The ESA’s attempt to land a probe on Comet 67P/Churyumov-Gerasimenko didn’t go as planned, but the mission has been far from a failure. A recent analysis of Philae’s harrowing journey across the comet has revealed some fascinating clues about its surface, while providing critical insights for future comet missions.
A meticulous analysis of Philae’s first several hours on comet 67P/Churyumov-Gerasimenko shows that its initial “landing” transpired on a rather precarious spot. When Philae first made touchtown, a spot dubbed Agilkia, its legs sunk to a depth of about 0.82 feet (0.25 meters), indicating that it made contact on a soft, granular surface. Eventually, the lander struck more solid ground causing it to bounce back up into space for nearly two hours before finally settling down on the surface. Philae’s final resting spot, called Abydos, appears to have a much harder surface. The details of these findings can now be found at the journal Science.
It’s impossible to study the finer details of this comet’s surface features from Earth, but Philae’s landing—or rather, more specifically, its bouncing—presented a unique opportunity for scientists to do just that. By studying its surface properties, scientists are gaining a better understanding of the formation, evolution, and activity of comets. At the same time, it will inform future comet missions and (hopefully) prevent the next comet lander from experiencing the same misfortunes.
On November 12, 2014, Philae reached the comet surface. The plan was to have it activate a cold gas system, which would push it down onto the surface. In addition, the lander was supposed to fire two anchoring harpoons to secure it to the ground. Regrettably, neither of these systems worked. When Philae reached the touchdown site, it was traveling at 1.0 m/s. It bounced several times before coming to rest after an eventful two hour journey.
Landing points (SONC) on a NAVCAM image are indicated. A second landing spot could not be modeled, so only an example is given. The ESA researchers believe Philae’s last hop was only a few meters. (Credits: ESA/ROSETTA/NAVCAM/SONC/DLR)
An ESA research team led by Jens Biele and Stephan Ulamec studied the precise circumstances under which Philae landed and bounced across the surface. By analyzing engineering and operational instrument data captured by the probe, the researchers were able to piece together a timeline of the lander’s journey, while also gleaning important insights into the comet’s mechanical properties, such as surface strengths and layering. The ESA researchers were able to analyze the bounce dynamics at surprisingly high resolutions, ranging from ~10 cm to 1 meter.
The ESA’s Lander Control Center first realized that Philae had bounced when data from its solar generator indicated that the lander was rotating. Data from its telemetry instruments, thermal mapper (MUPUS TM), and plasma monitor (ROMAP) also indicated movement. Despite the unexpected situation, mission planners proceeded by activating all scientific instruments. But because Philae settled in an apparent shadow of a cliff, it could not extract enough solar energy to survive; its batteries finally ran out of power on November 15.
“We refer to the event at ~16:20 as a collision, not a touchdown, because no vertical deceleration of the ROMAP boom was detected,” write the researchers in the study. “After this encounter, Philae started to tumble.” (Credits: J. Biele et al., 2015)
Philae’s exact location is still unclear, but its final resting place has been narrowed down to an area 150 meters long by 15 meters wide.
Philae’s final resting place? (Credits: ESA/Rosetta/NAVCAM/CONSERT – CC BY-SA IGO 3.0)
Two different teams independently reconstructed the descent and bouncing trajectory of Philae: RMOC (Rosetta Mission Operations Centre, Flight Dynamics) and SONC (the Lander Science Operations and Navigations Centre). These teams used similar data sets, but used slightly different comet models and constraints. Both reached similar conclusions about the final landing site. Though no visual confirmation has yet to be established, solar patterns, such like day length, sunrise, and sunset taken by solar cell output, seem to confirm the spacecraft’s surmised location.
The ESA scientists had other data to work with as well. The OSIRIS and NAVCAM imagers aboard Philae’s mothership, Rosetta, were used to reconstruct the trajectory. Incredibly, one of the images even shows the “footprints” at the initial touchdown site, while another image shows the dust cloud produced by the impact.
That Sinking Feeling
After piecing all this information together, the ESA scientists were able to make certain inferences about the comet surface.
Extrapolated lander position at the initial site, Agilkia. (Credits: J. Biele et al., 2015)
By performing an energy balance analysis, the researchers were able to determine the amount of mechanical energy dissipated into the comet soil during the initial impact. Philae released 49.5 J of energy, a fraction of which, 5.0 J, remained within the lander after the first bounce. Its velocity was reduced from 1.0 to 0.32 m/s as somewhere between 23 an d 39 J of impact energy was transferred to the ground (friction and damping inside the lander accounted for the rest).
This means that the comet surface is strongly damping, sucking up about 50 to 80% of the lander’s kinetic energy. This soft, mushy layer is estimated to have a maximum thickness about a quarter-meter thick.
This granular soft surface, or regolith, is estimated to have a compressive strength of one kilopascal, possibly on top of a more rigid layer. And indeed, after penetrating the soft layer, one of Philae’s feet appears to have struck a solid layer of unknown thickness and compressive strength.
Caught Between a Rock and a Hard Place
Philae’s ice screws, which are located at its feet, did not penetrate the surface, indicating that the ground at Abydos, Philae’s final resting site, is much harder than at Agilkia, where it first touched down. The ground at the former is estimated to have a crushing strength of at least 2 MPa. This hardness might explain why only one of Philae’s legs was able to anchor to this latter surface, and partially at that.
“[This] layer appears to be strong and/or thick enough to withstand the dynamical loads exerted by Philae’s feet, because breakage of that layer and transition to the presumably soft granular material beneath is not seen in the data,” write the researchers in the study.
The researchers speculate that
This layer may be similar to the possible subsurface “competent” layer at Agilkia. The unexpectedly high strength of the hard material is similar to that of the “sintered layer” found in the Kometensimulation (KOSI) laboratory experiments on insolated cometary analog materials in vacuum (1), suggesting that the cometary matter near the surface may be processed and thus not representative of the pristine state after formation.
Interestingly, the two sites are situated in two very different morphological regions. One appears to be covered by a smooth “dust” layer with a brittle underlayer, while the other has an exposed consolidated surface with minimal bouldering.
Fascinating stuff. This comet clearly features a dynamic and complex surface. Armed with this information, future planners will have to figure out a better way to land a probe on an object with such a challenging terrain.
Read the entire study at Science: “The landing(s) of Philae and inferences about comet surface mechanical properties”.