Wheel abrasion experiment
The Wheel Abrasion Experiment (German wheel wear experiment ) was one of a total of four experiments that NASA had developed in cooperation with European institutions, namely also in Germany, for the remote-controlled rover Sojourner . As part of the Mars Pathfinder mission in 1997, the vehicle was designed for a working period of one week and was able to move about ten meters away from the Pathfinder probe on its six independently driven wheels. The Wheel Abrasion Experiment was used specifically to obtain data on the surface of Mars .
The aim and principle of operation of the experiment
The narrowly limited aim of the wheel wear experiment was to obtain information about the hardness of the Martian surface.
Direct surface analyzes could only have been realized with a comparatively high technical effort; However, this would have required a larger payload , which, however, always causes very high costs in space travel.
However, the consideration that the hardness of the surface is directly related to possible wear and tear made it possible to use an indirect method: It was not necessary to examine the unknown surface of the Mars itself; rather, it was sufficient to merely analyze the abrasion caused as a consequence in a purely quantitative manner. So you could limit yourself to light as a relatively simple measured variable if you carried a suitably prepared object such as the specially coated wheel, which could translate the surface hardness into an amount of light. Light measurements are technically possible with comparatively little effort.
In order to determine the hardness of the surface of Mars it was sufficient to measure the signs of wear on one of the six wheels (each 13 cm in diameter and 8 cm in width) on the 10.6 kg heavy rover. This was done optically with a photo cell : It was able to record the steadily decreasing reflectivity of sunlight on three very thin layers of metal applied to the WAE wheel, caused by specifically controlled abrasion.
Carrying out the actual experiment
The special WAE wheel had a black anodized background on which a 20–100 nm thick aluminum , nickel and platinum layer was vapor-deposited. Without wear and tear, these thin layers could reflect a lot of sunlight, but with increasing wear, the black background came to light, which reduced the amount of reflected sunlight accordingly.
The light measurements carried out, however, were not carried out after Sojourner had driven around at will; Rather, for the purpose of measuring five of his wheels were exactly twice each specifically Martian blocked and simultaneously driven the WAE-wheel, so that this could dig a little into the regolith. The spinning of this special wheel led to a varying degree of abrasion of its reflective metal layers, so that the black background of the wheels increasingly came to light. The brightness values measured by the photocell were sent by Sojourner to the Pathfinder probe a few meters away, with which there was a direct radio link to the earth. Based on the different degrees of hardness of the metals, it was then possible to draw conclusions about the strength and material properties of the Martian soil.
Preparatory actions
In the run-up to the mission, feasibility studies had not only provided clues about the expected material removal: In addition, it was also observed that dust adhered to it when driving around on dry ground. The main cause of this fact was an electrostatic charge that could even exceed 100 volts and thus reached a level that was already threatening for the rover. In order to reduce the likelihood of undesired discharges through the Martian atmosphere, discharge points have therefore been installed after additional tests.
A calibration was also necessary on Mars itself before the actual wear experiment so that the results obtained would not be falsified from the outset: Sojourner drove over sand, dust and clay in its vicinity to first determine to what extent alone The mere adherence of such particles resulted in a reduced reflection - which consequently was not due to any form of wear and tear.
See also
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- NASA: Wheel Abrasion Experiment Conducted on Mars (English)
- Bernd Leitenberger: Mars Pathfinder