March 25, 2005
spirit got a bath
If any of you are following the Mars Rovers, you'll probably have already seen the great news from the Spirit rover team that the solar panels got a good cleaning about two weeks ago. (For those of you who don't know, the lifespan of the rovers is probably going to be determined by the amount of dust that accumulates on the rovers' solar panels. The dust accumulates and blocks the panels' capability to generate electricity and recharge the rovers' batteries. Without that energy, the rovers die.)
About two weeks ago, just as Spirit snapped some amazing photos of dust devils the team noticed a big jump in the power output of the solar panels. It turns out that some very helpful dust lifting winds, probably the same winds that created the dust devils, cleaned up the solar panels.
This week, the team released some before and after photos of Spirit's Pancam calibration target which clearly shows how much of a cleaning the rover got. This is great news because it improves the power situation and will help Spirit make a great climb up to "Husband Hill" summit and probably extend the rover livespan significantly.
update If you're wondering why they didn't just build a windshield wiper onto the solar panels, read helvick's explanation in the comments. I should have added a note about that with the original post. Thanks, helvick.
Posted by asa at March 25, 2005 09:24 PMSo NASA is sending some high-tech piece of equipment on Mars but they didn't spend time on dev/producing a cleaning engine on the robot ?!? ...
Posted by: Mathieu Pellerin on March 25, 2005 11:23 PMMathieu, I was just about to write the very same question.
Posted by: Jonathan Horak on March 26, 2005 01:27 AMMathieu: Just because you thought of it, doesn't mean the NASA engineers thought of it. :-)
Posted by: alfons on March 26, 2005 01:37 AMOops: I meant "doesn't mean the NASA engineers DIDN'T thought of it." ;-)
Posted by: alfons on March 26, 2005 01:38 AMIt was determined that the cost/complexity/ability to break down of the cleaning system (panel wipers, static repellant, flip the panels upside down) was more than the benefit of extending the lifetime of the rovers. Of course, now that they know how long the rovers can really go for if kept clean, maybe future rovers will have some sort of cleaner. Then again, why add a cleaner when you have friendly dust devils.
Posted by: Hanspeter on March 26, 2005 05:32 AMThe debate about whether the rovers should have had some sort of solar panel cleaners was well thrashed out in blogs and elsewhere way back at the start of the mission. There was little well informed debate though, mostly off hand criticism of NASA for not doing something that seemed obvious. The truth is that a cleaning mechanism was not needed and they knew it wasn't so why add weight and complexity to fix a non-problem.
The design assumptions (based primarily on Pathfinder's experience) assumed a 0.28% loss of solar panel efficiency per sol due to dust deposition. Starting at 900 Watt hours they should have had 700 Watt hours available at the end of 90 sols (ignoring the effects of winter's reduced insolation which is way too simplistic but they factored that in rest assured). The rovers need a minimum of 280Watt hours of power per day just to survive but in order to do useful science I believe they need to be able to maintain > 400Watt hours. Looking at the numbers it's fairly clear that there was very little risk of hitting a power budget problem within the primary mission timeline, and probably even within a mission three times longer. Unless Pathfinders ground truth was dead wrong but that's a risk with any data based on few samples. Extrapolating out to 400+ days though it is evident (now) that degradation due to dust deposition has been less than expected even ignoring Spirit's cleaning event. So future models will be better and we have the unexpected bonus of knowing that occassional cleaning events might happen. In any case the design's non inclusion of any solar panel cleaning system was clearly correct.
This paper by NASA and OIA which is an analysis of possible solar array's for the Mars Polar Lander shows how much thought goes into these things. This specifically details the tradeoffs that need to be considered between planned mission life time, power budget, weight and complexity.
You know, Energizer would have made a great sponsor for the Mars rovers. :-)
Posted by: Jeff Walden on March 26, 2005 09:29 AMgood reply helvick, tks
Posted by: Mathieu Pellerin on March 27, 2005 02:19 AM