Gullies in wall of Hale Crater. Credit: NASA/MROOut of about 40 robotic missions launched toward Mars since the early 1960's, about 17 of them have been successful (I say "about" to hedge my bets, because the "success" of some of those missions is a bit gray), and of all of those, three orbiters, two rovers, and maybe—MAYbe—one lander are still active.

Here's a quick status on the active ones:

Mars Reconnaissance Orbiter: The most recent arrival at Mars (2006), MRO's 5-year mission (hmmm—sound vaguely familiar?) is to study the history of water and climate on Mars, as well as to serve as a telecommunications relay for other spacecraft. Armed with a suite of powerful instruments to study the atmosphere, surface, and subsurface of Mars—including a camera, HiRISE, that can almost read the license plates on Martian automobiles—MRO has to date sent back more data than all other Mars missions combined. It's not been a glitchless flight, however: in 2009, MRO's computer reset itself four times for unknown reasons; the last reset, in August, was followed by a 4-month operational hiatus as Earth-side controllers performed some careful programming updates to help guard against effects of any future resets. MRO resumed operation in December.

Mars Odyssey 2001: NASA's 2001 Odyssey is still going fine after nine years—although the computer glitchiness out around Mars seems to be catching: Odyssey's computer put itself into a safe mode last November 2009 in response to a memory error. This was corrected and Odyssey has resumed doing science. Among Odyssey's major discoveries was the detection of huge expanses of water ice just under the surface of polar lowlands, and the surveying of deposits of water-related minerals in various locations around the planet.

Mars Express: Arriving at Mars in 2004, Mars Express became the European Space Agency's first mission to another planet, which was recently extended to 2012. Though the Beagle 2 lander component of the mission fell to Mars and was never heard from again, the Mars Express orbiter has sent back years of captivating images and important data, including the confirmation of methane in Mars' atmosphere (whose source is in all likelihood subsurface, and the origin of which—organic or inorganic processes—is being debated).

Mars Exploration Rover Opportunity: After six years of crawling around Meridiani Planum discovering chemical and geological evidence for past water on Mars, Opportunity is now on a long march to a large impact crater, which it will reach (if it can keep on running) in about two years. Currently, the rover has stopped to RAT out chemical and geological information from a rock called Marquette Island—the RAT is its rock abrasion tool, or rock grinder. Still going….

Mars Exploration Rover Spirit: Also still alive after six years—almost 25 times longer than it was planned to run—Spirit has been stuck in a sand bog for the last six months. With a couple of wheels on the fritz, Earth-side operators have been confounded in trying to free the robot—but Spirit has continued to make scientific measurements anyway…and in fact made a significant discovery in the course of trying to get unstuck.

Phoenix lander: Although it's been in the deep dark freeze of a Martian winter since November 2008, the Phoenix lander has an outside chance of survival. Now that light is returning to Phoenix's landing site, NASA is listening for the robot's radio signal, in case the return of solar power means that Phoenix will rise from the frost and live again! So far, no such signal….

Next up: The Mars Science Laboratory rover, "Curiosity."

NASA's rover Spirit has churned up sulfate minerals in the
attempt to free itself from loose soil.
(Credit: NASA/JPL/Cornell)Some robots are born to greatness, and some have greatness forced upon them by circumstance. NASA's Mars rover Spirit has recently fallen into the latter category by making an accidental discovery in the course of trying to free itself from a sand trap….

Has it been six years since the twin rovers, Spirit and Opportunity, landed on Mars to begin their careers? Just about—next month is their landing anniversary.

Both rovers have shown signs of wear and tear, but Spirit, in its exploration range in Gusev Crater, has had the harder knocks. In fact, Spirit has driven backward for a couple of years now, due to a wheel that stopped working and which it was forced to drag through the Martian soil—a robot's form of limping.

About six months ago, Spirit became bogged down in loose soil, spinning its wheels but unable to break free of the trap—not unlike what happened to my car in Death Valley one time…and Spirit can't call upon the assistance of a National Park Ranger with an SUV and winch to help….

Over the months of entrapment, Spirit's handlers on Earth have continued to make measurements with the rover's instruments while trying to free it from its soil trap by manipulating its wheels in different ways (probably not unlike some of the strategies I tried to get my car out of the sand). But to no avail (either for Spirit or my car). Alas, is Spirit destined to remain a stationary explorer until its lifetime finally comes to an end?

Perhaps—but as it turns out, this doesn't mean Spirit can't still make significant discoveries–like a recent one it in fact made. All that wheel spinning and grinding and rocking back and forth have chewed up the soil in which Spirit sits—and has broken through a layer of soil to expose a surprise hiding beneath: a crust of sulfate minerals.

Sulfates—compounds containing sulfur—can be formed in the presence of water, like boiling water or steam escaping from a hydrothermal vent. It may be that these sulfates formed in the distant past when the area was active with volcanism and hydrothermal steam vents. That was then.

This is now: the layer of sulfate (calcium sulfate) Spirit's churning wheels broke through is crusty—a property that may point to more recent water activity than the original sulfate-forming steam vents. Scientists think that the crust may have been formed by the seasonal shifting of water from the Martian polar regions when it warms up in its summer, sending the water toward the equator–where it can even fall out as snow. Then, soil beneath the layer of snow warms the bottom layer of ice and causes it to melt. In turn, the melt water seeps down into the soil, dissolving and carrying away water soluble iron sulfate and leaving behind the crust of calcium sulfate.

In one fell swoop trying to escape sure peril, Spirit appears to have uncovered clues about the nature of Martian water action in the distant past as well as more recent times.

As serendipitous as Spirit's entrapment is to this accidental discovery, it's even better: the rover is stuck square on the edge of a small meteorite crater, allowing it to compare the sulfate concentrations in the sulfate-rich crust and the more typical soil medium, side by side.

Way to go! That's the spirit!