NASA's Curiosity Mars rover completed the fourth year of its original two-year mission today and is looking forward to another two-year extension beginning in October. When the US$2.5 billion unmanned explorer landed on the Red Planet on August 5, 2012, it was the most complex lander to have visited any world. In its brief career it has provided mankind with a new understanding of Mars and the chances that life may have once or could still exist there. Let's retrace its history making tracks.

At the moment, there are seven missions exploring Mars, with more on the way. In the past 20 years, so much has been learned about the planet and so many images of its surface have been sent back that it's become as familiar to the casual reader as the Mojave Desert or the Sahara. But this familiarity often makes us forget how hostile a planet Mars is and how difficult it is to explore. It's easy to forget that over half of the missions to Mars end in failure.

Gale Crater as it may have looked filled with water 3.8 billion years ago(Credit: NASA)

Though it's only half the size of Earth, Mars has the same land area because there aren't any oceans. The Martian atmosphere is made up of carbon dioxide with traces of nitrogen and argon, but the surface pressure is only 0.6 kilopascals (0.087 psi) compared to Earth's 101.3 kilopascals (14.69 psi) or Mount Everest's 33.7 kilopascals (4.89 psi). This is far too low for an unprotected human to survive in and the temperature range of minus 199⁰ F (minus 128⁰ C) during polar night to 80⁰ F (27⁰ C) at the equator on a summer day doesn't help.

In addition, the thin atmosphere and lack of a magnetic field can't keep out the solarwinds, hard UV, or cosmic radiation that have stripped Mars of its atmosphere, left the surface drier than any place on Earth, and loaded it with superoxides that can destroy organic molecules in short order.

Mars Exploration Program

Despite all these negatives, NASA is still very keen on exploring Mars for signs of past or present life and this has formed one of the main objectives of the space agency's Mars Exploration Program since it was formed after the Mars Observer mission failed in 1992.

One reason is that since the Viking missions of the 1970s, tantalizing clues as to whether life might exist on Mars began to appear. In 1996, meteorites were found in the Antarctic and other regions with gases trapped inside that matched the Martian atmosphere tasted by the Viking landers. This indicated that they were formed on Mars, but were blasted into space after some cosmic collision. There were controversial claims that the meteorites contained fossil bacteria, but they were confirmed to hold organic molecules, which were clues to an ancient, wetter Mars.

Another reason for hope was a better understanding of extremophiles – simple Earth organisms that can survive in environments that would normally be thought deadly, such as extreme heat, high sulfur concentrations, extremely saline wars, vacuum, or hard radiation. In addition, NASA's Mars Pathfinder found evidence of rounded pebbles and sockets in larger rocks like water-formed conglomerates on Earth, the Mars Global Surveyor found hillside gullies and other signs of water erosion, and Mars Odyssey found evidence that Mars is emerging from a recent ice age.

Curiosity's objectives

During the planning for the Mars Science Laboratory, as Curiosity is formally called, NASA set a series of objectives for the mission. Foremost would be to find signs of life. The Viking missions had already shown that Mars is so different that looking for life directly isn't feasible, and that such life might be little more than the fossil remains of long-extinct microbes.

With that in mind, Curiosity was tasked with looking for and determining the nature of any organic compounds it might find, looking for the effects of biological activity, and learning more about whether Mars is habitable – especially in terms of any features that could involve water. In addition, since the superoxides on Mars make surface life impossible, to look for alternative sources of energy underground, such as chemical or thermal, that might sustain life.

Along with its biological objectives, Curiosity was also assigned to study the geology of Mars, the long-timescale evolution of the Martian atmosphere, and to characterize the spectrum of radiations bombarding Mars, such as solar radiation, galactic and cosmic radiation, solar proton events, and secondary neutrons.

Designing Curiosity

Compared to previous Mars rovers, Curiosity is massive. The size of a 4x4, it weighs in at 1,982 lb (899 kg) and is powered by a plutonium-fueled radioisotope thermoelectric generator and lithium-ion batteries that power the avionics, keep the spacecraft from freezing at night, and run the electric motors for the six-wheeled undercarriage that propels it across the Martian desert. Curiosity also has a 2.1 m (6.9 ft) long, three-jointed robotic arm with a turret-like "hand" that can turn through 350 degrees.

The purpose of all this is to support the 165 lb (75 kg) science payload consisting of cameras and ten instruments. The cameras are especially important. Where some early probes had one or two cameras, Curiosity has 17. These begin with the primary Mast Camera (MastCam), which, as the name implies, sits on the rover's extendable mast and takes high-resolution images. Along with this are the navigation cameras, the hazard avoidance cameras, and Mars Hand Lens Imager (MAHLI) that acts as a sort of digital magnifying glass for the robotic arm.

Other cameras are more exotic. The Chemistry and Camera complex (ChemCam), for instance, has a Remote Micro Imager (RMI) telescope and a Laser-Induced Breakdown Spectroscopy (LIBS) telescope. The latter is very sci-fi with its one-million watt laser that can vaporize tiny rock samples into luminous plasma while spectroscopes focus in to determine their composition.

The other instruments carried by Curiosity include the Rover Environmental Monitoring Station (REMS) to monitor the weather and UV radiation, the Alpha Particle X-Ray Spectrometer (APXS) that uses alpha particles to cause samples to flash back X-rays for spectra analysis, the Sample Analysis at Mars (SAM) instrument suite to seek organic molecules, and the Dynamic Albedo of Neutrons (DAN) instrument for measuring traces of ice and water on or just below the Martian surface.

Launch and landing

Curiosity lifted off on November 26, 2011 at 10:02 am EST from Launch Complex 41 at Cape Canaveral Air Force Station in Florida atop an Atlas V rocket. On August 5 at 10:31 pm PDT 2012 (August 6 at 5:31 GMT), Curiosity set down on Mars after what NASA called "seven minutes of terror" because with a 13-minute delay due to the distance to Earth, the probe had to handle the atmospheric entry and landing using only its onboard computer.

After separating from the Cruise Stage that tended the craft during the passage from Earth, Curiosity struck the thin Martian atmosphere at hypersonic speed. During the descent, a revolutionary heat shield that not only protected it, but also acted as a lifting body that allowed the craft to steer itself as it slowed down due to atmospheric drag.

After the aeroshell and heat shield were jettisoned, the rover was lowered by a skycrane, which is a rocket-propelled frame with a winch that dropped Curiosity to the surface. The skycrane then flew off and crashed some distance away when its fuel ran out.

Curiosity touched down in Gale Crater at 4.5895°S 137.4417°E, now called Bradbury Landing after the American fantasy writer Ray Bradbury. Offering a large enough flat, elliptical area for a safe landing, Gale Crater is thought to be an impact crater between 3.5 to 3.8 billion-years old and was chosen because it showed signs of having been filled by water and wind-deposited sediments. It also had slopes that had worn away to expose geological layers over a two-billion year period.

Gale Crater as it may have looked filled with water 3.8 billion years ago(Credit: NASA)

The first year

Curiosity's first year on Mars was also its most eventful, with Mission control spending the first three weeks running the rover in like a new car. Images were sent back for study and to calibrate the cameras while systems long dormant during the interplanetary voyage were brought online and tested against the Martian environment. Meanwhile, the probe's software was swapped over with surface exploration programs replacing the ones used during the passage from Earth to Mars.

Perhaps the most dramatic event was on August 19 when Curiosity fired its laser for the first time. As the million-watt laser vaporized its first target with pulses of light lasting five one-billionths of a second, it marked the first time such a device had been used on another planet. Then came the first drive as Curiosity rolled a mere 20 ft (6 m) from its landing site. Once it got the okay and set out to explore Gale Crater on its way to Mount Sharp, the rover could manage a top speed of 0.085 mph (0.137 km/h). That isn't exactly racetrack standard, but the wheels do put out very high torque to keep it from getting stuck in the sandy surface of Mars.

On August 28, 2012, Curiosity carried out another historical first as it streamed a human voice from the planet Mars back to Earth across 168 million miles (267 million km). The 500 kilobyte audio file contained a prerecorded message of congratulations for the engineers behind Curiosity from NASA administrator Charles Bolden, and was used to show the technological challenges of sending a signal directly to Earth over a 15-watt radio beam. Normally, Curiosity relies on the Odyssey, Mars Reconnaissance Orbiter, and Mars Express spacecraft to act as relays.

But it isn't just with radio that Curiosity sent messages. Every time the rover moves the treads on its six wheels spell out "JPL" (Jet Propulsion Laboratory) over and over in Morse code (.--- for J, .--. for P, .-.. for L). This acts as an odometer. By keeping Curiosity's cameras trained on its tracks, mission control uses the code to measure how many times the wheels turn and from that they can calculate the distance traveled.

Two days after the audio stream test, Curiosity made its first tests of its robot arm and the remarkable tool kit that makes up its hand or "turret."

As Curiosity made its first trek across the face of Mars in September 2012, its discoveries made the scientists back on earth sit up and take notice. The remains of an ancient stream bed consisting of water-worn gravel that was washed down from the rim of Gale Crater was a major achievement that was dramatic proof that ancient Mars was a wetter place.

Part of what made these discoveries possible is Curiosity's drill, which got its first try out in February 2013 when it conducted the first robotic drilling operation on another planet. Capable of both rotation and percussive drilling a part of Curiosity's sample collecting and testing system. Where previous probes were content with a bit of scooping and scraping, Curiosity not only collects loose soil, but also drills into rocks and sends the powdered samples to its inbuilt laboratory for analysis.

Another first in February was when Curiosity took its own panoramic self-portrait using its Mars Hand Lens Imager to take 130 high-resolution images, These were stitched together to form a 360⁰ panorama that included itself. Aside from a bit of robo-narcissism, the self-portrait allows NASA to periodically inspect the rover for signs of wear and tear.

Curiosity's first year had another nail-biter after its dramatic landing. Between April 4 and May 1, 2013, Mars moved to the opposite side of the Sun to Earth. With Mars and the Sun in conjunction direct radio communications were severely impaired, so Curiosity was put on autonomous control as it transmitted a beep to Earth each day and the Odyssey spacecraft continued to relay information from the rover. No commands were sent from mission control because of the chance of data corruption, which might have disrupted the onboard computer. .

The second year

Curiosity's second year on Mars began with it singing "Happy Birthday" to itself before getting down to business.

While year one was filled with interplanetary first and discoveries, the second year was a bit more routine. The rover got down to the hard slog of collecting samples and data as it made its way across the Martian landscape. It was also a time when some design flaws in Curiosity became apparent.

By late 2013, Curiosity had negotiated some rough terrain and had detoured around even rougher. Images sent back by the rover's cameras showed that its six aluminum wheels were showing signs of wear as they wore down faster than first estimated. In addition, the onboard A computer started to act up and mission control switched to the backup B computer.

On August 27, 2013, Curiosity went on autonomous navigation mode for the first time. In this, instead of following pre-programmed instructions from Earth on how to travel, the rover was allowed to assess and navigate new ground by itself. It continued to take and analyze samples and found that the Rocknest region of Aeolis Palus in Gale Crater had up to three percent by weight of water, which is very high for Mars.

By November, Mars was taking more of a toll on Curiosity. On November 17 electrical problems from a short circuit were noticed and the rover got a week off as NASA engineers corrected the fault. In December, it received another software upgrade.

On June 3, 2014, the rover watched as Mercury moved across the disk of the Sun. This was the first time a planetary transit had been observed on another planet.

By June 27, Curiosity had reached to the edge of its landing ellipse. That is, the area of Gale Crater that designated safest for the 2012 landing. Now the rover was moving into more rugged terrain with more exotic geology.

The third year

In its third year on Mars, Curiosity had completed its first mission objectives and was given an official extension to its original two-year mission. By September 11, 2014, it had reached the slopes of Mount Sharp, its primary objective, where it had hopes of finding more diverse geological deposits.

In March 2015, Curiosity had to stop for more repair work to correct short circuits in the robotic arm caused by the pounding from the percussive drill.

The fourth year

Curiosity's fourth year opened with the rover continuing to study the uplands of Mount Sharp. It found high levels of hydrogen in Marias Pass, which showed water-related minerals close to the surface. In addition, it found more evidence that Gale Crater was once home to lakes and streams as well as a recurrent alluvial flow with open water present less the four billion years ago.

By March, Curiosity might have still been working on its fourth Earth year, but celebrated its Second Martian year, which meant that it had sent back weather reports on two complete cycles of the Martian seasons. And just to show that Mars still had surprises, in July it sent back images of sand ripples different from any seen on Earth even as it came back into full operation after a software problem caused it to go into safe mode.

Fifth year and beyond

As Curiosity begins its fifth year on Mars, it's already looking forward to a two-year mission extension that begins on October 1, but how long can this plucky little explorer carry on? According to the specifications, the power supply will last at least another ten years, but running out of electricity isn't the only factor at play.

The intense radiation on Mars is slowly degrading the electronics and eating away at the rubber and plastic parts. In addition, drilling operations are taking their toll on the robotic arm as short circuits keep recurring. Then there's the software, which has been suffering from glitches and switching into safe mode since 2013. Finally, there are the wheels, which at some point might crack and collapse.

Comparison of Curiosity, Opportunity/Spirit, and Sojourner (Credit: NASA)

Even if Curiosity turns out to be built better than the engineers thought, there are outside factors that might end the mission. The rover might run into a ravine or get stuck in a dune. A wheel might wedge on a rock. Then there's always the threat of human error, such as the software update sent to Viking 1 that ended up cutting off communications for good.

However long Curiosity continues to operate (and NASA probes have a habit of carrying on for a very long time if they can manage to reach their destination safely), it has already rolled its way into the history books. It may not have completed a Marathon like the Mars Opportunity rover, but it has brought some very heavy duty science to bear on some very heavy duty problems.

Also, the legacy of Curiosity will carry on in the form of NASA'S Mars 2020 mission, which will use a rover based largely on Curiosity's successful architecture. If signs of life haven't been found on Mars by sometime in the next decade, it won't be for lack of trying.

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