Philae Lander Research Articles

Two-point observations of low-frequency waves at 67P/Churyumov-Gerasimenko during the descent of PHILAE: comparison of RPCMAG and ROMAP

Abstract. The European Space Agency's spacecraft ROSETTA has reached its final destination, comet 67P/Churyumov-Gerasimenko. Whilst orbiting in the close vicinity of the nucleus the ROSETTA magnetometers detected a new type of low-frequency wave possibly generated by a cross-field current instability due to freshly ionized cometary water group particles. During separation, descent and landing of the lander PHILAE on comet 67P/Churyumov-Gerasimenko, we used the unique opportunity to perform combined measurements with the magnetometers onboard ROSETTA (RPCMAG) and its lander PHILAE (ROMAP). New details about the spatial distribution of wave properties along the connection line of the ROSETTA orbiter and the lander PHILAE are revealed. An estimation of the observed amplitude, phase and wavelength distribution will be presented as well as the measured dispersion relation, characterizing the new type of low-frequency waves. The propagation direction and polarization features will be discussed using the results of a minimum variance analysis. Thoughts about the size of the wave source will complete our study.

Light on Chirality: Absolute Asymmetric Formation of Chiral Molecules Relevant in Prebiotic Evolution.

Homochirality is a distinct property of living matter manifested by the unichiral molecular units of genetic material and of proteins, namely, d-deoxyribose and l-amino acids. These molecular subunits or their precursors have been shown to form under various prebiotic conditions in racemic form. However, the nature of the chiral influence, which results in the first breakage of molecular symmetry, remains unclear. In this respect, the photochemical model of enantioselection has gained particular importance in recent years. In this model, the interaction of circularly polarized light with racemic molecules generated in the interstellar medium is considered to be the main driving force of enantiomeric discrimination in early prebiotic evolution. Cometary ice simulations, l-enantio-enriched amino acids in meteorites, and the detection of circularly polarized electromagnetic radiation in star-forming regions provide evidence for photochirogenesis. The recent discovery of aldehydes in the cometary nucleus of 67P/Churyumov-Gerasimenko by means of the Cometary Sampling and Composition (COSAC) experiment on board the Rosetta lander Philae, along with the detection of aldehydes and aldopentoses (including chiral ribose) in simulated interstellar ice analogues, provide a direct link between laboratory simulations and cometary composition. In the context of these new findings, this review will provide an overview of the role of chiral molecules in prebiotic evolution with special emphasis on their chiroptical properties and absolute asymmetric photosynthesis.

Estimating the strength of the nucleus material of comet 67P Churyumov–Gerasimenko

Consideration is given to the estimates for the strength of the consolidated material forming the bulk of the nucleus of comet 67P Churyumov–Gerasimenko and those for the strength of the surface material overlying the consolidated material at the sites of the first and last contact of the Philae lander with the nucleus. The strength of the consolidated material was estimated by analyzing the terrain characteristics of the steep cliffs, where the material is exposed on the surface. Based on these estimates, the tensile strength of the material is in the range from 1.5 to 100 Pa; the shear strength, from ∼13 to ⩾30 Pa; and the compressive strength, from 30 to 150 Pa, possibly up to 1.5 kPa. These are very low strength values. Given the dependence of the measurement results on the size of the measured object, they are similar to those of fresh dry snow at –10°C. The (compressive) strength of the surface material at the site of the first touchdown of Philae on the nucleus is estimated from the measurements of the dynamics of the surface impact by the spacecraft’s legs and the geometry of the impact pits as 1–3 kPa. For comparison with the measurement results for ice-containing materials in terrestrial laboratories, it needs to be taken into account that the rate of deformation by Philae’s legs is four orders of magnitude higher than that in typical terrestrial measurements, leading to a possible overestimation of the strength by roughly an order of magnitude. There was an attemp to put one of the MUPUS sensors into the surface material at the site of the last contact of Philae with the nucleus. Noticeable penetration of the tester probe was not achieved that led to estimation of the minimum compressive strength of the material to be ⩾4 MPa4 This fairly high strength appears to indicate the presence of highly porous ice with grains “frozen” at contacts.

Dust Impact Monitor (SESAME-DIM) on board Rosetta/Philae: Millimetric particle flux at comet 67P/Churyumov-Gerasimenko

The Philae lander of the Rosetta mission, aimed at the in situ investigation of comet 67P/C-G, was deployed to the surface of the comet nucleus on 12 Nov 2014 at 2.99 AU heliocentric distance. The Dust Impact Monitor (DIM) as part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME) on the lander employed piezoelectric detectors to detect the submillimetre- and millimetre-sized dust and ice particles emitted from the nucleus. We determine the upper limit of the ambient flux of particles in the measurement range of DIM based on the measurements performed with the instrument during Philae's descent to its nominal landing site Agilkia at distances of about 22 km, 18 km, and 5 km from the nucleus barycentre and at the final landing site Abydos. The geometric factor of the DIM sensor is calculated assuming an isotropic ambient flux of the submillimetre- and millimetre-sized particles. For the measurement intervals when no particles were detected the maximum true impact rate was calculated by assuming Poisson distribution of the impacts, and it was given as the detection limit at a 95% confidence level. Based on measurements performed with DIM, the upper limit of the flux of particles in the measurement range of the instrument was of the order of $10^{-8}-10^{-7}m^{-2}s^{-1}sr^{-1}$ during descent and $1.6\cdot10^{-9}m^{-2}s^{-1}sr^{-1}$ at Abydos on 13 and 14 Nov 2014. Considering particle speeds below escape velocity, the upper limit for the volume density of particles in the measurement range of DIM was constrained to $10^{-11}m^{-3}-10^{-12}m^{-3}$. Results of the calculations performed with the GIPSI tool on the expected particle fluxes during the descent of Philae were compatible with the non-detection of compact particles by the DIM instrument.

Rosetta following a living comet

The International Rosetta Mission was launched on 2nd March 2004 on its 10 year journey to rendezvous with comet 67P Churyumov-Gerasimenko. Rosetta performed comet orbit insertion on the 6th of August 2014, after which it characterised the nucleus and orbited it at altitudes as low as a few kilometres. In November 2014 Rosetta delivered the lander Philae to perform the first soft landing ever on the surface of a comet. The critical landing operations have been conducted with remarkable accuracy and will constitute one of the most important achievements in the history of spaceflight.After this critical operation, Rosetta began the escort phase of the comet in its journey in the Solar System heading to the perihelion, reached in August 2015. Throughout this period, the comet environment kept changing with increasing gas and dust emissions. A first phase of bound orbits was followed by a sequence of complex flyby segments which allowed the scientific instruments to perform in depth investigation of the comet environment and nucleus. The unpredictable nature of the comet activity forced the mission control team to implement unplanned changes to the flight plan prepared for this mission phase and to plan the whole mission in a more dynamic way than originally conceived.This paper describes the details of the landing operations and of the main comet escort phase. It also includes the mission status as achieved after perihelion and the findings about the evolution of the comet and its environment from a mission operations point of view. The lessons learned from this unique and complex operations phase and the plans for the next mission phases, which include a mission extension into 2016, are also described.

Rosetta lander Philae: Flight Dynamics analyses for landing site selection and post-landing operations

On the 12th of November 2014, The Rosetta Lander Philae became the first spacecraft to softly land on a comet nucleus. Due to the double failure of the cold gas hold-down thruster and the anchoring harpoons that should have fixed Philae to the surface, it spent approximately two hours bouncing over the comet surface to finally come at rest one km away from its target site. Nevertheless it was operated during the 57h of its First Science Sequence. The FSS, performed with the two batteries, should have been followed by the Long Term Science Sequence but Philae was in a place not well illuminated and fell into hibernation. Yet, thanks to reducing distance to the Sun and to seasonal effect, it woke up at end of April and on 13th of June it contacted Rosetta again. To achieve this successful landing, an intense preparation work had been carried out mainly between August and November 2014 to select the targeted landing site and define the final landing trajectory. After the landing, the data collected during on-comet operations have been used to assess the final position and orientation of Philae, and to prepare the wake-up. This paper addresses the Flight Dynamics studies done in the scope of this landing preparation from Lander side, in close cooperation with the team at ESA, responsible for Rosetta, as well as for the reconstruction of the bouncing trajectory and orientation of the Lander after touchdown.

Software on a Comet: The Philae Lander's Central Onboard Computer

For the Philae lander to land on a comet after 10 years of space travel, its software had to meet rigorous requirements.

Rosetta lander Philae – Landing performance and touchdown safety assessment

This paper describes the simulation and analysis work on Rosetta lander Philae׳s touchdown dynamics which was done to support its Landing Site Selection Process. The simulation part consists of a numerical multi-body simulation to describe Philae׳s touchdown dynamics. Suitable performance metrics in conjunction with Monte Carlo trajectory data from the flight dynamics analysis yields landing area specific landing gear performance and safety figures. These were then incorporated into the site selection process with regard to landing system performance margins and touchdown safety. While Philae finally made a nearly successful landing the actual flight data were used to review and discuss the applicability of the presented simulation and analysis scheme.

ROSETTA lander Philae: Touch-down reconstruction

The landing of the ROSETTA-mission lander Philae on November 12th 2014 on Comet 67P/Churyumov-Gerasimenko was planned as a descent with passive landing and anchoring by harpoons at touch-down. Actually the lander was not fixed at touch-down to the ground due to failing harpoons. The lander internal damper was actuated at touch-down for 42.6mm with a speed of 0.08m/s while the lander touch-down speed was 1m/s. The kinetic energy before touch-down was 50J, 45J were dissipated by the lander internal damper and by ground penetration at touch-down, and 5J kinetic energy are left after touch-down (0.325m/s speed). Most kinetic energy was dissipated by ground penetration (41J) while only 4J are dissipated by the lander internal damper. Based on these data, a value for a constant compressive soil-strength of between 1.55kPa and 1.8kPa is calculated.This paper focuses on the reconstruction of the touch-down at Agilkia over a period of around 20s from first ground contact to lift-off again. After rebound Philae left a strange pattern on ground documented by the OSIRIS Narrow Angle Camera (NAC). The analysis shows, that the touch-down was not just a simple damped reflection on the surface. Instead the lander had repeated contacts with the surface over a period of about 20s±10s.This paper discusses scenarios for the reconstruction of the landing sequence based on the data available and on computer simulations. Simulations are performed with a dedicated mechanical multi-body model of the lander, which was validated previously in numerous ground tests. The SIMPACK simulation software was used, including the option to set forces at the feet to the ground. The outgoing velocity vector is mostly influenced by the timing of the ground contact of the different feet. It turns out that ground friction during damping has strong impact on the lander outgoing velocity, on its rotation, and on its nutation. After the end of damping, the attitude of the lander can be strongly changed by the additional ground contacts even with the flywheel still running inside the lander.The simulation shows that the outbound velocity vector and the lander rotation were formed immediately at touch-down during the first 1.5s. The outbound velocity vector is found to be formed by the ground slope and the lander damping characteristic, especially the nearly horizontal flight out.

The Philae Lander: Science planning and operations

Rosetta is an ambitious mission launched in March 2004 to study comet 67P/Churyumov–Gerasimenko. It is composed of a space probe (Rosetta) and the Philae Lander. The mission is a series of premieres: among others, first probe to escort a comet, first time a landing site is selected with short turnaround time, first time a lander has landed on a comet nucleus. In November 2014, once stabilized on the comet, Philae has performed its “First Science Sequence”. Philae’s aim was to perform detailed and innovative in-situ experiments on the comet’s surface to characterize the nucleus by performing mechanical, chemical and physical investigations on the comet surface. The main contribution to the Rosetta lander by the French space agency (CNES) is the Science Operation and Navigation Center (SONC) located in Toulouse. Among its tasks is the scheduling of the scientific activities of the 10 lander experiments and then to provide it to the Lander Control Center (LCC) located in DLR Cologne. The teams in charge of the Philae activity scheduling had to cope with considerable constraints in term of energy, data management, asynchronous processes and co-activities or exclusions between instruments. Moreover the comet itself, its environment and the landing conditions remained unknown until separation time. The landing site was selected once the operational sequence was already designed. This paper will explain the specific context of the Rosetta lander mission and all the constraints that the lander activity scheduling had to face to fulfill the scientific objectives specified for Philae. A specific tool was developed by CNES and used to design the complete sequence of activities on the comet with respect to all constraints. The baseline scenario for the lander operation will also be detailed as well as the sequence performed on the comet to highlight the difficulties and challenges that the operational team faced.

Rosetta mission operations for landing

The International Rosetta Mission of the European Space Agency (ESA) was launched on 2nd March 2004 on its 10 year journey to comet Churyumov-Gerasimenko and has reached it early August 2014. The main mission objectives were to perform close observations of the comet nucleus throughout its orbit around the Sun and deliver the lander Philae to its surface. This paper describers the activities at mission operations level that allowed the landing of Philae.The landing preparation phase was mainly characterised by the definition of the landing selection process, to which several parties contributed, and by the definition of the strategy for comet characterisation, the orbital strategy for lander delivery, and the definition and validation of the operations timeline.The definition of the landing site selection process involved almost all components of the mission team; Rosetta has been the first, and so far only mission, that could not rely on data collected by previous missions for the landing site selection. This forced the teams to include an intensive observation campaign as a mandatory part of the process; several science teams actively contributed to this campaign thus making results from science observations part of the mandatory operational products. The time allocated to the comet characterisation phase was in the order of a few weeks and all the processes, tools, and interfaces required an extensive planning an validation. Being the descent of Philae purely ballistic, the main driver for the orbital strategy was the capability to accurately control the position and velocity of Rosetta at Philae’s separation. The resulting operations timeline had to merge this need of frequent orbit determination and control with the complexity of the ground segment and the inherent risk of problems when doing critical activities in short times.This paper describes the contribution of the Mission Control Centre (MOC) at the European Space Operations Centre (ESOC) to this mission phase and the lessons learned that can be derived from this experience.

Ptolemy operations at the surface of a comet, from planning to reality

Ptolemy is a Gas Chromatograph–Isotope Ratio–Mass Spectrometer (GC–IR–MS) aboard the Philae lander element of the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Developed to determine the chemical and stable light isotopic composition of cometary material, Ptolemy was conceived as a highly flexible instrument able to accommodate changes in operational functionality via software modification. This was considered essential to allow for different modes of operation not only in response to rapid/unexpected changes and opportunities, but also to longer-term shifts in priorities as the overall mission plan (and indeed cometary science in general) changed during the decades from initial concept to landing. Against the backdrop of events of the Philae landing, this paper describes the methods of instrument operation and rational behind them used to achieve the Ptolemy scientific results during the period 12–14th November 2014. In particular we demonstrate the importance of a flexible modular approach to the instrument architecture enabling complex instrument operations, especially in a situation where the environment of exploration is effectively unknown and some of the engineering solutions were being tested in the field for the first time.

Rosetta-Philae RF link, challenging communications from a comet

The Rosetta spacecraft reached the vicinity of the comet 67P/Churyumov–Gerasimenko in 2014 and released the lander Philae for an in-situ analysis through 10 scientific instruments. The analysis of the lander RF link telemetry reveals major information on the lander orientation and its near environment during the 50-h mission on the comet. The Philae waking-up in April 2015 led to new RF contacts whose analysis gives among others assumptions on the RF equipment status, even if further and deeper investigation will be necessary.

Performance of the mission critical Electrical Support System (ESS) which handled communications and data transfer between the Rosetta Orbiter and its Lander Philae while en route to and at comet 67P/Churyumov-Gerasimenko

The Electrical Support System (ESS), which was designed and built in Ireland, handled commands transmitted from the Rosetta spacecraft to the Command and Data Management System (CDMS) aboard its Lander Philae during a ten year Cruise Phase to comet 67P/Churyumov-Gerasimenko as well as at the comet itself. The busy Cruise Phase included three Earth flybys, a fly-by of Mars and visits to two asteroids, Steins and Lutetia. Data originating at the individual Lander experiments measured while en-route to and at the comet were also handled by the ESS which received and reformatted them prior to their transmission by Rosetta to Earth. Since the success of the Lander depended on the acquisition of scientific data, the ESS was defined by the European Space Agency to be Mission Critical Hardware. The electronic design of the ESS and its method of handling communications between the spacecraft and Philae are herein presented. The nominal performance of the ESS during the Cruise Phase and in the course of subsequent surface campaigns is described and the successful fulfilment of the brief of this subsystem to retrieve unique scientific data measured by the instruments of the Philae Lander demonstrated.

Command and data management system (CDMS) of the Philae lander

The paper covers the principal requirements, design concepts and implementation of the hardware and software for the central on-board computer (CDMS) of the Philae lander in the context of the ESA Rosetta space mission, including some technical details. The focus is on the implementation of fault tolerance, autonomous operation and operational flexibility by means of specific linked data structures and code execution mechanisms that can be interpreted as a kind of object oriented model for mission sequencing.

Philae locating and science support by robotic vision techniques

The ROLIS, CIVA-P and OSIRIS instruments on-board the Philae lander and the Rosetta orbiter acquired high-resolution images during the lander׳s descent towards the targeted landing site Agilkia, during its unexpected rebounds and at the final landing site Abydos on comet 67P/Churyumov–Gerasimenko. We, exploited these images, using robotic vision techniques, to locate the first touchdown on the surface of the comet nucleus, to reconstruct the lander׳s 3D trajectory during the descent and at the beginning of the first rebound, and to create local digital terrain models and depth maps of Agilkia and Abydos sites. Using the ROLIS close-up images we could also determine the actual movements of the lander between the beginning and the end of the First Science Sequence and we propose a new lander׳s bubble movement command meant to increase the probability for a successful drilling during a hypothetical future Long Term Science phase.

Attitude reconstruction of ROSETTA׳s Lander PHILAE using two-point magnetic field observations by ROMAP and RPC-MAG

As part of the European Space Agency׳s ROSETTA Mission the Lander PHILAE touched down on comet 67P/Churyumov–Gerasimenko on November 12, 2014. The magnetic field has been measured onboard the orbiter and the lander. The orbiter׳s tri-axial fluxgate magnetometer RPC-MAG is one of five sensors of the ROSETTA Plasma Consortium. The lander is also equipped with a tri-axial fluxgate magnetometer as part of the ROSETTA Lander Magnetometer and Plasma-Monitor package (ROMAP). This unique setup makes a two point measurement between the two spacecrafts in a relatively small distance of less than 50km possible. Both magnetometers were switched on during the entire descent, the initial touchdown, the bouncing between the touchdowns and after the final touchdown. We describe a method for attitude determination by correlating magnetic low-frequency waves, which was tested under different conditions and finally used to reconstruct PHILAE׳s attitude during descent and after landing. In these cases the attitude could be determined with an accuracy of better than ±5°. These results were essential not only for PHILAE operations planning but also for the analysis of the obtained scientific data, because nominal sources for this information, like solar panel currents and camera pictures could not provide sufficient information due to the unexpected landing position.

Characterization of the Abydos region through OSIRIS high-resolution images in support of CIVA measurements

On 12 November 2014, the European mission Rosetta delivered the Philae lander on the nucleus of comet 67P/Churyumov-Gerasimenko (67P). After the first touchdown, the lander bounced three times before finally landing at a site named Abydos. Aims. We provide a morphologically detailed analysis of the Abydos landing site to support Philae’s measurements and to give context for the interpretation of the images coming from the Comet Infrared and Visible Analyser (CIVA) camera system onboard the lander. Methods. We used images acquired by the OSIRIS Narrow Angle Camera (NAC) on 6 December 2014 to perform the analysis of the Abydos landing site, which provided the geomorphological map, the gravitational slope map, the size-frequency distribution of the boulders. We also computed the albedo and spectral reddening maps. Results. The morphological analysis of the region could suggest that Philae is located on a primordial terrain. The Abydos site is surrounded by two layered and fractured outcrops and presents a 0.02 km2 talus deposit rich in boulders. The boulder size frequency distribution gives a cumulative power-law index of −4.0 + 0.3/ − 0.4, which is correlated with gravitational events triggered by sublimation and/or thermal fracturing causing regressive erosion. The average value of the albedo is 5.8% at λ1= 480.7 nm and 7.4% at λ2= 649.2 nm, which is similar to the global albedos derived by OSIRIS and CIVA, respectively.

Planning and implementation of the on-comet operations of the instrument SD2 onboard the lander Philae of Rosetta mission

The lander Philae of the Rosetta mission landed on the surface of the comet 67P/Churyumov–Gerasimenko on November 12, 2014. Among the specific subsystems and instruments carried on Philae, the sampling, drilling and distribution (SD2) subsystem had the role of providing in-situ operations devoted to soil drilling, sample collection, and their distribution to three scientific instruments. After landing, a first sequence of scientific activities was carried out, relying mainly on the energy stored in the lander primary battery. Due to the limited duration and the communication delay, these activities had to be carried out automatically, with a limited possibility of developing and uploading commands from the ground. Philae׳s landing was not nominal and SD2 was operated in unexpected conditions: the lander was not anchored to the soil and leant on the comet surface shakily. Nevertheless, one sampling procedure was attempted. This paper provides an overview of SD2 operation planning and on-comet operations, and analyses SD2 achievements during the first science sequence of Philae׳s on-comet operations.

Dust Impact Monitor (SESAME-DIM) measurements at comet 67P/Churyumov-Gerasimenko

The Rosetta lander Philae successfully landed on the nucleus of comet 67P/Churyumov-Gerasimenko on 12 November 2014. Philae carries the Dust Impact Monitor (DIM) on board, which is part of the Surface Electric Sounding and Acoustic Monitoring Experiment (SESAME). DIM employs piezoelectric PZT sensors to detect impacts by sub-millimeter and millimeter-sized ice and dust particles that are emitted from the nucleus and transported into the cometary coma. The DIM sensor measures dynamical data like flux and the directionality of the impacting particles. Mass and speed of the particles can be constrained assuming density and elastic particle properties. DIM was operated during three mission phases of Philae at the comet: (1) Before Philae's separation from Rosetta at distances of about 9.6 km, 11.8 km, and 25.3 km from the nucleus barycenter. In this mission phase particles released from the nucleus on radial trajectories remained undetectable because of significant obscuration by the structures of Rosetta, and no dust particles were indeed detected. (2) During Philae's descent to its nominal landing site Agilkia, DIM detected one approximately millimeter-sized particle at a distance of 5.0 km from the nucleus' barycenter, corresponding to an altitude of 2.4 km from the surface. This is the closest ever dust detection at a cometary nucleus by a dedicated in-situ dust detector. (3) At Philae's final landing site, Abydos, DIM detected no dust impact which may be due to low cometary activity in the vicinity of Philae, or due to shading by obstacles close to Philae, or both. Laboratory calibration experiments showed that the material properties of the detected particle are compatible with a porous particle having a bulk density of approximately $250\, \mathrm{kg\,m^{-3}}$. The particle could have been lifted off from the comet's surface by sublimating water ice.