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Space Robotics

Philae

Program pages ESA: Rosetta

A full-scale Philae model suspended under a comet nucleus mock-up. The hexagonal body, about 0.7 by 0.7 by 0.9 m, is covered on all six sides by the solar panels that carried 1224 cells; the tripod landing gear with its three foot pads spans about 2.3 m across the feet, and the balcony structure at the base holds instrument apertures. None of the four hold-down systems that were to have anchored these feet worked at touchdown DLR German Aerospace Center. CC BY 2.0.

Overview

Section titled “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

Section titled “Specifications”
ParameterValue
Lander mass97.6 kg, 111 kg including the units left on Rosetta
Instrument complement mass27 kg over 10 experiments
Body dimensionsabout 0.7 x 0.7 x 0.9 m
Landing leg tripod foot spacingabout 2.3 m

Source: [1].

Mission profile

Section titled “Mission profile”
ParameterValueSource
HostRosetta orbiter[1]
Targetcomet 67P/Churyumov-Gerasimenko[1]
Separation12 November 2014, from 20.5 km altitude[1], [3]
Separation velocity0.1874 m/s, achieved to 1 percent[1]
Nucleus surface reachable by soft landing40 percent[1]
Descent duration7 h ballistic[1], [3]
First touchdown15:34:04 UTC, 12 November 2014, the first landing on a comet nucleus[1], [3]
Landing dispersion achieved112 m from the aim point [1], 120 m [3][1], [3]
Rest positionAbydos, at 17:31:17 UTC after about two hours of bouncing across the nucleus[3]
First Science Sequence duration56 h 28 min on primary battery[1]
Hibernation entered15 November 2014, after 00:02:36 UTC[1]
Intermittent contact13 June to 9 July 2015, never with enough link margin to command new science[1]
Total returned data13.88 Mbyte across ten instruments[1]

Design environment and why it failed

Section titled “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

Section titled “Landing gear and hold-down systems”

Three independent systems were to fix the lander at touchdown:

SystemDesign functionOutcome
Landing gear tripod with generatorConvert vertical kinetic energy to electrical energy, absorbing up to about 60 J, equivalent to a 1.1 m/s vertical touchdownAbsorbed about 10 percent of the arrival energy
Active Descent SystemCold gas thruster on the top plate, fired at touchdown to push the lander into the groundGas tank valve did not open; detected before separation
Two harpoons with cordsFired into the surface at touchdown, then tensioned by a rope and pulley to pull the lander downDid not fire despite nominal subunit status and correct command execution
Ice screws in each footEngage after the lander is held downNever 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

Section titled “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

Section titled “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

Section titled “Thermal environment measured at the surface”
QuantityValue at Abydos
Diurnal surface temperature90 to 130 K
Thermal inertia85 J/m2/K/s^0.5
Thermal conductivity0.02 to 0.06 W/m/K
Bulk porosity of the surface layer40 to 55 percent
Relative permittivity, first meter2.45 +/- 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

Section titled “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

Section titled “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

Section titled “Payload”
InstrumentMeasurementOperating timeData returned
APXSElemental composition of surface material6 h 51 min0.063 Mbyte
CIVAPanoramic and microscopic imaging, sample composition1 h 50 min2.338 Mbyte
CONSERTNucleus interior structure by radio transmission to the orbiter16 h 41 min2.630 Mbyte
COSACMolecular composition and chirality of samples2 h 08 min0.277 Mbyte
MUPUSDensity, porosity and thermal properties of surface and subsurface37 h 20 min0.347 Mbyte
PTOLEMYIsotopic composition of light stable elements1 h 09 min0.102 Mbyte
ROLISDescent and down-looking surface imaging1 h 44 min3.878 Mbyte
ROMAPMagnetic field and plasma monitoring32 h 33 min1.268 Mbyte
SD2Drill sample acquisition and transfer2 h 48 min0.045 Mbyte
SESAMEElectrical and acoustic sounding, dust impact monitoring15 h 17 min2.930 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

Section titled “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

Section titled “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

Section titled “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

Section titled “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 [13].

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

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    BibTeX
    @article{boehnhardt2017philae,
  title = {The Philae lander mission and science overview},
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}
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    BibTeX
    @article{snodgrass2024past,
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    BibTeX
    @article{baranyai2016partial,
  title = {Partial reconstruction of the rotational motion of Philae spacecraft during its landing on comet 67P/Churyumov-Gerasimenko},
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}
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    BibTeX
    @article{scheeres2010scaling,
  title = {Scaling forces to asteroid surfaces: The role of cohesion},
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    BibTeX
    @article{snodgrass2016perihelion,
  title = {The perihelion activity of comet 67P/Churyumov-Gerasimenko as seen by robotic telescopes},
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    BibTeX
    @article{gicquel2016sublimation,
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}
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    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, M. R. and Güttler, C. and Ip, W.-H. and Keller, H. U. and Lamy, P. and Marzari, F. and Massironi, M. and Naletto, G. and Pajola, M. and Sierks, H. and {{OSIRIS Team}}},
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  journal = {arXiv preprint},
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  url = {http://arxiv.org/abs/1505.07021v1}
}
  11. Schroeder, I. R. H. G., Altwegg, K., Balsiger, H., Berthelier, J.-J., De Keyser, J., Fiethe, B., Fuselier, S. A., Gasc, S., Gombosi, T. I., Rubin, M., Sémon, T., Tzou, C.-Y., Wampfler, S. F. and Wurz, P. (2018). The $^16$O/$^18$O ratio in Water in the Coma of Comet 67P / Churyumov-Gerasimenko measured with the Rosetta / ROSINA Double-Focusing Mass Spectrometer. arXiv preprint. Source
    BibTeX
    @article{schroeder2018o,
  title = {The $^{16}$O/$^{18}$O ratio in Water in the Coma of Comet 67P / Churyumov-Gerasimenko measured with the Rosetta / ROSINA Double-Focusing Mass Spectrometer},
  author = {Schroeder, Isaac R. H. G. and Altwegg, Kathrin and Balsiger, Hans and Berthelier, Jean-Jacques and De Keyser, Johan and Fiethe, Björn and Fuselier, Stephen A. and Gasc, Sébastien and Gombosi, Tamas I. and Rubin, Martin and Sémon, Thierry and Tzou, Chia-Yu and Wampfler, Susanne F. and Wurz, Peter},
  year = {2018},
  journal = {arXiv preprint},
  eprint = {1809.03798v2},
  url = {http://arxiv.org/abs/1809.03798v2}
}
  12. 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,
  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},
  title = {The Landing(s) of {Philae} and Inferences about Comet Surface Mechanical Properties},
  journal = {Science},
  volume = {349},
  number = {6247},
  pages = {aaa9816},
  year = {2015},
  doi = {10.1126/science.aaa9816}
}
  13. 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 and Astrophysics. Source
    BibTeX
    @article{heinisch2019compressive,
  author = {Heinisch, P. and Auster, H.-U. and Gundlach, B. and Blum, J. and Güttler, C. and Tubiana, C. and Sierks, H. and Hilchenbach, M. and Biele, J. and Richter, I. and Glassmeier, K. H.},
  title = {Compressive Strength of Comet {67P/Churyumov-Gerasimenko} Derived from {Philae} Surface Contacts},
  journal = {Astronomy and Astrophysics},
  volume = {630},
  pages = {A2},
  year = {2019},
  doi = {10.1051/0004-6361/201833889}
}

Further reading

  • 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
  • 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. Source
  • Spohn, T., Knollenberg, J., Ball, A. J., Banaszkiewicz, M., Benkhoff, J., Grott, M., Grygorczuk, J., Hüttig, C., Hagermann, A., Kargl, G., Kaufmann, E., Kömle, N., Kührt, E., Kossacki, K. J., Marczewski, W., Pelivan, I., Schrödter, R. and Seiferlin, K. (2015). Thermal and Mechanical Properties of the Near-Surface Layers of Comet 67P/Churyumov-Gerasimenko. Science. Source
  • Ulamec, S., Fantinati, C., Maibaum, M., Geurts, K., Biele, J., Jansen, S., Küchemann, O., Cozzoni, B., Finke, F., Lommatsch, V., Moussi-Soffys, A., Delmas, C. and O'Rourke, L. (2016). Rosetta Lander: Landing and Operations on Comet 67P/Churyumov-Gerasimenko. Acta Astronautica. Source
  • Wu, Y., Küppers, M., Grieger, B. and Shang, H. (2019). Characterization of the Agilkia Region through Discrete-Element Simulation of Philae's Rebound. Astronomy and Astrophysics. Source
  • (2026). ESA: Rosetta. esa.int/Science_Exploration/Space_Science/Rosetta
  • O'Neill, P. M., Golge, S. and Slaba, T. C. (2014). Implementing the Badhwar-O'Neill Galactic Cosmic Ray Model for Spacecraft Analysis. NASA. Source