Spacecraft exploration
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Spacecraft exploration
Since the beginning of the space age, Mars has been a focus of planetary exploration for three main reasons: (1) it is the most Earth-like of the planets; (2) other than Earth, it is the planet most likely to have developed indigenous life; and (3) it will probably be the first extraterrestrial planet to be visited by humans. Between 1960 and 1980 the exploration of Mars was a major objective of both the U.S. and Soviet space programs. U.S. spacecraft successfully flew by Mars (Mariners 4, 6, and 7), orbited the planet (Mariner 9 and Vikings 1 and 2), and placed lander modules on its surface (Vikings 1 and 2). Three Soviet probes (Mars 2, 3, and 5) also investigated Mars, two of them reaching its surface. Mars 3 was the first spacecraft to soft-land an instrumented capsule on the planet, on December 2, 1971; it landed during a planetwide dust storm and returned data for about 20 seconds.
Photo NASA/JPL/Caltech (NASA photo # PIA00563)
Mariner 9, the first spacecraft to orbit another planet, was placed around Mars in November 1971 and operated until October 1972. It returned a wide variety of spectroscopic, radio-propagation, and photographic data. Some 7,330 pictures covering 80 percent of the surface demonstrated a history of widespread volcanism, ancient erosion by water, and reshaping of extensive areas of the surface by internal forces.
The central theme of the Viking missions was the search for extraterrestrial life. No unequivocal evidence of biological activity was found (see below The question of life on Mars), but the various instruments on the two orbiters and two landers returned detailed information concerning Martian geology, meteorology, and the physics and chemistry of the upper atmosphere. Vikings 1 and 2 were placed into orbit during June and August 1976, respectively. Lander modules descended to the surface from the orbiters after suitable sites were found. Viking 1 landed in the region of Chryse Planitia (22° N, 48° W) on July 20, 1976, and Viking 2 landed 6,500 km (4,000 miles) away in Utopia Planitia (48° N, 226° W) on September 3, 1976.
Caltech—JPL/NASA
NASA
In 1988 Soviet scientists launched a pair of spacecraft, Phobos 1 and 2, to orbit Mars and make slow flyby observations of its two satellites. Phobos 1 failed during the yearlong flight, but Phobos 2 reached Mars in early 1989 and returned several days of observations of both the planet and Phobos before malfunctioning.
Amid failures of several U.S. spacecraft missions to Mars in the 1990s, Mars Pathfinder successfully set down in Chryse Planitia (19° N, 33° W) on July 4, 1997, and deployed a robotic wheeled rover called Sojourner on the surface. This was followed by Mars Global Surveyor, which reached Mars in September 1997 and systematically mapped various properties of the planet from orbit for several years beginning in March 1999. These included Mars’s gravity and magnetic fields, surface topography, and surface mineralogy. The spacecraft also carried cameras for making both wide-angle and detailed images of the surface at resolutions down to 1.5 metres (5 feet).
NASA/JPL/Caltech
Mars Odyssey safely entered Mars orbit in October 2001 and started mapping other properties, including the chemical composition of the surface, the distribution of near-surface ice, and the physical properties of near-surface materials. Neutron measurements suggested that the polar regions above latitude 60° contain huge subsurface reservoirs of water ice. Mars Odyssey also discovered caves on a volcano by using its infrared cameras to show that the temperatures of the cave entrances, which appeared as dark circular features, did not change as much as the surrounding surface.
JPL-Caltech—ASU/USGS/NASA
A wave of spacecraft converged on Mars in late 2003 and early 2004 with mixed outcomes. Nozomi, launched by Japan in 1998 on a leisurely trajectory, was the first to reach the vicinity of the planet, but malfunctions prevented it from being put into Mars orbit. In mid-2003 the European Space Agency’s Mars Express was launched on a half-year journey to the Red Planet. Carrying instruments to study the atmosphere, surface, and subsurface, it entered Mars orbit on December 25; however, its British lander, Beagle 2, which was to examine the rocks and soil for signs of past or present life, failed to establish radio contact after having landed on the Martian surface the same day. Within weeks of its arrival, the Mars Express orbiter detected vast fields of water ice as well as carbon dioxide ice at the south pole and confirmed that the southern summer remnant cap, like the northern one, contains permanently frozen water. It also detected large sulfur-rich deposits, mainly in Valles Marineris, and clay minerals in the heavily cratered terrains.
Also launched in mid-2003 was the U.S. Mars Exploration Rover mission, which comprised twin robotic landers, Spirit and Opportunity. Spirit touched down in Gusev Crater (15° S 175° E) on January 3, 2004. Three weeks later, on January 24, Opportunity landed in Meridiani Planum (2° S 6° W), on the opposite side of the planet. The six-wheeled rovers, each equipped with cameras and a suite of instruments that included a microscopic imager and a rock-grinding tool, analyzed the rocks, soil, and dust around their landing sites, which had been chosen because they appeared to have been affected by water in Mars’s past. Both rovers found evidence of past water. Perhaps the most dramatic was the discovery by Opportunity of rocks that appeared to have been laid down at the shoreline of an ancient body of salty water. Each rover was designed for a nominal 90-day mission but functioned well past that time. Spirit and Opportunity stopped transmitting to Earth in March 2010 and June 2018, respectively. Opportunity had covered 45 km (28 miles) over 14 years, which were records for distance driven and mission time spent on another planet.
NASA/JPL/Cornell University
In 2005 the U.S. launched the Mars Reconnaissance Orbiter, carrying an imaging system with a resolution of 20 cm (8 inches), a spectrometer to determine surface composition, and a ground-penetrating radar. The imaging system took photographs of dark streaks that appeared to be salty water flowing downhill after it had melted during the Martian spring. The spectrometer revealed that clay minerals and other alteration products indicative of a warm distant past are common in the ancient cratered terrains, while the radar determined the thickness of the ice at the poles and detected glaciers elsewhere.
In 2008 the U.S. probe Phoenix landed in the north polar region of Mars. Phoenix carried a small chemical laboratory to study the arctic soil. It found water ice underneath the surface of Mars and alkaline soil.
Max Planck Institute—University of Arizona/JPL-Caltech/NASA
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The U.S. Mars Science Laboratory rover, called Curiosity, landed in Gale crater in 2012. Weighing about 900 kg (2,000 pounds) and measuring about 3 metres (10 feet) long, it was the heaviest and longest rover on Mars. Gale crater is at a low elevation, so if Mars ever had surface water, it would have pooled there. Aeolis Mons (also called Mount Sharp), the crater’s central mountain, consists of many layers of sedimentary rock that were laid down over much of Mars’s geological history. Curiosity took pictures of water-transported gravel, which suggests that at one time Gale crater was likely the floor of an ancient stream. Curiosity also found that early Mars could have supported life. It discovered traces of organic molecules preserved in rock layers 3.5 billion years old and that the amount of methane in the Martian atmosphere varies with the seasons.
In September 2014 two probes entered Martian orbit. The U.S. Mars Atmosphere and Volatile Evolution (MAVEN) probe studied the upper atmosphere and found that Mars had lost most of its early atmosphere to the Sun’s ultraviolet radiation and solar wind. India’s Mars Orbiter Mission (MOM) probe was that country’s first to reach another planet. Its instruments included a colour camera, an ultraviolet spectrometer, and a sensor for methane.
The ExoMars mission was a joint project of the European Space Agency and Russia. The first part of the mission arrived at Mars in October 2016 and consisted of two spacecraft—the Trace Gas Orbiter (TGO) and the Schiaparelli lander. Schiaparelli ejected its parachute early and crashed into the surface. The TGO mapped the vertical distribution of dust and water vapour in the atmosphere. It did not detect any methane, which conflicted with Curiosity’s detection and suggests that some process destroys methane before it spreads throughout the atmosphere.
The U.S. InSight (Interior exploration using Seismic Investigations, Geodesy, and Heat Transport) lander touched down on Elysium Planitia in November 2018. InSight placed a seismometer on the surface that made the first detections of quakes on Mars and revealed the planet’s internal structure. The lander also deployed a probe that burrowed into the ground to study heat flow in the soil. However, the probe seemed to have hit some underground obstacle, such as a rock or gravel, because it stopped before reaching the desired depth. The same rocket that launched InSight also carried two “CubeSats,” small satellites whose basic unit is a modular cube measuring roughly 10 cm (4 inches) square per side. Each of the InSight CubeSats, called Mars Cube One (MarCO), consisted of six such units. The first CubeSats to be launched to another planet, they relayed communications to Earth from InSight during its landing.
In February 2021 three missions arrived at Mars. The United Arab Emirates orbiter Hope carried a camera and infrared and ultraviolet spectrometers to study the Martian atmosphere. The Chinese mission Tianwen-1 consisted of an orbiter and a small rover, Zhurong, which landed on Mars on May 14. The American Mars 2020 mission carried the Perseverance rover, which had a drill designed to collect core samples that could be taken to Earth for analysis. Perseverance landed on February 18 in Jezero crater near a dried-up river delta and was designed to search for signs of ancient microbial life. The rover also carried a small helicopter, Ingenuity, which on April 19, 2021, became the first machine to fly through the skies of another planet when it ascended to a height of 3 metres (10 feet).
NASA/JPL-Caltech
Future missions scheduled to arrive at Mars include the second part of the ExoMars mission, the Rosalind Franklin rover. Rosalind Franklin will arrive in 2023 and will carry a drill that can reach 2 metres (6 feet) underground to collect soil samples for onboard analysis.
The question of life on Mars
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From the beginnings of telescopic observations of Mars, people have speculated about whether life could have started on the planet and what that life might be like. Early observers were concerned mostly with intelligent life, but the focus now is on life’s origin, microbial communities, and limits to their survival.
Views on the prospects for life on Mars have varied greatly in recent decades. In the 1960s the possibility that changes seen at the telescope could have a biological cause led to Mariner 9’s attempts to monitor surface changes in 1972 and to the launching of the Viking landers to Mars in 1975. The Viking spacecraft had an array of sophisticated experiments to detect metabolism and organic molecules. The negative results from these experiments resulted in considerable pessimism that continued through the 1980s for the prospects for life.
However, several factors subsequently contributed to a more optimistic view. The first is recognition that life can survive in a far wider range of conditions than was formerly thought possible, including near deep-sea vents at temperatures well over 1,000 °C (1,800 °F), in basaltic rocks deep below the surface, and in very saline and acid environments. The second is the discovery that on Earth life started very quickly, possibly before the end of heavy bombardment, which possibly indicates that the origin of life is not an extremely low-probability event but rather will follow if the right conditions are present. The third is mounting evidence that conditions on early Mars, when life arose on Earth, were Earth-like. A fourth factor is recognition that Earth and Mars exchange materials. As indicated above, more than 30 pieces of Mars have been found on Earth, despite the difficulty of distinguishing Mars rocks from Earth rocks. It is more difficult to get Earth rocks to Mars. Nevertheless, during the period of heavy bombardment, when life may have already started on Earth and conditions on Mars were Earth-like, pieces of Earth may have been transported to Mars. Thus, life may have originated independently on Mars or been seeded from Earth.
In 1996 the scientific world was shocked when a group of scientists announced that they had found evidence of life in a Martian meteorite. In support of their conclusion, they listed (1) bacteria-like objects in electron microscope imagery, (2) detection of hydrocarbons, (3) mineral assemblages that were not produced in chemical equilibrium, and (4) magnetic particles similar to those produced by some terrestrial bacteria. The announcement triggered a vigorous scientific debate into the validity of the claims. The scientific consensus now is that there are plausible abiological explanations for all the observations and that the claims are likely invalid.
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Despite this setback, the main driver of the Mars exploration program is still the search for life. Because liquid water is so essential for life, the initial focus has been on the search for evidence of warm conditions that would enable the persistence of liquid water. The evidence for such conditions at least on early Mars is now compelling, and there is some evidence that liquid water sometimes flows on the surface in a few places. The exploration thrust will likely shift to search for more-direct evidence such as organic remains and isotopic signatures. It could be argued that the best strategy is to look for fossil remains from the early period in Mars’s history when conditions were more Earth-like. But the Martian meteorite debate and disagreements about early terrestrial life point to the difficulty of finding compelling evidence of microbial fossil life. Alternatively, it could be argued that the best strategy is to look for present-day life in niches, such as warm volcanic regions or the intermittent flows of what may be briny water, in the hope that life, if it ever started on Mars, would survive where conditions were hospitable.
Human exploration
Human exploration is still decades away despite optimism when the Apollo program ended in the early 1970s that Mars exploration would soon follow. The technical difficulties of getting people to Mars and back, while challenging, are not overwhelming. The main difficulty has been in coming up with a compelling rationale that would justify the tremendous costs and risks. Advocates have argued that exploring Mars and extending human reach beyond Earth–Moon space needs no practical rationale; to explore is an essential part of being human. Others have argued that practical benefits such as economic stimulus, scientific discovery, and technology feedback would result. Nevertheless, despite numerous strong advocates, human exploration of Mars now seems farther from reality than in the 1970s.
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Several studies have been undertaken to determine how human missions to Mars might be implemented. There are two basic classes of missions that follow from the orbital motions of Mars and Earth. In opposition-class missions, the round-trip time is 500–600 days, with 30 days spent on Mars. For conjunction-class missions, the round-trip time is about 900 days, and the time spent on Mars may be as much as 550 days. Most studies conclude that the short stay time for opposition-class missions could not justify the cost and effort of getting there and that, despite the greater resources needed, a conjunction-class mission is preferred.
There are many variants on how such a mission might be implemented. In some scenarios a cargo ship is sent ahead of the humans to establish a robotically operated base, and the humans follow, possibly years later. In other variants all the resources needed accompany the crew. Another issue is the extent to which resources at Mars could be used for supporting people during their stay and for making oxidants and propellants for the return trip. In situ resource units (ISRUs) could be sent to Mars well ahead of the people to extract and store both oxygen from the atmosphere and hydrogen from water, using solar or nuclear power. People would not be launched from Earth until adequate resources for mission success had accumulated at Mars. To save fuel, the atmosphere at both Mars and Earth could be used to decelerate (aerobrake) the outgoing and incoming spacecraft, respectively; an additional option is direct entry. Another issue is what the astronauts would do at Mars. Should they stay close to a base and operate remote robots at various locations around the planet, or should they go on extended trips from the base and explore, thereby incurring the risk of being stranded?
One issue that is not so readily susceptible to quantitative engineering analysis is crew health. Long times in space have various physiological effects. The circulatory and vestibular systems are affected, and bones tend to demineralize. Countermeasures to minimize degradation in human performance would have to be devised. The crew would also experience psychological pressures from several hundred days in a confined space far from Earth with no other company. Radiation effects would also be significant, particularly if solar storms occurred during the two–three year trip.
Planetary protection is another issue. The first concern is scientific: Mars should not be contaminated with terrestrial materials before the potential for indigenous life has been adequately assessed. Robotic spacecraft sent to Mars are subject to rigorous cleanliness requirements, and parts that touch the surface are sterilized to minimize the possibility of interfering with the detection of indigenous life. Clearly, similar protective measures cannot be followed for human missions. The second issue concerns back contamination. International protocols require that materials returned from Mars be quarantined until they have been proved to be safe. The astronauts may similarly have to be quarantined. Fortunately, most of the planetary-protection issues should be resolvable well ahead of human missions by robotic return of Martian samples.
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