Friday, August 17, 2012

Risk Assessment for a Vista Capture

  Before trying to capture a vista panarama on the other side of the crest one needs to assess the risks involved. The main problem on a slope is slippage. One needs to determine the angle at which slippage will occur. For this one needs to know the coefficient of friction between the rover's wheels and the surface it is on. The coefficient of friction, μ = Ffriction/Fweight, is the ratio of the force at which friction occurs to the force of the weight of the material on a level surface. Slippage occurs on a slope of angle θ when the component of an objects weight along the surface is greater than the frictional force or when,
  Fweight sin(θ) > μ Fweight cos(θ).
This simplifies to tan(θ) > μ. The angle of slippage as a function of the coefficient of friction is shown in the following plot.
  The rover was designed to climb 45° slopes but the operational limit is 30°. This requires that at least μ > 0.6. However there are some soft materials like graphite, gypsum and chalk which can have low coefficients of friction of about 0.2 or so. Graphite is used as a lubricant and materials at the lower end of the Mohs scale may cause a problem on slopes. Sand is also a problem since its cohesiveness is very low and the MER rovers got bogged down in "sand traps." If one does not know the strength of the materials involved one would have to assume the worst and the angle of slippage for the softest materials which limits how far down a slope that one could go safely. Curiosity needs to be sure she won't lose her footing.

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