What a difference a windy day can make. Before and after self-portraits dramatically show how gusty Martian winds swept away months of dust that had accumulated on the Opportunity rover's solar panels.
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Strong winds blowing over the rim of Endeavour Crater in mid-March removed almost all of the accumulated dust, increasing the power available from 375 watt-hours in January to more than 620 watt-hours per day by mid-April or about 80% of maximum. In fact, the rover now has cleaner solar arrays than in any Martian winter since it first landed on the Red Planet in 2005.
In a similar move, rains recently removed the accumulated road salt from my car deposited during the long winter. It's now about 70% back to its original appearance. Opportunity 'cleaning events' , as they're called, happen periodically, and have played a significant role in the rovers' longevity. Mars' winds typically blow 20 mph with maximum speeds recorded by the 1970s Viking landers of 60 mph.
With extra juice to power its instruments, Opportunity can continue exploring Murray Ridge on the western rim of Endeavor Crater. Can you believe the rover's been rolling around the Red Planet for more than 10 years now?
During Opportunity's first decade on Mars and the 2004-2010 career of its twin, Spirit, the hardy robots have shown that Mars was once wet with both acidic waters and milder, more neutral ones conducive to supporting life.
While we're on the topic of Mars, you may have noticed that the planet has been moving in the wrong direction since early March. The outer planets normally track east across the sky as they orbit the sun, but around the time of opposition, when they're closest to Earth, they reverse gears and travel west in retrograde motion.
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Of course, Mars didn't really stop and change directions. It only appears that way as the faster Earth laps the slower-moving planet. Like passing a slower car, Mars appears for a time to travel backwards from our perspective. Once Earth moves far enough ahead of Mars - rounding the corner of our orbit as it were - the Red Planet resumes its normal eastward movement.
All the outer planets move more slowly around the sun than Earth and show retrograde motion, but the farther the planet, the smaller the loop. Mars scrawls a bigger loop because it's closest.
This year's retrograde amble takes Mars only a degree from Porrima, an attractive, closely paired double star in Virgo.
They'll be closest later this week on May 2-3. Small telescope owners can use Mars to find Porrima. At low magnification, it looks like a single star, but increase the power to 100x and higher and you'll see twin pearls of starlight.
Porrima's stellar twins are similar to the sun but hotter and nearly identical in size. They go around each other every 169 years in an elliptical orbit that makes their separation narrow and widen as seen from Earth. The stars were closest in 2005 when they were separated by about 500 million miles, equal to Jupiter's distance from the sun. They'll be over 7 billion miles apart around the year 2080.
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