Mars rovers braving severe dust storms
Fri Jul 20, 2007 at 20:36 UTC
Having explored Mars for three-and-a-half years in what were missions originally designed for three months, NASA's Mars rovers are facing perhaps their biggest challenge.
For nearly a month, a series of severe Martian summer dust storms has affected the rover Opportunity and, to a lesser extent, its companion, Spirit. The dust in the Martian atmosphere over Opportunity has blocked 99 percent of direct sunlight to the rover, leaving only the limited diffuse sky light to power it. Scientists fear the storms might continue for several days, if not weeks.
We're rooting for our rovers to survive these storms, but they were never designed for conditions this intense.
-- Alan Stern, NASA Science Mission Directorate
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Before the dust storms began blocking sunlight last month, Opportunity's solar panels had been producing about 700 watt hours of electricity per day, enough to light a 100-watt bulb for seven hours. When dust in the air reduced the panels' daily output to less than 400 watt hours, the rover team suspended driving and most observations, including use of the robotic arm, cameras and spectrometers to study the site where Opportunity is located.
On Tuesday, July 17, the output from Opportunity's solar panels dropped to 148 watt hours, the lowest point for either rover. On Wednesday, Opportunity's solar-panel output dropped even lower, to 128 watt hours.
NASA engineers are taking proactive measures to protect the rovers, especially Opportunity, which is experiencing the brunt of the dust storm. The rovers are showing robust survival characteristics. Spirit, in a location where the storm is currently less severe, has been instructed to conserve battery power by limiting its activities.
Image Credit: NASA/JPL-Caltech/Cornell
High resolution image
This image is a time-lapse composite where each horizon-survey image has been compressed horizontally (but not vertically) to emphasize the sky. The relative brightness and darkness of the sky from sol to sol (over a 30-sol period beginning June 14, 2007) is depicted accurately in these images, which view roughly the same part of the plains southwest of the rover. The images are approximately true color composites, generated from calibrated radiance data files using the panoramic camera's 601-nanometer, 535-nanometer and 482-nanometer filters.
The rovers' atmospheric science team is concerned that smaller, regional dust storms could expand into a larger, globe-encircling storm. That could extend the time the sun stays obscured, challenging the capability of Opportunity's solar panels to produce enough electricity for the rover to function.
Fortunately, as of July 19, 2007, the Opportunity site is clearing slightly. When the storm ends, atmospheric scientists hope to review data from the rovers that will help them determine what sort of dust was being lifted and distributed.
The numbers across the top of the image report a measurement of atmospheric opacity, called by the Greek letter tau. The lower the number, the clearer the sky. Both Opportunity and Spirit have been recording higher tau measurements in July 2007 than they had seen any time previously in their three and a half years on Mars. The five sol numbers across the bottom correspond (left to right) to June 14, June 30, July 5, July 13 and July 15, 2007.
Image Credit: NASA/JPL/MSSS
High resolution image
The two maps shown here are mosaics of images acquired by the Mars Reconnaissance Orbiter (MRO) Mars Color Imager (MARCI) on two days separated by about 3 and a half weeks. The first, on 22 June, shows that there was a dust storm occurring near the east end of the Valles Marineris trough system (left of the label for "Opportunity" in the map). This was the first in the series of storms. The second mosaic shows how Mars appeared on 17 July, after dust was lofted high into the atmosphere by several regional storms and countless smaller, local dust storms.
Each map was constructed from 13 pole-to-pole image swaths at red, green, and blue wavelengths acquired by the MRO MARCI. The maps are simple cylindrical projections, with north at the top and south at the bottom. Each image swath was acquired at about 3 p.m. local time on Mars over the course of 13 orbits. The black gaps occur in the MARCI data at places where the MRO spacecraft was slewed east or west to point its instruments at a specific target of scientific interest.
The north polar region is not shown because winter began on 4 July and the north polar region is in wintertime darkness. Key features labeled on the maps include the Tharsis Montes and Olympus Mons volcanoes, the Hellas impact basin, Noachis Terra, Sinus Meridiani, and the two Mars Exploration Rover (MER) landing sites, Opportunity and Spirit. The dust storms, and the planet-encircling dust veil they generated, has greatly reduced the amount of sunlight available to run the two solar-powered rovers.
Image Credit: NASA/JPL/MSSS
High resolution image
This sequence of Mars Reconnaissance Orbiter (MRO) Mars Color Imager (MARCI) daily mosaics shows some of the dust storm activity that occurred near the Mars Exploration Rover (MER) Opportunity landing site between 21 June 2007 and 18 July 2007. The Opportunity rover is located near the martian prime meridian and equator. The top and middle rows of images show the first six days of dust storm activity near the rover site as dust advanced from the west to the south and passed south of the rover over the course of a week. By the end of that first week, storm activity strengthened and continued to move east, eventually passing over nearly half of the martian southern hemisphere. Other storms spawned by this atmospheric disturbance affected the MER Spirit rover on the other side of the planet, while new storms developed, approached, and affected Opportunity.
The bottom three images show dust activity over the MER Opportunity site on 3, 14, and 18 July. By 19 July, most of the martian surface was obscured by the dust lofted from these storms. As with previous large dust-raising events on Mars, once the active storms die down, many weeks to months will pass before the dust settles out and the atmosphere clears. The white circle indicates the location of the Opportunity landing site, the black gaps are caused by slewing the spacecraft east or west to image specific science targets, and north in each picture is toward the top, west is to the left.
"We are taking more aggressive action with both rovers than we needed before," said John Callas, project manager for the twin rovers at the Jet Propulsion Laboratory, Pasadena, Calif.
By Opportunity's 1,236th Martian day, which ended Tuesday, driving and all science observations had already been suspended. The rover still used more energy than its solar panels could generate on that day, drawing down its battery. "The only thing left to cut were some of the communication sessions," Callas said.
To minimize further the amount of energy Opportunity is using, mission controllers sent commands on Wednesday, July 18, instructing the rover to refrain from communicating with Earth on Thursday and Friday. This is the first time either of the rovers has been told to skip communications for a day or more in order to conserve energy. Engineers calculate that skipping communications sessions should lower daily energy use to less than 130 watt hours.
A possible outcome of this storm is that one or both rovers could be damaged permanently or even disabled. Engineers will assess the capability of each rover after the storm clears.
