10October 24, 1601: Tycho Dies, Torch Passed to Kepler
It may seem morbid to herald the death of a famous astronomer as an important date. Undoubtedly, Tycho Brahe was extremely valuable to science while he lived. He created the most precise observational instruments of his time, the best until the invention of the telescope, and with them conducted meticulous observations of the sky. However, Tycho jealously guarded his data, especially from his assistant, Johannes Kepler, whom he set to the task of fitting Mars’s orbit into his celestial model (in which Earth was the center of the universe). After Tycho’s death, Kepler was able to obtain that data (although he did not use the most legal means). Using Tycho’s observations, Kepler discovered that the orbit of Mars—and those of all the other planets—was an ellipse, not a circle. From there Kepler crafted his laws of planetary motion, which describe how the planets orbit the Sun in the solar system and set the stage for Newton’s description of gravity.
9August 6, 1672: Polar Ice Cap Observed
Dutch scientist Christiaan Huygens and his DIY-better-than-Galileo’s telescope brought clarity to many of the mysterious features of the solar system, including the rings of Saturn. In August 1672 Huygens observed and illustrated a bright spot on Mars, which was later discovered to be a polar ice cap. The question of Martian water would plague scientists centuries later.
8September 5, 1877: Opposition and Exciting Discoveries
Astronomers had been observing Mars for hundreds of years, always concluding that the planet was moonless. It was not until 1877, as Mars was nearing opposition—when it makes its closest approach to the Sun and is on the opposite side of our sky from the Sun, a great time for seeing Mars up close—that Asaph Hall finally spotted one. He discovered Deimos on August 12 and, several days later while observing Deimos, spotted Phobos on August 18. During that same perihelic opposition, Giovanni Schiaparelli mapped the features of Mars and observed the linear structures he named canali (”channels”). Public imagination ran wild with those canali, mistranslated into English as “canals,” and Earthlings began to wonder if they might have Martian cousins gathering around red-planet watering holes. After decades of theorizing about those features and what they meant for possible life, the canals were discovered to be optical illusions, the result of astronomers looking for features at the limit of visual resolution.
7April 12, 1963: The Air up There
In April 1963 a group of scientists used spectrographic analysis to determine that Mars’s atmosphere contained water, long speculated on account of the polar caps found centuries before. In the grand scheme of things, there was almost no water at all—much, much less than in the air above Earth’s driest deserts. Mars’s atmosphere is also very thin and composed almost entirely of carbon dioxide. The hope of having Martian cousins was growing dimmer.
6July 14, 1965: Encounter with Mariner 4
In 1965, finally, humans made their best contact with Mars to date when a spacecraft from Earth, Mariner 4, flew by the planet. Mariner 4 took the first photos of the Martian surface, which were in fact the first-ever photos of another planet taken from deep space. Observers on Earth finally got to see the red planet in all its glory, craters and all. There were no canals, no water, and no Martian inhabitants—merely a Moon-like cratered world.
5November 14, 1971: Mariner 9 Comes to Visit
On November 14, 1971, Mariner 9 became the first spacecraft to orbit a planet when it entered the orbit of Mars. Unexpectedly, Mariner 9 got front-row seats to a planet-wide dust storm. It also discovered major features like volcanoes, canyons, weather, and ice clouds. One canyon, 2,500 miles (4,000 km) long, was named Valles Marineris in honor of the pioneering spacecraft. In nearly a year of orbiting, Mariner 9 was able to capture more than 7,000 photos of Mars and imaged about 80 percent of its surface.
4July 20, 1976: Viking 1 Makes Contact
Viking 1 was the first American spacecraft to land on the surface of Mars. From its Martian home, Viking 1 and later its twin, Viking 2, beamed back images and weather data and conducted experiments for six years—even though the mission had been planned for only 90 days! Scientists discovered that Mars has different types of rocks, potentially from different points of origin, and that Mars has seasons and calm winds at night. For the first time, Earthlings could imagine what it might be like to crunch along the planet’s rocky soil and feel its tumultuous winds.
3August 7, 1996: LIFE!…or Something
While orbiters and landers proved definitively that Mars harbored no humanoids, speculation remained as to whether tiny life-forms such as microbes might be lurking on or beneath Mars’s surface. A revelation seemed to come when a group of scientists announced on August 7, 1996, that they had found a meteorite from Mars in Antarctica that contained microscopic Martian fossils. Obviously that announcement launched much fanfare, public debate, and speculation. Intense study of the meteorite and its contents revealed that the “fossils” were likely the result of some natural process and not the remains of life. Nevertheless, the claimed finding spurred discussion about whether we would know how to recognize alien life if we did find it and the mother of all questions—What is life, really?
2July 4, 1997: Pathfinder Blazes a Trail
Much had been learned about Mars from orbit and from landers, but until July 4, 1997, nothing had trod the planet’s surface. On that date Mars Pathfinder landed and released a tiny robotic rover, Sojourner, the first object to cruise the planet. Sojourner was designed to operate for seven days but ended up going for twelve times that long, sending back images and data about Mars’s wind and weather and conducting experiments on its soil. More importantly, the Pathfinder mission proved that landers could be more economical than the astronomically (pun intended) expensive Viking mission and paved the way for future rovers in subsequent decades.
1September 28, 2015: Liquid at Last
Another orbiter made history on September 28, 2015, when NASA scientists announced that spectra taken by the Mars Reconnaissance Orbiter showed liquid water flowing on the planet’s surface. It was thought that the water was uninhabitable, but questions remained about its source. Was it coming from underground, or perhaps condensing from the air? With the idea of manned missions to Mars buzzing around in the popular consciousness and popular media, maybe the first human explorers to Mars will be the ones to find out.