Image credit:ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Philae was supposed to land at a site called Agilkia on the comet’s head on Nov. 12th but its harpoons failed to fire to secure it and it bounced twice over two hours before coming to rest in a region called Abydos. Images were captured by both Rosetta’s OSIRIS and NAVCAM cameras showing it in flight after the first bounce.

Further data revealing its movements was gained from other instruments, including magnetic field measurements from ROMAP on board Philae, and radio signals from both spacecraft that were part of the CONSERT experiment. CONSERT suggested that Philae would be found within an ellipse measuring 16 metres by 160 metres.

The Rosetta team found a number of bright spots in the vicinity of the ellipse. Some are transient because they glint when catching sunlight, then go dark in other images. Most of the bright spots appear to be surface features on the comet’s nucleus, but one in particular caught the interest of scientists working with OSIRIS team member Philippe Lamy at the Laboratoire d’Astrophysique de Marseille (LAM) and the Institut de Recherche en Astrophysique et Planétologie (IRAP) in France, Emily Baldwin reports today on ESA’s Rosetta blog.

Lamy said in a statement: “Although the pre- and post-landing images were taken at different spatial resolutions, local topographic details match well, except for one bright spot present on post-landing images, which we suggest is a good candidate for the lander.

“This bright spot is visible on two different images taken in December 2014, clearly indicating that it is a real feature on the surface of the comet, not a detector artefact or moving foreground dust speck.”

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Images from Rosetta's OSIRIS camera show the lander drifting in front of the comet after its first bounce on Nov. 12th. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

ESA’s blog stresses that they cannot yet be sure that the candidate really is their lost lander. Analysis by the French space agency CNES’s Science Operations and Navigation Center (SONC) say it satisfies a number of conditions, such as illumination and radio visibility in this region. But it is also just outside the ellipse identified by CONSERT. It is also possible that the bright spot is actually material in the nucleus that has been freshly exposed as it becomes more active.

To confirm that the object is Philae will require Rosetta to get closer to the comet to obtain higher-resolution images, but the increased activity from its spurting jets of gas and dust mean that such close flybys might not be possible for some months. The other possibility is for Philae to wake up and to confirm where it is with its radio signals.

The Philae team are still hopeful that this will happen. Lander Project Manager Stephan Ulamec tells the blog: “The conditions for Philae’s wake-up are becoming more and more favourable as the comet approaches the Sun. The team at DLR’s Lander Control Center has continued to prepare long term operations for Philae and its instruments in the hope that it does wake up soon.”

A zoom in on the ellipse in which Philae was expected to be found. Credit:ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA