Philae

DLR German Aerospace Center. CC BY 2.0.

Overview

Philae was the lander element of the Rosetta mission and the first spacecraft to land on a comet nucleus, touching down on 67P/Churyumov-Gerasimenko on 12 November 2014 after a seven-hour ballistic descent from 20.5 km [1]. It was built by a consortium led by DLR with CNES, ASI and the Max Planck Institute for Solar System Research. Every one of its four hold-down systems failed at touchdown, so it bounced for about two hours before coming to rest at Abydos in a shadowed attitude that ended the mission when the primary battery ran down [1], [3].

Specifications

Parameter	Value
Lander mass	97.6 kg, 111 kg including the units left on Rosetta
Instrument complement mass	27 kg over 10 experiments
Body dimensions	about 0.7 x 0.7 x 0.9 m
Landing leg tripod foot spacing	about 2.3 m

Source: [1].

Mission profile

Parameter	Value	Source
Host	Rosetta orbiter	[1]
Target	comet 67P/Churyumov-Gerasimenko	[1]
Separation	12 November 2014, from 20.5 km altitude	[1], [3]
Separation velocity	0.1874 m/s, achieved to 1 percent	[1]
Nucleus surface reachable by soft landing	40 percent	[1]
Descent duration	7 h ballistic	[1], [3]
First touchdown	15:34:04 UTC, 12 November 2014, the first landing on a comet nucleus	[1], [3]
Landing dispersion achieved	112 m from the aim point [1], 120 m [3]	[1], [3]
Rest position	Abydos, at 17:31:17 UTC after about two hours of bouncing across the nucleus	[3]
First Science Sequence duration	56 h 28 min on primary battery	[1]
Hibernation entered	15 November 2014, after 00:02:36 UTC	[1]
Intermittent contact	13 June to 9 July 2015, never with enough link margin to command new science	[1]
Total returned data	13.88 Mbyte across ten instruments	[1]

Design environment and why it failed

Philae was designed against a surface whose Young modulus and strength were unknown to within three to four orders of magnitude, so the lander was sized to cover that entire range [3]. When the Rosetta target changed from 46P to 67P, the landing gear was requalified for a body about three times larger in radius and therefore roughly thirty times more massive.

The nucleus that was found is at the weak end of the design envelope. Bulk density is 470 kg/m3 [2]. Derived from OSIRIS shape and imaging data, tensile strength of overhangs is 3 to 15 Pa with an upper limit of 150 Pa, shear strength of fine material and boulders is 4 to 30 Pa, and compressive strength of overhangs is 30 to 150 Pa with an upper limit of 1500 Pa. The overall tensile and compressive strength of 67P turned out to be below the minimum assumed when Philae was built [3].

The force scaling is what drives the anchoring problem. At the accelerations that hold on a body of this size, weight is not the largest force acting on surface material, and cohesion between grains dominates over a wide particle size range [6]. Bond number reaches unity at a grain radius of about 2 cm at 1 milli-g and about 65 cm at 1 micro-g, so the material a foot rests on is held to its neighbors far more strongly than to the body. A lander cannot press a tool against such a surface using its own weight, and cannot generate friction proportional to it. The strength-to-gravity ratio of 67P material is comparable to that of weak terrestrial rocks even though the absolute strengths are three orders of magnitude lower [2].

Landing gear and hold-down systems

Three independent systems were to fix the lander at touchdown:

System	Design function	Outcome
Landing gear tripod with generator	Convert vertical kinetic energy to electrical energy, absorbing up to about 60 J, equivalent to a 1.1 m/s vertical touchdown	Absorbed about 10 percent of the arrival energy
Active Descent System	Cold gas thruster on the top plate, fired at touchdown to push the lander into the ground	Gas tank valve did not open; detected before separation
Two harpoons with cords	Fired into the surface at touchdown, then tensioned by a rope and pulley to pull the lander down	Did not fire despite nominal subunit status and correct command execution
Ice screws in each foot	Engage after the lander is held down	Never reached engagement, because the lander was already rebounding

Sources for the table: [1] for all four systems and outcomes, [3] for the screw engagement sequence.

The redundancy was serial rather than parallel: the ice screws required the lander to still be on the surface, which required either the thruster or the harpoons to have worked, and neither did [3]. With the thruster known failed before separation, firm fixation depended exclusively on the harpoons. Neither friction nor screw engagement can be generated against a surface whose grains are bound to each other more strongly than to the body [6].

Touchdown and rebound

At first touchdown the feet sank into soft regolith and dug five holes, ejecting a dust cloud imaged by OSIRIS and the Rosetta navigation cameras [1]. The lander made repeated ground contacts over about 20 s, absorbed roughly 10 percent of its kinetic energy in the landing gear, and rebounded. The subsequent hop lasted almost 2 h, with at least one and possibly two further surface contacts, ending at Abydos [3].

Attitude through the whole sequence was reconstructed from the electrical power output of the six solar panels, which carry 254, 162, 162, 162, 254 and 230 cells respectively, fitted against a seven degree of freedom dynamic model [4]. The single-axis stabilizing gyroscope ran at about 865 rad/s through descent and was commanded off automatically at first touchdown, then spun down over the following 42 minutes on internal friction alone. That decay is a torque source on the lander body for the entire duration of the hop, and the reconstruction shows the resulting motion was a complex combination of rotation and precession in which any part of the lander, solar panels included, could have contacted the surface. The lander survived essentially intact: only one foot mounting on the tripod was deformed, and a ROMAP sensor entrance window may have been blocked by soil [1].

Measured surface mechanics

The touchdown itself is the only mechanical test ever run on a comet nucleus. Finite element analysis of the touchdown dynamics against lander and orbiter data gives a mean compressive strength at Agilkia of 1.5 to 2 kPa for a depth-independent strength parameter, or 3 kPa/m for strength increasing linearly with penetration depth, which the authors prefer; lateral strength should not exceed 10 Pa [1]. Foot and leg excavation holes were at least 20 cm deep.

An independent estimate uses the footprint geometry directly. The largest footprint is about 150 cm across and about 20 cm deep by shape-from-shading, made by a single leg of a 100 kg lander arriving at 1 m/s onto a contact area of 0.016 m2 per foot, which gives a compressive strength of 15.6 kPa [2]. That figure is an upper limit, both because the footprint proves the material yielded and because penetration was arrested by a harder buried layer.

At Abydos the MUPUS hammer could not penetrate deeply and possibly not at all, which implies a compressive strength above 2 MPa in the hammering area [1]. That number should be treated with caution: it is orders of magnitude above every other observational, experimental and theoretical constraint on 67P, and it is unresolved whether the surface was genuinely that hard or the hammer failed to deploy in the lander’s final attitude [3]. The reconciliation offered for the strength gap is a sintered water ice layer beneath the dust deposit, produced by sublimation and redeposition cycles, a few centimeters to several meters thick and with strengths reaching 1 MPa, which is not representative of the bulk nucleus [2].

Thermal environment measured at the surface

Quantity	Value at Abydos
Diurnal surface temperature	90 to 130 K
Thermal inertia	85 J/m2/K/s^0.5
Thermal conductivity	0.02 to 0.06 W/m/K
Bulk porosity of the surface layer	40 to 55 percent
Relative permittivity, first meter	2.45 +/- 0.2
Nucleus interior permittivity (CONSERT)	1.27
Nucleus interior porosity (CONSERT)	75 to 85 percent

Source: [1].

The Abydos thermal inertia of 85 J/m2/K/s^0.5 is well above the MIRO orbital average of 10 to 50, which implies a thinner regolith cover at the final site than over the nucleus as a whole [1]. Surface porosity of 40 to 55 percent is below the 75 to 85 percent found for the interior, consistent with thermal sintering having compacted the near surface. Diurnal forcing of this magnitude drives thermal fracturing in the top tens of centimeters of a cometary surface. Modeled diurnal swings on 67P reach about 230 K at perihelion, and the resulting stress penetrates about 0.25 m and reaches tens of MPa seasonally, enough to fracture the near-surface layer wherever thermal inertia is at or above 50 J/m2/K/s^0.5 [7]. The Abydos value of 85 is above that threshold.

Power and thermal

Power came from six body-mounted solar panels carrying 1224 cells in total [4], backed by a primary battery sized for the First Science Sequence and a rechargeable secondary battery. The final attitude, tilted in a shaded rocky wedge, left insufficient illumination to recharge, and the lander entered hibernation when the batteries were exhausted [1]. During hibernation the lander experienced temperatures of about -100 C or below, well outside its designed operating and survival limits, and still woke: solar cells, battery and computer were all reported healthy in the June 2015 housekeeping. Wake-up conditions were reached in the last week of April 2015 and recurred on almost every cometary day thereafter [3]. Contact was re-established on 15 June 2015 when Rosetta passed over Abydos at 200 km, and ended on 9 July 2015 [3].

Communications

Communication was a two-way relay through the Rosetta orbiter only; there is no direct-to-Earth path. The link was quasi-continuous through descent and the hopping phase, which is why the payload returned data from both Agilkia and Abydos and why ROMAP collected magnetometer data across most of the traverse [3]. After final landing the link was interrupted for geometric reasons about 27 minutes later, and was re-established four more times before the primary battery was exhausted on 15 November 2014. CONSERT ranging between the lander and orbiter units narrowed the final landing location to an area of 22 x 10^6 m2; direct imaging by OSIRIS on 2 September 2016 fixed it at 358.45 deg longitude and -8.15 deg latitude to about 3 m [1].

Payload

Instrument	Measurement	Operating time	Data returned
APXS	Elemental composition of surface material	6 h 51 min	0.063 Mbyte
CIVA	Panoramic and microscopic imaging, sample composition	1 h 50 min	2.338 Mbyte
CONSERT	Nucleus interior structure by radio transmission to the orbiter	16 h 41 min	2.630 Mbyte
COSAC	Molecular composition and chirality of samples	2 h 08 min	0.277 Mbyte
MUPUS	Density, porosity and thermal properties of surface and subsurface	37 h 20 min	0.347 Mbyte
PTOLEMY	Isotopic composition of light stable elements	1 h 09 min	0.102 Mbyte
ROLIS	Descent and down-looking surface imaging	1 h 44 min	3.878 Mbyte
ROMAP	Magnetic field and plasma monitoring	32 h 33 min	1.268 Mbyte
SD2	Drill sample acquisition and transfer	2 h 48 min	0.045 Mbyte
SESAME	Electrical and acoustic sounding, dust impact monitoring	15 h 17 min	2.930 Mbyte

All values from [1]. SD2 was operated but the lander attitude prevented sample collection and drilling from reaching the surface [3].

ROLIS is the instrument that returned the close-range surface characterization. Its detector is 1024 x 1024 pixels over a 57.7 by 57.7 degree field, nominally focused at 30 cm and illuminated by four LED arrays spanning 400 to 950 nm, with 14-bit data and wavelet compression at a factor of about 8 [5]. At Abydos the foreground lay at 0.7 to 0.9 m rather than the nominal 30 cm, giving 0.8 mm per pixel with the local horizon about 1.5 m away, and the camera operated at -72 C. The lander was inclined by roughly 60 degrees from the ROLIS boresight normal, which is the geometry that also cost the drill its access to the surface.

Surface characterization and site selection

Landing site selection ran over about two months from mid-August 2014 against four criteria: safe descent and landing with instruments intact, ability to run the First Science Sequence at 3 AU, ability to run the long-term science phase between 3 and 2 AU pre-perihelion, and at least part-time solar illumination [1]. Ten candidates were reduced to five, then to a primary and a single backup. Only 40 percent of the nucleus surface was reachable by a soft landing at all, given the orbiter trajectory and the body shape and gravity model available before landing [1].

Slope estimates carry an error of about 5 degrees below 60 degrees and up to 20 degrees above it, which bounds how finely a site can be screened from orbit [2].

The terrain classification derived from gravitational slopes is the practical design input for any successor. Low-slope terrain from 0 to 20 degrees is covered by fine material with a few isolated boulders above 10 m; intermediate slopes from 20 to 45 degrees are fallen consolidated material and debris fields with boulders from below 1 m to 10 m; slopes from 45 to 90 degrees are cliffs exposing consolidated material with neither boulders nor fine material [2]. The angle of repose sits at 45 +/- 5 degrees. ROLIS imaging of Agilkia resolved a granular surface at centimeter to decimetre scale, with regolith thickness up to 50 cm in the smooth terrain [1].

Ground operations

Operations were split between the Lander Control Center at DLR Cologne and the Science Operations and Navigation Center at CNES Toulouse, with ESA responsible for orbiter navigation and the release trajectory [1]. The First Science Sequence had to be adapted, reshuffled and supplemented in near real time, because the lander’s location and attitude were unknown while it was executing; the sequence was designed with an emergency execution block and with in-flight modification treated as a nominal mode of operation rather than a contingency. That design choice is what preserved the science return.

Technologies developed and lessons

The three-way hold-down architecture failed as a set, not as three independent items, because two of the three required the third to have worked first. The published record treats this as the primary design lesson for comet and asteroid landers [3].

Philae also produced the only in-situ mechanical and thermal measurements of a cometary surface: compressive strength at two sites separated by three orders of magnitude, thermal inertia, thermal conductivity, porosity and permittivity, and the demonstration that a nucleus interior at 75 to 85 percent porosity underlies a compacted surface layer [1], [2].

Radiation and the wider comet environment

No lander-borne radiation measurement was made. The design environment for a body at 3 AU is the interplanetary galactic cosmic ray field with the nucleus providing the only shielding mass, taken from the Badhwar-O’Neill model rather than from in-situ data [14].

The comet itself is an active environment that changes on the timescale of a surface mission. Ground-based robotic telescope monitoring tracked the rise in 67P activity through perihelion [8], OSIRIS detected sublimating icy aggregates in the coma [9], and ROSINA measured the water oxygen isotope ratio [11]. The nucleus origin constraints from OSIRIS observations bound the material the lander was standing on [10]. A permanent coating of the solar cells by redeposited dust from this activity is one of the three scenarios proposed for the loss of the lander after July 2015 [1]. Mission-level description and imagery are held on the ESA Rosetta pages [12].

References

Boehnhardt, H., Bibring, J.-P., Apathy, I., Auster, H.-U., Ercoli Finzi, A., Goesmann, F., Klingelhöfer, G., Knapmeyer, M., Kofman, W., Krüger, H., Mottola, S., Schmidt, W., Seidensticker, K. J., Spohn, T. and Wright, I. P. (2017). The Philae lander mission and science overview . Philosophical Transactions of the Royal Society A. Source

BibTeX

@article{boehnhardt2017philae,
  title = {The Philae lander mission and science overview},
  author = {Boehnhardt, H. and Bibring, Jean-Pierre and Apathy, I. and Auster, Hans-Ulrich and Ercoli Finzi, A. and Goesmann, Fred and Klingelhöfer, G. and Knapmeyer, Martin and Kofman, Wlodek and Krüger, H. and Mottola, Stefano and Schmidt, Walter and Seidensticker, Klaus J. and Spohn, Tilman and Wright, Ian P.},
  journal = {Philosophical Transactions of the Royal Society A},
  volume = {375},
  pages = {20160248},
  year = {2017},
  doi = {10.1098/rsta.2016.0248},
  abstract = {The Philae lander accomplished the first soft landing and the first scientific experiments of a human-made spacecraft on the surface of a comet. Planned, expected and unexpected activities and events happened during the descent, the touch-downs, the hopping across and the stay and operations on the surface. The key results were obtained during 12–14 November 2014, at 3 AU from the Sun, during the 63 h long period of the descent and of the first science sequence on the surface. Thereafter, Philae went into hibernation, waking up again in late April 2015 with subsequent communication periods with Earth (via the orbiter), too short to enable new scientific activities. The science return of the mission comes from eight of the 10 instruments on-board and focuses on morphological, thermal, mechanical and electrical properties of the surface as well as on the surface composition. It allows a first characterization of the local environment of the touch-down and landing sites. Unique conclusions on the organics in the cometary material, the nucleus interior, the comet formation and evolution became available through measurements of the Philae lander in the context of the Rosetta mission. This article is part of the themed issue ‘Cometary science after Rosetta’.}
}

Groussin, O., Jorda, L., Auger, A.-T., Kührt, E., Gaskell, R., Capanna, C., Scholten, F., Preusker, F., Lamy, P., Hviid, S., Knollenberg, J., Keller, U., Huettig, C., Sierks, H., Barbieri, C., Rodrigo, R., Koschny, D., Rickman, H., A'Hearn, M. F., Agarwal, J., Barucci, M. A., Bertaux, J.-L., Bertini, I., Boudreault, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., De Cecco, M., El-Maarry, M. R., Fornasier, S., Fulle, M., Gutiérrez, P. J., Güttler, C., Ip, W.-H., Kramm, J.-R., Küppers, M., Lazzarin, M., Lara, L. M., Lopez Moreno, J. J., Marchi, S., Marzari, F., Massironi, M., Michalik, H., Naletto, G., Oklay, N., Pommerol, A., Pajola, M., Thomas, N., Toth, I., Tubiana, C. and Vincent, J.-B. (2015). Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations . Astronomy & Astrophysics. Source

BibTeX

@article{groussin2015gravitational,
  title = {Gravitational slopes, geomorphology, and material strengths of the nucleus of comet 67P/Churyumov-Gerasimenko from OSIRIS observations},
  author = {Groussin, Olivier and Jorda, Laurent and Auger, A.-T. and Kührt, Ekkehard and Gaskell, R. and Capanna, C. and Scholten, Frank and Preusker, Frank and Lamy, P. and Hviid, Stubbe and Knollenberg, J. and Keller, U. and Huettig, C. and Sierks, Holger and Barbieri, C. and Rodrigo, R. and Koschny, Detlef and Rickman, H. and A'Hearn, M. F. and Agarwal, J. and Barucci, M. A. and Bertaux, J.-L. and Bertini, I. and Boudreault, S. and Cremonese, Gabriele and Da Deppo, V. and Davidsson, B. and Debei, S. and De Cecco, M. and El-Maarry, Mohammed R. and Fornasier, Sonia and Fulle, M. and Gutiérrez, P. J. and Güttler, Carsten and Ip, Wing-Huen and Kramm, J.-R. and Küppers, Michael and Lazzarin, M. and Lara, L. M. and Lopez Moreno, J. J. and Marchi, S. and Marzari, F. and Massironi, M. and Michalik, H. and Naletto, G. and Oklay, Nilda and Pommerol, A. and Pajola, Maurizio and Thomas, N. and Toth, I. and Tubiana, Cecilia and Vincent, Jean-Baptiste},
  journal = {Astronomy & Astrophysics},
  volume = {583},
  pages = {A32},
  year = {2015},
  doi = {10.1051/0004-6361/201526379},
  abstract = {Aims. We study the link between gravitational slopes and the surface morphology on the nucleus of comet 67P/Churyumov-Gerasimenko and provide constraints on the mechanical properties of the cometary material (tensile, shear, and compressive strengths).}
}

Snodgrass, C., Feaga, L., Jones, G. H., Kueppers, M. and Tubiana, C. (2024). Past and Future Comet Missions . arXiv preprint. Source

BibTeX

@article{snodgrass2024past,
  title = {Past and Future Comet Missions},
  author = {Snodgrass, Colin and Feaga, L. and Jones, Geraint H. and Kueppers, Michael and Tubiana, Cecilia},
  journal = {arXiv preprint},
  volume = {2208.08476},
  year = {2024},
  doi = {10.48550/arxiv.2208.08476},
  abstract = {We review the history of spacecraft encounters with comets, concentrating on those that took place in the recent past, since the publication of the Comets II book. This includes the flyby missions Stardust and Deep Impact, and their respective extended missions, the Rosetta rendezvous mission, and serendipitous encounters. While results from all of these missions can be found throughout this book, this chapter focuses on the questions that motivated each mission, the technologies that were required to answer these questions, and where each mission opened new areas to investigate. There remain a large number of questions that will require future technologies and space missions to answer; we also describe planned next steps and routes forward that may be pursued by missions that have yet to be selected, and eventually lead to cryogenic sample return of nucleus ices for laboratory study.}
}

Baranyai, T., Balázs, A. and Várkonyi, P. L. (2016). Partial reconstruction of the rotational motion of Philae spacecraft during its landing on comet 67P/Churyumov-Gerasimenko . arXiv preprint. Source

BibTeX

@article{baranyai2016partial,
  title = {Partial reconstruction of the rotational motion of Philae spacecraft during its landing on comet 67P/Churyumov-Gerasimenko},
  author = {Baranyai, Tamás and Balázs, András and Várkonyi, Péter L.},
  journal = {arXiv preprint},
  year = {2016},
  doi = {10.48550/arxiv.1604.04414},
  abstract = {This paper presents a partial reconstruction of the rotational dynamics of the Philae spacecraft upon landing on comet 67P/Churyumov-Gerasimenko as part of ESA's Rosetta mission. We analyze the motion and the events triggered by the failure to fix the spacecraft to the comet surface at the time of the first touchdown. Dynamic trajectories obtained by numerical simulation of a 7 degree-of-freedom mechanical model of the spacecraft are fitted to directions of incoming solar radiation inferred from in-situ measurements of the electric power provided by the solar panels. The results include a lower bound of the angular velocity of the lander immediately after its first touchdown. Our study also gives insight into the effect of the programmed turn-off of the stabilizing gyroscope after touchdown; the important dynamical consequences of a small collision during Philae's journey; and the probability that a similar landing scenario harms the operability of this type of spacecraft.}
}

Schröder, S. E., Mottola, S., Arnold, G., Grothues, H.-G., Jaumann, R., Keller, H. U., Michaelis, H., Bibring, J.-P., Pelivan, I., Koncz, A., Otto, K., Remetean, E., Souvannavong, F. and Dolives, B. (2017). Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera . Icarus. Source

BibTeX

@article{schroder2017close,
  title = {Close-up images of the final Philae landing site on comet 67P/Churyumov-Gerasimenko acquired by the ROLIS camera},
  author = {Schröder, Stefan E. and Mottola, Stefano and Arnold, G. and Grothues, H.-G. and Jaumann, Ralf and Keller, Horst Uwe and Michaelis, H. and Bibring, Jean-Pierre and Pelivan, I. and Koncz, A. and Otto, K. and Remetean, Emile and Souvannavong, F. and Dolives, Benoit},
  journal = {Icarus},
  volume = {285},
  pages = {263-274},
  year = {2017},
  doi = {10.1016/j.icarus.2016.12.009}
}

Scheeres, D. J., Hartzell, C. M., Sánchez, P. and Swift, M. (2010). Scaling forces to asteroid surfaces: The role of cohesion . Icarus. Source

BibTeX

@article{scheeres2010scaling,
  title = {Scaling forces to asteroid surfaces: The role of cohesion},
  author = {Scheeres, Daniel J. and Hartzell, C. M. and Sánchez, Paul and Swift, M.},
  journal = {Icarus},
  volume = {210},
  pages = {968--984},
  year = {2010},
  doi = {10.1016/j.icarus.2010.07.009}
}

Attree, N., Groussin, O., Jorda, L., Rodionov, S., Auger, A.-T., Thomas, N., Brouet, Y., Poch, O., Kührt, E., Knapmeyer, M., Preusker, F., Scholten, F., Knollenberg, J., Hviid, S. and Hartogh, P. (2018). Thermal fracturing on comets: Applications to 67P/Churyumov-Gerasimenko . Astronomy & Astrophysics. Source

BibTeX

@article{attree2018thermal,
  title = {Thermal fracturing on comets: Applications to 67P/Churyumov-Gerasimenko},
  author = {Attree, N. and Groussin, Olivier and Jorda, Laurent and Rodionov, S. and Auger, A-T. and Thomas, N. and Brouet, Y. and Poch, O. and Kührt, Ekkehard and Knapmeyer, Martin and Preusker, Frank and Scholten, Frank and Knollenberg, J. and Hviid, Stubbe and Hartogh, Paul},
  journal = {Astronomy & Astrophysics},
  volume = {610},
  pages = {A76},
  year = {2018},
  doi = {10.1051/0004-6361/201731937},
  abstract = {We simulate the stresses induced by temperature changes in a putative hard layer near the surface of comet 67P/Churyumov-Gerasimenko with a thermo-viscoelastic model. Such a layer could be formed by the recondensation or sintering of water ice (and dust grains), as suggested by laboratory experiments and computer simulations, and would explain the high compressive strength encountered by experiments on board the Philae lander. Changes in temperature from seasonal insolation variation penetrate into the comet’s surface to depths controlled by the thermal inertia, causing the material to expand and contract. Modelling this with a Maxwellian viscoelastic response on a spherical nucleus, we show that a hard, icy layer with similar properties to Martian permafrost will experience high stresses: up to tens of MPa, which exceed its material strength (a few MPa), down to depths of centimetres to a metre. The stress distribution with latitude is confirmed qualitatively when taking into account the comet’s complex shape but neglecting thermal inertia. Stress is found to be comparable to the material strength everywhere for sufficient thermal inertia (≳50 J m −2 K −1 s −1∕2 ) and ice content (≳45% at the equator). In this case, stresses penetrate to a typical depth of ~0.25 m, consistent with the detection of metre-scale thermal contraction crack polygons all over the comet. Thermal fracturing may be an important erosion process on cometary surfaces which breaks down material and weakens cliffs.}
}

Biele, J., Ulamec, S., Maibaum, M., Roll, R., Witte, L., Jurado, E., Muñoz, P., Arnold, W., Auster, H.-U., Casas, C., Faber, C., Fantinati, C., Finke, F., Fischer, H.-H., Geurts, K., Güttler, C., Heinisch, P., Herique, A., Hviid, S., Kargl, G., Knapmeyer, M., Knollenberg, J., Kofman, W., Kömle, N., Kührt, E., Lommatsch, V., Mottola, S., Pardo de Santayana, R., Remetean, E., Scholten, F., Seidensticker, K. J., Sierks, H. and Spohn, T. (2015). The Landing(s) of Philae and Inferences about Comet Surface Mechanical Properties . Science, 6247. Source

BibTeX

@article{biele2015landings,
  title = {The Landing(s) of {Philae} and Inferences about Comet Surface Mechanical Properties},
  author = {Biele, Jens and Ulamec, Stephan and Maibaum, Michael and Roll, Reinhard and Witte, Lars and Jurado, Eric and Muñoz, Pablo and Arnold, Walter and Auster, Hans-Ulrich and Casas, Carlos and Faber, Claudia and Fantinati, Cinzia and Finke, Felix and Fischer, Hans-Herbert and Geurts, Koen and Güttler, Carsten and Heinisch, Philip and Herique, Alain and Hviid, Stubbe and Kargl, Günter and Knapmeyer, Martin and Knollenberg, Jörg and Kofman, Wlodek and Kömle, Norbert and Kührt, Ekkehard and Lommatsch, Valentina and Mottola, Stefano and Pardo de Santayana, Ramon and Remetean, Emile and Scholten, Frank and Seidensticker, Klaus J. and Sierks, Holger and Spohn, Tilman},
  journal = {Science},
  volume = {349},
  number = {6247},
  pages = {aaa9816},
  year = {2015},
  doi = {10.1126/science.aaa9816},
  abstract = {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.}
}

Gicquel, A., Vincent, J.-B., Agarwal, J., A'Hearn, M. F., Bertini, I., Bodewits, D., Sierks, H., Lin, Z.-Y., Barbieri, C., Lamy, P. L., Rodrigo, R., Koschny, D., Rickman, H., Keller, H. U., Barucci, M. A., Bertaux, J.-L., Besse, S., Cremonese, G., Da Deppo, V., Davidsson, B., Debei, S., Deller, J., De Cecco, M., Frattin, E., El-Maarry, M. R., Fornasier, S., Fulle, M., Groussin, O., Gutierrez, P. J., Gutierrez-Marquez, P., Guettler, C., Hoefner, S., Hofmann, M., Hu, X., Hviid, S. F., Ip, W.-H., Jorda, L., Knollenberg, J., Kovacs, G., Kramm, J.-R., Kuehrt, E., Kueppers, M., Lara, L. M., Lazzarin, M., Moreno, J. J. L., Lowry, S., Marzari, F., Masoumzadeh, N., Massironi, M., Moreno, F., Mottola, S., Naletto, G., Oklay, N., Pajola, M., Pommerol, A., Preusker, F., Scholten, F., Shi, X., Thomas, N., Toth, I. and Tubiana, C. (2016). Sublimation of icy aggregates in the coma of comet 67P/Churyumov-Gerasimenko detected with the OSIRIS cameras onboard Rosetta . arXiv preprint. Source

BibTeX

@article{gicquel2016sublimation,
  title = {Sublimation of icy aggregates in the coma of comet 67P/Churyumov-Gerasimenko detected with the OSIRIS cameras onboard Rosetta},
  author = {Gicquel, A. and Vincent, Jean-Baptiste and Agarwal, J. and A'Hearn, M. F. and Bertini, I. and Bodewits, D. and Sierks, Holger and Lin, Z.-Y. and Barbieri, C. and Lamy, P. L. and Rodrigo, R. and Koschny, Detlef and Rickman, H. and Keller, Horst Uwe and Barucci, M. A. and Bertaux, J.-L. and Besse, S. and Cremonese, Gabriele and Da Deppo, V. and Davidsson, B. and Debei, S. and Deller, J. and De Cecco, M. and Frattin, E. and El-Maarry, Mohammed R. and Fornasier, Sonia and Fulle, M. and Groussin, Olivier and Gutierrez, P. J. and Gutierrez-Marquez, P. and Guettler, C. and Hoefner, S. and Hofmann, M. and Hu, Xiangyun and Hviid, S. F. and Ip, Wing-Huen and Jorda, Laurent and Knollenberg, J. and Kovacs, Gabor and Kramm, J.-R. and Kuehrt, E. and Kueppers, Michael and Lara, L. M. and Lazzarin, M. and Moreno, J. J. Lopez and Lowry, S. and Marzari, F. and Masoumzadeh, N. and Massironi, M. and Moreno, Francisco and Mottola, Stefano and Naletto, G. and Oklay, Nilda and Pajola, Maurizio and Pommerol, A. and Preusker, Frank and Scholten, Frank and Shi, X. and Thomas, N. and Toth, I. and Tubiana, Cecilia},
  journal = {arXiv preprint},
  year = {2016},
  doi = {10.48550/arxiv.1608.08774},
  abstract = {Beginning in March 2014, the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) cameras began capturing images of the nucleus and coma (gas and dust) of comet 67P/Churyumov-Gerasimenko using both the wide angle camera (WAC) and the narrow angle camera (NAC). The many observations taken since July of 2014 have been used to study the morphology, location, and temporal variation of the comet's dust jets. We analyzed the dust monitoring observations shortly after the southern vernal equinox on May 30 and 31, 2015 with the WAC at the heliocentric distance Rh = 1.53 AU, where it is possible to observe that the jet rotates with the nucleus. We found that the decline of brightness as a function of the distance of the jet is much steeper than the background coma, which is a first indication of sublimation. We adapted a model of sublimation of icy aggregates and studied the effect as a function of the physical properties of the aggregates (composition and size). The major finding of this article was that through the sublimation of the aggregates of dirty grains (radius a between 5 microm and 50 microm) we were able to completely reproduce the radial brightness profile of a jet beyond 4 km from the nucleus. To reproduce the data we needed to inject a number of aggregates between 8.5 x $10^{13}$ and 8.5 x $10^{10}$ for a = 5 microm and 50 microm respectively, or an initial mass of $H_2O$ ice around 22kg.}
}

Heinisch, P., Auster, H.-U., Gundlach, B., Blum, J., Güttler, C., Tubiana, C., Sierks, H., Hilchenbach, M., Biele, J., Richter, I. and Glassmeier, K. H. (2019). Compressive Strength of Comet 67P/Churyumov-Gerasimenko Derived from Philae Surface Contacts . Astronomy & Astrophysics. Source

BibTeX

@article{heinisch2019compressive,
  title = {Compressive Strength of Comet {67P/Churyumov-Gerasimenko} Derived from {Philae} Surface Contacts},
  author = {Heinisch, Philip and Auster, Hans-Ulrich and Gundlach, Bastian and Blum, J. and Güttler, Carsten and Tubiana, Cecilia and Sierks, Holger and Hilchenbach, M. and Biele, Jens and Richter, Ingo and Glassmeier, K. H.},
  journal = {Astronomy & Astrophysics},
  volume = {630},
  pages = {A2},
  year = {2019},
  doi = {10.1051/0004-6361/201833889},
  abstract = {Context. The landing and rebound of the Philae lander, which was part of the ESA Rosetta mission, enabled us to study the mechanical properties of the surface of comet 67P/Churyumov-Gerasimenko, because we could use Philae as an impact probe. Aims. The aim is to approximate the descent and rebound trajectory of the Philae lander and use this information to derive the compressive strength of the surface material from the different surface contacts and scratches created during the final touchdown. Combined with laboratory measurements, this can give an insight into what comets are made of and how they formed. Methods. We combined observations from the ROMAP magnetometer on board Philae with observations made by the Rosetta spacecraft, particularly by the OSIRIS camera system and the RPC-MAG magnetometer. Additionally, ballistic trajectory and collision modeling was performed. These results are placed in context using laboratory measurements of the compressibility of different materials. Results. It was possible to reconstruct possible trajectories of Philae and determine that a pressure of ~100 Pa is enough to compress the surface material up to a depth of ~20 cm. Considering all errors, the derived compressive strength shows little dependence on location, with an overall upper limit for the surface compressive strength of ~800 Pa.}
}

Jurado, E., Martin, T., Canalias, E., Blazquez, A., Garmier, R., Ceolin, T., Gaudon, P., Delmas, C., Biele, J., Ulamec, S., Remetean, E., Torres, A., Laurent-Varin, J., Dolives, B., Herique, A., Rogez, Y., Kofman, W., Jorda, L., Zakharov, V., Crifo, J.-F., Rodionov, A., Heinish, P. and Vincent, J.-B. (2016). Rosetta Lander Philae: Flight Dynamics Analyses for Landing Site Selection and Post-Landing Operations . Acta Astronautica. Source

BibTeX

@article{jurado2016rosetta,
  title = {{Rosetta} Lander {Philae}: Flight Dynamics Analyses for Landing Site Selection and Post-Landing Operations},
  author = {Jurado, Eric and Martin, Thierry and Canalias, Elisabet and Blazquez, Alejandro and Garmier, Romain and Ceolin, Thierry and Gaudon, Philippe and Delmas, Cedric and Biele, Jens and Ulamec, Stephan and Remetean, Emile and Torres, Alex and Laurent-Varin, Julien and Dolives, Benoit and Herique, Alain and Rogez, Yves and Kofman, Wlodek and Jorda, Laurent and Zakharov, Vladimir and Crifo, Jean-François and Rodionov, Alexander and Heinish, P. and Vincent, Jean-Baptiste},
  journal = {Acta Astronautica},
  volume = {125},
  pages = {65--79},
  year = {2016},
  doi = {10.1016/j.actaastro.2016.03.030},
  abstract = {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 57 h 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.}
}

O'Rourke, L., Heinisch, P., Blum, J., Fornasier, S., Filacchione, G., Van Hoang, H., Ciarniello, M., Raponi, A., Gundlach, B., Blasco, R. A., Grieger, B., Glassmeier, K.-H., Küppers, M., Rotundi, A., Groussin, O., Bockelée-Morvan, D., Auster, H.-U., Oklay, N., Paar, G., Perucha, M. D. P. C., Kovacs, G., Jorda, L., Vincent, J.-B., Capaccioni, F., Biver, N., Parker, J. W., Tubiana, C. and Sierks, H. (2020). The Philae Lander Reveals Low-Strength Primitive Ice inside Cometary Boulders . Nature, 7831. Source

BibTeX

@article{orourke2020philae,
  title = {The {Philae} Lander Reveals Low-Strength Primitive Ice inside Cometary Boulders},
  author = {O'Rourke, Laurence and Heinisch, Philip and Blum, Jürgen and Fornasier, Sonia and Filacchione, Gianrico and Van Hoang, Hong and Ciarniello, Mauro and Raponi, Andrea and Gundlach, Bastian and Blasco, Rafael Andrés and Grieger, Björn and Glassmeier, Karl-Heinz and Küppers, Michael and Rotundi, Alessandra and Groussin, Olivier and Bockelée-Morvan, Dominique and Auster, Hans-Ulrich and Oklay, Nilda and Paar, Gerhard and Perucha, Maria del Pilar Caballo and Kovacs, Gabor and Jorda, Laurent and Vincent, Jean-Baptiste and Capaccioni, Fabrizio and Biver, Nicolas and Parker, Joel Wm. and Tubiana, Cecilia and Sierks, Holger},
  journal = {Nature},
  volume = {586},
  number = {7831},
  pages = {697--701},
  year = {2020},
  doi = {10.1038/s41586-020-2834-3}
}

Rickman, H., Marchi, S., A'Hearn, M. F., Barbieri, C., El-Maarry, M. R., Güttler, C., Ip, W.-H., Keller, H. U., Lamy, P., Marzari, F., Massironi, M., Naletto, G., Pajola, M., Sierks, H. and OSIRIS Team. (2015). Comet 67P/Churyumov-Gerasimenko: Constraints on its origin from OSIRIS observations . arXiv preprint. Source

BibTeX

@article{rickman2015comet,
  title = {Comet 67P/Churyumov-Gerasimenko: Constraints on its origin from OSIRIS observations},
  author = {Rickman, H. and Marchi, S. and A'Hearn, M. F. and Barbieri, C. and El-Maarry, Mohammed R. and Güttler, Carsten and Ip, Wing-Huen and Keller, Horst Uwe and Lamy, P. and Marzari, F. and Massironi, M. and Naletto, G. and Pajola, Maurizio and Sierks, Holger and {OSIRIS Team}},
  journal = {arXiv preprint},
  year = {2015},
  doi = {10.7892/boris.81702},
  abstract = {Context. One of the main aims of the ESA Rosetta mission is to study the origin of the solar system by exploring comet 67P/Churyumov-Gerasimenko at close range. Aims. In this paper we discuss the origin and evolution of comet 67P/Churyumov-Gerasimenko in relation to that of comets in general and in the framework of current solar system formation models. Methods. We use data from the OSIRIS scientific cameras as basic constraints. In particular, we discuss the overall bi-lobate shape and the presence of key geological features, such as layers and fractures. We also treat the problem of collisional evolution of comet nuclei by a particle-in-a-box calculation for an estimate of the probability of survival for 67P/Churyumov-Gerasimenko during the early epochs of the solar system. Results. We argue that the two lobes of the 67P/Churyumov-Gerasimenko nucleus are derived from two distinct objects that have formed a contact binary via a gentle merger. The lobes are separate bodies, though sufficiently similar to have formed in the same environment. An estimate of the collisional rate in the primordial, trans-planetary disk shows that most comets of similar size to 67P/Churyumov-Gerasimenko are likely collisional fragments, although survival of primordial planetesimals cannot be excluded. Conclusions. A collisional origin of the contact binary is suggested, and the low bulk density of the aggregate and abundance of volatile species show that a very gentle merger must have occurred. We thus consider two main scenarios: the primordial accretion of planetesimals, and the re-accretion of fragments after an energetic impact onto a larger parent body. We point to the primordial signatures exhibited by 67P/Churyumov-Gerasimenko and other comet nuclei as critical tests of the collisional evolution.}
}

Philae

Overview

Specifications

Mission profile

Design environment and why it failed

Landing gear and hold-down systems

Touchdown and rebound

Measured surface mechanics

Thermal environment measured at the surface

Power and thermal

Communications

Payload

Surface characterization and site selection

Ground operations

Technologies developed and lessons

Radiation and the wider comet environment

References

Further reading