Lars Witte

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.

Experimental Investigations of the Comet Lander Philae Touchdown Dynamics

The comet lander Philae (as part of Europe’s Rosetta mission) is en route to its target, 67/P Churyumov-Gerasimenko. With landing operations coming up at the end of 2014, a partial retesting of the Philae lander’s touchdown system was carried out in spring of 2013. Intensive testing was performed as part of Philae’s design and verification program approximately 10 years ago. However, the new test series specifically addresses touchdown conditions that have been out of capability of the pendulum test facility used at those times. Thus, the follow-up tests focus on touchdown conditions such as asymmetric loads, effects from terrain undulation, and the effect of granular soil mechanics, which could not be studied sufficiently in the original tests. This paper provides insight into the touchdown system of the Philae lander, the characteristics of the used test facility, its weight offloading operating mode, and the specific application to a small-body landing test. The results of the study are presented and d...

Stochastic Modeling of a Hazard Detection and Avoidance Maneuver – The Planetary Landing Case

Hazard Detection and Avoidance (HDA) functionalities, thus the ability to recognize and avoid potential hazardous terrain features, is regarded as an enabling technology for upcoming robotic planetary landing missions. In the forefront of any landing mission the landing site safety assessment is an important task in the systems and mission engineering process. To contribute to this task, this paper presents a mathematical framework to consider the HDA strategy and system constraints in this mission engineering aspect. Therefore the HDA maneuver is modeled as a stochastic decision process based on Markov chains to map an initial dispersion at an arrival gate to a new dispersion pattern affected by the divert decision-making and system constraints. The implications for an efficient numerical implementation are addressed. An example case study is given to demonstrate the implementation and use of the proposed scheme.

The Network Infrastructure for the ROBEX Demomission Space

The demonstration mission space of the alliance Robotic Exploration of Extreme Environments (ROBEX) was a campaign on Mt. Etna in summer 2017. The network infrastructure and parts of the ground segment for this demonstration mission space were set up by the Mobile Rocket Base (MORABA) of the DLR. The ground segment included a control center which was based near Catania, Italy. Various terminals allowed for controlling a lander, a robotic rover and several experiment carriers which have been placed in 23 km airline distance on Mt. Etna. The distance was bridged by a radio link between the control center and a base camp at the demonstration site. From the base camp a shorter radio link of several hundreds of meters to the lander was established, and from there, the signal was distributed using several access points.