Conference PaperPDF Available

A rover for the JAXA MMX Mission to Phobos

October 2019

Conference: IAC 2019

Authors:

S. Ulamec

German Aerospace Center (DLR)

Patrick Michel

Observatoire de la Côte d'Azur

Matthias Grott

German Aerospace Center (DLR)

Show all 38 authorsHide

The Martian Moons eXploration (MMX) is a mission by the Japan Aerospace Exploration Agency, JAXA, to the Martian moons Phobos and Deimos. It will primarily investigate the origin of this moon by bringing samples back from Phobos to Earth and deliver a small (about 25 kg) Rover to the surface. The Rover is a contribution by the Centre National d’Etudes Spatiales (CNES) and the German Aerospace Center (DLR). Its currently considered scientific payload consists of a thermal mapper (miniRAD), a Raman spectrometer (RAX) a stereo pair of cameras looking forward (NavCAM) and two cameras looking at the interface wheel-surface (WheelCAM) and consequent Phobos’ regolith mechanical properties. The cameras will serve for both, technological and scientific needs. The MMX rover will be delivered from an altitude of <100 m and start uprighting and deploying wheels and a solar generator after having come to rest on the surface. It is planned to operate for three months on Phobos and provide unprecedented science while moving for a few meters to hundreds of meters. MMX will be launched in September 2024 and inserted into Mars orbit in 2025, the Rover delivery and operations are planned for 2026-2027

Schematic view of NavCAM.

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Image sensor of a WheelCAM (CASPEX module) [14].

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Camera emplacement. Schematic showing the respective locations and viewing angles of the NavCAMs and WheelCAMs.

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Locomotion system. The image shows one leg including the wheel and the actuators IV.V Communication

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RF communication on the spacecraft : RF SBand and patch antenna This communications subsystem allows communication up to 100 km, which covers all mission phase of the Rover while the MMX spacecraft is in visibility of the Rover. The data rate for the telecommand is 32 kbit/s whereas the data rate for the telemetry can go up to 512 kbit/s.

…

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IAC-19-A3.4.8 Page 1 of 8

IAC-19-A3.4.B8

A ROVER FOR THE JAXA MMX MISSION TO PHOBOS

Stephan Ulamec1*, Patrick Michel2, Matthias Grott3, Ute Böttger3, Heinz-Wilhelm Hübers3, Naomi Murdoch4

Pierre Vernazza5, Özgür Karatekin6, Jörg Knollenberg3, Konrad Willner3, Markus Grebenstein7, Stephane Mary8,

Pascale Chazalnoël8, Jens Biele1, Christian Krause1, Tra-Mi Ho9, Caroline Lange9, Jan Thimo Grundmann9, Kaname

Sasaki9, Michael Maibaum1, Oliver Küchemann1, Josef Reill7, Maxime Chalon7, Stefan Barthelmes7, Roy

Lichtenheldt7, Rainer Krenn7, Michal Smisek7, Jean Bertrand8, Aurélie Moussi8, Cedric Delmas8, Simon Tardivel8,

Denis Arrat8, Frans IJpelaan8, Laurence Mélac8, Laurence Lorda8, Emile Remetean8, Michael Lange10, Olaf

Mierheim10, Siebo Reershemius9, Tomohiro Usui11, Moe Matsuoka11, Tomoki Nakamura12, Koji Wada13, Hirdy

Miyamoto14, Kiyoshi Kuramoto15, Julia LeMaitre8, Guillaume Mas8, Michel Delpech8, Loisel Celine8, Arthur

Rafflegeau8, Honorine Boirard8, Roseline Schmisser8, Cédric Virmontois8, Celine Cenac-Morthe8, Dominique

Besson8, Fernando Rull16

1 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 51147 Cologne, Germany

2 Université Côte d’Azur, Observatoire de la Côte d'Azur, CNRS, Laboratroire Lagrange, 06304 Nice, France

3 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 12489 Berlin, Germany

4 Institut Supérieur de l'Aéronautique et de l'Espace (ISAE-SPAERO), 31055 Toulouse, France

5 Laboratoire d’Astrophysique de Marseille (LAM), 13388 Marseille, France

6 Royal Observatory of Belgium (ROB), Bruxelles, Belgium

7 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 82234 Oberpfaffenhofen, Germany

8 Centre National d’Etudes Spatiales (CNES), 31401 Toulouse, France

9 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 28359 Bremen, Germany

10 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), 38108 Braunschweig, Germany

11 Japan Aerospace Exploration Agency (JAXA), ISAS, 252-5210 Sagamihara, Japan

12 Tohoku University, 980-8578 Sendai, Japan,

13 Chiba Institute of Technology, 275-0016Narashino, Japan

14 University of Tokyo, 113-0033 Tokyo Japan

15 Hokkaido University, Sapporo 060-0810, Japan

16 Universidad de Valladolid – Centro de Astrobiología, 47151 Valladolid, Spain

* Corresponding author: Stephan Ulamec, DLR, stephan.ulamec@dlr.de

The Martian Moons eXploration (MMX) is a mission by the Japan Aerospace Exploration Agency, JAXA,

to the Martian moons Phobos and Deimos. It will primarily investigate the origin of this moon by bringing samples

back from Phobos to Earth and deliver a small (about 25 kg) Rover to the surface.

The Rover is a contribution by the Centre National d’Etudes Spatiales (CNES) and the German

Aerospace Center (DLR). Its currently considered scientific payload consists of a thermal mapper (miniRAD), a

Raman spectrometer (RAX) a stereo pair of cameras looking forward (NavCAM) and two cameras looking at the

interface wheel-surface (WheelCAM) and consequent Phobos’ regolith mechanical properties. The cameras will

serve for both, technological and scientific needs.

The MMX rover will be delivered from an altitude of <100 m and start uprighting and deploying

wheels and a solar generator after having come to rest on the surface. It is planned to operate for three

months on Phobos and provide unprecedented science while moving for a few meters to hundreds of meters.

MMX will be launched in September 2024 and inserted into Mars orbit in 2025, the Rover delivery

and operations are planned for 2026-2027.

I. INTRODUCTION

The MMX Rover is a contribution by the Centre

National d’Etudes Spatiales (CNES) and the German

Aerospace Center (DLR) to the Mars Moon Explorer

(MMX), a mission by the Japan Aerospace Exploration

Agency, JAXA, to the Martian Moons Phobos and

Deimos [1,2]. It will be delivered to the surface of

Phobos to perform in-situ science but also to serve as a

scout, preparing the landing of the main spacecraft. The

IAC-19-A3.4.8 Page 2 of 8

MMX Rover will deliver information on the regolith

(geometrical and mechanical) properties by high

resolution imaging of the interface of the wheels with

the surface. Even movies are foreseen.

Phobos exploration with spacecraft goes back to the

1970s, when Mariner 9 took the first images of the

Martian moons. Since then several spacecraft have

taken images (or spectral data) of Phobos, from orbit or

from the Martian surface, respectively [3]. The Origin

of both Phobos and Deimos, however, are still not clear

[4]. Two possible scenarios are currently discussed

among the science community: asteroid capture [5] or

formation after a major impact on Mars [6-8]. The

MMX Mission targets to solve this puzzle by measuring

in great details the properties of Phobos through

measurements from orbit and on the surface and by

returning samples to Earth.

Despite three attempts (Phobos 1 and 2 in 1988 and

Phobos-Grunt in 2011, all were lost before arrival at

Phobos) and several mission studies [e.g. 9-13], there

has been no dedicated mission to the Martian moons,

yet.

II. SCIENTIFIC OBJECTIVES

The Scientific Objectives of the MMX Rover are

defined in line with those of the overall MMX Mission,

however, complementing the science which can be

performed with the instruments onboard the main

spacecraft or the samples returned.

The data provided by the rover thanks to its

instrument suit (see below) are of high interest for the

communities interested in the regolith dynamics in the

low gravity environment of Phobos, in surface processes

and in the geological history of Phobos, in the

composition of Phobos and in its thermal properties.

This data set obtained in-situ is of high value for the

interpretation of data obtained remotely by the

spacecraft by adding ground truth, and to provide a

geological context to the samples that will be returned

to Earth in order to clarify the origin and history of

Phobos.

The Rover will perform:

 Regolith science (e.g. dynamics,

mechanical properties like surface strength,

cohesion, adhesion; geometrical properties

like grain size distribution, porosity)

 Close-up and high resolution imaging of

the surface terrain

 Measurements of the mineralogical

composition of the surface material (by

Raman spectroscopy)

 Determination of the thermal properties of

the surface material (surface temperature,

thermal capacity, thermal conductivity)

This will allow determination of the heterogeneity of

the surface material and thus will also help defining the

landing and sampling strategy.

Characterization of the regolith properties shall

considerably reduce the risk of the landing (and

sampling) of the main spacecraft, as the touch-down

strategy can be adapted accordingly.

III. INSTRUMENTS

The Rover design allows the accommodation of four PI

(Principal Investigator) instruments, which are:

 NavCAMs

 WheelCAMs

 Raman Spectrometer – RAX and

 miniRAD

During phase A also a gravimeter (GRASSE), provided

by the Royal Observatory of Belgium as well as a

lightweight ground penetrating radar (GRAMM) to be

provided by the Technical University Dresden together

with the University Tokyo have been studied. However,

the current limitations regarding mass and volume led to

a removal of those instruments from the baseline

configuration.

III.I NavCAM

The NavCAMs will be used for autonomous

navigation but also to image the landscape and place

some constraints on the level of heterogeneity of the

regolith both in terms of composition and space

weathering alteration with a higher spatial resolution

with respect to the orbiter. Figure 1 shows a schematic

view of the stereo pair of cameras, Figures 2 and 4 the

position aboard the rover.

They will thereby provide context information for

the remaining rover instruments.

Fig. 1: Schematic view of NavCAM.

IAC-19-A3.4.8 Page 3 of 8

Figure 2: Rover in fully deployed configurations with

positions of MiniRad, rear WheelCAM and NavCAM

stereo bench

III.II WheelCAM

There will be two identical WheelCams on the rover

(Figures 3 and 4). These panchromatic cameras have a

2048 x 2048 pixel resolution and a spatial resolution of

approximately 35µm at the center of the image. The

WheelCAMs will be used to image the wheels and their

interaction with the regolith. By observing the

properties of the regolith compaction and flow around

the wheels it will be possible to characterise the

mechanical properties of the regolith itself. In addition,

the WheelCAMs spatial resolution will be sufficient to

characterise the size distribution of regolith particles

and their angularity. Such information is not only

important for understanding the surface properties of

Phobos and how regolith behaves in a low gravity

environment, but this knowledge about the surface will

also lower the risk of the MMX mission.

The CMOS camera sensor for the WheelCAMs, as

well as the NavCAMs has been developed by 3DPLUS

under CNES contract [14].

Figure 3: Image sensor of a WheelCAM (CASPEX

module) [14].

Figure 4: Camera emplacement. Schematic showing the

respective locations and viewing angles of the

NavCAMs and WheelCAMs.

III.III RAX

RAX (Raman spectrometer for MMX) is a compact,

low-mass Raman instrument with a volume of

approximately 81х98х125 mm³ and a mass of less than

1.4 kg developed by DLR, INTA/UVA and

JAXA/UTOPS/Rikkyo. It will perform Raman

spectroscopic measurements to identify the mineralogy

of the Phobos surface. The RAX data will support the

characterization of a potential landing site for the MMX

spacecraft and the selection of samples for their return

to Earth. The RAX measurements will be compared

with Raman measurements obtained from the RLS

instrument during the ExoMars 2020 mission, to

provide evidence for the Martian or non-Martian origin

of the surface minerals of Phobos. Furthermore, RAX

measurements will be compared with Raman

measurements performed in Earth laboratories on the

returned samples for better interpretation of the data

acquired. Figure 5 shows a schematic of the

spectrometer module.

IAC-19-A3.4.8 Page 4 of 8

Figure 5: RAX spectrometer module (RSM)

III.IV miniRAD

The miniRAD instrument will investigate the

surface temperature and surface thermo-physical

properties of Phobos by measuring the radiative flux

emitted in the thermal infrared wavelength range.

miniRad will use thermopile sensors to measure flux in

6 wavelength bands and obtain information on regolith

[15,16] and boulder [17] thermo-physical properties.

The instrument will thus directly address or contribute

to addressing fundamental MMX science objectives and

provide a basic picture of surface processes on airless

small body. Furthermore, miniRAD will help to

characterize the space environment and the surface

features of Phobos, thus enabling a comparison with

asteroids. The instrument has strong heritage from the

MASCOT radiometer MARA [18].

Figure 6: Conceptual design of the miniRAD sensor

head showing the fields of view of the six individual

infrared channels. The instrument’s field of view will

overlap that of the rover’s navigation stereo cameras.

IV. SYSTEM OVERVIEW

The overall Rover system consists of 2 ground segments

(one in Germany and one in France) and the flight

hardware. The communication between the Rover

ground segment and the flight segment is linked via the

JAXA MMX mission (JAXA ground segment, ground

stations and MMX main spacecraft).

The MMX Rover flight system consists of the

Mechanical Support System (MECSS) which provides

interface (mechanical fixation, electrical link…)

between the Rover and the main spacecraft and allows

the release and the ejection of the Rover. The RF

communication hardware (antenna and RF electronic

box) aboard the spacecraft is also part of the Rover

flight hardware.

The Rover itself has an internal module referred to

as Service Module (SEM), thermally insulated from the

outside and supporting the electronics, batteries, attitude

sensor and instruments. The service module is

embedded into the chassis (outside walls) which support

the locomotion system and solar generator.

The overall mass of the Rover Flight System (incl.

the units on the main S/C) is 29.1 kg. (see Table 1 for a

mass breakdown). The payload, including the cameras

has an overall mass of about 2.6 kg. Figure 7 shows the

Rover, attached to the MECSS, wheels and solar

generator folded, as in launch/cruise configuration.

Figure 8 shows the rover assembly.

Mass [kg]

Electronics (incl. OBC, battery …)

4.7

Attitude sensor

1.1

Service module, SEM (structure,

harness…)

3.8

Cameras (all cameras, bench…)

1.0

RAX

1.5

miniRad

0.3

Chassis (structure, shutters, harness…)

4.4

Mobility (motors, legs, wheels)

5.6

Solar array

3.4

∑ Rover

25.8

MECSS

2.3

RF hardware on main spacecraft

1.0

∑

29.1

Table 1: Mass breakdown of MMX Rover

IAC-19-A3.4.8 Page 5 of 8

Figure 7: MMX Rover in launch configuration

Figure 8: Exploded view of Rover assembly, showing

Service Module, chassis, locomotion system and solar

generator.

IV.I Structure

The two rover system primary structures, Chassis

and MECSS, feature a carbon fibre-reinforced plastic

(CFRP) sandwich designs.

The MECSS is a single sandwich plate design with a

30 mm aluminium honeycomb core and 1.5 mm CFRP

face sheets. Next to providing an interface to the MMX

S/C, the four corners are part of an integrated rover

bearing concept (see Fig. 9). This combines the

interface to the MMX main spacecraft and the rover

bearing as well as the Hold-Down and Release

Mechanism (HDRM) in one integral insert. Thus, the

rover is clamped right at its four bearings, which are

aligned concentrically to the four HDRM frangibolts.

Further the MECSS features a Push-Off Mechanism that

is based on the MASCOT design [19]. After releasing

the HDRM, the Push-Off mechanism will stabilize the

rover and provide it with the required separation energy.

Similarly to the MECSS, the Rover Chassis has two

structural main interfaces, one towards the MECSS and

the other towards the Service Module. During launch

the chassis must remain secured on the MECSS and also

support the Service Module. Once separated from the

MECSS and the MMX spacecraft, respectively, the

Rover chassis must sustain the landing impact on

Phobos surface keeping its structural integrity. Thus, the

chassis features an integral base element and separable

panels closing the base element. Additionally, there are

titanium flanges next to each locomotion unit, which are

part of the base element and redirect loads into the

removable front and rear panel. A top panel, which also

supports the solar generator, is closing the chassis.

Except for the top frame and the locomotion interface

flanges, all parts are sandwich plates (5 mm aluminium

honeycomb core and 0.5 mm CFRP face sheets on each

side). Only the chassis bottom plate, which includes the

main interface with the SEM and the MECSS, as well as

three shutters and the umbilical, is a sandwich plate

including a 10 mm aluminium honeycomb core and

1 mm face sheets.

Figure 9: Top view of the MECSS with central Push-

Off Mechanism. In the four corners a combined

interface to the MMX S/C and the Rover Chassis,

respectively, including the HDRM.

Rover I/F

incl. HDRM

Push-Off

Mechanism

MMX S/C I/F

IAC-19-A3.4.8 Page 6 of 8

IV.II Thermal Concept

The thermal control of the MMX Rover is quite

challenging due to the cold environment of Phobos and

the low level of electrical power available for rover

operations and control. In addition, some internal

equipment temperature requirements are difficult to

reach.

Power dissipated by the units inside the service

module has to be kept inside as much as possible to

reduce the need of additional heating power.

Consequently, all the thermal leaks between the internal

module, the chassis and the environment have to be

minimized.

During cruise, the Rover will be heated by the

orbiter via an umbilical to power cruise heaters and read

out thermal sensors for regulation.

On the surface of Phobos the operational modes

have to be chosen carefully to keep the instruments and

electronics within the thermal limits and minimize the

required heating power.

Thermal and power requirements limit the latitudes

on Phobos, where the Rover can be operated.

IV.III Power System

The MMX Rover power architecture is composed

of 3 deployable solar panels and one fixed on the top

panel of the rover, one rechargeable battery and one

power conversion and distribution unit (PCDU). A

particularity for this rover is a connection with MMX

spacecraft umbilical via MECSS equipment. This will

enable the rover to be powered by the spacecraft during

cruise phase. The umbilical and its separation connector

pair are based on technologies already used on PHILAE

and developed further for MASCOT [19].

Power management on board during the operation

phase on Phobos is critical because the solar flux is

limited. To guarantee a positive power budget, the

Rover shall point the solar panels toward the Sun, using

the locomotion subsystem while stationary.

IV.IV Locomotion Concept

The technology for locomotion on the surface of Phobos

is one of the major endeavors of the MMX rover. As the

rover is very compact and light weight, the locomotion

concept is based on four wheels only. No additional

steering actuators are used. The rover features four legs

to bring itself into nominal orientation from any initial

position after landing phase. The sequence of

positioning the four legs to the ground depends on the

achievement of nominal orientation and needs to be

selected autonomously. To increase flexibility on that,

the upper joint is able to perform one full rotation.

Figure 10 shows the main module of the locomotion

system, one leg of the rover.

The wheels have been designed in order to

maximize the likelihood of success even in case of very

soft soil thanks to advanced particle simulations.

The drive train is designed to fulfill three purposes:

 execute the self-righting moves required to

bring the rover in an upward position after

landing,

 drive the rover to a location on the surface

of Phobos,

 orient the body of the rover in order to

maximize the performance of the science

instruments and to maximize the energy

flux on the panels.

The actuators and sensors for the four wheels and

legs are all placed inside the rover, to provide a more

advantageous thermal configuration at the cost of a

more complex mechanical design. The actuators are

three phase brushless motor, with a nominal six-step

commutation mode and a stepper mode commutation in

case of sensor error. The leg position is measured

redundantly with two potentiometer technology, a

classical resistive track with grabber and a foil

potentiometer. Finally, a very sensitive joint torque

sensor is used as a safety mechanism to detect

extraordinary torques applied to the legs, most likely

indicating a large rock or an unexpected wheel sinkage.

The motion controller electronics is divided into

three PCBs. Two of them include the power inverter

electronics for providing the motor phase voltages. One

PCB is going to measure the sensor signals, ensure

communications link to OBC via SpaceWire and

compute the control algorithms for all eight actuators.

The MMX Rover is equipped with all the software

and hardware needed to perform autonomous navigation

to overcome the limitations of receiving telemetry and

sending commands between Phobos and Earth, leading

to a drastic reduction of effective speed by “manual”

operations. The rover can create digital elevation maps,

traversability maps and detect obstacles. Therefore,

once it is confirmed that the rover can drive on the

surface, combined with its path planning and scheduling

software, the rover will be able to safely navigate on the

surface of Phobos, covering much greater distances and

collecting more scientific data. One of the important

activities on ground will be the modeling of the ground

properties based on the collected data in order to drive

faster and more efficiently. Autonomous driving is a

prerequisite in the design of roving vehicles to explore

celestial bodies even more distant than Mars.

IAC-19-A3.4.8 Page 7 of 8

Figure 10: Locomotion system. The image shows one

leg including the wheel and the actuators

IV.V Communication

The RF system for the Rover – main spacecraft

communication link will be managed on Rover and

spacecraft side by the rover system.

On board the main spacecraft an S-Band

communication system with a patch antenna will

communicate with the Rover (see Fig. 11).

Figure 11: RF communication on the spacecraft : RF S-

Band and patch antenna

This communications subsystem allows

communication up to 100 km, which covers all mission

phase of the Rover while the MMX spacecraft is in

visibility of the Rover.

The data rate for the telecommand is 32 kbit/s

whereas the data rate for the telemetry can go up to 512

kbit/s.

V. OPERATIONS CONCEPT

The MMX Rover will be commanded via the MMX

main spacecraft, operated at JAXA. All TMTC will be

linked via the main spacecraft as relay. During cruise,

the rover is connected to MMX via an umbilical, after

separation an S-band RF system will be used.

The Rover itself will be operated from two control

centers, at CNES Toulouse and at DLR/MUSC in

Cologne, respectively.

Operations will be divided into several mission phases:

 Launch and Cruise (incl. commissioning, health

checks)

 Separation-Landing-Upright-Deployment (SLUD)

phase (from separation from the main spacecraft, to

descent, bouncing-phase to the quasi-autonomous

up-righting and solar generator deployment)

 Phobos commissioning phase, to check the

functionality of the Rover, its subsystems and

payload.

 Phobos Operational (Driving & Science) Phase

with a life-time of >100 days (including previous

commissioning phase) on the surface of Phobos.

During this phase different locomotion modes

(including automated) will be tested and

instruments will perform measurements at different

locations on Phobos´ surface.

 End of Mission phase (finally passivating the

Rover).

Figure 12 shows the Rover with its solar generator

deployed as it would be in the Phobos operational phase

on the surface of Phobos.

Figure 12: MMX Rover with deployed wheels and solar

panels in on-surface configuration

VI. CONCLUSIONS AND OUTLOOK

The Rover is currently in its phase B and will go

through development and qualification in the coming

years. The logic of development of the Rover is based

on a protoflight approach. The Rover AIT shall start in

2022 and the Flight Model will be delivered to JAXA in

early 2023. The MMX mission is to be launched in

2024.

IAC-19-A3.4.8 Page 8 of 8

Rover operations are currently foreseen for at least

100 days, in 2026. It is currently planned, that MMX

will leave Mars orbit to return to Earth in 2028 [2].

ACKNOWLEDGEMENTS

MMX is a JAXA mission with contributions

from NASA, CNES and DLR. The Rover will be

provided by CNES and DLR with science

contributions from Japan and Spain.

The authors would like to thank the teams of MMX

and the Rover as well as the programmatic support to

start this project.

DLR Bremen acknowledges the contribution of

nano-spacecraft developments locally shared with

GOSSAMER-1, ROBEX, and AISAT-1. P.M.

acknowledges funding support from CNES.

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Knollenberg, and M. Hamm (2018): Possibility of

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MARA temperature measurement, Icarus, Vol.

333, pp. 318-322, 2019.

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Knollenberg: A Method to Derive Surface

Thermophysical Properties of Asteroid (162173)

Ryugu (1999JU3) from In-Situ Surface Brightness

Temperature Measurements. Planet. and Space

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Expansion and Experimental Evaluation of Scaling Relations for the Prediction of Wheel Performance in Reduced Gravity

Article

Full-text available

Nov 2023
MICROGRAVITY SCI TEC

Traversing granular regolith, especially in reduced gravity environments, remains a potential challenge for wheeled rovers. Mitigating hazards for planetary exploration rovers requires testing in representative environments, but direct Earth-based testing fails to account for the effect of reduced gravity on the soil itself. Granular scaling laws (GSL) have been proposed in the literature to predict performance of a larger wheel based on tests with a smaller wheel, or to predict performance in one gravity level based on tests in another gravity level. However, this is the first work to experimentally validate GSL in reduced gravity. Here, an expanded version of existing GSL was evaluated experimentally by measuring performance of a single wheel driving through cohesionless lunar soil simulant GRC-1 aboard parabolic flights that reproduce the effects of lunar gravity, and comparing those results to scaled tests performed on the ground. This scaled-wheel testing achieved less than 10% prediction error on three measured output metrics: drawbar pull (i.e. net traction), sinkage, and power draw. Predictions also erred on the conservative side. Subsurface soil imaging revealed similar soil behavior between scaled tests. GSL thus offers an accurate and conservative method for predicting wheel performance in reduced gravity based on 1-g experiments, at least in cohesionless soil.

Evaluation of Microlenses, Color Filters, and Polarizing Filters in CIS for Space Applications

Article

Full-text available

Jun 2023
SENSORS-BASEL

For the last two decades, the CNES optoelectronics detection department and partners have evaluated space environment effects on a large panel of CMOS image sensors (CIS) from a wide range of commercial foundries and device providers. Many environmental tests have been realized in order to provide insights into detection chain degradation in modern CIS for space applications. CIS technology has drastically improved in the last decade, reaching very high performances in terms of quantum efficiency (QE) and spectral selectivity. These improvements are obtained thanks to the introduction of various components in the pixel optical stack, such as microlenses, color filters, and polarizing filters. However, since these parts have been developed only for commercial applications suitable for on-ground environment, it is crucial to evaluate if these technologies can handle space environments for future space imaging missions. There are few results on that robustness in the literature. The objective of this article is to give an overview of CNES and partner experiments from numerous works, showing that the performance gain from the optical stack is greater than the degradation induced by the space environment. Consequently, optical stacks can be used for space missions because they are not the main contributor to the degradation in the detection chain.

The wedge-pentahedra method (WPM): Topographic reduction of local terrain in the context of solar system surface gravimetry and robotic exploration

Article

Full-text available

Sep 2022

In classical gravimetry, different corrections are applied, e.g. to correct for the measurement elevation above a reference plane and the gravitational attraction of the material lying between the measurement point and reference plane. Additionally, and especially in non-flat regions, a correction for the topography is generally needed. While this contribution is relatively small on spherical celestial objects, it can be more important for irregularly shaped bodies, such as small bodies or some natural satellites. With the surface gravity being much smaller, the relative importance of the topographic correction increases, while the approximation errors of the surface will become larger. In this work, the novel Wedge-Pentahedra Method (WPM) for topographic correction for (near-) surface gravimetric measurements and simulations is presented that allows precise topographic corrections for asteroids and natural satellites. For a first study, the WPM is applied to the Martian moon Phobos. Taking an exemplary surface location, a high-resolution artificial terrain is added to the surrounding, and the gravitational influence of this topography compared to the original surface is assessed. It is found that the influence of topography on the surface gravity of a small body such as Phobos can be in the order of a few percent, making it an important correction not only for surface gravity science, but likewise for landing and surface operations, to best ensure the mission success. Therefore, the here presented WPM opens a manifold of possible future applications in the context of Solar System exploration, regarding both space science and space technology.

The importance of Phobos simulants: a review on our current knowledge

Article

Full-text available

Jun 2023

Phobos, a satellite of Mars, was successfully studied by flyby, orbiter, and landing missions to the Red Planet, but several questions remain about its origin, composition, and relationship to Mars. It is suggested that Phobos is either a captured body from the asteroid belt or the outer Solar System (capture scenario), or a consequence of re-accreted ejecta from Mars ( in situ formation/giant impact). So far, Phobos has been characterized by its two spectral units - blue and red - with different compositional restrains. The red unit represents most of the surface, while the blue unit is focused on the Stickney crater and surroundings. In the absence of samples returned from this satellite, simulant regolith must be studied to infer various proprieties, and complement in situ studies. To date, there are three simulants of this satellite: Phobos-1C, Phobos Captured Asteroid-1 (PCA-1), and Phobos Giant Impact-1 (PGI-1). Since Phobos may have a Mars-like composition, terrestrial analogues of Mars should also be analysed. The data retrieved from the various assays performed with these planetary field analogues may be used as a database to complement future space missions to Phobos, but, ultimately, the composition of Phobos will have to be analysed by a sample-return mission.

Testing for the MMX Rover Autonomous Navigation Experiment on Phobos

Conference Paper

Mar 2023

Fault Detection, Isolation and Recovery in the MMX Rover Locomotion Subsystem

Conference Paper

Mar 2023

In any mechatronic system, faults can occur. Likewise also in the MMX rover, which is a wheeled rover mutually developed by CNES (Centre national d’ ́etudes spatiales) and DLR (German Aerospace Center), intended to land on Phobos. An essential part of the MMX rover is the locomotion subsystem which includes several sensors and eight motors actuating the four legs and the four wheels. In each of these components and their interfaces, there is a possibility that faults arise and lead to subsystem failures, which would mean that the rover cannot move anymore. To reduce this risk, the possible faults of the MMX locomotion subsystem were identified in a FMECA study and their criticality was classified, which is presented in here. During this examination, the criticality was graded depending on different mission phases. With the help of this study, the hardware, firmware and software design were enhanced. Fur- ther, certain fault detection, isolation and recovery strategies were implemented in the locomotion firmware and software as well as in the full rover software.

MMX Rover Simulation - Robotic Simulations for Phobos Operations

Conference Paper

Mar 2022

The MMX Rover, developed by CNES and DLR, will fly to and explore the surface of the Martian Moon Phobos within the JAXA Martian Moon Exploration Mission. It will be the first wheeled locomotion system in a milli-g environment. In the development of the rover, simulations have been used to test and develop its robotic activities. This paper presents the multi-physics simulations that are being used. The overall simulator setup and its main components are discussed. To provide appropriate simulations for the var-ious topics while maintaining a unified simulator, a modular approach was required. The different modules and their role will be outlined. For this, Dymola's implementation of the Mod-elica modeling language provides the basis, especially regarding multi-body dynamics, and the possibility to include external libraries, e. g. for environment interaction, control logic and visualization. Finally, examples for the simulator used in driving, uprighting, alignment and separation will be presented. These examples illustrate the approach on experiment design, setup and result evaluation. To date the MMX Rover simulator is regarded as an indispensable development and analysis tools, especially since representative lab experiments are much limited when designing a robotic system for milli-g operations. It is also planned to be used during operations phase for planning and analysis.

Toolchain for a Mobile Robot Applied on the DLR Scout Rover

Conference Paper

Mar 2022

This paper motivates and details the tools used to control a planetary exploration rover prototype and its simulation twin. The novelty is the coupling of the prototype and the simulation. The prototype and simulation of the rover use the same software tools and code for communication, signal processing, data storage and drive control algorithms. The presented toolchain is unprecedented for mobile robot to the best of the authors' knowledge, yet generic and versatile enough that it can easily be adapted to different kinds of robotic systems as the full paper will show. Advantages and drawbacks as well as comparison to other solutions are also key content of this text. DLR's Institute of System Dynamics and Control (SR) is developing a new mobile robot called \textquotedblScout\textquotedbl that will serve as the example of the toolchain. The Scout rover is a robust cave exploration rover, based on rimless wheels and biologically inspired locomotion mechanisms. It consists of segments interconnected by elastic \textquotedblvertebrae\textquotedbl in longitudinal direction. Some of the special features of this rover design can only reasonably be unfold thanks to the toolchain, e.g. the bionic phase shift control or simulative exploration of different rover and wheel configurations. The development of the Scout rover relies on modeling, simulation and optimization and follows the \textquotedblV\textquotedbl-shaped paradigm of system development. Elements of rapid control prototyping are implemented and commercial of the shelf sensors, actuators and computation units integrated. This has rapidly led to a robust mobile robot that is capable to cross obstacles that are larger than the radius of the wheels. The Scout rover development relies on the Modelica language for guiding and safeguarding virtual design decisions as well as for rapid control prototyping using the commercial Modelica modeling and simulation environment Dymola. Using this toolchain is a good compromise for satisfying the requirements in terms of rapid control prototyping. It has effective facilities for modeling the challenging physics for reliable Model-in-the-Loop simulations, at the same time the model of the controller can be directly used on the prototype's hardware. A well-thought-out model structure allows a transparent and seamless swapping between virtual model components and model components which drive physical drives and sensors. The actuators of the prototype are addressed using the EtherCAT protocol, communication with sensors and human operators goes through a light-weight in-house middleware and logged data is stored in the popular HDF5 format. Coordination of the actuators and reaction to user input in a state machine like manner is implemented with scripts in the Lua language. Steps for further work include integration of cameras and implementation of obstacle detection, additional control algorithms and more autonomy. Operations in the field of a space mission is envisioned around 2030. The presented toolchain is thought-out enough that this won't require major changes even when COTS parts need to be replaced by space qualified hardware. Likewise, the simulation of the rover will remain an important activity.

Trajectory-based generic chassis control framework for the MMX-Rover

Conference Paper

Mar 2022

Martian Moons eXploration (MMX) is a joint project of the French, German and Japanese space agencies Centre national d'etudes spatiales (CNES), German Aerospace Center (DLR) and Japan Aerospace Exploration Agency (JAXA) for an exploration mission to the moons of Mars, i.e. Phobos and Deimos. CNES and DLR are providing a rover payload to the mother spacecraft of JAXA which is designated to be the first rover to drive in milli-g-environment. Locomotion in milli-g-environment on potentially hazardous terrain with steep slopes and high sinkage raises increased demands on the locomotion subsystem and control algorithms. The MMX-Rover chassis control algorithms (CCA) are embedded as part of the MMX-Rover locomotion software partition in a three layer architecture between the Command and Control Software and the hardware-related Basic Software. This paper proposes a trajectory-based generic chassis control framework as extended chassis control variant for the MMX-Rover. The rover locomotion possibilities are formulated upon a generic rover motion interface to derive analytical solutions for the specialized MMX-Rover locomotion modes. The resulting algebraic chassis control algorithms (A-CCA) are mathematically formulated in detail. A connection to meta-modelling for kinematic locomotion functions is established. The generic chassis control framework can be used to formulate the onboard control algorithms as minimal application code or generic application code. The decision between the minimal or generic code formulation variant practically influences the control software architecture and implementation. The generic chassis control framework is used as a practical analysis and verification tool to ensure the qualification of a geometrically inspired and kinematically simplified control algorithm variant (G-CCA) as flight software candidate. In particular, the algebraic equations originating from the generic chassis control framework are compared to the geometric control algorithms regarding their functional scope, kinematic formulation and practical software implementation. Results for the analysis and verification of G-CCA are shown and their qualification in terms of the locomotion functional coverage and rover motion behavior discussed. The generic chassis control framework contributes to ensure and enhance the functioning and quality of the control algorithms in the MMX-Rover mission context.

MMX Rover Locomotion Subsystem - Development and Testing towards the Flight Model

Conference Paper

Mar 2022

Wheeled rovers have been successfully used as mobile landers on Mars and Moon and more such missions are in the planning. For the Martian Moon eXploration (MMX) mission of the Japan Aerospace Exploration Agency (JAXA), such a wheeled rover will be used on the Marsian Moon Phobos. This is the first rover that will be used under such low gravity, called milli-g, which imposes many challenges to the design of the locomotion subsystem (LSS). The LSS is used for unfolding, standing up, driving, aligning and lowering the rover on Phobos. It is a entirely new developed highly-integrated mechatronic system that is specifically designed for Phobos. Since the Phase A concept of the LSS, which was presented two years ago [1], a lot of testing, optimization and design improvements have been done. Following the tight mission schedule, the LSS qualification and flight models (QM and FM) assembly has started in Summer 2021. In this work, the final FM design is presented together with selected test and optimization results that led to the final state. More specifically, advances in the mechanics, electronics, thermal, sensor, firmware and software design are presented. The LSS QM and FM will undergo a comprehensive qualification and acceptance testing campaign, respectively, in the first half of 2022 before the FM will be integrated into the rover

Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu

Article

Full-text available

Nov 2019
Nat. Astron.

C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1–3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias⁶ in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry⁷.

A method to derive surface thermophysical properties of asteroid (162173) Ryugu (1999JU3) from in-situ surface brightness temperature measurements

Article

Full-text available

Mar 2018
PLANET SPACE SCI

The MASCOT radiometer MARA on board the Hayabusa2 mission will measure surface brightness temperatures on the surface of asteroid (162173) Ryugu in six wavelength bands. Here we present a method to constrain surface thermophysical properties from MARA measurements. Moreover, uncertainties when determining surface thermal inertia as well as emissivity are estimated. Using data from all filters and assuming constant emissivity, thermal inertia of a homogeneous surface can be determined with an uncertainty range of 250 ±16 Jm⁻²K⁻¹s−1/2, while the emissivity uncertainty is below 6%. Similar results are obtained if emissivity is allowed to vary as a function of wavelength and if the MARA channels with the best signal-to-noise ratio are used to constrain thermal inertia. If the observed surface is heterogeneous and two morphologically different units are present in the instrument's field of view, thermal inertia of the subunits can be retrieved independently if their contrast in terms of thermophysical properties is large enough. If, for example, the surface is covered by equal area fractions of fine-grained and coarse-grained material, then thermal inertia is found to be retrievable with uncertainties of 658 ±78 and 54 ±22 Jm⁻²K⁻¹s−1/2 for the coarse-grained and fine-grained fraction, respectively.

MASCOT - The Mobile Asteroid Surface Scout onboard the Hayabusa2 Mission

Article

Full-text available

Jul 2017
SPACE SCI REV

On December 3rd, 2014, the Japanese Space Agency (JAXA) launched successfully the Hayabusa2 (HY2) spacecraft to its journey to Near Earth asteroid (162173) Ryugu. Aboard this spacecraft is a compact landing package, MASCOT (Mobile Asteroid surface SCOuT), which was developed by the German Aerospace Centre (DLR) in collaboration with the Centre National d’Etudes Spatiales (CNES). Similar to the famous predecessor mission Hayabusa, Hayabusa2, will also study an asteroid and return samples to Earth. This time, however, the target is a C-type asteroid which is considered to be more primitive than (25143) Itokawa and provide insight into an even earlier stage of our Solar System. Upon arrival at asteroid Ryugu in 2018, MASCOT will be released from the HY2 spacecraft and gently descend by free fall from an altitude of about 100 m to the surface of the asteroid. After a few bounces, the lander will come to rest at the surface and perform its scientific investigations of the surface structure and mineralogical composition, the thermal behaviour and the magnetic properties by operating its four scientific instruments. Those include an IR imaging spectrometer (MicrOmega, IAS Paris), a camera (MASCAM, DLR Berlin), a radiometer (MARA, DLR Berlin) and a magnetometer (MASMAG, TU Braunschweig). In order to allow optimized payload operations the thermal design of MASCOT is required to cope with the contrasting requirements of the 4-year cruise in cold environment versus the hot conditions on the surface of the asteroid. Operations up to 2 asteroid days (∼16 hours) based on a primary battery are currently envisaged. A mobility mechanism allows locomotion on the surface. The mechanism is supported by an attitude and motion sensing system and an intelligent autonomy manager, which is implemented in the onboard software that enables MASCOT to operate fully independently when ground intervention is not available.

Are Phobos and Deimos the result of a giant impact?

Article

Full-text available

Feb 2011

Robert Craddock

Despite many efforts an adequate theory describing the origin of Phobos and Deimos has not been realized. In recent years a number of separate observations suggest the possibility that the martian satellites may have been the result of giant impact. Similar to the Earth-Moon system, Mars has too much angular momentum. A planetesimal with 0.02 Mars masses must have collided with that planet early in its history in order for Mars to spin at its current rate (Dones, L., Tremaine, S. [1993]. Science 259, 350-354). Although subject to considerable error, current crater-scaling laws and an analysis of the largest known impact basins on the martian surface suggest that this planetesimal could have formed either the proposed 10,600 by 8500-km-diameter Borealis basin, the 4970-km-diameter Elysium basin, the 4500-km-diameter Daedalia basin or, alternatively, some other basin that is no longer identifiable. It is also probable that this object impacted Mars at a velocity great enough to vaporize rock (>7 km/s), which is necessary to place large amounts of material into orbit. If material vaporized from the collision with the Mars-spinning planetesimal were placed into orbit, an accretion disk would have resulted. It is possible that as material condensed and dissipated beyond the Roche limit forming small, low-mass satellites due to gravity instabilities within the disk. Once the accretion disk dissipated, tidal forces and libration would have pulled these satellites back down toward the martian surface. In this scenario, Phobos and Deimos would have been among the first two satellites to form, and Deimos the only satellite formed - and preserved - beyond synchronous rotation. The low mass of Phobos and Deimos is explained by the possibility that they are composed of loosely aggregated material from the accretion disk, which also implies that they do not contain any volatile elements. Their orbital eccentricity and inclination, which are the most difficult parameters to explain easily with the various capture scenarios, are the natural result of accretion from a circum-planetary disk.

Dose and Single Event Effects on a Color CMOS Camera for Space Exploration

Article

Dec 2018

This paper focuses on the radiation-induced dose and single-event effects (SEEs) on a color CMOS camera designed for space missions. The γ -ray and proton tests are used to evaluate the tolerance against cumulative dose effects. The dark current of the image sensor is the main parameter impacted by dose effects. Heavy ions testing is performed to evaluate SEEs. single-event upset, single-event functional interrupt, and single-event latchup have been observed and mitigation techniques were proposed for specific space missions.

DePhine – The Deimos and Phobos Interior Explorer

Article

Jan 2018
ADV SPACE RES

DePhine – Deimos and Phobos Interior Explorer – is a mission proposed in the context of ESA’s Cosmic Vision program, for launch in 2030. The mission will explore the origin and the evolution of the two Martian satellites, by focusing on their interior structures and diversity, by addressing the following open questions: Are Phobos and Deimos true siblings, originating from the same source and sharing the same formation scenario? Are the satellites rubble piles or solid bodies? Do they possess hidden deposits of water ice in their interiors? The DePhine spacecraft will be inserted into Mars transfer and will initially enter a Deimos quasi-satellite orbit to carry out a comprehensive global mapping. The goal is to obtain physical parameters and remote sensing data for Deimos comparable to data expected to be available for Phobos at the time of the DePhine mission for comparative studies. As a highlight of the mission, close flybys will be performed at low velocities, which will increase data integration times, enhance the signal strength and data resolution. 10 – 20 flyby sequences, including polar passes, will result in a dense global grid of observation tracks. The spacecraft orbit will then be changed into a Phobos resonance orbit to carry out multiple close flybys and to perform similar remote sensing as for Deimos. The spacecraft will carry a suite of remote sensing instruments, including a camera system, a radio science experiment, a high-frequency radar, a magnetometer, and a Gamma Ray / Neutron Detector. A steerable antenna will allow simultaneous radio tracking and remote sensing observations (which is technically not possible for Mars Express). Additional instrumentation, e.g. a dust detector and a solar wind sensor, will address further science goals of the mission. If Ariane 6-2 and higher lift performance are available for launch (the baseline mission assumes a launch on a Soyuz Fregat), we expect to have greater spacecraft mobility and possibly added payloads.

An ongoing satellite–ring cycle of Mars and the origins of Phobos and Deimos

Article

Mar 2017

The Martian moons Phobos and Deimos may have accreted from a ring of impact debris, but explaining their origin from a single giant impact has proven difficult. One clue may lie in the orbit of Phobos that is slowly decaying as the satellite undergoes tidal interactions with Mars. In about 70 million years, Phobos is predicted to reach the location of tidal breakup and break apart to form a new ring around the planet. Here we use numerical simulations to suggest that the resulting ring will viscously spread to eventually deposit about 80% of debris onto Mars; the remaining 20% of debris will accrete into a new generation of satellites. Furthermore, we propose that this process has occurred repeatedly throughout Martian history. In our simulations, beginning with a large satellite formed after a giant impact with early Mars, we find that between three and seven ring–satellite cycles over the past 4.3 billion years can explain Phobos and Deimos as they are observed today. Such a scenario implies the deposition of significant ring material onto Mars during each cycle. We hypothesize that some anomalous sedimentary deposits observed on Mars may be linked to these periodic episodes of ring deposition.

PADME (Phobos and Deimos and Mars Environment): A Proposed NASA Discovery Mission to Investigate the Two Moons of Mars

Conference Paper

Mar 2015

PADME is a proposed NASA Discovery mission to investigate the origin of two remarkable and enigmatic small bodies, Phobos and Deimos, the two moons of Mars.

Formation of Phobos and Deimos via a Giant Impact

Article

May 2015
ICARUS

Although the two moons of Mars, Phobos and Deimos, have long been thought to be captured asteroids, recent observations of their compositions and orbits suggest that they may have formed from debris generated by one or more giant impacts of bodies with ∼0.01× target mass. Recent studies have both analytically estimated debris produced by giant impacts on Mars and numerically examined the evolution of circum-Mars debris disks. We perform a numerical study (Smoothed Particle Hydrodynamics simulation) of debris retention from giant impacts onto Mars, particularly in relation to a Borealis-scale giant impact ( J) capable of producing the Borealis basin. We find that a Borealis-scale impact is capable of producing a disk of mass ∼ kg (∼1–4% of the impactor mass), sufficient debris to form at least one of the martian moons according to recent numerical studies of martian debris disk evolution. While a Borealis-scale impact may generate sufficient debris to form both Phobos and Deimos, further studies of the debris disk evolution are necessary. Our results can serve as inputs for future studies of martian debris disk evolution.

MERLIN: Mars-Moon Exploration, Reconnaissance and Landed Investigation

Article

Mar 2012
ACTA ASTRONAUT

Mars' moons Phobos and Deimos are low-albedo, D-type bodies that may preserve samples of outer solar system material that contributed organics and volatiles to the accreting terrestrial planets. A Discovery-class mission concept described in this paper, the Mars-Moon Exploration, Reconnaissance and Landed Investigation (MERLIN), will obtain in situ measurements from Deimos to test models for the moon's origin. The measurement objectives of MERLIN are to determine Deimos' elemental and mineralogical composition, to investigate its volatile and organic content, and to characterize processes that have modified its surface. To achieve these objectives, a landed payload will provide stereo imaging and measurements of elemental and mineralogical composition and interior structure. An orbital payload will acquire global high-resolution and color imaging, putting the landing site in context by characterizing Deimos' geology. Following MOI the spacecraft flies in formation with Deimos, and uses small changes in its orbit around Mars to investigate Deimos from a range of altitudes and illuminations over 4 months. Data taken during 1- to 2-km altitude flyovers will certify a landing site. The spacecraft will be delivered to a point several kilometers above Deimos, and will navigate to landing on a fresh exposure of regolith using onboard imaging. 90 days of baseline landed operations will provide a complete set of measurements, with schedule reserve, and there is sufficient propellant to repeat the measurements at a second site.

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