Why Have The Rovers Lasted So Long?
Living Beyond the Extended WarrantyWith a mission planned for 90 sols (Martian days), the Mars Exploration Rovers Spirit and Opportunity lived way past their "warranty dates." Spirit lasted thousands of days longer than the original plan. Opportunity has lasted more than 50 times as long and is still going! So, how have these rovers survived the rocky terrain, the frigid cold, and the wind-blown dust scouring the surface of Mars?
So Why Have the Rovers Lasted so Long?A combination of sturdy construction, creative solutions for operating the rovers and even a little luck!
Built to be ToughEngineers designed the rovers' aluminum and titanium mobility systems to handle both the tough terrain of Mars and the wild temperature swings of up to minus 180 degrees Fahrenheit (minus 100 degrees Celsius) each day.
Hard to Tip OverEach rover has six-wheel drive for getting around and a "rocker-bogie" suspension designed for stability. The center of gravity for the rover is at the pivot point, where the rocker-bogie attaches to the body. This design means the rover can tilt up to 45 degrees in any direction without toppling over.
Spreading the LoadA differential, a shaft that connects both sets of wheels, uses gears to spread the rover's weight across all six wheels. If the left side wheels tilt up, the right side presses down, providing the balance to all six wheels to get out of a sticky situation.
Keeping Warm When It's Really ColdInside the "belly" of each rover is a "warm electronics box" to keep batteries and other cold-sensitive equipment warm enough to keep working. Eight radioisotope heater units each produce about one watt of heat from about 2.7 grams (0.1 ounce) of plutonium dioxide.
Surviving the Cold NightsWith nighttime temperatures dipping as low as minus 157 degrees Fahrenheit (minus 105 degrees Celsius), the heaters have allowed the rovers to operate for years in the harsh Martian cold.
Driving BackwardsTwo years into her mission, Spirit's right front wheel motor stopped. Pushing a stuck wheel across sandy soil was too hard. The best choice: drive backwards to take the pressure off. Spirit spent the rest of her over six-year mission driving backwards, dragging her front wheel.
Leaning Toward the LightBecause Mars is almost twice as far from the sun as Earth, less sunlight reaches the surface, even in the summer. The rovers needed more creative solutions to survive the especially gloomy Martian winters when even less sunlight was available for power.
Working in WinterParking for months on north-facing slopes both saved energy and allowed the dust-caked solar panels to be tilted toward the weak winter sun. With the small boost in power, the rovers could keep working while standing still.
Working While ParkedIn 2006, for example, Spirit took 119 images to create a high-resolution panorama of her winter haven, resulting in the McMurdo Panorama that you see here.
Opportunity Loves 'Lily Pads'During its eighth Martian winter, the Opportunity rover has been able to drive despite low amounts of sunlight. Engineers on Earth drive the rover from one small hill to another, called "lily pads," always keeping the solar panels tilted toward the sun.
Solar Panels to Collect the Sun's RaysThe rovers' unique wing-shaped solar panels provide more surface area to collect the weak Martian sunlight. That also means more area to get covered in the fine, reddish dust! Engineers expected some dust to accumulate on the panels.
Dust-Covered Solar PanelsAs the years passed, the rovers appeared to blend in with the landscape as the dust covered them. The amount of energy generated by the dusty panels declined.
Dust Swept AwayFrom time to time, whirling columns of air called dust devils swept across and removed dust from the rovers' panels. Both rovers took images of "mini-martian twisters" skidding across the landscape.
Weathering Martian StormsMars is known for large and powerful dust storms that can grow to the size of continents. The storms bring dark skies, reducing the amount of sunlight reaching the rover's solar panels, draining its batteries.
Powering Through the StormDuring storms, controllers on Earth monitor a rover's power levels closely and can reduce activities to save energy. Some actions can draw a lot of battery power, and lower a rover’s temperature. In such situations, rovers often have to limit activities, even stop science observations.
The Dust Storm of 2018A local dust storm started near Opportunity's location on June 1, 2018. In a few days' time, the storm ballooned to about 15.8 million square miles (41 million square kilometers), the area of North America and Russia combined. This is the harshest storm the rover has ever faced, and it’s taking several precautions to weather it.
Opportunity Hunkers DownIn this situation, the rover has hunkered down and suspended science observations to conserve its battery, limiting the amount of power it uses. Storms can be rough, even for an overachieving little rover. The team is monitoring Opportunity closely, and with a bit of luck, we may see the rover weather this latest storm.