In its orbit around the Sun, Mars comes within 56 million kilometers (35 million miles) of Earth, which is closer than any other planet but Venus. Because it’s farther from the Sun than the Earth, its sunlit side faces Earth when it is closest to us. (You can draw a simple diagram of the two orbits to confirm this fact.) This illumination, when Mars is relatively close, offers Earth-based observers the opportunity to study its surface features.
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The first studies of Mars were soon after Galileo first used the astronomical telescope in 1610. Dutch physicist Christian Huygens first sketched some dark, patchy markings on the planet in 1659. A few years later, Italian astronomer Giovanni Cassini tracked the markings to determine that Mars turns once on its axis in 24 hours and 37 minutes. In other words, the length of the Martian day is almost the same as that on Earth.
By the end of the 1700s, astronomers had observed even more features that resemble Earth’s. Mars has scattered clouds and bright, white polar caps. The polar ice caps of Mars shrink in summer and grow in winter, like the polar ice caps of Earth. The surface markings observed by Cassini and others underwent seasonal changes, growing darker in the summer and changing shape from year to year. Some observers even thought they might be patches of vegetation.
Early observations of Mars supported a Renaissance idea called the plurality of worlds. This hypothesis suggested that other planets are actually worlds like the Earth. To see how revolutionary this idea was at the time, think about the progression of ideas about other planets through history. In Greek times, the planets seemed to be supernatural entities or mystical, crystalline lights that traveled in a celestial realm above the mundane Earth. Then, in the 1600s, the Copernican revolution established that the planets and the Earth were all bodies moving around the Sun. The idea of the plurality of worlds took the Copernican revolution one step further: other planets might be entire worlds like the Earth, with familiar geological surfaces, climates, clouds, weather - and perhaps even life!
The concept of intelligent life on other planets, and specifically Percival Lowell's theory of canal-building Martians, had a profound effect on our culture for nearly a century. Lowell’s ideas forced people to expand their imaginations to consider a civilization on another planet. English poet Alfred Lord Tennyson wrote a poem about whether the Martians could detect civilization on Earth. British writer H.G. Wells published an early science fiction novel in 1898, The War of the Worlds, in which Martians invade Earth, only to be defeated by terrestrial germs. Wells noted that the native populations of the Pacific and the Americas had been decimated in the 1700s and 1800s by European diseases (and syphilis went in the other direction, from America to Europe). He raised the possibility that contact between worlds could prove fatal to one species or the other, either through disease or conflict. Wells wrote, "The Tasmanians, in spite of their human likeness, were entirely swept out of existence in a war of extermination by European immigrants... Are we such apostles of mercy as to complain if the Martians warred in the same spirit?"
H.G. Wells was exploring the possible consequences of scientific ideas, but there was also more fevered speculation. A UFO scare in the 1890s, after the publication of The War of the Worlds, produced a wave of reports of Martian spaceships. They were described as looking like the first dirigibles, which were then flying. UFOs have historically been reported to have the form of the advanced aerial technology of the day: dirigibles in the 1890s, and then sleek shiny metallic objects since the 1950s, when the first spy planes and jet aircraft were being produced. The number of reported UFO sightings peaked concurrently with several major events in the space program: The Soviet Union shocked the United States with the launch of Sputnik in 1957 and made many people concerned over the prospect of weapons in space. The next major peak in UFO sightings occurred when humans reached the Moon in 1969. These sightings were almost certainly not a product of real alien visitations, but of public interest in other worlds.
From the 1910s to the 1940s, Edgar Rice Burroughs (creator of Tarzan), Ray Bradbury, and others wrote many more science fiction novels about Martian civilizations. Around Halloween in 1938, the brilliant young movie director Orson Welles aired a radio dramatization of The War of the Worlds, in which Martians landed in New Jersey and advanced on New York. It caused a panic on the East Coast when listeners mistook it for a real news broadcast. Civilizations on Mars captured the world’s imagination and became deeply embedded in our popular culture.
In the 20th century, scientific information about Mars continued to evolve. Better astronomical techniques and new data proved Mars to have a more forbidding environment than was first thought. Mars is a very cold planet, and the air is thin. Nevertheless, some argued that evolution might have produced Martian vegetation that had adapted to these difficult conditions. Many scientists believed that such vegetation explained the seasonally changing dark marks. But data from Earth-based telescopes in the 1960s made Mars seem even more hostile: spectroscopic measurements revealed that the air pressure on Mars was comparable to that 100,000 feet above the Earth, higher than commercial jets can fly. In 1965, Mariner 4 became the first space probe to reach the red planet. As it zipped past Mars, it snapped a few close-up photos of local regions showing an impact-cratered surface that looked like the Moon. Most scientists then concluded that Mars must be a frozen, dead, Moon-like world. They were closer to the truth than Lowell was, but Mars turned out to be more interesting than they thought.