Why Have The Rovers Lasted So Long?

With 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 5,000+ days on Mars! So, how have these rovers survived the rocky terrain, the frigid cold, and the wind-blown dust scouring the surface of Mars?

A combination of sturdy construction, creative solutions for operating the rovers and even a little luck!

Engineers 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.

Each 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.

A 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.

Inside 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.

With 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.

Two 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.

Because 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.

Parking 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.

In 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.

During its eighth Martian winter, the Opportunity rover drove despite low amounts of sunlight. Engineers on Earth drove the rover from one small hill to another, called "lily pads," always keeping the solar panels tilted toward the sun.

The 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.

As 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.

From 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.

Mars 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.

During 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.

A 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 has overcome significant challenges during its 14-plus years on Mars. The rover lost use of its right-front wheel years ago and drives backwards, dragging that wheel. Despite this, Opportunity has driven more than any other robot-- 28 miles (45 kilometers).

During the storm, Opportunity 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 has sent over 800 commands to the rover. Despite these efforts, Opportunity has not communicated with Earth since June 10, 2018.