The Philae lander, the first spacecraft designed to make a controlled landing on a comet, failed to latch onto the surface, bouncing off the frozen body. New research reveals the vehicle likely scraped the side of a crater as it came to rest on the nucleus of the comet.
Data from the Rosetta Lander Magnetometer and Plasma Monitor (Romap) on-board the Philae lander is being studied, in an attempt to reconstruct what happened as the spacecraft bounced off the comet.
As Philae glided toward Comet 67P/Churyumov-Gerasimenko, mission planners monitored magnetic fields between the vehicle and its robotic companion. Small magnetic fields are generated by the vehicle and Rosetta, the orbiting "mother ship" for Philae, and their interaction can be seen in data. Astronomers normally remove this effect from readings, in order to eliminate "noise" from cometary effects and solar wind.
"Any motion of Philae in a magnetic field - even if it is small - can be seen by field changes in the measured magnetic field direction," Hans-Ulrich Auster of the Technische Universität Braunschweig, in Germany, and Romap co-principal investigator, said.
According to the new analysis, Philae successfully separated from Rosetta, rotating once every five minutes. The rotation later achieved a planned deceleration to one rotation every 8.5 minutes. At this point, the landing gear successfully deployed. Soon after, Romap was deployed on a boom extending from Philae. For seven hours, landing appeared to be going according to plan. At 10:34 a.m. EST, instruments recorded initial touchdown on the cometary surface.
Readings from Romap indicate the rotational rate of the lander suddenly increased after touchdown, as the vehicle bounced off the surface. A flywheel in the lander was turned off by control electronics, and its angular momentum was transferred to the vehicle. For the next 40 minutes, the wayward spacecraft was spinning once every 13 seconds.
At 11:20 EST, Philae came into contact once more with the cometary surface. The patterns seen in the Romap data are consistent with collision between the vehicle and a crater rim or other outcropping of material.
"We think that Philae probably touched a surface with one leg only - perhaps grazing a crater rim - and after that the lander was tumbling," Hans-Ulrich told the press.
Philae stayed above the comet, tumbling once every 24 seconds, until striking the icy surface once more, at 12:25 EST. The vehicle likely came to rest on all three feet, near the site where it struck the comet for the final time.
"It was really an exciting and almost unbelievable excursion," Hans-Ulrich said.
Mission planners are still searching for the final resting spot of Philae, which is currently in hibernation mode, unable to gather enough solar energy to conduct experiments. Researchers hope the vehicle may come back to operation in March 2015, as the spacecraft and comet approaches the Sun.
Correction: Second contact made by Philae at 11:20 EST.
