Exploration of Mars

Human exploration of Mars has been underway since the 1960s, although the planet has captured people’s imaginations for thousands of years. As the place in our solar system that most closely resembles Earth, Mars has been the subject of more human study than any other planet. By August 2022, the National Aeronautics and Space Administration (NASA) had launched forty-eight missions to Mars, with ten successfully landing on its surface. Dozens more have photographed and mapped the planet from orbit. Many of those missions have attempted to find evidence of microbial life, a theorectical possibility considering the presence of some liquid water on the planet. Through the first three decades of the twenty-first century, the exploration of Mars was exclusively carried out by spacecraft and robotic rovers. However, several planned crewed missions to Mars were in the developmental stages, with the eventual goal of establishing colonies on the planet.

Background

Mars is the second-smallest planet in the solar system with a diameter of about 4,212 miles (6,780 kilometers)—just slightly less than half the size of Earth. With its smaller size and mass, Mars exerts a gravitational force about 37.5 percent of that of Earth. The planet’s atmosphere is about one hundred times thinner than Earth’s and is made up mostly of carbon dioxide. At the closest point in its orbit, Mars is about 34 million miles (55 million kilometers) from Earth. At its farthest distance, Mars is about 250 million miles (402 million kilometers) away. A day on Mars corresponds to twenty-four hours, and thirty-seven minutes on Earth, with a Martian year lasting for 687 Earth days. Temperatures on Mars average –81 degrees Fahrenheit (–63 Celsius), with a range of 86 Fahrenheit (30 Celsius) to –284 Fahrenheit (–140 Celsius).

On the surface, Mars appears brown, gold, and tan; however, Martian rocks and soil contain high amounts of iron, which reacts with oxygen in the atmosphere to form iron oxide or rust. The Martian dust gets kicked up into the atmosphere and spread around, making the planet appear red from Earth. About four thousand years ago, the ancient Egyptians were the first civilization to record the planet, calling it Her Desher or “the red one.” To the ancient Chinese, Mars was known as the “fire star.” The Greeks noted its blood-red color and named it after Ares, the god of war. In turn, the Romans named it after their own god of war—Mars.

While much attention has been given to the planet's surface, scientists have also made efforts to study the planet's core. For decades, scientists believed that Mars had a large core that was not particularly dense, which was considered unusual for a planet of its size. However, two studies published in October 2023 debunked this long-held belief and determined that the Martian core was surrounded by a sea of radioactive magma. Researchers involved in these studies noted the unusual nature of Mars's interior, which was different than Earth's core.

Overview

In the nineteenth century, improved telescopic technology allowed astronomers to get their first glimpses of the Martian surface, although their observations led to more confusion than answers. In 1877, Italian astronomer Giovanni Schiaparelli observed a series of dark and light regions on the Martian surface. He called these areas “seas” and “continents” and used the term canali to refer to dark streaks that he saw crisscrossing the planet. In Italian, canali means “channels,” but to many people, the word was misinterpreted as “canals.”

In 1895, American astronomer Percival Lowell seemed to confirm that Mars did in fact have canals and recorded his findings in a series of detailed maps. Since the term “canals” implies some sort of intelligent organization, Lowell theorized that a civilization had built the canals to transport water from the polar ice caps to the dry Martian equator. His “discovery” led to the widespread belief that Mars was inhabited by intelligent life. However, later astronomers discovered that Lowell’s “canals” were actually just optical illusions created by the human mind’s tendency to see patterns in nature.

The First Attempts. By the mid-twentieth century, astronomers were aware that Mars was likely a bone-dry desert world with little chance of being home to an intelligent civilization. Still, the planet became a possible target for exploration in the early days of the “Space Race” between the United States and the Soviet Union. US officials proposed a crewed mission to the Red Planet in the 1950s, but the technology to accomplish that goal was well out of reach at the time. The Soviets were the first to attempt to send a probe to Mars when they launched Korabl 4 in 1960 but the craft failed to reach Earth’s orbit. The US launched the first successful Mars mission with its Mariner 4, which flew by the planet in 1965 and transmitted twenty-one images to Earth. NASA had more success in 1969 with the flybys of Mariner 6 and Mariner 7, but it was Mariner 9 in 1971 that returned a wealth of information about Mars. Mariner 9 reached orbit around Mars and sent back more than seven thousand images of the Red Planet. Among its discoveries was Olympus Mons, the largest volcano in the solar system, and Valles Marineris, a rift valley almost four times as long as the Grand Canyon.

In 1971, the Soviets attempted the first landings on the planet, but their Mars 2 lander crashed into the surface, and Mars 3 transmitted only twenty seconds of data before going silent. In 1976, the United States successfully landed the first spacecraft on Mars when Viking 1 and Viking 2 touched down within two months of each other. The spacecraft photographed the Martian surface, studied its geology, and tested the soil for signs of microbial life. One of those tests seemingly returned evidence of such life, but the results were later revealed as a chemical reaction in the Martian soil.

Failure and Success. Martian exploration efforts in the 1990s produced mixed results, with some notable triumphs. In 1992, the United States launched the Mars Observer spacecraft on a mission to study Martian geology and weather. Mars Observer had been in development since 1984 and cost about $813 million. Mars Observer was supposed to reach the planet in August 1993, but NASA lost contact with the craft just before it entered orbit. In the wake of the failure, NASA shifted its focus to more cost-effective space missions. The first of these to land on the Red Planet was the Mars Pathfinder, which touched down in July 1997. Pathfinder carried a small, six-wheeled vehicle called Sojourner, which was the first robotic rover ever deployed on another planet. Pathfinder and Sojourner both lasted well beyond their planned mission times, sending back billions of bits of data and thousands of pictures. The most noteworthy of the craft’s many findings was evidence that ancient Mars was once much warmer and liquid water was common on its surface.

Although it was launched before Pathfinder, the Mars Global Surveyor arrived at Mars in September 1997. The orbiting spacecraft studied Martian geology, mapped its surface, and observed its weather. Before ceasing transmission in 2006, the Mars Global Surveyor detected further evidence of ancient water on the planet. NASA’s Mars Odyssey entered orbit around the planet in 2001 and passed the record for the longest operational spacecraft on Mars in 2010. During the early 2020s, Odyssey remained operational, and NASA expected it to continue transmissions at least until the end of 2025.

In 2003, the European Space Agency (ESA) met with mixed success when it launched its first planetary mission to Mars. The ESA’s orbiter, Mars Express, successfully reached the planet and, in 2024, was still transmitting data back to Earth. However, a planned lander, Beagle 2, crashed into the planet and was lost. In 2018, the Mars Express orbiter found evidence of an underground lake of liquid water buried beneath the ice at the Martian south pole.

NASA landed a pair of rovers on Mars about six months apart in 2004. Spirit and Opportunity were initially designed to study the Martian surface and atmosphere for ninety days but ended up far outlasting their expected lifespan. Spirit got stuck in a sand dune in 2010, and Opportunity was lost in a 2018 sandstorm. The data from the rovers provided scientists with definitive proof that liquid water once existed on Mars.

NASA’s Mars Phoenix lander touched down on Mars in 2008 and found evidence that water ice existed beneath Mars’ surface. Four years later, the Curiosity rover landed on Mars with the aid of an innovative “sky crane” landing system, which used rockets to hover above the surface and lower the rover to the ground. Curiosity continued NASA’s study of the planet’s climate and geology, detecting even more evidence of water and signs of organic compounds on Mars. Organic compounds are not proof that life exists but are the chemical building blocks that make life possible. Traces of organic compounds are common in the universe. Curiosity continued actively exploring the Red Planet, and by May 2024 had been operational for nearly twelve years.

In 2018, NASA’s Mars InSight landed on the planet on a geological mission to study its crust, mantle, and core; InSight remained active until 2022. In February 2021, the Perseverance rover, which was modeled after the successful Curiosity rover, touched down on Mars using a similar sky crane landing. Like many past missions, Perseverance looked for signs of water and microbial life, but the rover was also able to collect and store samples for a future robotic mission that would retrieve those samples and return them to Earth.

Other Missions and Future of Mars Exploration: Since 1960, humans have launched roughly fifty missions to the Red Planet, although more than half have ended in failure. As of 2024, NASA, the ESA, the Soviet/Russian space agency, India’s space agency, China, and the United Arab Emirates (UAE) have successfully launched missions to Mars. Both the Chinese Tianwen-1 and UAE Hope spacecraft reached Mars orbit in February 2021 and were the first successful missions for both nations. The majority of missions to Mars have been orbiting vehicles, with only a few countries, including the US, China, and the Soviet Union/Russia successfully landing any spacecraft on the surface by 2024.

Mars also remains the target of a robust series of planned missions throughout the 2020s and beyond. The ESA planned to launch its own rover—the Rosalind Franklin—to the Martian surface in 2022, although it later delayed this launch until 2028. Japan planned to send an orbiter and a lander to Mars called the Tera-hertz Explorer (TEREX) in 2022, although this was also delayed until 2026. Both spacecraft will study the Martian atmosphere. Japan also planned to launch a probe in 2025 to land on Phobos, the larger of the planet’s two moons, collect samples, and return to Earth. The craft was also expected to by the smallest moon, Deimos. The Soviet Union attempted to send two probes to Phobos in the late 1980s but those attempts failed. In 2011, Russia and China made a cooperative effort to reach the Martian satellite, but that mission also failed.

In addition to searching for evidence of the Martian past, NASA is also collecting information on the feasibility of future crewed missions to the Red Planet. For example, part of Perseverance’s mission is to gather data on the Martian weather, winds, radiation levels, and atmospheric dust to develop technology for human habitation. NASA has announced plans to send humans back to the moon's surface by 2026 on Artemis III, with the intent to use the information from that effort to plan for a future trip to Mars. A very tentative timeline suggests 2030 as the earliest NASA would set out for Mars.

However, the private aerospace company SpaceX, founded by South African entrepreneur Elon Musk in 2002, announced a more ambitious plan to reach Mars than the US space agency. Musk was very vocal about his wish to establish a human colony on Mars, including the construction of a self-sustaining city on the planet. According to Musk, SpaceX initially planned to launch a crewless supply ship to Mars by 2022 and follow that with the first crewed launch in 2024 or 2026.

While these plans were later delayed, the company did hit a number of key milestones during the early 2020s; for example, in 2020, SpaceX's Crew Dragon became the first private spaceship to transport human beings to the International Space Station (ISS). To get to Mars, SpaceX would rely on a launch vehicle called Starship, an ultra-powerful, reusable spacecraft that could be refueled in orbit. SpaceX conducted a number of test flights on its Starship throughout the early 2020s, and during its third test flight in March 2024, the spacecraft reached orbit for the first time.

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