Mars Exploration Rover Technical Data

The Mars Exploration Rover Maintenance Manual

Compiled and maintained by Rupert Scammell

1. Preface

This page is the outgrowth of a growing frustration with the paucity of flight hardware and software details that NASA/JPL have released about the MER. In an attempt to learn more about the operational parameters of these two fine pieces of machinery, this page has been created to serve as a running notepad for myself, and hopefully will be of interest to others (especially the crew on's #maestro channel.

2. High level MER hardware specifications

2.1 MER hardware features:

MER image

2.2 Major dimensions:
MER Dimensions

Image courtesy of Mike Morrow

2.3 Power
2.4 Science instruments, aka Athena Package:
2.5 Cameras:
2.6 Antennae:
2.7 Major MER structural components

Specifications and image credit: NASA/JPL/Cornell

3. MER Power Subsystem

3.1 Solar arrays:
The MER solar arrays are composed of 55 parallel strings of 20 cells each of 2cm x 4cm GaAs/Ge cells. Each string within the array is diode isolated. The total solar array area on the MER is 1.2 square meters.

3.2 Battery configuration:
Note to self: Primary battery configuration?? The rover's primary battery array is composed of lithium thionyl-chloride batteries.

The MER's secondary battery array consists of 3 strings of 5 amp-hr Li-ion cells, producing a voltage of 11-16 volts each. At the end of the mission, the secondary battery is capable of 10 amp-hrs of output, with 50% redundancy. The secondary battery weighs less than 3 kg, and has a volume of 2 liters.

Typical rechargeable Li-ion battery aboard the MER:

MER battery MER battery
A color photo shows a cell composed of two black cylinders stacked on top of each other, in contact along their long axis, and wired together. An orange power cord with an electrical connector at its end extends from the pair of cylinders. -- NASA PLLS #1410 MER battery pack image from AFRL/ (I assume that this is the packaged version of the battery shown on the left hand side)

3.3 Power subsystem functions
The power subsystem onboard MER provides conditioned power for internal power loads. Additionally, the PS performs load monitoring, provides mission and alarm clock functionality when the rover CPU is powered down, and controls the charging of the secondary battery array.

Charging of batteries by the PS is via a continuously operating, dedicated battery charger board, which draws 200mw of power, and performs battery temperature monitoring, cell charging, power switching, and panel power shunt functions.

3.5 Thermal considerations for the PS
The PS nominally operates from a minimum temperature of -40C, to a maximum temperature of 40C within the Warm Electronics Box. However, the PS is qualified using a temperature range of -50C to 50C within the WEB.

Outside the WEB, nominal PS operating temperatures are -90C to 40C, with a qualification temperature range of -105C to 55C.

Thermal regulation of the battery charger board is accomplished via a shunt radiator mounted on the rocker-bogie structure.

3.6 MER Power utilization

Power subtotalCPU and I/O powerComponent 1 powerComponent 2 power
38 W19 W9W (accel and gyro)10W drive wheel motors
75 W19 W55 W (transmission) MO/MGS commsn/a
30 W19 W10 W (transmission) lander relay comms ??n/a
55 W19 W33 W (peak motor) - drilling ?? is this the RAT ??n/a
29 W23 W6W (cameras)n/a
1.4 Wn/a1.4 W (APXS spectrometer)n/a
2.3 Wn/a2.3 W for nighttime spectroscopyn/a
141 Whr/dayn/a141 Whr/day engineering housekeepingn/a
75 Whrn/a75 Whr nightime ops limitn/a

3.7 Subsystem contractors
Ball Aerospace built the MER power subsystem.
MER's Li-ion batteries were developed in a joint venture between the AFRL, JPL, and the Glenn Research Laboratory.

Power subsystem information credit NASA/JPL/Cornell.
Much of the information in this section was distilled from the Power Aware Computing and Communication presentation.

Left-hand MER battery image courtesy of NASA's Public Lessons Learned System Database, Lesson #1410.
Right-hand MER battery image courtesy of AFRL, via

4. Panoramic Camera (pancam)

4.1 Introduction
By virtue of being under the auspices of the Cornell team, as well as being one of the more 'accessible' scientific instruments aboard the MER, information abounds on the 'Net concerning how the pancam aboard the MER operates. The pancam is mounted on a camera bar atop the PMA (Pancam Mast Assembly). Major components are visible in the image below:

The primary site for the pancam is

Not linked on the Athena or JPL sites as far as I can see, is the incredibly detailed, definitive research paper about the pancam, which hasn't been dumbed down for public consumption --
Bell, J.F., S.W. Squyres, K.E. Herkenhoff, J.N. Maki, H.M. Arneson, D. Brown, S.A. Collins, A. Dingizian, S.T. Elliot, E.C. Hagerott, A.G. Hayes, M.J. Johnson, J.R. Johnson, J. Joseph, K. Kinch, M.T.Lemmon, R.V. Morris, L. Scherr, M Schwochert, M.K. Shepard, G.H. Smith, J.N. Sohl-Dickstein, R.J. Sullivan, W.T. Sullivan, and M. Wadsworth (2003), The Mars Exploration Rover Athena Panoramic Camera (Pancam) Investigation, submitted to the Journal of Geophysical Research special issue on the Mars Exploration Rover missions.

4.2 Pancam CCDs
The pancam contains dual (L/R) 1024x2048 pixel CCD array detectors, designed by Dr. Mark Wadsworth. The CCDs were fabricated by Mitel Semiconductor. Each CCD has a 1024 x 1024 pixel region used for imaging, and a second 1024 x 1024 pixel region, which is utilized as a frame transfer buffer. Pixels within the CCD are continuous, and have a 12 micrometer pitch in both directions. The CCDs have an exposure interval that ranges from 0-30s. The 0s exposure capability of the CCDs is used to define the 'readout smear' signal that occurs during the approximately 5ms that it takes to transfer the image from the active imaging region to the frame transfer buffer. Dark current is minimal at -55C, and rises to <1200 electrons/sec at 0 C. Analog to digital converters provide 12-bit encoded output from each CCD. CCD response has a linearity of >99% for signals between 10-90% of full well.

Pancam CCDs:

4.3 Pancam optics
Both pancam cameras share identical optics. Camera optics are made up of 3-element symmetrical lenses, which possess an effective focal length of 38mm, and a focal ratio of f/20, yielding an IFOV of 0.28 mrad/pixel, and a square FOV of 16.8 degrees by 16.8 degrees per eye. A sapphire window covers the front of the pancam camera optics barrel, protecting the optics and filters within.The pancam is able to maintain focus from 1.5m to infinity. Defocus blur occurs at shorter ranges. At a range of 80cm, a defocus blur of approximately 10 pixels is experienced.

In the image below, you can see a pancam lens and filter wheel assembly (see Sec. 4.4 for pancam camera filter information):

Rear view of the pancam lens barrel. The CCD housing assembly is attached to the rear panel of the lens assembly:

4.4 Pancam filters
In addition to the pancam optics discussed above in Section 4.3, each pancam camera has an eight position filter wheel that can be rotated in order to position the appropriate filter in front of the pancam optics within the lens barrel.

A fantastic explanation of the various pancam filters can be found here.

Credits: Much of the information for this section was transcribed and paraphrased from Cornell's Pancam Technical Briefing. All images are courtesy of NASA/JPL/Cornell .

Note: I'm going to gloss over the rest of the science instruments for now. There's a ton of documentation available on already, so I'll fill the following sections out later on, in favour of working on and documenting more obscure aspects of the MER system.

5. Miniature thermal emission spectrometer (Mini-TES)

Cornell Mini-TES instrument page

Research paper on Mini-TES: Christensen, P.R., G.L. Mehall, S.H. Silverman, S. Anwar, G. Cannon, N. Gorelick, R. Kheen, T. Tourville, D. Bates, S. Ferry, T. Fortuna, J. Jeffryes, W. O'Donnell, R. Peralta, T. Wolverton, D. Blaney, R. Denise, J. Rademacher, R.V. Morris, and S. Squyres (2003), The Miniature Thermal Emission Spectrometer for the Mars Exploration Rovers, submitted to the Journal of Geophysical Research special issue on the Mars Exploration Rover missions.

MER Mini-TES images (interior/exterior)

Mini-TES images courtesy NASA/JPL/Cornell/ASU

6. Mossbauer spectrometer (MB)

Research paper on MB: Klingelhöfer, G., R.V. Morris, B. Bernhardt, D. Rodionov, P.A. de Souza Jr., S.W. Squyres, J. Foh, E, Kankeleit, U. Bonnes, R. Gellert, Ch. Schröder, S. Linkin, E. Evlanov, B. Zubkov, and O. Prilutski (2003), The Athena MIMOS II Mössbauer Spectrometer Investigation, submitted to the Journal of Geophysical Research special issue on the Mars Exploration Rover mission.

7. Alpha particle X-ray spectrometer (APXS)

Cornell APXS instrument page

Research paper on APXS: Rieder R. Gellert R. Bruckner J. Klingelhofer G. Dreibus G. Yen A. Squyres SW (2003), The new Athena alpha particle X-ray spectrometer for the Mars Exploration Rovers, published in the Journal of Geophysical Research-Planets. 108(E12):8066, 2003 Nov 11.

Grrrr. The AGU page containing the PDF and HTML versions of this paper requires a login.

MER APXS images and schematics

APXS operational schematic

APXS sensor head

8. Microscopic imager (MI)

The definitive source for technical information about the MI tool is Herkenhoff, K.E., S.W. Squyres, J.F. Bell III, J.N. Maki, H.M. Arneson, P. Bertelsen, D.I. Brown, S.A. Collins, A.Dingizian, S.T. Elliot. W. Goetz, E.C. Hagerott, A. G. Hayes, M.J. Johnson, R.L. Kirk, S. McLennan, R.V. Morris, L.M. Scherr, M.A. Schwochert, L.R. Shiraishi, G.H. Smith, L.A. Soderblom, J.N. Sohl-Dickstein, M.V. Wadsworth, and the Athena Science Team (2003), The Athena Microscopic Imager Investigation, submitted to the Journal of Geophysical Research special issue on the Mars Exploration Rover missions.

9. Rock abrasion tool (RAT)

An excellent and detailed discussion of the technical specifications, operational considerations, and algorithms used for the RAT can be found in Gorevan, S.P., T. Myrick, K. Davis, J.J. Chau, P. Bartlett, S. Mukherjee, S, Stroescu, C. Batting, R. Anderson, S.W. Squyres, R.E. Arvidson, M.B. Madsen, P. Bertelsen, W. Goetz, C.S. Binau (2003), The Rock Abrasion Tool: Mars Exploration Rover Mission, submitted to the Journal of Geophysical Research special issue on the Mars Exploration Rover missions.

10.Magnet arrays


11. High Gain Antenna subsystem (HGA)

11.1 Introduction to the HGA subsystem

The HGA carried aboard the MERs is used for bidirectional Mars-Earth communication. Due to power, and thermal constraints, the HGA is only usable for a maximum of 3h per sol, although communications opportunities may persist for much longer periods of time. The HGA subsystem is made up of two primary sub-subsytems; the HGA antenna element proper, and the High Gain Antenna Gimbal (HGAG), used for positioning the HGA.

11.2 High gain antenna hardware specifications

The HGA is a unidirectional antenna, whose circular face has a diameter of 0.28m. The unidirectional nature of the antenna necessitates the two-axis HGAG positioning system, which is discussed in detail below. The HGA transmits and receives in the X-Band (8-12 Ghz), with a throughput of 1850 bits/sec. A coaxial cable with a rotating fitting runs from the HGA antenna element, through a center cavity of the HGAG, and down to communications hardware located within the MER WEB. In the picture below, the face of the HGA antenna is visible as the circle in the bottom right corner of the image:

11.3 High gain antenna gimbal system specifications

The HGAG is a two axis gimbal positioning system for the HGA, which employs identical azimuth and elevation drives. Each drive consists of a maxon motor 34-Vdc REO 20 ironless-core brush motor. Each REO 20 rotates an integral three-stage, 81.37:1 planetary gearbox which, in turn, drives a 1.333:1 spur-gear stage. The spur-gear stage then powers an HD Systems Size-14 SHF 50:1 harmonic drive, for a final reduction ratio of 5,425:1.

Engineering schematic of HGAG subsystem mechanics (from Machine Design -

11.3.1 HGAG system elevation and azimuth drives

Unfortunately, I wasn't able to locate images of the exact maxon motors (company name is lower case) REO-20 brush motor. However, I have managed to locate images and schematics for a similar mm RE-series motor, which is shown below:

maxon motors example RE-series brushless motor imagemaxon motors RE-16 series motor schematic, probably? similar to REO-20 on MER

11.3.2 HGAG spur gear

Little is known about this component, other than as previously stated, it's a spur gear with a gear ratio of 1.333:1. As far as I can tell, although maxon manufactures spur gearheads, none of their products have such a gear ratio. So it's most likely from another vendor, or custom manufactured by NASA/JPL or the contractor for the HGAG.

11.3.3 HGAG system harmonic drive

The HD Systems SHF 50:1 gear ratio harmonic drive has the following specifications:
Below, you can see an image, along with a schematic of the HD Systems SHF HD component:

Image of HDS SHF 50:1 HDSchematic of HDS SHF 50:1 HD

Harmonic drives are ideally suited for use when high precision servo or actuator operation is required. Additionally, harmonic drives provide a very high degree of repeatability of movement, since the circular spline element of the HD (a thick-walled, rigid ring with internal spline teeth, which normally functions as the non-rotating member) has zero freeplay or backlash with the HD flexspline element (externally toothed, non-rigid (flexible), thin-walled, cylindrical cups which are smaller in circumference and normally have two less teeth than the circular splines. The flexspline is normally the rotating output element). More about the engineering advantages and disadvantages of the harmonic drive system can be found in the page linked at the beginning of this paragraph.

11.3.4 MER HGAG Operational Tolerances

Operations on the martian surface present rapidly cycling extremes of hot and cold, along with a strong potential for particulate contamination and delubrication of mechanical parts. To take these conditions into account, mechanical components within the HGAG are constructed at especially high levels of operational tolerance.

MER specifications require that the HGAG A/E drives must supply double the amount of output torque needed to climb a 40 degree slope with an outside temperature of -70°C and minimal battery power.

A torque margin (about 5.4 N-m) is required. Motor revolution count is limited to 2.5 million rotations over the course of the mission.

Harmonic drive lubrication during subzero temperatures was also a significant design concern. Tests conducted by JPL to measure actuator torsion and efficiency resulted in the determination that it was more effective to coat moving actuator components with a light coating of grease, rather than to fill gearbox and driveshaft cavities with a larger amount of grease.

Quoting from "...Torsional stiffness and ratcheting were other considerations. The drives must meet the HGAG pointing spec but also be compliant enough to limit loads when commanded into hardstops at the motor's 34-Vdc maximum operating voltage. Then, harmonic-drive peak torque may reach 52 N-m. Radially flexible members -- circular spline, wave generator, and housing -- must be adequately stiff to prevent ratcheting. Ratcheting happens when the flex spline doesn't mesh properly with the fixed outer ring gear in the housing. A finite-element model from engineers at HD Systems estimated minimum ratcheting torque at 77.4 N-m, well within the peak torque spec, and significantly above the worst-case operating torque."

Credits: Much of the excellent and detailed technical information about the HGAG's mechanical components and operational parameters has been distilled or transcribed from Mars Rover to Earth: "Where to now?" published by Machine Design
RE-series motor images and schematics are from maxon motor. Harmonic drive images, details, and schematics are from HD Systems. HGAG image is courtesy of NASA/JPL/Cornell/Michael Lyle.

12. Rocker-Bogie Mobility System

12.1 Introduction

Spirit and Opportunity are equipped with a Rocker-Bogie type suspension and chassis system, similar to the one used on the 1997 Mars Pathfinder Rover. The RB system permits each wheel to independently conform to uneven terrain, allowing the rover to traverse obstacles twice the diameter of the rover's wheels. The RB system also provides exceptional stability when the rover is operating on steeply sloped surfaces.

The RB suspension system was invented by Don Bickler, and patented by NASA/JPL in 2000 and 2001 (US Patents 6,112,843 and 6,267,196.

Side view of Rocker-Bogie suspension system on MER

12.2 Rocker-Bogie Mobility System Components

One side of the RB system, with primary structural and mechanical components illustrated.

The MER provides a separate RB system for the left and right wheels. Each RB consists of a forward and aft bogie, to which are attached corresponding forward and aft bogie wheel struts. The end of each wheel strut provides an attachment point for a drive motor and wheel assembly. The aft bogie structure slides into the forward bogie's structural cavity, and is secured to the FB by way of a square bolt, which can be inserted into one of four available positioning holes. The aft rocker arm is affixed to the forward bogie, near the AB/FB attach point through a hinge and actuator assembly. A U shaped adapter bracket fits onto the other end of the AR. The bracket holds a structural member that provides a mount for the Rocker Deployment Actuator, which permits the forward bogie that it's attached to to be rotated around the longitudinal axis of the rover. The end of the forward bogie has an attach point for a drive motor and wheel assembly.

12.3 Rocker-Bogie Specifications
Structural materialCNC machined titanium
Maximum tilt angle (about lateral axis)45 degrees
Maximum tilt angle (about longtudinal axis)45 degrees
Maximum forward wheel drop (tested)25 cm
Rated mobility load (wheel drop, suspended mass)3.4g with an angular velocity (alpha) of 82 rad/sec^2 (this is probably at the forward rocker/aft bogie hinge point, but the document isn't clear)

Schematic illustrating 25 cm forward wheel drop test

MER prototype undergoing forward wheel drop testing

Credits: The history of rocker-bogie suspension system development was distilled from BrickVista Tech-Notes' page on the topic. Detailed structural specifications, schematics, and the rocker-bogie image are from Lee, D. "Design and Verification of the Mars Exploration Rover Primary Payload", S/C & L/V Dynamic Environments Workshop, 2003.

13. Warm Electronics Box (WEB)

13.1 Introduction to the WEB

The MER WEB is a thermoregulated environment, topped by the Rover Equipment Deck (RED), in which the MER's core flight and science electronics reside. The WEB's bottom and sidewalls are made up of honeycomb shear panel material. Bonded titanium fittings are used to join the bottom and side walls of the WEB together.

Schematic view of the MER WEB, illustrating instrument positions within the enclosure

13.2 WEB Specifications

WEB length (along +X axis)~34"
WEB width (along +Y axis)~21.6"
WEB height (along +Z axis)~14.4"
WEB bottom/sidewall material5056 Aluminum Honeycomb Panel
WEB bottom/sidewall density3.1 pcf
WEB fastener constructionBonded titanium, with Astroquartz softening layer used at fittings to reduce bondline peaking stress
WEB Landing Design Load41g
WEB MAC Design Load10g (VLC: 9.1g)
WEB minimum rated temperature-55C

Schematic diagram of WEB/RED dimensions and construction

This page is under construction. More to come!