Exploration of Mars
author Paul Boșcu, June 2017

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At the same time as the successes of the United States’ Project Apollo, which ultimately sent astronauts to the Moon were mounting, both the U.S. and the Soviet Union developed a large suite of robotic space probes that traveled to other planets. The Soviets had a series of failed missions to Mars, but the Americans sent missions that successfully flew past, orbited, and eventually landed on Mars.

These early missions were exploration in the truest sense of the word — before NASA’s Mariner 4 mission in 1964, for example, some scientists still thought that Mars might have canals and other signs of an ancient civilization.

Although not tremendously successful, the Soviet Union made several attempts at sending probes to Mars from 1960 through 1973. The Soviets deserve credit for getting the ball rolling in terms of Mars exploration, but not one of these missions achieved its goal. Of course, with as new an enterprise as building interplanetary spacecraft, some failures can be anticipated.

In addition to a series of flops belonging to the numbered Mars program, the Soviet Union launched several other unsuccessful missions to explore the Red Planet. Sending a spacecraft to another planet is a difficult task, and the Soviet rockets and boosters of the 1960s suffered from a proliferation of designs that weren’t systematically tested. These failures weren’t addressed until the 1970s, when existing designs were consolidated and thoroughly tested, a strategy that has resulted in some extremely reliable spacecraft and rocket designs, such as Soyuz.

Despite a number of attempts in the 1960s, the Soviets didn’t have any luck with an operational Mars probe until the 1970s. Even then, the few Soviet spacecraft that were modest successes couldn’t match the data quality and quantity from contemporary NASA missions.

Marsnik 1 and 2, both launched in October 1960, would’ve attempted Mars flybys, but both missions failed on launch. Marsnik 1 never made it into orbit at all, having self-destructed on launch. Marsnik 2 lifted off, but the third rocket stage wasn’t successful in launching the spacecraft into Earth orbit.

The Sputnik Mars missions constituted three more failed attempts. Sputnik 22 was launched in 1962 with the intent of doing a Mars flyby, but the spacecraft broke apart in orbit. Sputnik 23, also called Mars 1 (1962), launched successfully, but this mission lost communication with Earth before reaching Mars. Sputnik 24 (also 1962) would’ve landed on Mars had the mission been successful; instead, the spacecraft broke up during flight.

The failures unfortunately continued with the Zond 2 mission (1964), which aimed to conduct a Mars flyby. The Zond 2 spacecraft was equipped with cameras and ultraviolet spectrometers, as well as an infrared spectrometer that would’ve helped identify the presence of methane. But Zond 2 lost communication with Earth and was unable to return any data.

Mars 2 and Mars 3 featured both a lander and an orbiter. Both orbiters succeeded and sent back a few observations of the Martian surface and atmosphere. However, transmitter problems, less-than-ideal orbits, and a global dust storm on Mars substantially reduced the quality and quantity of the data. The Mars 2 lander crash-landed, and the Mars 3 lander succeeded with a soft landing on the Martian surface, but returned just 20 seconds worth of data to Earth before mysteriously going silent. However, the working orbiters and one successful soft landing qualify Mars 2 and Mars 3 as some of the more successful Soviet Mars missions.

Mars 4 (1973) suffered from equipment failures that doomed its scientific productivity. The spacecraft did reach Mars, but a computer malfunction kept the spacecraft from entering orbit. Only one batch of images was returned from this mission.

The Mars 5 mission (1973) made it into orbit around Mars, and in a surprising turn of events for the Soviet Mars program, the lander’s cameras worked and returned about 60 images during the nine days it survived on the planet. The other onboard instruments relayed information about the planet’s surface temperature (a maximum of approximately 32 degrees Fahrenheit, or 0 degrees Celsius), atmosphere, ozone layer, and soil composition.

The Mars 6 mission (1973) reached Mars and actually did well enough to start sending data back home to Earth as the Mars 6 lander descended through the atmosphere. Unfortunately, the transmitted data was mostly useless due to a computer chip problem, and all contact was lost just before the lander reached the Martian surface. Mars 7 (1973) arrived at Mars, but its landing probe was prematurely disconnected from the rest of the spacecraft. The probe (which was intended to land on the surface) missed the planet entirely.

Early planetary exploration didn’t belong solely to the Soviet Union. American planetary research started with the Mariner Program, the United States’ first long-running interplanetary spacecraft program, which was intended to start exploring the strange new worlds of Mercury, Mars, and Venus. This program consisted of ten space missions, seven of which were successes

The Mars-bound Mariner 3 spacecraft was a flop due to launch failure, but Mariner 4 (1964) successfully became the first spacecraft to fly by Mars at close range. This mission was the first time scientists got pictures of the surface of another planet, and they were very surprised to see an old, cratered surface that looked much more like the Moon than the Earth. Mariner 4’s images thus revolutionized society’s view of Mars as a planet.

An American tag team of octagonal spacecraft, Mariner 6 and 7, gained valuable information from their flybys of Mars in 1969. Launched about a month apart, the two spacecraft took about 200 pictures of Mars, some from their approach and others at closer range. These photos showed a number of details about the north and south poles of Mars, in addition to providing images of Mars’s moon Phobos.

A pair of twin ships, Mariner 8 and 9, would’ve become the world’s first Martian satellites. They were intended to continue the data collection begun by the Mariner 6 and 7 missions and perform a detailed mapping of the Martian surface. However, Mariner 8 failed during launch and fell back into the ocean. The honor of being the first spacecraft to orbit another planet fell to Mariner 9 alone. Mariner 9 entered orbit around Mars in November 1971. From there, it prepared to photograph the Martian surface and took atmospheric measurements using instruments much like those aboard its immediate predecessors.

Ultimately, Mariner 9 was able to map more than 80 percent of the Martian surface from orbit. A full geologic picture emerged as a result, complete with images of valleys, huge volcanoes, canyons, and evidence of geologic features carved by both wind and water. This mission rekindled interest in Mars as a possible location for past, or current, life.

NASA’s Viking missions to Mars in the 1970s provided a wealth of data from two orbiters and two landers. Not only that, but these landers actually performed the first search for life on another planet.

NASA was inspired to create an ambitious Martian exploration program that was motivated as much by scientific goals as by political showmanship (the Soviet Union had yet to have a successful Mars landing). Martian features such as canyons and valleys that appeared to have been carved by liquid water, combined with the hot surface temperatures and noxious clouds of Venus that could in no way support life, rekindled interest in Mars as a possible location for life in the solar system. So began the United States’ efforts to find evidence of life on Mars.

Taking a page from the successful Soviet strategy of twin spacecraft NASA built Viking 1 and 2. The two spacecraft included both an orbiter, for global mapping, and a lander, for landing on another planet and searching for life. The Viking Program didn’t come out of nowhere. It built on the past achievements of the Mariner Program. The concept behind the mission was to send two spacecraft to Mars, separated by about a month. Each would land at a different site while their orbiters remained in space to complete their missions of imaging the planet and conducting other studies from space.

The Viking 1 mission departed on August 20, 1975, and arrived in Mars orbit nearly a year later. The orbiter and lander stayed united for about a month while they photographed the planet and looked for a landing site. Meanwhile, the Viking 2 mission launched on September 9, 1975, reaching Mars orbit in August 1976. The Viking 1 lander detached from its orbiter on July 20, 1976, and reached the Mars surface at Chryse Planitia. The Viking 2 lander wasn’t far behind, landing at another area of Mars called Utopia Planitia on September 3, 1976.

The orbiters from the Viking 1 and 2 missions led productive lives: The Viking 1 orbiter made more than 1,400 orbits around Mars over the course of four years, and the Viking 2 orbiter completed about half that many orbits in two years. The images taken from these spacecraft gave scientists the most complete and detailed image maps of the Martian surface until the Mars Global Surveyor arrived at Mars 20 years later, in 1997.

The Viking landers were also very long-lived: The Viking 2 lander successfully sent back data for more than three and a half years until its battery failed in 1980, and the Viking 1 lander operated for more than six years until 1982, when a faulty software update mistakenly deleted essential data used by the lander to point its antenna at Earth, shutting down communication.

The scientific goal of the Viking missions was clear: Study the biological, physical, chemical, and other properties of Mars’s atmosphere and surface in an effort to better understand the planet and its potential to support life. Each Viking lander housed numerous instruments, including cameras, sample-collection devices, and temperature and wind sensors, to help it fulfill this goal. None of these tests proved conclusively that there was life (as we know it, at least) on Mars.

The lander cameras were able to image the surface using a high-tech version of smoke and mirrors. Movable mirrors were aimed at vertical strips of the landscape; photodetectors then measured the light reflected from those mirrors back into the cameras. The cameras were then moved for the next scan, and these strips were recombined later to form a complete image.

The Viking 1 and 2 landers didn’t just rely on imaging in their search for Martian life. Both landers had their own robotic arm with a soil scoop that could dig small trenches on the surface of Mars and bring the soil into the spacecraft for chemical analysis.

The intense excitement surrounding the search for life — and the failure to successfully detect it — meant that Mars exploration basically came to a standstill at the end of the Viking missions. After an expenditure of close to a billion dollars on the Viking Program, no successful missions were sent to Mars for more than 20 years.

At the dawn of the Space Age, science fiction blended with science fact to produce a view of the solar system that included life on other planets. Although the first missions to the Moon, Mars, and Venus revealed harsh conditions that seemed unlikely to support life as we know it, scientists knew life could still exist elsewhere in the solar system. In 1966, the U.S. and several other nations signed a space exploration treaty, part of which requires that nations protect other planets and the Earth from contamination due to space exploration.

Planetary protection is designed into space missions for two reasons: to avoid contaminating the planets being explored and to prevent any byproduct of the mission from causing reciprocal harm to Earth. Keeping the other planets free from biological contaminants helps preserve them in pristine form for future research. Keeping the Earth free from potential extraterrestrial contaminants is, similarly, a clear preventative measure against the unknown. Although all missions have some form of planetary protection, it becomes most critical when astronauts or robotic landers bring samples back to Earth.

The first few Apollo crews to land on the Moon were actually kept in isolation following their return to Earth. The Apollo 11 astronauts, for example, climbed out of their floating Command/Service Module (CSM) after it splashed down and went straight into a decontamination raft. They were washed with a bleach like cleanser and hustled into a quarantine unit for the next several weeks. Their decontamination raft was actually sunk so as to eliminate the potential spreading of any lunar microbes they may have picked up, and their CSM was washed with an antiseptic before it was hauled onto the USS Hornet.

Fortunately for the astronauts, the decontamination procedures were substantially reduced after Apollo 14 when it became clear that returning astronauts didn’t succumb to any sort of lunar flu.

The Viking landers, with their life-detection experiments, took extraordinary precautions to avoid contaminating the Martian surface. In many ways they set the gold standard for planetary protection. The hydrazine used in the landing rockets, for example, was purified in order to try to reduce the chance of contaminating the Martian surface as the chemical burned. As the landers separated and traveled into Martian orbit, they were covered with an aeroshell that prevented microbes and organisms from flying off into the Martian atmosphere.

Robotic exploration of Mars took a long hiatus following the failure of NASA’s 1976 Viking missions to detect life, although not for lack of trying. The Soviet Union’s Phobos spacecraft failed due to computer glitches in the late 1980s, and the United States’ Mars Observer spacecraft was lost in the early 1990s, further depressing the status of NASA’s planetary exploration program.

Despite the Viking landers’ suites of instruments specifically designed to look for signs of life, nothing was sent back to Earth to indicate life exists on Mars. If the idea of potential life on Mars had excited the public worldwide, the Viking missions’ negative results definitely soured public support for future Mars exploration. Building scientific and political momentum for a new crop of Mars missions took time, and those plans suffered further setbacks from a series of failed Soviet efforts in the late 1980s and disappointing American attempts in the early 1990s.

Space exploration is a difficult business, and not all missions are successful. Mars missions in particular have had a bad track record: Between 1960 and 2008, 42 missions were launched to Mars, but only 18 have been completely successful, with 4 more partial successes (missions that either landed or reached orbit successfully but stopped transmitting data after sending back only a few pictures or measurements).

Renewed exploration of the Red Planet didn’t start out well. The first post-Viking Mars missions were a pair of ambitious spacecraft developed by the Soviet Union. As part of the Phobos program, the Soviets sent a pair of robotic spacecraft, Phobos 1 and Phobos 2, to Mars to study both the planet itself and its two small moons, Phobos and Deimos. The missions launched a few days apart in July 1988. Phobos 1 never made it into Mars orbit. Phobos 2 transmitted some 40 images before contact was lost.

Both Phobos spacecraft featured a new design that consisted of an instrumentation section surrounded by electronics, sphere-shaped tanks that contained fuel for controlling the spacecraft’s altitude and orbit, and winglike solar panels for power.

Phobos 1 seemed to go as planned until it dropped communication less than a month after launch. The Soviets eventually discovered that a software glitch had turned off the spacecraft’s attitude thrusters, which were responsible for controlling the orientation of the spacecraft so that it could use its solar panels to keep the batteries charged. Because Phobos 1 never made it into orbit around Mars, no useful mission data was collected.

Phobos 2 successfully orbited Mars and managed to return about 40 images before losing contact with Earth in March 1989, just before it was supposed to deploy a small lander onto the surface of Phobos. This time a computer failure was to blame.

Future Soviet probes to Mars and other planets were all put on hold thanks to the 1991 breakup of the Soviet Union. The Russian Federal Space Agency took over the Soviet Union’s space work and began putting all of its dwindling resources into keeping Russia’s cosmonaut program and Mir space station running.

NASA’s Mars Observer was supposed to study the surface material of Mars in depth, in addition to determining the nature of Mars’s gravitational and magnetic fields and obtaining details on Martian weather and atmosphere. It was also meant to be the first of a series of missions, much like the Mariner and Pioneer mission series of two decades earlier. The Observer missions were intended to study the inner solar system, but Mars Observer was the only one of these missions to launch. The spacecraft was lost in transit towards Mars.

At first all seemed normal, and Mars Observer successfully traveled through interplanetary space toward Mars. However, the spacecraft lost contact with Earth a few days before it was scheduled to arrive in Martian orbit. All future attempts to contact it failed, and a panel was ultimately convened to determine the cause of the failure. Scientists later decided that the most likely problem was a rupture of a contained propellant line after the system was pressurized just before the planned rocket burn to enter orbit.

The mission was a failure in terms of gathering data, but NASA engineers were able to upgrade future missions to account for what they think derailed Mars Observer.

The failure of Mars Observer, which was a big flagship mission, left NASA’s planetary exploration program in an uproar. Scientists and engineers had lost large chunks of time and effort, and the agency itself had lost funding from the federal government.

As part of NASA’s Discovery Program, the Mars Pathfinder mission was seen mainly as a testbed for trying out technologies, innovations, and equipment intended for later missions. It was also responsible for conducting experiments designed to reveal more about the structure, topography, and composition of the Martian surface.

The mission design of Mars Pathfinder was ambitious and risky, including many technologies never before attempted by NASA. Most notably, it included not only a lander but also a small rover, the iconic Sojourner.

The Pathfinder spacecraft itself also featured an innovative landing technique. Instead of using the standard retrorockets that had aided the landing of the Viking spacecraft two decades before, Pathfinder slowed its descent with parachutes. It then inflated giant airbags so it could bounce along the Martian surface before coming to a rest.

On July 4, 1997, seven months after its launch, the solar-powered Pathfinder bounced to a safe landing in the Red Planet’s Ares Vallis area. After the Sojourner rover rolled out of the lander, it got right to work by driving over to interesting rocks to examine them. Valuable data was sent back over a three month period, about two months longer than the mission was expected to last.

Mars Pathfinder, and in particular the Sojourner rover, marked one of the first NASA missions to fully take advantage of the emerging Internet. Millions of people from around the world followed the mission via the Web site, logging in daily to see the latest pictures. The novelty of watching a rover drive around the surface of another planet captured the imagination of the general public, and Sojourner had a huge impact on society’s renewed interest in Mars exploration.

The Mars Global Surveyor (MGS) mission, a separate mission from the Discovery Program, actually marked the beginning of the United States’ return to Mars because it launched a month before the Mars Pathfinder mission. However, it arrived in orbit around the Red Planet later than Pathfinder. The MGS mission had several major scientific goals: study and map the surface characteristics of Mars, discover the true shape of the planet, plus details on its topography, find out about Mars’s magnetic field, record weather patterns and atmospheric details.

Launch proceeded on November 7, 1996, and the orbiter reached Mars orbit on September 11, 1997 (around the time that the Mars Pathfinder mission was reaching its conclusion). In order to get closer to the surface, the spacecraft used a technique called aerobraking to lower its altitude; this is a fuel saving maneuver that uses atmospheric drag rather than the spacecraft’s engines to slow down and change the shape of the spacecraft’s orbit.

The MGS mission made significant scientific discoveries, including observations of surface features that changed over the course of the mission and could, perhaps, be due to liquid water near the surface of Mars. Other significant contributions of the MGS mission include its study of key facets of the Martian climate and the info it gathered that could one day lead to human Mars exploration.

Despite being scheduled for completion in January 2001, MGS continued returning data to Earth for almost ten years until communications were lost in late 2006. During the extended portion of the mission, the spacecraft was able to take images of potential landing sites for future Mars missions and continue recording weather and atmospheric data.

In 1996, a Russian Mars mission, called Mars 96, was resurrected. Its goals were to explore the surface of Mars and create maps of the Red Planet’s structure, topography, and composition. Mars 96 also was charged with studying Mars’s magnetic field and examining gamma rays and other phenomena on its way to the planet. Although liftoff initially succeeded, the spacecraft’s launch vehicle failed in its final stage, leaving Mars 96 unable to leave Earth orbit.

Following the breakup of the Soviet Union in 1991, the Russian space program was in chaos. Although an infusion of funds from NASA was able to help sustain Russian human spaceflight to the space station Mir, and later the International Space Station, Russia’s robotic space program wasn’t so fortunate. Many of these missions were canceled or delayed for years.

The Mars 96 spacecraft was similar in design to the earlier Soviet Phobos spacecraft, and it was supposed to have made some improvements based on the Phobos failures. The spacecraft carried two landers, which in turn carried a series of instruments to study the surface of Mars; Mars 96 also carried two penetrators designed to hit the Martian surface at a high speed to drive the pointed probe into the surface. Both penetrators were equipped with cameras, sensors, and other measuring equipment for analyzing the soil and material underneath the Martian surface.

The spacecraft fell back to Earth about five hours after the launch. Initially, Russian scientists thought that the spacecraft, with its radioactive plutonium fuel, had burned up safely in the atmosphere, with any remnants falling into the Pacific Ocean. However, reanalysis of the trajectory, as well as reports from witnesses on the ground, suggest that Mars 96 may have crashed in the Andes Mountains somewhere in Chile or Bolivia. It has yet to be found.

Publication of scientific papers does not often generate headlines. But when news leaked that the August 16, 1996, issue of the prestigious journal Science would contain a paper offering proof of life on Mars, NASA Administrator Dan Goldin had to schedule a press conference to stop wild speculations by the media. “I want everyone to understand that we are not talking about ‘little green men,’ ” he said in announcing the press conference. “These are extremely small single-cell structures that somewhat resemble bacteria on Earth.”

Journalists packed the room to hear from Johnson Space Center scientists David McKay, Everett Gibson, and Kathie Thomas-Keprta. Millions listened in as they described their two-year study of a 4.5-billion-year-old meteorite from Mars, designated ALH84001. They claimed that complex organic molecules, carbonate globules, and tiny structures similar to fossilized remains of organisms on Earth were evidence that microscopic bacteria once lived on Mars.

Though some scientists argued for nonbiological explanations of the evidence, they did not dispute that the meteorite came from Mars. Its chemistry matched that measured by the Viking spacecraft. The rock was apparently knocked free of Mars by an impact about 15 million years ago. It fell to Antarctica about 13,000 years ago and was found in 1984. The new technology of a scanning electron microscope and a special laser mass spectrometer had made the discovery of the previously hidden tiny structures possible.

The Mars Pathfinder and Mars Global Surveyor missions set the stage for what was to be an ambitious program of Mars exploration by NASA. However, that program was derailed almost before it began thanks to two mission failures. The Mars Climate Orbiter was part of a two-spacecraft program with the Mars Polar Lander. These two Mars missions were intended to provide detailed studies of the weather, atmosphere, water, and general climate on Mars, but neither fulfilled its goals. The loss of Mars Climate Orbiter and Mars Polar Lander resulted in severe setbacks for the just-rebounding NASA Mars exploration program.

Both Mars Climate Orbiter and Mars Polar Lander were lost due to minor, yet fatal, errors that would’ve been found with sufficient time and staff to perform quality checks and troubleshooting. Future Mars missions, such as Mars Odyssey and Phoenix Mars Lander, successfully learned from the mistakes of these two failed missions, but the tradeoff was that these later missions were far more costly.

The Mars Climate Orbiter was designed with equipment for monitoring weather, noting wind strength and other atmospheric conditions, recording the amount of water vapor in the atmosphere, and generally giving scientists an understanding of how climate works on Mars. The orbiter contained cameras and other recording equipment, antennae for data relay, solar panels for power, and a host of other instruments.

The Mars Climate Orbiter made it to Mars on schedule but lost contact while entering Martian orbit. A study of the failure revealed that the spacecraft mistakenly entered orbit far closer to Mars than was safe. Consequently, forces on Mars Climate Orbiter from friction and heating in the Martian atmosphere resulted in the spacecraft’s destruction.

The Mars Polar Lander was a 1999 mission scheduled to study the climate, weather, and atmosphere of Mars. It had similar goals to the Mars Climate Observer mission, but the spacecraft itself consisted of a lander that would’ve landed near the South Pole of Mars.

Mars Polar Lander was sent to Mars alongside two small probes designed to burrow into the Martian surface. These probes were quite small, around 2 kilograms each and surrounded by an aeroshell (a protective exterior shield) that would’ve allowed them to impact the surface, separate from the shells, and penetrate the soil.

The spacecraft and probes reached Mars orbit successfully following their launch on January 3, 1999, but they lost contact with Earth before landing. The failure of Mars Polar Lander was ultimately blamed on a software design error that caused the descent rockets to cut off too far above the surface of Mars, resulting in a crash-landing. The probes were also never heard from again.

Mars Odyssey, which reached Mars orbit in 2001, was the first robotic spacecraft to successfully reach Mars in the 21st century. Its purpose? To start looking for evidence of volcanic activity and water on the surface of Mars. Perhaps the spacecraft’s most significant finding is the existence of places on the Martian surface that may hold chloride minerals such as salt. Additionally, data gathered by Mars Odyssey has revealed concentrations of hydrogen that are thought to indicate near-surface water, which could indicate the more-recent presence of habitable locations on the Red Planet.

A repeat of many experiments on the failed Mars Climate Orbiter, Mars Odyssey was also designed to relay communications from future Mars landers. Because landers typically rely on an orbiting spacecraft to relay their info all the way back to Earth, having Mars Odyssey around for this purpose is critical.

The Mars Odyssey spacecraft fulfilled its mission goals as planned in 2001 and helped relay the vast majority of images from three different Mars landers. Because the spacecraft and its equipment were still functioning (and because Mars Odyssey was already out in space) NASA officials decided to extend the Mars Odyssey mission to allow the spacecraft to study climate phenomena, such as ice and dust, and continue relaying information from other Mars missions.

In 2003, the European Space Agency (ESA) got into the act of Mars exploration with its ambitious Mars Express mission. Intended to send both a lander and an orbiter to study the geology and biology of Mars, Mars Express was also charged with taking climatic and atmospheric measurements and investigating the Martian surface and subsurface. Despite the failure of its lander, Beagle 2 to land on Mars, the Mars Express orbiter reached orbit successfully and has been returning valuable scientific images and measurements.

The spacecraft left Earth via a Soyuz/Fregat launch vehicle (thanks to an agreement with the Russian Federal Space Agency). It arrived at Mars and successfully launched the lander, called Beagle 2 after Charles Darwin’s famous ship. Beagle 2 managed to descend into the Martian atmosphere, but it stopped communicating with Earth immediately afterward and was officially declared lost.

The landing site chosen for Beagle 2 was the Isidis Planitia, an area where scientists had been hoping to search for signs of life. The site wasn’t a particularly dangerous one, as far as planetary landings go, and scientists speculate that Beagle 2 may have been irreparably damaged during landing if the atmosphere on Mars was thinner than expected due to recent dust storms.

Two of the mission’s most amazing scientific results to date are its beautiful three-dimensional stereo views of Mars’s surface and its unexpected detection of methane on the Red Planet’s surface. This detection was unexpected because on Earth, methane usually comes from biological sources; as far as we know, there is no life on Mars, so finding methane there was something of a surprise.

The ability to move around on the surface of Mars has revolutionized people’s understanding of the wide variety of environments present on the Red Planet. Lessons learned from the tiny yet successful Sojourner rover helped NASA engineers design the two Mars Exploration Rovers: Spirit and Opportunity. These rovers are fully mobile, solar-powered labs capable of sending their data to an orbiting spacecraft for relay back to Earth. If necessary, they can even contact Earth directly from the Martian surface. The initial goals of the MER mission were to explore and start defining the surface characteristics of Mars.

Spirit (also known by the far more boring name of MER-A) launched on a Delta II rocket on June 10, 2003, followed by Opportunity (also known as MER-B) on July 7, 2003. Spirit landed on Mars first, hitting the large Gusev Crater on January 4, 2004. Opportunity followed on January 24, 2004, landing in the Meridiani Planum.

The rovers are designed to do just about everything a geologist would do if he or she were to walk the surface of Mars. Cameras mounted to them take 360-degree panoramic images of the Red Planet’s terrain, and robotic arms extend instruments in order to touch and study rocks up close. Both rovers are capable of grinding the surfaces of rocks to expose fresh, new material that can then be scanned for thermal emissions and bombarded with X-rays to determine the rocks’ composition.

From orbit, Spirit’s landing site in Gusev Crater looked like a geologic wonderland of features formed by water. However, once on the ground, Spirit found rock after rock that had been formed by volcanic, not fluvial (flowing liquid), processes. It turned out that any water-related features in Gusev Crater had long ago been covered over by lava flows.

Opportunity’s landing site, by contrast, had geologic features that seemed less interesting from orbit, but the appearance of large deposits of hematite was too good to pass up because hematite usually only forms in the presence of liquid water on Earth. After landing on the other side of Mars from its twin lander, Opportunity’s airbags rolled it into a small crater with fascinating layers that included small round spheres dubbed “blueberries” that turned out to be the mysterious hematite. Scientists used their studies of the fine details of these and other rock types to determine that the Meridiani Planum was once covered with water.

Many images of Mars have been taken from orbit, but none boast the impressive degree of detail provided by the images taken from Mars Reconnaissance Orbiter. MRO’s mission goals were to view the surface of Mars at as high a resolution as possible from orbit to help NASA better understand how the landing sites for the rovers Spirit and Opportunity fit into the overall picture of Mars (see the previous section for details on the rovers), as well as to help select sites to which future missions could be directed.

MRO left Earth on August 12, 2005, and entered orbit around Mars about seven months later with the help of aerobraking (using the atmosphere of a planet to change the orbit of a spacecraft). MRO was intended as a follow-up to the Mars Global Surveyor mission of 1996, and it has a wide variety of state-of-the-art instruments aboard, including several cameras.

The HiRISE camera has revealed layered deposits near Mars’s North Pole that may hold data on how the planet’s climate has changed over the last few million years. Images of salt deposits — a key indicator of areas where liquid water could’ve evaporated over time — near the southern highlands have also been sent back. Additionally, the HiRISE camera has observed Spirit and Opportunity from orbit, as well as both Viking landers, and captured a dramatic image of the Phoenix Mars Lander while it was landing.

The Phoenix Mars Lander was a robotic spacecraft sent to Mars to find definitive evidence of water. And find water it did! In June 2008, the lander dug a trench that was filled with a bright material scientists thought could’ve been ice. Several days later, the bright clumps had vaporized, suggesting that they were probably water ice. Additional evidence of water was gleaned from water vapor measurements sent back in late July 2008. All previous Mars landers have set down near the Martian equator, but Phoenix was targeted for terrain near the North Pole.

Like the poles on Earth, both poles of Mars have ice caps, though the ice on Mars is primarily made of carbon dioxide rather than water. The North Pole was chosen for Phoenix’s landing site because the northern polar cap is larger than the southern polar cap (and because more water ice is thought to exist there).

The primary mission goals for Phoenix were to find water, examine the history of water on Mars, and, in the process, discover more about the mechanisms of climate change. A related goal was to determine how habitable Mars was, or had been, as a result of the presence (or absence) of liquid water at various points in its history.

The Phoenix Mars Mission launched successfully on August 4, 2007. The lander made it to Mars, setting down in Vastitas Borealis (one of the areas that promised a high concentration of water ice). Initial pictures showed a flat surface scattered with pebbles and polygonal cracks thought to be due to the freezing and thawing of ice just below the surface.

Unlike the previous few Mars missions, Phoenix was a stationary lander rather than a rover. Because the lander itself couldn’t move, one of its most important tools was a robotic arm that could dig about 0.5 meters into the surface of Mars, bringing up soil and ice samples that could then be analyzed and studied. A scoop on the end of the robotic arm delivered soil samples into a variety of chemical analysis tools aboard the spacecraft, which wetted and heated the soil to perform various compositional analyses.

The Mars Science Laboratory and its rover centerpiece, Curiosity, is the most ambitious Mars mission yet flown by NASA. The rover's primary mission is to find out if Mars is, or was, suitable for life. Another objective is to learn more about the red planet's environment. The MSL spacecraft arrived on Mars on Aug. 6, 2012, after a daring landing sequence that NASA dubbed "Seven Minutes of Terror." Because of Curiosity's weight, NASA determined that the past method of using a rolling method with land bags would probably not work. Instead, the rover went through an extremely complicated sequence of maneuvers to land.

The rover has a few tools to search for habitability. Among them is an experiment that bombards the surface with neutrons, which would slow down if they encounter hydrogen atoms: one of the elements of water. Curiosity's 7-foot arm can pick up samples from the surface and cook them inside the rover, sniffing the gases that come out of there and analyzing them for clues as to how the rocks and soil formed. The Sample Analysis of Mars instrument, if it does pick up evidence of organic material, can double-check that.

Curiosity's prime mission is to determine if Mars is, or was, suitable for life. While it is not designed to find life itself, the rover carries a number of instruments on board that can bring back information about the surrounding environment. Scientists hit something close to the jackpot in early 2013, when the rover beamed back information showing that Mars had habitable conditions in the past.

Powder from the first drill samples that Curiosity obtained included the elements of sulfur, nitrogen, hydrogen, oxygen, phosphorus and carbon, which are all considered "building blocks" or fundamental elements that could support life. While this is not evidence of life itself, the find was still exciting to the scientists involved in the mission.