[X] My Assistant

[Astro0]

So it seems for the amazing Mars Exploration Rovers that another sol has come and another milestone has been reached.

Opportunity has passed the Viking 1 lander for operational time on Mars - 2245 sols.
While Spirit remains in hibernation, there's no way to know whether she has the record.
I'm sure that Opportunity will happily stand aside for Spirit when we hear from her again

Here's a banner logo and desktop image to mark the milestone.





Enjoy

[MarsEngineer]

Oerstad, we did consider wind. We even considered going up to the "Mars" low pressure wind tunnel up at NASA Ames with a bag of dust and a fan in hand. The problem was that we did not have a Mars dust stimulant that we trusted. Our expectations were that the dust - at least the first few microns of it - would be "sticky" due to Van der Waals forces or other triboelectric effect. We suspected (and still suspect) that our intuition about materials interaction goes out the window when in a low pressure, highly desiccated environment with materials we do not understand.

One factor that dampened our enthusiasm for testing on MER was that we knew that the dust was NOT cleaned on MPF's (nor Sojourner's) solar panels after dust devils went by in the summer of 1997. Although we could not see them, the dust devils were measured using the atmosphere pressure transducer and the hot wire anemometer (part of the ASI/MET experiment). We could see the pressure drop as it went by and see the change in wind direction at the same time. (Look up Dr. Tim Schofield.)

So..... dust devils, no cleaning. We could not count on it. Instead we made the solar arrays bigger. But why does the wind successfully (completely?) clear the arrays on MER? We don't know. Perhaps it is because the rover is higher off the ground than MPF and higher above the boundary layer. Perhaps it is the airspace below the panels that allow the wind to flow faster over the arrays. Is it perhaps that MER winds bring dust with them and scour (sand blast) off the existing dust? Perhaps the dust devils are NOT the cause of the MER cleaning and that some other wind phenomena is doing it (like a passing front that has sustained winds). Without an atmosphere science experiment on board MER, we simply do not know.

In our work, we often have to learn to live with simply not knowing and instead learn to imagine and assume the worst case. The worst case seemed to be the 1/4 percent per sol of dust with no end in sight. That is how we design these beasties. The trouble is, even our worst imaginings might not be good enough ... fortunately we tend to err on the conservative side just enough to win in the end.

People ask me why the rover's lasted far far more than the original 90 sols .... did we over-design them? One could say "well of course!", but ... think about it, if NASA had asked us to design to last exactly 90 sols (with no designed-in life-limiting "timer" like MPF had with its Ag-Zn battery), could we have? No. If we had tried, the rovers would have lasted either 3 sols or 300. When it comes to reliability, we can only design for inequalities: we design for MORE than 90 sols (90 sols plus various design-in and tested margin). If we had designed for ZERO margin above 90 sols, then we might have lost them on sol 3. That would not do. Our contract with NASA and Steve was to last 90 sols or more. If MER had died on Sol 91, we still won (just), but if it died on sol 80 we would not.

On MSL we do have a a life-limiting timer: the MMRTG. We know that no amount of wind will make it generate more power as it ages. But at least we have a guaranteed supply of trickle-charging into the batteries. We have traded limited life for reliable power over a wide latitude band and over a Mars year.

You asked about lessons from MER to MSL (and from Phoenix to MSL). The list of lessons is too long to type. Interestingly, due to design and mission differences there are some aspects of MSL that just are not well informed by any previous surface mission. In some aspects we are in new engineering territory (again). MSL really is a big challenge, easily as big as MER.

Good night,

-Rob

[Fozzie]

This might be a silly question, but why wasn't it possible to make the solar panels tiltable so the dust would simply slide off when they were set vertically?