Photo of Comet 67P/C-G taken by Rosetta on August 6, 2014. Credit: ESA
In August 2014, the ESA’s Rosetta spacecraft arrived at its destination, Comet 67P/Churyumov-Gerasimenko, after a 10 year journey. Rosetta carried a small companion, the Philae Lander. On November 12th, Philae was sent to the surface of Comet 67P. Unfortunately, things didn’t go exactly as planned, and the lander’s mission lasted only 63 hours.
During that time, it gathered what data it could. But mission scientists weren’t certain of its precise location, meaning its data was difficult to interpret accurately. Only when scientists knew precisely where Philae was located on the comet, could they make best use of all of its data.
Left image from Rosetta’s OSIRIS narrow-angle camera shows the Philae lander on November 12, 2014 after it left the spacecraft for the comet's nucleus. Right: Close-up of a promising candidate for the lander photographed on December 12. Copyright: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
It’s only a bright dot in a landscape of crenulated rocks, but the Rosetta team thinks it might be Philae, the little comet lander lost since November.
The Rosetta and Philae teams have worked tirelessly to search for the lander, piecing together clues of its location after a series of unfortunate events during its planned landing on the surface of Comet 67P/Churyumov-Gerasimenko last November 12.
Mosaic photo capturing Philae’s flight above the comet’s nucleus and one of its three touchdowns on November 12, 2014. The images cover a 30 minute period spanning the first touchdown. The Greenwich Mean Time time of each of image is marked on the corresponding insets. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Philae first touched down at the Agilkia landing site that day, but the harpoons that were intended to anchor it to the surface failed to work, and the ice screws alone weren’t enough to do the job. The lander bounced after touchdown and sailed above the comet’s nucleus for two hours before finally settling down at a site called Abydos a kilometer from its intended landing site.
No one yet knows exactly where Philae is, but an all-out search has finally turned up a possible candidate.
Approximate locations of five lander candidates initially identified in high-resolution photos taken in December 2014, from a distance of about 12.4 miles (20 km) from the comet’s center. The candidates identify Philae-sized features about 3-6 feet (1-2 meters) across. The contrast has been stretched in some of the images to better reveal the candidates. All but one of them (top left) have subsequently been ruled out. The candidate at top left lies near to the current CONSERT ellipse (see below). Credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0; insets: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Rosetta’s navigation and high-resolution cameras identified the first landing site and also took several pictures of Philae as it traveled above the comet before coming down for a final landing. Magnetic field measurements taken by an instrument on the lander itself also helped establish its location and orientation during flight and touchdown. The lander is thought to be in rough terrain perched up against a cliff and mostly in shadow.
High resolution images of the possible landing zone were taken by Rosetta back in December when it was about 11 miles (18 km) from the comet’s surface. At this distance, the OSIRIS narrow-angle camera has a resolution of 13.4 inches (34 cm) per pixel. The body of Philae is just 39 inches (1-meter) across, while its three thin legs extend out by up to 4.6 feet (1.4-meters) from its center. In other words, Philae’s just a few pixels across — a tiny target but within reach of the camera’s eye.
The current 50 x 525 feet (16 x 160 m) CONSERT ellipse overlaid on an OSIRIS narrow-angle camera image of the same region. It’s believed Philae is located within or near this ellipse. Copyright Ellipse: ESA/Rosetta/Philae/CONSERT; Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The candidates in the photo above are “all over the place.” To narrow down the location, the Rosetta team used radio signals sent between Philae and Rosetta as part of the COmet Nucleus Sounding Experimentor CONSERT after the final touchdown. According to Emily Baldwin’s recent posting on the Rosetta site:
“Combining data on the signal travel time between the two spacecraft with the known trajectory of Rosetta and the current best shape model for the comet, the CONSERT team have been able to establish the location of Philae to within an ellipse roughly 50 x 525 feet (16 x 160 meters) in size, just outside the rim of the Hatmehit depression.”
Zooming in to the CONSERT ellipse, a number of bright dots are seen in the region. Since only one could be the lander, the majority must be associated with surface features on the comet nucleus. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
So what can we see there? Zooming in closer, a number of glints or bright spots appear, and they change depending on the viewing angle. But among those glints, one might be Philae. What mission scientists examined images of the area under the same lighting conditions before Philae landed and then put them side by side with those taken after November 12. That way any transient glints could be eliminated, leaving what’s left as a potential candidate.
‘Before’ and ‘after’ comparison images of a promising candidate located near the CONSERT ellipse as seen in images from Rosetta. Each box covers roughly 65×65 feet (20 x 20 m) on the comet. The left-hand image shows the region as seen on 22 October (before the landing of Philae) from a distance of about 6 miles from the center of the comet, while the center and right-hand images show the same region on December 12 and 13 from 12 miles (20 km) after landing. The candidate is only seen in the two later pictures. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
In photos taken on December 12 and 13, a bright spot is seen that didn’t appear in the earlier photos. Might this be Philae? It’s possible and the best candidate yet. But it may also be a new physical feature that developed between November and December. Comet surfaces are forever changing as sunlight sublimates ice both on and beneath the surface
For now, we still can’t be sure if we’ve found Philae. Higher resolution pictures will be required as will patience. The comet’s too close to the Sun right now and too active. Rubble flying off the nucleus could damage Rosetta’s instruments. Mission scientists will have to wait until well after the comet’s August perihelion (closest approach to the Sun) for a closer look.
Magnificent! Comet 67P/Churyumov-Gerasimenko photographed by Rosetta from about 125 miles away on June 5, 2015. Now only two months from perihelion, the comet’s crazy with jets of dust and gas. One wonders what the chances are of a gassy geyser erupting beneath or near Philae and sending it back into orbit again. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Meanwhile, mission teams remain hopeful that with increasing sunlight at the comet this summer, Philae’s solar panels will recharge its batteries and the three-legged lander will wake up and resume science studies. Three attempts have been made to contact Philae this spring and more will be made but so far, we’ve not heard a peep.
For the time being, Philae’s like that lost child in a shopping mall. The search party’s been dispatched, clues have been found and it’s only a matter of time before we see her smiling face again.
Philae's MUPUS probe took temperature measurements and hammered into the surface at the landing site to discover the lander alighted on some very hard ice. Credit: ESA
An uncontrolled, chaotic landing. Stuck in the shadow of a cliff without energy-giving sunlight. Philae and team persevered. With just 60 hours of battery power, the lander drilled, hammered and gathered science data on the surface of comet 67P/Churyumov-Gerasimenko before going into hibernation. Here’s what we know.
Despite appearances, the comet’s hard as ice. The team responsible for the MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science) instrument hammered a probe as hard as they could into 67P’s skin but only dug in a few millimeters:
Close-up of the first touchdown site before Philae landed (left) and after clearly shows the impressions of its three footpads in the comet’s dusty soil. At the final landing site, it’s believed that Times are CST. Philae’s 3.3 feet (1-m) across. Credit: ESA
“Although the power of the hammer was gradually increased, we were not able to go deep into the surface,” said Tilman Spohn from the DLR Institute of Planetary Research, who leads the research team. “If we compare the data with laboratory measurements, we think that the probe encountered a hard surface with strength comparable to that of solid ice,” he added. This shouldn’t be surprising, since ice is the main constituent of comets, but much of 67P/C-G appears blanketed in dust, leading some to believe the surface was softer and fluffier than what Philae found.
This finding was confirmed by the SESAME experiment (Surface Electrical, Seismic and Acoustic Monitoring Experiment) where the strength of the dust-covered ice directly under the lander was “surprisingly high” according to Klaus Seidensticker from the DLR Institute. Two other SESAME instruments measured low vaporization activity and a great deal of water ice under the lander.
As far as taking the comet’s temperature, the MUPUS thermal mapper worked during the descent and on all three touchdowns. At the final site, MUPUS recorded a temperature of –243°F (–153°C) near the floor of the lander’s balcony before the instrument was deployed. The sensors cooled by a further 10°C over a period of about a half hour:
The location of Philae’s first touchdown on the surface of Comet 67P/C-G. Although covered in dust in many areas, Philae found strong evidence for firm ice beneath the comet’s surface. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
“We think this is either due to radiative transfer of heat to the cold nearby wall seen in the CIVA images or because the probe had been pushed into a cold dust pile,” says Jörg Knollenberg, instrument scientist for MUPUS at DLR. After looking at both the temperature and hammer probe data, the Philae team’s preliminary take is that the upper layers of the comet’s surface are covered in dust 4-8 inches (10-20 cm), overlaying firm ice or ice and dust mixtures.
The ROLIS camera (ROsetta Lander Imaging System) took detailed photos during the first descent to the Agilkia landing site. Later, when Philae made its final touchdown, ROLIS snapped images of the surface at close range. These photos, which have yet to be published, were taken from a different point of view than the set of panorama photos already received from the CIVA camera system.
During Philae’s active time, Rosetta used the CONSERT (COmet Nucleus Sounding Experiment by Radio wave Transmission) instrument to beam a radio signal to the lander while they were on opposite sides of the comet’s nucleus. Philae then transmitted a second signal through the comet back to Rosetta. This was to be repeated 7,500 times for each orbit of Rosetta to build up a 3D image of 67P/C-G’s interior, an otherworldly “CAT scan” as it were. These measurements were being made even as Philae lapsed into hibernation. Deeper down the ice becomes more porous as revealed by measurements made by the orbiter.
Rosetta’s Philae lander includes a carefully selected set of instruments to analyze a comet’s surface. Credit: ESA, Composite – T.Reyes
The last of the 10 instruments on board the Philae lander to be activated was the SD2 (Sampling, Drilling and Distribution subsystem), designed to provide soil samples for the COSAC and PTOLEMY instruments. Scientists are certain the drill was activated and that all the steps to move a sample to the appropriate oven for baking were performed, but the data right now show no actual delivery according to a tweet this morning from Eric Hand, reporter at Science Magazine. COSAC worked as planned however and was able to “sniff” the comet’s rarified atmosphere to detect the first organic molecules. Research is underway to determine if the compounds are simple ones like methanol and ammonia or more complex ones like the amino acids.
Stephan Ulamec, Philae Lander manager, is confident that we’ll resume contact with Philae next spring when the Sun’s angle in the comet’s sky will have shifted to better illuminate the lander’s solar panels. The team managed to rotate the lander during the night of November 14-15, so that the largest solar panel is now aligned towards the Sun. One advantage of the shady site is that Philae isn’t as likely to overheat as 67P approaches the Sun en route to perihelion next year. Still, temperatures on the surface have to warm up before the battery can be recharged, and that won’t happen until next summer.
Let’s hang in there. This phoenix may rise from the cold dust again.
