# Mars rovers roving again, for now

Giant dust storms that have pinned the Mars rovers down for the last six weeks are beginning to lift, allowing the two explorers to restart their slow crawl over the red planet's surface. Opportunity is once again rolling along. Credit: NASA/Opportunity The storms have blotted out the light from the sun, leaving the craft …

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#### Units

Watt-hours is a measure of energy, not effect. 300 Wh over how long time? A second? An hour? A day? There's a rather big difference.

#### Why not tell us the power of the solar panels in teranewton furlongs per fortnight?

At least that would actually be a unit of power. Unlike watt hour.

#### Watt Hour is perfectly acceptable measure...

Since they are simply talking about the energy produced by the solar panels, watts/hour is perfectly fine. That is stating that when exposed to sunlight, they produce 300 watts in one hour. It is implied that this will continue whenever their current condition is maintained, and they are exposed to sunlight at the level they currently recieve.

NOW - to discuss the energy budget of the craft, you would then need to analyze amount of sunlight per day, battery efficiency, and possibly even the variableness of that 300 w/hr figure by panel orientation. This then leads into the time domain, and gets a bit trickier...but still, 300 w/hr. would be one of the inputs to that calculation.

#### No!

Robert, "300 watts in one hour" is the derivative of energy production, or in other words, Joule per second per second, J/s^2, an acceleration. To be used in phrases like "In the first instants of the Chernobyl accident, the reactor developed a high J/s^2, going from 0 to something fierce in half a second".

We want to hear the J/s unit, or Watt, which is what is also written on lightbulbs. Or simply Joule if we want to hear about stored energy.

Yes, Watt-Hour is a unit of ENERGY, and 300 Watts/hour is DUMB. Power divided by time doesn't make any sense. Anyway 300 watts of solar panels even on earth is quite a bit. Since Mars is further away from the Sun, even more area is needed to make that much power.

I suspect that the proper term is about 300 Watt-hours/(mars)day [sol], or the solar panels are doing about 25watts (in sunlight for 1/2 day[sol]) for the 12+ hours they have sunlight.

#### Units of power cf. units of energy

I have to agree with Olof P and the anonymous post...

A Watt Hour (Wh) is a unit of energy, not power. Domestic electricity meters record the energy you use in kWh (kilo Watt hours). The Watt is a unit of power, which is energy consumed over a period of time.

It's not logical to say watts per hour (watts/hour) i.e. "300 watts in one hour" any more than it is to say the solar cells produced "300 Watt-hours" of power.

It would make more sense that the power systems can store 300Wh of energy per day, as can be read in the article at:

http://www.nasa.gov/mission_pages/mer/mer-20070824.html

#### @Robert Hill

You obviously also don't know about units. A Watt is a unit of power, equal to one Joule per second, talking about "watts/hour" is meaningless (a rate of increase or decease of power?). Similarly, a "watt-hour" is the energy of one watt (power) exerted over the period of an hour, and is equivalent to 3600 joules.

The correct unit for the power is the watt. Saying that the rover is getting 300W of power (at best/typical orientation) would make sense. Or saying that it gets a total energy of 300W-Hr over the period of a (Martian) day would make sense.

#### RE: Watt Hour is perfectly acceptable measure...

Dear Robert: Watt Hour is a perfectably acceptable measure of _energy_. And a measure of energy doesn't tell you anything about the solar panels. Even the tiniest most inefficient solar power in the universe will generate you 199293.9281 terawatt-hours if you give it enough time. Or just 12733 gigawatt-hours, given slightly less time. You get the drift. A measure of energy can be converted to a measure of power by specifying the _time_ during which the energy was produced/dissipated/transferred/whatever. A straightforward unit measured from watt-hour would be watt-hours per hour. 1 Watt-hour per hour = 1 Watt-furlong-fortnights per fortnight-furlong = 1 Watt. Which is incidentally the unit I believe the original author actually meant to be using.

#### Watts or not is not the problem here

Just imagine, there are hundreds, if not thousand people working at the space agency who's responsible for the robots and they never thought about a anti-dust device for the solar-panels and lenses ??

C'mon give me a break, how hard can that be ? A tiny broom on top of the solar panel ?? Even I could think about it. Does that mean now I am a rocket scientist ? Or does it mean they are only thinking from noon to midday ??

TJ Ay

on the Islands "Ebersberg" and "Kreiss" in Second Life

#### @TJ Ay

No, it means you don't realize that the rovers were specified for a sixty day mission, during which dust buildup was far less a concern than using the weight and space a dust removal device would have taken for doing more science.

The rovers are still running because they were designed extraordinarily WELL - and not failing now because they were designed badly, as you implied.

It just goes to show - exceed all expectations in your job and some git will accuse you of incompetence for it. Great.

#### Guys, guys

- look at us. Squabbling, bickering, like children. What's happening to us? We never used to be like this.

#### Quality of design

Obviously if dust clearance had been factor in the original design, it would have been sorted.

On another note...

While I find it impressive that the rovers are still functional after all this time, it could well be argued that this is evidence of *bad* design.

If the mission is 60 days, then the devices should be engineered to be reliable for 60 days and no more - obviously there should be some margin to ensure reliability is maintained for the whole mission but that doesn't have to be huge.

That the rovers are still (more or less) functional suggests someone seriously over-engineered them. This is almost as bad as under-engineering them as it adds cost, complexity and (most likely) weight, none of which is a good thing particularly for a space mission.

A properly engineered rover design would have lasted 60 days working perfectly, and disintegrated at dawn on day 61. (paraphrasing Colin Chapman)

#### wow

If I recall correctly these little long range remote control toys were only expected to work for a few months...... So they're doing pretty well considering the original estimate of their lifespan!

#### Re: TJ Ay

I gotta side with NASA on the Rover program. They really nailed this project. The units are operating far past their design life and have managed some pretty neat unplanned maneuvers.

Not having a little broom (or something) is not a design error or "bug". Please submit a feature request and your idea will be considered during our next upgrade phase.

yes we did.

#### Ah, the accountants

Of course we COULD have engineered it to last exactly 61 days but then we would have had to explain why the wheels started to fall off on the 59th day.

#### Attn; amanfrommars

AMFM, could you please get out the swiffer and give our probes a once-over?

TIA,

Teh Earthers

#### Rovers roving....

"Traversed the planet when heaven sent me. I saw the kings who rule them all

Still, by the firelight and purple moonlight, I hear the rested rivers call,

And the wind is crying, from a love that won't grow cold.

My lover, she is lying, on the dark side of the globe."

#### I love those little guys

Its amazing that those things are still going. They're on another PLANET for cocks sake!! I hope they trundle on for decades.

Come on, we have little robots exploring that planet in Total Recall, thats gotta count for something!

#### long term utility and function must be considered always

its is amazing that these little bots have lasted for sure....

however it can only be logical even to the layman that even at less than 60 days they could have needed dust removal so it should have been added.

more so, this simple device could have been dual purpose as in a simple directional vacume or blow pipe design that both directs the native wind to were its required to remove the dust and also as a wind power generator combined.

why...? , simple , if as it turns out the bots can keep going and collect far more data, then you have a fallback position with power generation and use, or if the roving parts fail in some way and the bots are still able to generate and store power that can be used later if we send other devices/man missions to the planet.

why waste weight an your later missions if ou know that you have

a device were your going that have power waiting for you to utilise later......

it always seems stange that when they crash these long range devices into things on perpose they dont release the power generation and storage devices into an orbit that can be collected and used later if its needed, why detroy it or set it adrift never to be seen again.

whatever happened to build it better make it last that the victorians mastered and gave to the world.......

#### Excuse me, nerds

You've missed something. The Mars rovers aren't actually on Mars. How else could they be 'not out of the woods yet'?

Proof at last that Apollo was a scam!

#### 700Wh per sol

For those concerned about the energy figures, for comparison NASA says that the normal figure for the rovers is 700Wh per sol (martian day).

http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20070807a.html

#### A modest proposal...

...to take care of the dust: just ramp the little robots up to whatever top speed you can crank out of them and aim them at a large enough nearby rock to cause them to bounce abruptly, thus shaking the dust loose from cameras/solar panels.

I've had similar experiences when hitting rocks at speed on various motorcycles that have managed to dislodge far heavier objects than dust particles...

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