MMX Rover (IDEFIX)

DLR, from [1]. CC BY 3.0.

Overview

IDEFIX is a four-wheeled rover built by DLR and CNES to be delivered to Phobos by the JAXA Martian Moons eXploration spacecraft. It is released during an MMX landing rehearsal from about 40 m altitude, falls to the surface, probably bounces several times, and then uprights itself with an autonomous sequence of leg deployments before deploying its solar generator [1]. The whole Separation, Landing, Uprighting and Deployment sequence begins before separation and runs autonomously; the landing point is known only to an uncertainty ellipse of up to 100 m until the first post-uprighting image is compared against orbiter terrain models [5].

Specifications

Parameter	Value
Rover flight segment mass, including units on the spacecraft	27.5 kg
Rover mass	23.47 kg
Scientific payload mass	2.44 kg
Locomotion subsystem mass	4.52 kg
Solar generator mass	3.76 kg
Battery mass	1.43 kg
Nominal driving velocity	1 mm/s
Weight on Phobos	about 0.1 N

Source: [1].

Mission profile

Parameter	Value	Source
Carrier	JAXA Martian Moons eXploration spacecraft
Target	Phobos
Release altitude	about 40 m, during an MMX landing rehearsal
Landing point knowledge before the first post-uprighting image	uncertainty ellipse of up to 100 m	[5]
Traverse requirement	100 m
Nominal surface operation	at least 100 Earth days; three months on the DLR program page	[11]
Launch	2026, H-3 from Tanegashima Space Center	[11]
Landing on Phobos	early 2029	[11]
Sample return to Earth	2031	[11]
Integration status	final work and tests at the CNES site in Toulouse; carbon structure delivered to CNES in November 2022	[11]

Unless marked, the values above are from [1].

Operating environment

Quantity	Value
Dimensions	26.06 x 22.80 x 18.28 km
Mean radius	10.993 km
Mass	1.065e16 +/- 0.015e16 kg
Mean bulk density	1.860 +/- 0.013 g/cm3
Surface gravity	0.0030 to 0.0068 m/s2
Escape velocity	11.39 m/s
Rotation period	7 h 39 min 19.47 s
Equatorial rotation velocity	2.97 m/s
Orbital semi-major axis	9375 km, 2.76 Mars radii
Orbital eccentricity	0.015
Geometric albedo	0.071
Infrared emissivity	0.98
Thermal inertia	20 to 70 J/m2/K/s^0.5
Surface temperature	60 to 330 K within 60 degrees of the equator
Surface slopes	0 to 40 degrees, mostly below 40 with many below 10

Source: [1], [2].

Rows with no marker are from [2].

Regolith particle size is inferred rather than measured. Estimated average particle diameter is below 2 mm, and below 1 mm if thermal inertia is 55 J/m2/K/s^0.5, with surface porosity of 30 to 60 percent and most probably above 53 percent [2]. Fine material below about 300 micrometer may be depleted. The internal friction angle of the particles is estimated at 30 to 50 degrees [2]. A three-layer regolith model is used for design: micrometer-sized fluffy dust from 0 to 3 cm, high-porosity debris from 10 cm to 3 m, and denser regolith below about 10 m. The mission assumes a particle size range of 30 micrometer to 10 cm for wheel interaction analysis [3], against boulders of 2 to 85 m seen from flyby imaging.

The gravity regime is the design problem. At 0.003 to 0.007 m/s2 the rover weighs about 0.1 N, so available traction is correspondingly tiny [1]. Phobos gravity is roughly 1/2000 of Earth’s, 1/750 of Mars’ and 1/250 of the Moon’s, and the Moon is the lowest-gravity body a wheeled rover had driven on before IDEFIX [10]. Scaling free-fall time by the square root of the gravity ratio gives a factor of about 50 between Earth and Phobos, so a rover moving at the roughly 5 cm/s typical of Yutu-2 or Perseverance corresponds to about 50 times slower motion at Phobos gravity [10]. Cohesion between grains exceeds their weight over the relevant particle size range at this acceleration, because van der Waals forces between regolith grains dominate over particle weight, electrostatic forces and solar radiation pressure at asteroid-surface gravity levels [8]. This scaling argument treats terrestrial experiments on cohesive powders as relevant analogues for asteroid surfaces rather than as inapplicable 1-g artifacts [8]. The bulk cohesive strength of a regolith of this grain size is of order tens of pascals [9], which is the same order as the loads a wheel can apply. The same weak inter-grain forces that give a rubble-pile asteroid a small but finite tensile strength are the ones a wheeled rover must overcome to move through the regolith rather than compact or shear it in place [9]. Proposed techniques to measure that strength rely on cratering theory, and have been applied to the Hayabusa2 cratering experiment as a worked example of what an impact or excavation event on a rubble pile can reveal about its regolith [9].

Simulant

The UTPS family is the Phobos simulant used for testing [2].

Simulant	Poured bulk density	Poured porosity	Angle of repose	Tapped density	Tapped porosity
UTPS-S1	0.82 g/cm3	68.7 percent	55.1 degrees	1.51 g/cm3	42.4 percent
UTPS-S2	1.33 g/cm3	53.5 percent	48.4 degrees	1.44 g/cm3	49.7 percent
UTPS-S3	1.67 g/cm3	41 percent	47.1 degrees	1.85 g/cm3	34.6 percent

All values from [2]. The Tagish Lake based variant UTPS-TB has a grain density of 2.8 to 3.0 g/cm3, a bulk density of 1.68 +/- 0.03 g/cm3 and a microporosity of 40.8 +/- 1.0 percent. A factor of two in poured bulk density across three simulants of the same family is the measure of how loosely the target is constrained.

Reduced-gravity penetrometry into Phobos simulant on parabolic flight gives the complementary data: penetration at 3 to 35 mm/s, 42 mm of penetration at 3 mm/s, Froude numbers of 0.18 to 1.8, and the finding that cohesive material returns reaction forces of nearly the same magnitude as at 1 g [6]. Penetrometer tip shape matters at these scales: flat tips give reaction forces that differ from conical and hemispherical tips of the same diameter, which is why the parabolic-flight campaign compared several tip geometries rather than assuming a single design generalizes [6]. Sampler penetration modeling at the Phobos value of 5.7e-3 m/s2 uses friction angles of 20, 42 and 60 degrees against cohesion of 0, 0.5 and 1 kPa [7].

Mobility

The locomotion subsystem comprises four individually actuated wheels of about 200 mm diameter, each mounted on an individually actuated leg 275 mm long, giving skid steering [1]. The sequence for unfolding, standing up, driving, aligning and lowering the rover was designed specifically for Phobos rather than adapted from a planetary rover [3]. It has four modes [1]:

Mode	Function
Driving	Four wheels differentially driven with legs stationary, for straight lines, curves and point turns
Alignment	Legs and wheels moved to change chassis height and orientation with minimal change in position, for aiming instruments or pointing the array at the Sun
Uprighting	Leg angles adjusted while wheels rotate in coordination so the wheels keep rolling on the surface
Passthrough	Manual commanding of each wheel and leg individually

Source: [1].

The open-loop uprighting sequence, frame by frame: the legs unfold and refold repeatedly, each repetition having a high probability of rotating the rover from whatever face it came to rest on onto its belly. No sensor determines the starting orientation, which is why the sequence is identical from every attitude. Source: [1]. CC BY 4.0.

Uprighting is a universal open-loop sequence of repeated leg unfolding and refolding, each repetition having a high probability of rotating the rover from any face onto its belly [1]. It requires no sensor to determine the rover’s current orientation. The rover’s final attitude after the bounces cannot be predicted, and the WheelCams are activated during the uprighting sequence rather than after it [3], [5].

The nominal 1 mm/s driving velocity is not a mechanism limit [1]. The locomotion system can go faster; the cap exists because at Phobos gravity an abrupt stop risks toppling the rover about its front axis [1]. Reduced-gravity penetrometry gives the reason the wheel loads cannot simply be scaled down with gravity: in cohesive material the reaction force is nearly the same magnitude as at 1 g [6]. Entrapment is handled by using the individually actuated legs and wheels to shift weight distribution and lift single wheels out of the regolith, and particularly soft regolith is crossed by inching rather than driving [1].

Wheel sinkage is the quantity the WheelCams exist to measure. Sinkage has a static component from compaction under load and a dynamic component from shearing as the wheel rotates, and its extent is set directly by the shear strength and cohesion of the surface material [3]. Greater sinkage increases the wheel-regolith contact area and is read together with slip ratio, defined so that 0 is perfect rolling and 1 is a wheel spinning in place with no forward motion, as an indicator of trafficability [3]. Wheel motor current, which rises with resistance to motion, is a secondary indicator of the same sinkage and slip behavior. The planned analysis fits sinkage and slip against a Bekker-family terramechanics model, of which the Reece-Wong variant is one candidate because it accounts for the dynamic slip-sinkage behavior expected on Phobos, to back out the regolith’s shear strength and stiffness [3].

Landing dynamics are instrumented. Four three-axis ADXL356 MEMS accelerometers log the impact and bounce sequence to internal memory, with an abridged version relayed in real time to the ascending mothership to confirm a successful landing [1]. Two Silicon Sensing CRM200 gyroscopes give roll and pitch rates during descent and detect excessive roll or pitch while driving. Together with spacecraft imagery these reconstruct the landing and bouncing trajectory and constrain the mechanical properties of the landing site.

Power and thermal

The solar generator has four panels, three deployable and one fixed to the top panel, with a total area of 0.36 m2, feeding one rechargeable battery through a power control and distribution unit [1]. An umbilical from the MMX spacecraft supplies power and communication lines during the cruise, and heats the rover. At a geometric albedo of 0.071 the surface returns very little light to the array, so the available flux is essentially direct solar [2].

Operations are power critical. After an energy-consuming activity such as driving, several Phobos days are needed for recharging, where one Phobos day is 7.65 h [1]. An attitude control sub-unit called SKA reorients the rover around Phobos noon to point the cells at the Sun, which forces locomotion activities into the Phobos morning.

Thermal design is insulation rather than active control: the internal module is inside multilayer insulation and every thermal leak between the units, the chassis and the environment has to be minimized to keep heater power down [1]. Operating modes on the surface are chosen to hold instruments and electronics within thermal limits and minimize heating power. Thermal and power requirements constrain the landing site latitude. The limits themselves are instrument-specific: the miniRAD electronics box is qualified only from -45 to +60 C while its sensor head runs from -140 to +50 C, so the two sit in different thermal zones [4].

Communications

Communication after separation is via an S-band radio to the MMX spacecraft; there is no direct Earth link [1].

Link	Rate
Telecommand	32 kbit/s
Telemetry	64 kbit/s or 512 kbit/s

Rates are from [1]. During the descent itself only a compressed subset of data reaches the spacecraft before it loses contact with the rover [5]. The operational cadence is one Earth-to-spacecraft pass per Earth day and two spacecraft-to-rover contacts per Earth day, and because all commanding and telemetry relay through the mothership, a ground loop that depends on the full downlink of a previously executed activity takes at least 2 Earth days [1].

Payload

Instrument	Mass	Key parameters	Additional source
NavCams (stereo pair)	0.4 kg	122 degree diagonal field of view, 1 mrad angular resolution, 400 to 800 nm, focal length about 7.9 mm, 68 mm stereo baseline, depth of field 35 cm to infinity	[5]
WheelCams	0.22 kg	2048 x 2048 pixels at 5.5 um pitch, 32.5 degree field, about 100 um per pixel at 30 cm, depth of field +/- 5 cm, white and 590 / 720 / 880 nm LEDs	[3]
RAX (Raman spectrometer)	1.51 kg	532 nm laser at 20 mW, 50 um spot, 535 to 680 nm, Raman shift 90 to 4000 cm-1 at about 10 cm-1 resolution, 80 mm autofocus working range
miniRAD (radiometer)	0.34 kg	Six channels over 4.7 to 100 um, 32 degree FWHM field, 25 to 150 cm observation distance	[4]

Mass and key parameters are from [1] throughout.

miniRAD splits into a 96 g sensor head, 209 g of electronics and a 27 g calibration target [4]. Its channel set is a 4.7 to 6.2 um short-wave channel, narrow bandpasses at 8.2, 8.9 and 9.5 um of 0.5 um width, and long-wave channels at 14 to 25 um and 15 to 100 um. Calibration covers 100 to 330 K, with a temperature uncertainty of 1 K above 270 K rising to 5 K on the long-wave channels at 100 K, and a noise-equivalent temperature difference of 6 mK at 270 K on the bandpasses and 40 mK at 100 K on the long-pass channels [4]. Sampling is 0.06 Hz in science and calibration mode and 1 Hz in fast science mode. Power draw is 0.45 W standby, 0.55 W diagnostic, 1.0 to 1.4 W science and 4.0 W calibration [4]. Sensor head operating range is -140 to +50 C, electronics -45 to +60 C, and the calibration target -155 to +60 C.

RAX is placed on the underside of the rover looking down at the ground [1]. Above 18 C it triggers thermal heat switches. miniRAD carries heritage from the Rosetta MUPUS thermal mapper [11], and its purpose extends beyond rover science: the regolith properties the rover characterizes are meant to reduce the risk of the MMX mothership’s own landing and sampling, since the rover is the only source of direct surface-response measurements before those operations take place [11].

The thermal measurement is sensitive to a very thin layer. The diurnal thermal skin depth is close to 5 mm in regolith and of order a few centimeters in boulders, so miniRAD can detect a dust layer of 50 micrometer on regolith and 500 micrometer on a boulder [4]. Its main objective is Phobos’ nighttime surface brightness temperature, from which thermal inertia is derived; secondary objectives are the slope of thermal-infrared emissivity, the location of the Christiansen feature, and surface roughness [4].

The NavCams support digital terrain models at about 1 mm ground sampling over the first meter with an accuracy of a few millimeters at 1 m, which is also how the rover is localized by comparison against the orbiter’s high-resolution terrain models [5].

Beyond localization, the NavCam science goals are framed as open questions rather than confirmed results: whether space weathering on Phobos runs Lunar-like or Tagish-Lake-like, the origin of Phobos’ color dichotomy, whether exogenous material is present on the surface, and whether dust is being transported or levitated across it [5]. The color dichotomy appears concentrated around Stickney crater, which is taken as a hint of a genetic link between the crater and the dichotomy rather than as an explanation of it [5].

Autonomy and ground operations

Driving is commanded day by day from the ground within the MMX planning cycle [1]. Two experimental autonomous navigation systems are being developed in parallel, one by DLR and one by CNES, intended to extend the driving range beyond what daily manual commanding reaches [1], [5]. Autonomous guidance uses the navigation camera stereo pair [5].

Commissioning of the locomotion system is treated as a distinct mission phase, because the mechanical behavior of natural regolith at Phobos gravity is uncertain and wheel-soil traction must be extremely small at a rover weight near 0.1 N [1]. The total distance the rover will actually cover is stated as difficult to estimate; 100 m is the engineering requirement, not a prediction.

The first drives are planned as a validation exercise in their own right: every assumption, tool and method used to design the locomotion system was checked only against experiments and simulations on Earth, with limited knowledge of regolith behavior, so the early on-surface drives are meant to confirm, validate and adapt the design basis rather than simply execute it [10]. That framing follows from the entrapment risk in soft soil at even gentle slopes, which the design team identified as the leading hazard of driving on Phobos before the rover was built [10].

Trafficability in a blocky terrain is an open concern for both landing and driving, which is part of why the wheel-facing cameras are carried at all [3]. Basic driving actions, forward, backward, skid steering and point turns, are deliberately tested early in the surface mission so that a robust set of proven commands exists before more complex behavior such as inching is attempted [10].

Spacecraft activities constrain the rover. During MMX sampling and landing the spacecraft has limited ability to communicate with the rover, so the rover is placed in a safe configuration performing no critical operations and guaranteed not to become power critical for several days without contact [1]. Landing, sampling and especially departure eject substantial surface material, so the rover will if possible move away from the spacecraft landing site to keep dust off its solar cells, and the landing site selection and separation maneuver are designed to limit that exposure.

Radiation

No rover-specific radiation design data is published. Phobos has no atmosphere or magnetic field of its own; the cruise environment is the interplanetary galactic cosmic ray field taken from the Badhwar-O’Neill model [12]. Published dust environment figures for the surface exist but are quoted in the source without a resolvable exponent, so they are not reproduced here.

Technologies developed

IDEFIX is the first attempt at wheeled locomotion on a body in the milligravity regime, and the sensorless uprighting sequence is the direct design consequence of a landing whose final attitude cannot be predicted or measured [1]. The accelerometer and gyroscope record of the landing and bounce is intended as a measurement of the surface, not only as engineering telemetry, which places it in the same category as the OSIRIS-REx contact record as a source of in-situ small body geotechnical data. The wheel sinkage and slip record is the second such product, and is the first direct measurement of wheel-regolith interaction at milligravity rather than an inference from a simulant test at 1 g [3]. Everything the design was sized against on the Phobos side, particle size, porosity and friction angle, is at present inferred from thermal inertia and simulant work rather than measured [2].