Saturday, June 13, 2015

Wayward Philae: The Search for ESA’s Stranded Comet Lander Continues

Is the bright spot really the Philae lander? Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Seven months without any word from Philae and counting. ESA scientists are still trying to determine at least the current location of the lander, after it touched down on the comet 67P/Churyumov-Gerasimenko on Nov. 12, 2014 and all of a sudden, it went silent three days later. “We have identified several possible lander candidates in OSIRIS images, both inside the CONSERT [COmet Nucleus Sounding Experiment by Radiowave Transmission] region of interest and nearby,” said Holger Sierks, principal investigator of the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS). The OSIRIS instrument is onboard the Rosetta spacecraft that released Philae and orbits the comet, continuously imaging it.

Philae’s final landing spot may be far more distant than it was estimated before Rosetta arrived at the comet. During the landing, Philae’s harpoons did not deploy and the lander rebounded for an additional two-hour flight before finally coming to rest at an unknown location. The scientists believe that it has travelled more than a kilometer from its initial contact point.

For months ESA has been studying high-resolution imagery of the comet’s terrain to locate the lander. Now, the mission scientists think they may have found the lander, resting on a frigid terrain of the comet.

“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,” said Philippe Lamy, member of the OSIRIS team. “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.”

But the scientists are not absolutely sure that this bright spot is really Philae. Rosetta’s closer flybys will be needed to provide higher-resolution imaging. However, due to the recent high cometary activity, when jets of gas and dust could be hazardous for the spacecraft’s navigation systems, the idea has been put on hold.

‘Before’ and ‘after’ comparison images of a promising lander candidate located near the CONSERT ellipse as seen in images from the OSIRIS Narrow-Angle Camera. Each box covers roughly 20 x 20 m on Comet 67P/Churyumov–Gerasimenko.  The left-hand image shows the region as seen on 22 October (before the landing of Philae) from a distance of about 10 km from the centre of the comet, while the centre and right-hand images shows the same region on 12 and 13 December from 20 km (after landing). The candidate is seen only in the two later images.  The illumination conditions are broadly similar in the three images and the same topography can be recognised in each case. The difference in distance at which the images were taken yields a difference in resolution and thus the December images have been resampled and interpolated to match the scale of the October image. As a result, the candidate covers more pixels calculated for a Philae-sized object seen by the OSIRIS narrow-angle camera from a distance of 18 km to the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
‘Before’ and ‘after’ comparison images of a promising lander candidate located near the CONSERT ellipse as seen in images from the OSIRIS Narrow-Angle Camera. Each box covers roughly 20 x 20 m on Comet 67P/Churyumov–Gerasimenko. The left-hand image shows the region as seen on 22 October (before the landing of Philae) from a distance of about 10 km from the centre of the comet, while the centre and right-hand images shows the same region on 12 and 13 December from 20 km (after landing). The candidate is seen only in the two later images. The illumination conditions are broadly similar in the three images and the same topography can be recognised in each case. The difference in distance at which the images were taken yields a difference in resolution and thus the December images have been resampled and interpolated to match the scale of the October image. As a result, the candidate covers more pixels calculated for a Philae-sized object seen by the OSIRIS narrow-angle camera from a distance of 18 km to the surface. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

"It is extremely difficult to locate the lander in the uneven terrain and to know for sure if 'that is Philae'," said Ekkehard Kührt, a planetary scientist at DLR (German Aerospace Center) and a member of the OSIRIS team. "In addition, the Rosetta orbiter has had to increase its distance from the comet for safety reasons, due to the rising activity of the comet."

The other possibility of determining Philae’s location would come if the lander were to receive enough power to wake-up from its hibernation and resume its scientific study. Then, the CONSERT instrument could be used to perform additional ranging measurements and gives us a clue to the lander’s location.

For this, it needs to be able to generate at least five watts of power and have an operating temperature above minus 45 degrees Celsius. Then, the lander will switch itself into operating mode. Slightly more energy – a total of 19 watts – is needed to communicate with the DLR team on the ground.

"The conditions for Philae's wake-up are becoming more and more favorable as the comet approaches the Sun," said Lander Project Manager Stephan Ulamec. "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."

In the coming weeks, the team will continue to search through all available data. For now, Rosetta is constantly observing the comet from a range of distances as the comet’s activity increases.

Rosetta is an ESA mission with contributions from its Member States and NASA. Rosetta's Philae lander is funded by a consortium headed by DLR, the Max Planck Institute for Solar System Research (MPS), the French Space Agency (CNES) and the Italian Space Agency (ASI).

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