Spirit, Opportunity, and the Mars Missions: A Guide for Newswatchers

January 2010 has so far been a big month for astronomy. In addition to major announcements from some of the major NASA space telescopes, especially the Kepler Mission, there was some deeply disappointing news from Mars: Spirit is on its way out. The 400-pound automated probe was driving on southern Mars when it got stuck in a sand pit several months ago, and so far has been unable to extricate itself. Unable to move, its solar panels are being gradually coated with blowing dust, and eventually, in the cold Martian winter, the probe will literally freeze to death. It will be an ignominious end for one of the twin exploration rovers which, in the past five years, have revolutionized human exploration of the red planet and pointed the way toward the future of space exploration.

Mars holds a special place in popular and scientific attention, as the closest planet to Earth and the one most easily visited (at least in theory) by human spacecraft. The more optimistic pipe dreams of the National Aeronautic and Space Administration (NASA) still feature a manned mission to Mars, although perennial budget cuts at this point make the space agency unable even to make a return to the Moon, something we were able to accomplish in the 1960s. (So much for progress.)

Nevertheless, Mars still features prominently in space news because of the variety of unmanned probes which have been sent to its harsh, dry, dusty surface. This article is a quick overview of the most important Martian probes, so that the next time you come across a mention of them while you're reading and watching the news, you will know what is being talked about. As of today, there are two operational probes on the surface of Mars -- the Spirit and Opportunity rovers -- as well as a small number of orbiting spacecraft.

Early Exploration of Mars

Our interest in Mars isn't just scientific, or pragmatic (it's the closest), it's also cultural (it's the coolest). There have long been a variety of crackpot theories that Mars was inhabited, beginning with misreading of early astronomical observations of supposed "canals" on the surface of Mars. This cultural heritage is why so many people were willing to give at least a little credence to the spurious story that the Phoenix probe had encountered evidence of Martian life so compelling and disturbing that the President of the United States was summoned to an urgent briefing.

Since it's comparatively close, Mars is easy to get to, with launch windows occurring roughly every two years (the next ones will be in late 2011 and early 2014). Plus, the Martian surface is a lot more hospitable than that of our next close neighbour, Venus. Mars is essentially a rocky desert, subject to intense heat and cold and dust storms, but that's about it. Venus's atmosphere, by contrast, is uniformly hot, heavy and corrosive. The Soviet Venera 13, which touched down in 1982, survived for a little over two hours, and scientists actually considered that a surprisingly good result (initial estimates were that it would last just half an hour).

Not that Mars is an easy task, either. Roughly one-third of American probes have suffered some sort of catastrophic failure during their transit to Mars, or shortly after their arrival. The Russians were particularly unsuccessful; historically, most of their craft have failed. (Other than simple bad luck, it's not clear why; their exploration of Venus, by comparison, was actually much more successful than the Americans'.) Japan's Nozomi probe suffered an electrical failure before entering orbit and the European Beagle lander crashed and is presumed destroyed. In the most embarrassing mess-up to date, NASA wrecked its $300 million Mars Climate Orbiter in 1999 by confusing metric and imperial measurements and sending it plunging into the atmosphere, where it almost certainly disintegrated.

The Soviets first attempted to send a pair of Marsnik probes to the red planet in 1960, and succeeded with Mars 1 in 1962, with which we lost contact only a year later. NASA's Mariner program joined the race to Mars in 1964; these were followed by the revolutionary Viking program, which landed two probes on the surface and operated them for several years. Viking 1, which survived for 6.5 years, still holds the record for longest survival time on that planet, although if Opportunity may yet beat that record.

The following probes are still believed to be operational, on or in orbit of Mars.

Mars Odyssey (2001-present) -- http://marsprogram.jpl.nasa.gov/odyssey/ -- Odyssey was sent to Mars to search for evidence of present and past water on the surface, as well as to study volcanic activity. It promptly detected large reserves of hydrogen, possibly associated with subsurface ice. Odyssey is still in operation in orbit, studying polar ice and dust storms, although it has lost its gamma ray equipment to overheating. The orbiter is expected to survive until 2015 on its remaining fuel reserves, and, in addition to its own mission, surveys as a relay transmitter for the Spirit and Opportunity probes on the surface.

Mars Exploration Rover "Opportunity" (2003-present) -- http://marsrovers.jpl.nasa.gov -- In 2003, NASA launched a pair of advanced landers based on the previous Pathfinder mission, but much larger and more advanced, roughly the size of a car. The mission parameters required them to operate for 90 days, but they operate entirely on solar power, meaning that barring mechanical breakdowns, then can actually continue to work indefinitely. Most of this research involves identifying minerals, soil textures, and searching for iron, but it also attempts to find evidence of what the environment was like when Mars last had water.

Opportunity was the second of the probes to land, and so far has been the most successful. It has discovered the first meteor to land on Mars (or any planet other than Earth, for that matter), and spends most of its time studying craters. (It finished with the Victoria Crater in 2007 and is currently en route to Endeavour.)

Mars Exploration Rover "Spirit" (2003-present) -- http://marsrovers.jpl.nasa.gov -- Spirit's mission is essentially similar to Opportunity, although it is working on the far side of the planet. This has involved numerous discoveries. Unfortunately, Spirit seems cursed with a streak of bad luck. Just over two years into its extended mission, Opportunity's front right wheel broke; how, it's not clear, but it's been locked in place ever since. Scientists made use of this by turning it into an improvised digging tool. Unfortunately, more recently the rear wheel on that side broke, as well, and the "Rock Abrasion Tool" (used to dig deep scratches into rocks to study minerals) has worn out, as well.

Most seriously, last May Spirit ran into a sand drift and has not moved since. NASA has been running simulations using its rovers on Earth, but so far has had no luck actually getting the probe to move again. It's not unprecedented -- Opportunity spent two months buried in sand in 2005 -- but this event is particularly serious because of previous wheel problems. The most likely outcome is that Spirit will never move again, and will lose power and die some time this spring.

Now that it's stuck, scientists are rushing to get all the scientific value they can out of the mission by analyzing the sand trap that it's currently stuck in. Its discovery of sulfates implies there used to be steam vents, and therefore, just maybe, environments where life could once have thrived.

Mars Reconnaissance Orbiter (2006) -- http://marsprogram.jpl.nasa.gov/mro/ -- The Reconnaissance Orbiter studies the surface of Mars, researching minerals, ice, and geology. Its transmitter has sent back to Earth more scientific information than all previous space probes put together, and, as it is solar-powered, it is expected to survive as an interplanetary relay transmitter to assist future space missions. Its powerful cameras have spotted the dead Viking 1 and Phoenix probes in their final resting place on the surface, as well as the still-functional Opportunity.

In 2009, software problems shut down the craft for four months, but it resumed operations last month. The latest research by the Orbiter indicates that several geological formations may in fact be the remains of ancient lakes.

Phoenix (2008-present) -- http://phoenix.lpl.arizona.edu/ -- The Phoenix lander was sent to Mars with the goal of research water as well as searching for evidence of micro-organic life. It is the first lander to achieve a successful touchdown in the polar regions, and found basic nutrient compounds believed to be necessary in order to sustain life. Currently, Phoenix is out of contact and possibly lost: solar power dropped precipitously during the winter, and scientists have not been able to contact the probe since November 2008. The next window for re-establishing contact will be early this year, but scientists are not optimistic. If no signal is received, the mission will be closed permanently, but during its brief time active on Mars, Phoenix did achieve all of its objectives.

Future Mars Missions

Several NASA projects are still in the pipeline. In 2011, NASA plans to launch Curiosity, the next-generation rover which will replace Spirit and Opportunity. It will be five times as large as those probes, with ten times the scientific equipment payload. After that will be MAVEN in 2013, another orbiting probe intended to study the Martian atmosphere as well as the solar wind. The next launch window, in 2016, may see another American-European lander, currently designated the Mars Science Orbiter. As with all space missions, however, the more distant the established launch date, the more likely the program will be significantly delayed or even cancelled due to budget cuts.

Other countries are getting into the new wave of Martian exploration, as well. The Russians are expected to launch the Phobos-Grunt probe in 2011, bound for the Martian moon of Phobos. Phobos-Grunt is designed to collect soil samples and return to Earth, which has thus far been done successfully only from Earth's own Moon. China, Canada, India and Finland are planning probes, too.

The most optimistic mission, if it ever launches, will be an as-yet-unnamed joint European-American Sample Return Mission intended to land on Mars and then return with Martian rocks. For the moment, this mission is tentatively set to launch in 2018. Landing and returning from another planet would be an unprecedented achievement and a necessary first step before manned missions can begin.