segunda-feira, 23 de julho de 2007

Futuro dos rovers Spirit e Opportunity em grave risco devido à tempestade global em Marte

A tempestade global de areia em Marte (de que já aqui falei) continua a piorar, e a NASA está a ponderar a possibilidade agora bem real de vir a perder um ou ambos os rovers Spirit e Opportunity.

Esta imagem compara a distribuição do pó na atmosfera de Marte
antes da tempestade (22 de Junho) e agora, durante a tempestade.

Os rovers tinham previstas missões que não iam além dos 90 dias. No entanto já estão a funcionar em Marte há mais de 3 anos! Mas o fim pode estar próximo, já que estes robots dependem da luz solar para carregar as suas baterias, e também para os manter quentes - muitos dos circuitos electrónicos dos rovers não podem ser expostos a temperaturas excessivamente baixas.

Esta tempestade é tão intensa que apenas chega à superfície 1% da radiação solar!

Esta sequência de imagens foi captada a partir do rover Opportunity, entre
14 de Junho e 19 de Julho de 2007. Os números no topo representam a
opacidade da atmosfera.

Mars rovers struggle to weather dust storm
3:05 PM, 7/22/07,

NASA's twin Mars rovers are struggling to weather widespread dust storms that have reduced solar power generation to critically low levels. If conditions worsen, one or both robots could succumb to frigid temperatures and "bite the dust" after three-and-a-half years of unprecedented exploration.

"The team is concerned. We all have very strong bond with these two rovers," said John Callas, rover project manager at the Jet Propulsion Laboratory in Pasadena, Calif. "We think of them as our children and we're worried for them right now because they're in this dangerous environmental condition on Mars.

"We're trying to help them as much as we can, but there's only so much we can do from the ground," he said by telephone Friday. "Ultimately it comes down to that. But these things are such intrepid, feisty vehicles. So the mood is 'guarded optimism.' We're watching the weather every day."

The dust storms flared up over the past month just as engineers were preparing to order Opportunity into Victoria Crater to study exposed bedrock that might hold valuable clues about the role of water on the red planet in the distant past.

Instead, the rover remains parked outside the crater, its electronic systems shut own to conserve power under a deeply overcast sky. Half a world away, the Spirit rover was in much the same state, although slightly more power positive than Opportunity.

"The figure of merit on the weather that we track is atmospheric opacity," Callas said. "The higher that number is (known by the Greek letter tau), the more sunlight is lost. Our last estimates of tau for Opportunity was around 5, which means that less than 1 percent of the direct sunlight is getting through the atmosphere. That means that basically the solar array energy is coming from the diffuse sunlight. So it's like a very overcast day and all you have is diffuse light.

"The tau for Spirit is right now at about 4, so it's not as high but it's still exceedingly high. And these numbers are higher than any numbers ever measured by Viking. The highest number Viking measured was about 4. Mariner 9, when it arrived at (Mars) the entire planet was covered with a massive global dust storm that obscured the surface. This is not quite at that scale, but this is probably the worst we've seen in, I don't know, over a decade, I guess."

Spirit and Opportunity landed on opposite sides of Mars three weeks apart in January 2004. They were designed to operate for at least 90 days and while hopes were high they would survive the harsh martian environment beyond their design life, no one expected them to last as long as they have. As of last week, Opportunity had logged some 1,240 days on Mars and the rovers are considered among the most successful interplanetary missions ever launched.

At present, the ongoing martian dust storm is best described as "hemispheric." It doesn't shroud the entire planet but Spirit and Opportunity, on opposite sides of Mars, are both affected.

"A possible outcome of this storm is that one or both rovers could be damaged permanently or even disabled," NASA said in a statement Friday. "Engineers will assess the capability of each rover after the storm clears."

The forecast, Callas said, is "not good. More of the same."

"And it's been variable. We've been spoofed a couple of times where the tau would spike up and then it would turn around and drop down a little bit and we'd think oh, well the worst is past. And then it would just turn around again and spike back up to a new record."

Conditions are slightly worse over Opportunity. Solar panel output stood at around 720 watt hours of electricity before the dust storm began. Seven hundred watt hours is enough electricity to power a 100-watt bulb for seven hours.

When the solar panel output dropped below 400 watt hours a day, Opportunity was parked and its science observations terminated. On July 17, power output dropped to 148 watt hours and one day later, to 128 watt hours, barely enough for the heaters needed to keep the electronics warm enough to prevent problems.

"What we've had to do is pretty much take everything out of the sequence the rover normally does," Callas said. "So it just wakes up and goes back to sleep after being up for a short amount of time. We do have communications sessions, but right now we're sprinkling them about every three days."

Flight controllers sent commands to Opportunity Friday but did not plan to check in again until Monday.

Callas said the electronics in each rover is mounted inside an insulated box and that is thermostatically protected. The avionics system is designed to operate at temperatures as low as minus 40 degrees Fahrenheit and to survive, powered down, at temperatures as low as minus 67 degrees. Below that, the rover would be at risk of failures from cracked wires or damage to other components.

So far, ground temperatures near Opportunity are running at around minus 26 degrees. And even if one or both orbiters ultimately had to be powered down and temperatures dropped below allowable limits, "it would not be certain death," Callas said. "There's some expectation that if we got outside of our operating regimes that the hardware would hang together."

When the sky eventually clears, "the rover would power back up, it would wake up on its own and it would boot up and it would start phoning home."

Callas said as long as the rovers remain power positive - generating more electricity than they consume - "we could ride this out forever."

"But if it gets just a little bit worse, then that could tip us over the edge and then we'd start running negative," he said. "What you do is, you're taking more energy out of the batteries each day than you're putting back in."

Eventually, the batteries would be drained and no power would be available for the heaters. Engineers are hopeful it won't come to that.

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