http://www.tnni.net/~dustymars/Observing_Mars_6

Since 1971, the year of the "Great Dust Storm of Mars," the ALPO Mars Recorders have suspected that these disturbances came in pairs. We have seen that in 1971 a major dust storm occurred on 213° Ls, followed by a "planet encircling" dust storm on 259° Ls. Again, in 1973 a major storm began on 244° Ls and was followed by a "planet encircling" storm on 300° Ls and [Martin, 1974]. The Viking Lander recorded two "planet encircling" storms on 204° and 268° [Tillman, 1988]

This is only the first big storm this season. At present, it's 282° Ls. And the big 1971 storm began to dissipate by 314° Ls. Still a long way to go...

Almost entirely off-topic: Still A Long Way To Go - an uncannily appropriate lyric.

It's truly extraordinary that both rovers are fully charged after the last couple of weeks. Unless the dustfall at the end of the storm covers the solar panels, it looks like MERs are virtually immortal - if they can survive this, even a prolonged, full-on global storm might be survivable.

Thanks for the info, OWW...

So now the question would be, once this storm's abated, do the Opportunity planners take a dip into Victoria, with the risk of getting caught somewhere less pleasant towards the end of a potential 50-60 day window, or sit still for two months with at least the potential for no more bad weather (is storm #2 chance or certainty?) and the risks that accompany an aging rover?

Andy, glad to not be decision-making.

I'm not a MER mission planner, nor do I play one on TV, but I would not hesitate to get in there and get valuable science done as soon as conditions permit. A follow-up storm might or might not happen. I don't see them being that far in that they couldn't quickly retrace their steps and get back out unless a massive storm suddenly took hold right in this area. I haven't seen that kind of bad luck yet on this mission (touch wood).

Brian

Why would being in the crater be worse than being outside?

Because being flat on the plain you get the maximum indirect illumination of the arrays by the dust. On a slope - that will drop off (which every direction the slope is)

With clear skies and 800 Whrs, it doesn't really matter given that the slope runs down to the East (better power in the morning, but lower power in the evening) but with almost all the power currently coming from the difuse route - you want to be flat as you can.

Doug

Reading over the original mission description, I found this chestnut:


How things have changed from then!
The bestest words I have heard this last week are not "Deathly Hallows" - They are "Power Positive!" *




*Though it does sound like some cheesy self help course or weightlifter diet additive when you say it by itself.

I expect it wouldn't have amounted to a great loss of power if Oppy had been on the planned ingress route to the white layer. Being inside the crater would mean that the apparent horizon as viewed from the rover would be higher than the true horizon, so less sky would be visible. But the white layer is only 4 metres or so vertically inside the crater. The nearest cliffs to the expected ingress route are on Cabo Verde, about 30 m away (recall that they don't plan to drive near any cliffs!). That means (allowing for the height of the solar panel deck above the ground) that only 6 or 7 degrees of the horizon towards Verde would be obscured. Similarly, because of the tilt of the crater slope, a "sliver" of sky very roughly 15 degrees maximum width at the near rim and tapering to zero width at the far rim would be obscured.

You could imagine working out this way what percentage of sky, or how many steradians, would be obscured at the expected white layer study sites. No doubt an accurate elevation model would help here. I very crudely estimate a fraction 0.13 of the sky would be obscured at the white layer.

But there are more factors than just solid angle obscured. The solar arrays are much less sensitive to sky illumination from near the horizon than from overhead, just because light from near the horizon strikes the arrays at a glancing angle. Thus to estimate the power loss you must multiply that 0.13 fraction by a small geometrical factor, which is going to be something like the sine of the typical altitudes of obscured sky - that works out to another factor or order 1/10. Also, from the latest pancam images the sky appears to grow darker near the horizon. Therefore you get down to the 1% order of magnitude for total loss of power at the white layer.

Of course, if levels are truly critical, then even a couple percent reduction (like from 100 Whrs to 98 Whrs) could conceivably be fatal. Still, this likely wouldn't have been very important. I could see us entering Victoria soon after power levels allow mobility.

But, when the storm(s) end, wouldn’t more dust settle on Oppie inside the sheltered crater than on the rim? And wouldn’t there be less wind down there to remove said dust?

This is interesting fredk. On a much lighter tone but using your idea, in the absolute, we can also demonstrate that having put solar cells under the solar panels could have brought some energy too! ...and even basicaly having put them anywhere on the rovers.

Does anyone have quantifiable differences?

To the best of my understanding, the difference in power would amount to about 3.5% if it were all diffuse.

Updated: I didn't read Fredk's post prior to this response. It seems consistant with my estimations.

Months, not weeks...
The storm started in Aug or Sept 71. When Mariner arrived in November, most of the surface was hidden except for south polar cap and the top of the Tharsis volcanos and Olympus. Craters were visible as bright circular spots due to increased scatter from the deeper dustier atmosphere compared with adjacent higher terrain, as was Valles Marineris. After proving that their pre-planned mappign and observation sequences were near worthless, they replanned repetitive storm observation sequences and mapping tests to see how things were clearing. They finally started systematic mapping of the planet in January 72, starting with the least dusty high southern latitudes and progressively moving northwards.

Are they calling this a global event yet?

I haven't heard that term used in public yet, Ed. This storm seems not to have engulfed the entire planet so far. It has, however, encircled the planet.

The Odyssey Themis dust storm monitoring page has been updated with an atmospheric opacity map for July 22-24. Per the latest map, it appears that opacity has improved somewhat for both rovers. But it's not obvious yet that the storm is dying as of Opportunity sols 1243-1244.

Big storms used to be called global, but Viking and later data showed that that was not strictly true as polar region dust levels are low enough the surface remains visible. Some papers <late 80's?> called the 1971 greatest-ever storm as truely global, while calling other storms as "globe encirclig"

Most pre-1971 storms were not observed well enough to make a clear distinction, and it's likely storms occured during perehelic solar conjunctions and the long periods when Mars is on the far side of the sun and until CCD imaging and advanced amateurs, was essentially not being observed at all.

The basic fact is that the historical record of storm statistics stinks.