[X] My Assistant

[Cugel]

Does anybody know how much time Oppy actually can spend on driving on a single sol?

I remember that when she travelled from Eagle to Endurance crater it was no more than 1.5 hours per sol (or so), due to power limitations. I have no idea what the current power situation and battery status of Oppy allows these days...

[edstrick]

Every time you wish for a solar panel cleaning system, consider 1.) weight, 2.) complexity, and ask: "Which scientific instrument on these rovers would I trade in return for the mass and volume for a dust-removal system?".

[Nirgal]

Every time you wish for a solar panel cleaning system, consider 1.) weight, 2.) complexity

then what about a simple "windscreen wiper" ?, lightweight (maybe 0.1 percent of rover mass), negligible complexity and quite effective

[dvandorn]

A "windscreen wiper" (or "windshield wiper," as we Americans call it) woud end up, over time, scratching the coatings of the solar cells so badly that they would lose effectiveness. (Try tossing fine rock dust on your windscreen and clearing it with the wipers -- you end up with thousands of tiny scratches in the glass. And that's glass, which is more resistant to such scratching than the materials from which solar cells are made.)

You would also have to design your solar cell surfaces so that they have no cracks or crevices, and nothing that protrudes up above them -- they would have to be very flat, smooth surfaces. Ever seen a solar cell array? They're not all that smooth -- and making them smooth would probably add weight, too.

I think the best dust-clearing device you could come up with would be a small compressor that takes in Martian air, compresses it into a bottle, and (when you need a cleaning) is commanded to blow the air out through vents positioned to duct it across the arrays. Or perhaps just a bottle of compressed CO2 instead of the compressor. A compressor would let you clean your arrays as many times as you want over the lifetime of the vehicle, while a pre-loaded CO2 bottle would only be good for "X" number of cleanings.

But Doug is right -- such a system would likely weigh as much as one of the major sensor packages you want to put on your probe. So you either have to 1) accept the sacrifice of one of your sensors, or 2) increase the mass you can land on the surface by *just* enough to include your cleaning system. Since the second option makes the whole mission more expensive, you'll probably end up getting stuck with the first.

And while you're deliberating on solar cell array cleaning systems, you have to face up to the fact that any solar-powered Martian lander could have its power cut down to below-survival levels by a planet-wide dust storm, whether or not you can clean the arrays. If a really energetic global dust storm occurs and the atmospheric opacity increases beyond a given level for too many sols, a solar-powered probe may simply die. And you've sacrificed your sensor for nothing.

So I'm inclined to agree with Doug -- for longer-duration missions, RTGs are the way to go. For shorter-duration missions, solar power without array cleaning is acceptable, since you're going to get your primary science return very early on. Besides, it seems like Mars is willing to provide enough wind to clean off solar arrays at frequent enough intervals to make extended missions for even these short-duration probes more likely.

There are ways of "tweaking" solar-powered probes, though, to try and get the longest life out them. For example, I believe the MERs use tiny radioisotopic heaters within their main "boxes" to keep the electronics and batteries comfortably warm. Those provide heat without using electricty, thus reducing power consumption needs. The MERs only have electric heaters in places where you can't easily use a passive heating system, like on the IDD. I'm sure there are other ways tweak solar-powered landers to enhance their lifetimes that are both easier to implement and weigh a lot less than solar array cleaning systems.

-the other Doug

[Marcel]

Right, i agree on almost everything said. Except: the payload being unable to increase. Ofcourse MER could have been made 5 kg's more heavy.

It's a matter of money. I see it like this:

Better a 1200 sol mission for 900 mln (an extra 100 million for 5 kg is enough isn't it ?), then a 800 sol for 800 mln. It's not only about $'s, it's about $ per sol.

[dvandorn]

It's not necessarily linear -- the easiest way to increase payload is to use a bigger booster. The MERs pretty well maxed out the capabilities of the Delta IIs used to launch them, and as of 2003, there really were no larger boosters available.

Delta IIs are proven technology, relatively inexpensive (compared to most of the rest of the options), and let you send something about the size of the MERs to the Martian surface.

Heavier-lift booster configurations (like some variations of the Delta IV and the Atlas V) are just starting to come on line, and will probably be used to launch MSL. But they're new, the companies that produced them spent a *lot* of money to develop them, and they're going to be quite a bit more expensive than the Delta IIs, especially at first. Whether that's $100 million or less, it's still significant.

Also, increasing the size of your lander package by a few kg isn't just a matter of adding a little more fuel to your rocket. Every pound you land on Mars costs in increased mass of *all* of the previous stages -- in MSL's case, the Skycrane would have to carry a little more fuel, the wheel suspension will have to be rated to survive touchdown with 5 kg of increased mass, the TMI stage will have to be a little more powerful, and since it will have to carry more fuel, the stage(s) of the booster will have to be more powerful and therefore heavier -- as you backtrack to the rocket sitting on the pad, every pound of extra payload you want to land on Mars can add hundreds and even thousands of pounds of additional fuel required to get it there. (To land less than 20,000 pounds on the Moon, Apollo had to launch a rocket that weighed nearly seven *million* pounds at lift-off. And it takes less overall energy to get a pound of mass to the lunar surface than it does to get it to the Martian surface.)

And on top of all of that, when you increase your lander mass and move to a new, bigger booster, you have to design all of the interfaces from scratch, you have to redesign your cruise stage, you have to validate and possibly redesign your parachute, you have to validate your heat shield configuration -- all of those things can make moving from a Delta II up to a Delta IV or an Atlas V more expensive than the simple increased cost of the booster.

So, maybe an extra $100 million would cover it. Maybe it wouldn't. We'll eventually transition to heavier-lift boosters, but y'all have to understand that this will inevitably lead to increased development and mission costs.

-the other Doug