Koen Geurts

The landing(s) of Philae and inferences about comet surface mechanical properties

The Philae lander, part of the Rosetta mission to investigate comet 67P/Churyumov-Gerasimenko, was delivered to the cometary surface in November 2014. Here we report the precise circumstances of the multiple landings of Philae, including the bouncing trajectory and rebound parameters, based on engineering data in conjunction with operational instrument data. These data also provide information on the mechanical properties (strength and layering) of the comet surface. The first touchdown site, Agilkia, appears to have a granular soft surface (with a compressive strength of 1 kilopascal) at least ~20 cm thick, possibly on top of a more rigid layer. The final landing site, Abydos, has a hard surface.

Rosetta Lander: On-Comet Operations Preparation and Planning

The ESA Rosetta mission is a European cornerstone mission that will study the evolution of comet 67P/Churyumov-Gerasimenko on its trajectory around the Sun and will include the delivery of the Philae lander for in-situ investigation of the comet nucleus and sub-surface. Philae is funded and operated by a European consortium, and consists of an independent spacecraft platform including 10 scientific instruments. Philae operations are scheduled and implemented by the Science Navigation and Operations Center (SONC) at CNES/Toulouse and the Lander Control Center (LCC) at DLR/Cologne. Rosetta was launched in March 2004 and conducted a 7 year cruise phase, after which a 3 year long deep space hibernation phase followed, in order to cope with the large Sun distances around the trajectory aphelion. In January 2014 Rosetta was woken up and the comet rendezvous phase commenced. Landing is foreseen for November 2014 to a landing site which will be selected during the preceding three month landing site selection process. The final descent trajectory and postdelivery Rosetta trajectory, with associated Philae – Rosetta communication windows, will be computed 30 days prior lander delivery. The Philae operations team must ensure that the baseline descent and science timelines developed over the timeframe 2011 – 2013 are adaptable in order to cope with the intrinsic uncertainties. Furthermore, Philae will conduct a long term science phase during which various scientific measurements are performed in order to characterize the evaluation of the nucleus and subsurface as a function of the decreasing Sun distance. The operational process and procedure developed by the Philae operations team will allow for the rapid adaptation to landing site specific features such as the comet local landing epoch, day/night variations and communication window uncertainties.

Rosetta-Lander: On-Comet Operations Execution and Recovery after the Unexpected Landing

Philae’s landing on comet 67P/Churyumov–Gerasimenko on 12 November 2014 was one of the main milestones of the European Rosetta mission. The nature of Philae’s mission, to land, operate and survive on comet 67P, required a high degree in autonomy of the on-board software and of the operations scheduling and execution concept. Philae’s baseline operations timeline consisted of predefined and validated blocks of instrument deployments and scientific measurements. These were supported by subsystem activities such as rotation and lifting of the main body relative to the landing gear to allow for specific instrument deployment or in order to cope with the unknown attitude after landing. The nominal descent was followed by an unforeseen rebound at touchdown, lifting Philae again from the comet surface to enter a two-hour phase of uncontrolled flight over the comet surface. Philae’s unknown final landing site, unfavorable attitude with respect to the local surface, bad illumination and lack of anchoring required a complete rescheduling of the baseline timeline. The autonomy offered by the system and the predefined contingency operations were exploited by the operations team to maximize output despite this undesirable state. Implementation of the rescheduling to allow a maximum scientific output, despite the limitations due to unknown communication windows, unknown orientation with respect to the comet surface, the associated risks of any mechanisms activation, the lack of sufficient solar power and limited battery lifetime, is described and elaborated.