More than four months after they landed on Mars, both of NASA's twin rovers have suffered setbacks, hinting at potential problems that could eventually lead to their demise in the Red Planet winter.
For the Spirit rover, in the midst of a weeks-long trek to the Columbia Hills in Gusev Crater, the setbacks have taken the form of computer glitches, apparently resulting from a couple of unlucky breaks.
Meanwhile, on the other side of the planet, the Opportunity rover has suffered a power drain that has reduced its working capacity to as little as one or two hours a day. To stretch the robot's power supply, controllers on Thursday will begin what they expect to be frequent use of an overnight "deep sleep" mode.
Shutting down more completely overnight will conserve enough battery charge to add several hours of science operations during the day, Jim Erickson, the rover mission's deputy project manager, said in a status report issued Wednesday by NASA's Jet Propulsion Laboratory in Pasadena, Calif.
But the "deep sleep" strategy has a potential downside, NASA said: Without an overnight heater running, one of the six scientific instruments might be disabled by the cold.
The susceptible instrument is Opportunity's miniature thermal emission spectrometer, called the Mini-TES. It makes infrared observations used for identifying minerals from afar to help the science team decide where to send the rover. Its observations also provide close-up evaluation of rock and soil targets, and thermal information about surface materials and the atmosphere.
"The Mini-TES gives us vital insight into the minerals in rocks and the role of liquid water in their formation, so this choice is a carefully considered decision to weigh the risk of losing this capability against the benefit of continuing and increasing Opportunity's ability to do all the other exploration-oriented things this rover can do," Jim Garvin, lead scientist for Mars and lunar exploration at NASA Headquarters, said in the status report.
Both Opportunity and its twin rover, Spirit, have already provided several weeks of bonus operations after successfully completing their primary missions: three months of examining geological evidence about past environments at their landing sites.
Opportunity is currently examining a stadium-sized impact crater from vantage points around the rim. Mission managers are considering whether to send the rover on a risky trip inside the crater, nicknamed Endurance.
As winter advances in Mars' southern hemisphere and dust accumulates on the solar panels, the amount of electricity the rovers can generate is decreasing. The decline is more serious for Opportunity, because the robotic arm of that rover has a heater with a malfunctioning switch. The switch cannot be turned off.
A properly functioning thermostat turns the heater off during the day, but the heater stays on overnight even when it’s not needed. The amount of energy wasted was not enough to hinder Opportunity from succeeding in its primary mission, but it is now sapping about one-third of the rover's diminished amount of solar-generated electricity.
"Deep sleep gives us a way to turn that heater off overnight," said Matt Wallace, Opportunity's mission manager. The capability to do so results from a software upgrade transmitted to both rovers in April. The first use of deep sleep, on Opportunity on May 6, verified its benefit to the useful power supply.
After that deep-sleep tryout, scientists and engineers decided not to use the procedure again until the spectrometer had completed high-priority observations from two different overlook points of Endurance Crater. Those observations were completed Tuesday.
Turning points ahead
Tests on Earth indicate that a key component of the Mini-TES would become ruined somewhere in the temperature range of minus-58 to minus-76 degrees Fahrenheit (-50 to -60 degrees Celsius). Computer analysis indicates that in deep-sleep mode, the instrument would experience temperatures of about 54 degrees below zero Fahrenheit (-48 degrees Celsius).
"That has me concerned because it's getting close," said Phil Christensen of Arizona State University, lead scientist for the Mini-TES. But Christensen concurred with the decision to take the risk in order to free up adequate power for the rover's other activities.
Meanwhile, engineers and scientists are assessing how well Opportunity would be able to climb out of Endurance Crater. The assessment will aid in deciding whether to send the rover into the crater for up-close examination of rock layers there. Opportunity may complete a circuit around the crater's rim by mid-June and be ready for a decision about entering the crater.
Spirit, halfway around Mars, resumed normal operations Sunday after engineers diagnosed a software glitch that halted the rover's activities last Friday. The symptoms resembled a problem seen about a week earlier, when the computer encountered a conflict between two onboard tasks. However, NASA said the errors were understood and the two incidents were unrelated. If they recur, neither would pose a threat to the rovers' health, NASA said.
Spirit is now less than a half-mile (700 meters) from the base of the Columbia Hills, having traveled more than 1.5 miles (2.5 kilometers) since landing. Controllers are optimistic that Spirit will reach the base of the hills by mid-June.
Spirit landed on Mars on Jan. 3, and Opportunity arrived three weeks later. The original $820 million mission was scheduled to last for 90 days, but last month that mission was extended until September, at an additional budgeted cost of $15 million.
This report is based on information from NASA's Jet Propulsion Laboratory.