Launched in June and July, respectively, of 2003, the twin rovers Spirit and Opportunity became the second successful mission(s) to land mobile planetary probes on the surface of Mars. In the intervening years since tiny Sojourner spent 84 days exploring the planet’s surface, two US Mars missions failed to reach their objectives. In 1998, Mars Climate Orbiter burned up in the planet’s atmosphere as it attempted to enter orbit. In 1999, Mars Polar Lander failed to successfully deploy its main payload, the Deep Space 2 penetrator.
In 2001, things began looking up (down?), with the success of the Mars Odyssey orbiter, which continues to stream back a rich trove of surface sensing data (and is expected to keep doing so until 2025). Nonetheless, in 2003 the Mars Explorer Rovers Spirit [MER-A] and Opportunity [MER-B] began to stir the red dust once more. Spirit and Opportunity follow the six-wheeled, independent bogie format of their mini-predecessor, Sojourner. In the twins’ case, each wheel is powered by its own motor, with the steerable wheelsets at front and rear. Onboard solar arrays generated about 140 watts for up to four hours per Martian day (sol) while rechargeable lithium ion batteries stored energy for use at night.
Spirit was targeted to land at a site that may have played host to liquid water in its past, the crater Gusev, a possible former lake within a larger impact crater. Spirit returned the above photo of its touchdown site. In 2004, NASA announced that Spirit had found hints of water history on Mars in a rock dubbed “Humphrey”. During a NASA press conference, mission scientist Raymond Arvidson of Washington University in St. Louis said, “If we found this rock on Earth, we would say it is a volcanic rock that had a little fluid moving through it.” Spirit stayed on the move for five years, until 2009, and stopped communicating back to JPL in 2010.
Later in summer 2003, Opportunity lifted off aboard NASA’s first Delta II (Heavy). It soon returned a photo of its own landing site, now named Challenger Memorial Station, after the shuttle that was lost with all hands in 1986. Opportunity landed on an open plain, near an intact meteorite.
In late April 2005, the Opportunity rover became mired in a dune, burying several of its wheels in the sand. Over a six-week period, Earth-based scientists performed physical simulations to decide how best to extract the rover without risking a permanent immobilization of the valuable vehicle. Successful maneuvering a few centimeters at a time eventually freed the rover, which resumed its travels. Opportunity then traversed Erebus, Victoria, and Endeavour Craters. The panorama below shows Erebus Crater.
Opportunity continues to add to the global Mars dataset, and has traveled semi-autonomously over more than 28 miles of terrain as of early 2017. Both Spirit and Opportunity employ(ed) an onboard computer using a 20MHz CPU, with 128 MB of RAM.