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Spirit and Opportunity, the twin Martian rovers

Dr Amanda Doyle, Feature writer
Jun 6, 2012, 23:00 UTC, Updated Sep 25, 2014, 9:17 UTC

Sen—The Mars Exploration Rovers, called Spirit and Opportunity, embarked on a journey to Mars intending to complete a three month primary mission to learn about the history of water on Mars.

The Mars Exploration Rovers were launched on 10 June 2003 and 7 July 2003, landing on Mars on 4 and 25 January 2004. Spirit touched down on Mars at Gusev crater, which is an impact crater that might have once held a lake. Meridiani Planum, on the opposite side of the planet, was the landing site for Opportunity as mineral deposits indicated water might have once been present. 

Over a decade since they landed Opportunity is still chugging away. Spirit succumbed to the harsh Martian environment in 2010, but still far exceeded expectations by driving 12 times further than was originally envisioned. 

In July 2014, Opportunity broke the record for the most distance covered by a rover on another planet. Since landing in 2004, Opportunity has travelled over 40 kilometres across the surface of the red planet. The rover was only designed to travel for about one kilometre.

If Opportunity makes it to a marathon distance of  42.2 kilometres, it will have reached a location that has been nicknamed "Marathon Valley". Space observations suggest this region might have exposed clay minerals which are worthy of investigation.

According to principle investigator Steve Squyres, it is unknown how long Opportunity will continue to function. “The way you predict the lifetime of space flight hardware is by doing tests to failure of critical components. We've never done such testing... there was never any reason to. So Opportunity could fail tomorrow, or could keep going for years... we have no idea.”

Scientific tools

The twin rovers are kitted out with two cameras; the Panoramic Camera (Pancam) and the Microscopic Imager, which serve opposite purposes as the names suggest. Three different spectrometers are used for analysing the alien red planet. The Miniature Thermal Emission Spectrometer (Mini-TES) is an infrared spectrometer that lays the ground work by identifying interesting rocks that deserve further inspection. Mini-TES can also look up and discern the temperature of the atmosphere above. The Mössbauer Spectrometer (MB) performs the close up inspections of the mineralogy of iron-bearing rocks and soil, while the Alpha Particle X-Ray Spectrometer (APXS) peeks at the abundances of the rocks and soil.

The rovers are equipped with magnets to help gather magnetic dust particles for analysis by the spectrometers. There is also a RAT on board each rover, in the form of the Rock Abrasion Tool. The RAT is responsible for nibbling away weathered surfaces of rocks to expose the fresh material beneath for analysis. 

Power is achieved through the solar panels, which have cells stacked in a triple layer on the solar array. This is in order to accumulate more energy than predecessor rovers such as Sojourner. The solar panels on Opportunity were coated in a layer of dust for a number of years, which blocked out some of the sunlight. Luckily, in March 2014 winds blew away most of the dust that was coating Opportunity's solar panels, boosting the amount of energy available to the rover.

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Opportunity's selfies taken in late March 2014 (bottom) shows that much of the dust on the rover's solar arrays has been removed since a similar self-portrait from January 2014 (top). Both were taken by Opportunity's panoramic camera (Pancam). Image Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

In December 2009, Opportunity was sent a software upgrade for Pancam – the Autonomous Exploration for Gathering Increased Science (AEGIS) system. Previously, interesting geologic targets were selected by scientists looking at the images that the rover produced. The AEGIS system however, allows for automatic target selection and detection of interesting features such as crater ejecta and outcrops. This negates the need for constant communication with Earth.

In August 2014, engineers successfully reformatted Opportunity's flash memory, which was needed after the rover suffered some memory problems.

Meeting the objectives, and much more

The Mars Exploration Rovers have successfully completed their scientific objectives, along with plenty of other unexpected achievements. The main goal was to learn about water in Mars’ history by examining rocks and soil, as well as discover the distribution of minerals in the localities. Another goal was to determine the geological processes that carved the terrain and to seek out clues as to what conditions persisted when liquid water flowed over the now barren landscape. Meridiani was confirmed to have once had water, perhaps in the form of a shallow sea and Spirit discovered overwhelming evidence for past hydrothermal activity on Mars, some of which could have occurred relatively recently.

The Mars Exploration Rovers have shown that the red planet is still somewhat active. Unlike the footprints of the Apollo astronauts left on the Moon, the tracks left by Spirit and Opportunity fade away as the Martian winds cause the tracks to dissipate. Other changes in the landscape observed by the rovers were a change in the reflectivity of the land as dust settles on to rocks or is eroded from them. Subtle ripples have also been seen to move across the sand. 

In its travels across the Martian landscape, Opportunity happened upon several large iron meteorites. The rover deduced that these meteorites impacted onto a soft and wet Mars millions of years ago, and the buried meteorites were subsequently exposed by erosion after the water dried up. A large iron-nickel meteorite was reproduced as a plastic replica by means of a 3D printer back on Earth.

While Opportunity remained stationary at Greeley Haven to wait out the Martian winter in 2012, the Deep Space Network telescopes on Earth were used to track signals from the rover. By combining this information with historic data from the Viking and Pathfinder rovers, scientists were able to refine measurements of the rotation of Mars. This led to improved knowledge of the planet's precession, and also gives an insight into the interior structure of the red planet.

Opportunity has concluded that ancient Mars was warmer and wetter than previously thought, which gives scientists hope of finding evidence of past microbial life. 

According to Squyres: “There's an enormous amount of possible exploration ahead if the rover holds together.”

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