From their vantage point 1.5 m above the Martian surface, the Pancam is able to make multi-spectral exposures of up to 30 seconds. Their angular resolution of 0.0164° rivals the acuity of the human eye and each camera is able to focus from 1.5 m to infinity over a field of view over 16° by 16° in angular size. Each camera
Table 27.1. A summary of the MER 'Athena' payloads and their data products
Sensor mass and power
(Pancam) Miniature thermal emission spectrometer (mini-TES) Mossbauer spectrometer (MB)
Alpha particle X-ray spectrometer (APXS) Microscopic imager (MI) Magnetic adhesion arrays Rock abrasion tool (RAT)
8 or 12-bit deep images of 1024 X 1024 pixels 1024 samples at 16 bit
512 data points per spectrum, each of 3 bytes
256 channels for a-particle spectra, 512 channels for X-ray sectra 8 or 12-bit deep images of 1024 X 1024 pixels None, analysed by other sensors Motor current
Dual cameras, each of 270 g mass, drawing 3 W in operation Mast-mounted optics feeding internally accommodated package. Total mass 2.4 kg, drawing 5.6 W in operation Arm-mounted sensor of 500 g, drawing an average of around 2 W Arm-mounted sensor of 640 g, drawing around 1.5 W
Arm-mounted sensor of 210 g, drawing 2.15 W Body and arm mounted items, total mass of —50 g, no power use Arm-mounted device of 685 g, drawing 11 W
is equipped with a different filter wheel, each with eight filter-positions that cover the near-UV (430 nm) through to IR (980 nm). The platform carrying the Pancam can slew through 360° and pitch by ± 90° with respect to the horizontal axis of the MER. The Pancam was developed at Cornell University in the USA.
Built at Arizona State University in the USA, the mini-TES (e.g. Christensen et al., 2003) is a Fourier transform spectrometer that obtains spectra in the wavelength range of 5 to 29.5 microns. Different minerals can be distinguished by their infrared emission spectra in this, with particular ability to discriminate between anion groups (CO3, SO4, SiO4, etc.). The imaging capability is provided by having the mini-TES input optics mounted on the Pancam platform. Accordingly, mini-TES has the same azimuthal angular range as the optical imagers, but space restrictions on the optics give a restricted elevation span: 30° above, and 50° below the nominal horizon. The mast supporting the Pancam is hollow and internal mirrors direct the light gathered from the 6.3 cm diameter aperture into the mini-TES which is physically located in the MER body. The mini-TES is able to make spectral images with two levels of angular resolution, 20 mrad and 8 mrad, and the Pancam and mini-TES boresights are essentially parallel, allowing images from these systems to be overlaid.
Attached to the exterior of each rover are three arrays of magnets, provided to the Athena payload by the University of Copenhagen. The arrays are positioned on the front of each rover, on the RAT of the arm, and on the exposed deck of the rover. It is expected, on the basis of similar measurements by the Viking and Sojourner craft, that much of the Martian regolith contains a magnetic component. Dust-sized mineral particles continually rain out from the Martian atmosphere, after being lofted by winds. Only the forward-facing array can be reached by the arm, so that the trapped material may be analysed by the arm microscope, MB, and APXS sensors. The deck and RAT-located arrays are imaged by the Pancam.
27.2.4 Mossbauer spectrometer (MB)
The MER Mossbauer device, built at the Johannes Gutenberg University in Mainz, Germany, is an improved version of the device flown by that group on the Mars Pathfinder mission (Klingelhofer et al., 2003). Two cobalt-rhenium sources provide the gamma rays used to irradiate, and detect the presence of, iron nuclei and its distribution among the various oxidation states (Fe2+, Fe3+, Fe6+). This instrument consists of two parts: the head (sources, collimation structures, and silicon detectors) which is carried on the rotating turret at the end of the rover's arm, and an electronics box (power supply, processing, memory) held within the MER. Limitations on the size and strength of sources available for use on the mission result in integration times typically lasting several hours, mitigated somewhat by the ability of the head to be manoeuvred to within a few cm of a target. The MB has a working field of view of around 1.5 cm .
27.2.5 Alpha-particle X-ray spectrometer (APXS)
Successful operation of their APXS on the Mars Pathfinder project led the team at the Max-Planck-Institut für Chemie, Mainz, Germany to develop a modified version for the Athena payload of the MERs. Like the MB, the APXS is arm-mounted and irradiates targets with alpha particles and X-rays from a curium source. The backscattered alpha particles are detected by an annular detector array, providing concentration measurements of light elements (C, O). X-rays emitted from heavier rock-forming elements (Mg, Al, Si, Ca, Fe, etc.) are detected by a high-resolution silicon drift detector, and the whole instrument has a roughly circular field of view a little over 12 cm2 in area.
27.2.6 Microscopic imager (MI)
The MI is positioned 180° around from the Mossbauer spectrometer on the arm of each MER. This instrument uses a charge-coupled device CCD imager with a resolution of 1024 X 1024 pixels to cover an area of 30 X 30 mm. The optical depth of field is ±3 mm and the monochromatic CCD is covered with a bandpass filter which transmits light in the spectral range between 400 nm and 680 nm. The MI relies on ambient lighting to illuminate the target and a transparent shield is used to cover the MI optics when the device is not imaging. The rod that extends from the face of the MI is a contact sensor which allows the MI to be accurately positioned and also prevents accidental impacts with target rocks. The MI is provided by Cornell University and uses the same camera body as the Pancam, giving the MI the same radiometric performance (signal to noise and sensitivity).
27.2.7 Rock abrasion tool (RAT)
Opposite the APXS on the platform carried by each rover's arm, is the RAT. This unique device is a combined grinding and coring tool that is able to remove up to
5 mm of rock from a circular region 45 mm in diameter. By exposing interior portions of rocks the RAT offers the other tools on the MER arm unprecedented access to material that will have suffered no exposure to the present Martian surface environment. Strictly, the RAT itself generates relatively little data; by monitoring the current drawn by the motors in the tool it is possible to infer the relative toughness of the rock (e.g. Bartlett et al., 2005). The MER arm provides a down-force of up to 80 N for the tool to engage with the target rock, and cutting is performed by rotating an armature carrying two grinding points made of industrial diamond matrices at 3000 rpm.
The mobility system used by the two rovers borrows significantly from the design used for the Sojourner rover in the Mars Pathfinder mission. Like the Sojourner, the MER rovers are equipped with two sets of three 23 cm diameter wheels, each of which houses a bidirectional motor and 1500:1 gearbox mounted in the wheel hub. Additionally, the front and aft wheels of each rover can be yawed around a near-vertical axis, allowing the rovers to turn on the spot. On hard, level ground this drive system allows the rovers to move at a speed of 50 mm s- , and in doing so the six motors expend a total power of over 100 W. Articulation for the wheels is achieved with a so-called 'rocker-bogie' system that enables all three wheels on each side of a rover to maintain good contact with the ground in the event of encountering obstacles. Packaging constraints on the rover delivery capsule meant that the suspension was delivered to Mars in a geometry different from that used when the rover was being driven. Specifically, the forward rocker was rotated with the front wheel tucked in front of the rover, and the joint between the aft and forward rockers (indicated by a black mark in Figure 27.3 (b)) was locked flat to permit the rover to squat closer to the floor of its lander package.
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