Ivanka Pelivan

Thermal and mechanical properties of the near-surface layers of comet 67P/Churyumov-Gerasimenko

Thermal and mechanical material properties determine comet evolution and even solar system formation because comets are considered remnant volatile-rich planetesimals. Using data from the Multipurpose Sensors for Surface and Sub-Surface Science (MUPUS) instrument package gathered at the Philae landing site Abydos on comet 67P/Churyumov-Gerasimenko, we found the diurnal temperature to vary between 90 and 130 K. The surface emissivity was 0.97, and the local thermal inertia was 85 ± 35 J m−2 K−1s-1/2. The MUPUS thermal probe did not fully penetrate the near-surface layers, suggesting a local resistance of the ground to penetration of >4 megapascals, equivalent to >2 megapascal uniaxial compressive strength. A sintered near-surface microporous dust-ice layer with a porosity of 30 to 65% is consistent with the data.

The structure of the regolith on 67P/Churyumov-Gerasimenko from ROLIS descent imaging

The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle “splashing” is responsible for the observed formations.

Collaborative Development and Cataloging of Simulation and Calculation Models for Space Systems

The application of modeling and simulation in the design, development and validation process of complex systems has significantly increased in the last decades. Creating high quality models is a time-consuming task. Particularly, if models should be shared and reused in future projects. In this paper, results of the project Simulation Model Library (SimMoLib) are presented that address the issues concerning the preservation of knowledge that lies within simulation and calculation models. SimMoLib provides modeling guidelines and best practices to help the developer to prepare models that can be reused in other contexts. Validation and verification of these models is covered by proposing a set of guidelines and two test frameworks as well as by promoting peer reviews of models by other experts. To archive, catalogue, and distribute models, a software framework is under development which supports the creation, management, retrieval, and utilization of models. In order to allow the collaborative editing of calculation and simulation models a simplified version control mechanism is established. SimMoLib is currently targeted to support the development of space systems. In the future it will be opened to other domains.

Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera

After coming to rest on the night side of comet 67P/Churyumov-Gerasimenko, the ROLIS camera on-board Rosetta’s Philae lander acquired five images of the surface below the lander, four of which were with the aid of LED illumination of different colors. The images confirm that Philae was perched on a sloped surface. A local horizon is visible in one corner of the image, beyond which we can see the coma. Having spent a full day on the surface Philae was commanded to lift and rotate, after which a final, sixth, LED image was acquired. The change in perspective allowed us to construct a shape model of the surface. The distance to the foreground was about 80 cm, much larger than the nominal 30 cm. This caused stray light, rather than directly reflected LED light, to dominate the image signal, complicating the analysis. The images show a lumpy surface with a roughness of apparently fractal nature. Its appearance is completely different from that of the first landing site, which was characterized by centimeter to meter-sized debris (Mottola et al., 2015). We recognize neither particles nor pores at the image resolution of 0.8 mm per pixel and large color variations are absent. The surface has a bi-modal brightness distribution that can be interpreted in terms of the degree of consolidation, a hypothesis that we support with experimental evidence. We propose the surface below the lander to consist of smooth, cracked plates with unconsolidated edges, similar to terrain seen in CIVA images.

The Space-Time Asymmetry Research (STAR) program

Space-Time Asymmetry Research (STAR) is a proposed satellite mission that aims for significantly improved tests of fundamental space-time symmetry and the foundations of special and general relativity. In the current concept, STAR comprises a series of three subsequent missions with increasingly advanced instruments performing clock to clock comparisons. While the first STAR missions will perform Kennedy-Thorndike (KT) and Michelson-Morley (MM) experiments, later missions will focus on fundamental gravitational physics by precision measurement of gravitational redshift, time dilation and Local Position Invariance (LPI). Compared to previous experimental accuracy, STAR aims for an improvement of at least two orders of magnitude. The STAR1 mission will measure the constancy of the speed of light to one part in 10-17 and derive the Kennedy Thorndike coefficient of the Mansouri-Sexl test theory to 7 × 10-10. The KT experiment will be performed by comparison of an atomic or molecular frequency reference with a length reference (highly stable cavity made e.g. from ultra low expansion (ULE) glass ceramics) during flight around Earth with an orbital velocity of 7 km/s. The corresponding sensitivity to a boost dependent violation of Lorentz invariance as modeled by the KT term in the Mansouri-Sexl test theory or a Lorentz violating extension of the standard model (SME) will be significantly enhanced as compared to Earth-based experiments. The space environment will enhance the measurement precision such that an overall improvement by a factor of 400 over current Earth bound experiments is expected. The STAR1 philosophy is to realize a fast, small - and therefore cheap - mission with a high scientific output, also providing the instrument technology and the spacecraft for the subsequent STAR missions, which plan to use different optical frequency standards. The 180 kg small satellite will be attitude, vibration and temperature controlled. The power consumption of the whole spacecraft will be less than 185 W. The launch of STAR1 is foreseen for 2015, the follow-on missions will be flown with an overlap with the previous mission by two to three years. Each mission has a maximum duration of 5 years (from mission set up to data acquisition) which permits students to experience the full mission lifecycle. Education and training of undergraduate and graduate students is a specific mission goal.