Why Mars?

Any discussion of human exploration of Mars must begin with a description of the reasons why this planetary destination has continually reemerged during the post-Apollo period as the “next logical step” for the American space program. Understanding the deep-rooted human interest in Mars provides some insight into the space program’s recurring focus on it as an objective for both robotic and human missions. Crewed Mars exploration has been seriously considered three times during the past 35 years, but our fascination with the red planet began a great deal earlier. For thousands of years, the human race has been drawn to Mars—our celestial neighbor fuels the imagination unlike any other planet in the solar system. Ancient humans examined the red planet as they attempted to unlock the mystery of the heavens. To primitive humans, the fourth planet from the sun was nothing more then a reddish point of light dancing across the night sky. Early civilizations gave it many names: the Egyptians called it Har decher (the Red One), the Babylonians named it Nergal (the Star of Death), the Greeks designated it Ares and the Romans called it Mars (both representing the God of War). While the early Babylonians made extensive astronomical observations, it was the Greeks that first categorized Ares as one of five wandering “planets” among the fixed stars (the others being Mercury, Venus, Jupiter, and Saturn). Greek astronomers observed that Ares did not always move from east to west, but sometimes moved in the opposite direc­tion. Due to the existing belief that the Earth was the center of the universe, this astronomical oddity would baffle sky watchers for centuries to come. By 250 B. C., Aristarchus of Samos had developed a complete heliocentric system that viewed Earth as an ordinary planet circling the sun once every year. This theory held the key to understanding the unusual movements of Ares. Later Greek and Roman astronomers did not follow Aristarchus’s lead, however, choosing to hold onto the geocentric system. Claudius Ptolemy made the greatest elaboration of this system during the second century A. D.—his geocentric model remained the predominant astronomical theory for more than a millennium.[10] [11]

Seventeen hundred years after Aristarchus first developed it, a Polish canon named Nicolaus Copernicus reintroduced the heliocentric model. Like Aristarchus, however, Copernicus could not exactly predict the motions of the planets using simple circular orbits. As a result, his contemporaries largely ignored his theories. While Copernicus had been primarily a theoretician, it would take two dedicated observational astronomers to discover the true movements of the planets—their names were Tycho Brahe and Johannes Kepler. Starting in 1576, Tycho spent 20 years studying the motions of the stars and planets, including Mars. In 1600, Kepler joined him and began examining the apparent retrograde motion of Mars. When Tycho died the next year, Kepler was appointed to succeed him as Imperial Math­ematician to the Holy Roman Emperor (although he was Lutheran).11

Using Tychos scrupulous observations, Kepler went to work trying to explain Mars’ apparent backward motion. Kepler argued that the planets revolved around the sun, but at different distances and therefore different speeds. While Earth orbited the sun in 365 days, it took Mars 687 days. Thus, the retrograde movement of Mars could be explained because the Earth was overtaking the slower-moving Mars. To an observer on Earth, it would appear that Mars was slowing down and then reversing course. Kepler proved, however, that this was simply an illusion. In 1609, Kepler published On the Motion of Mars, which expounded his first two laws of planetary motion—stating that planetary orbits about the Sun were elliptical (as opposed to circular as Aristarchus and Copernicus had assumed) and that a planets speed increases as it approaches the sun and decreases proportionally as it moves farther away. As a result of Tycho and Keplers observations and theories, the heliocentric system finally overcame Ptolemy’s geocentric model.[12]

In 1609, the same year that Kepler published On the Motion of Mars, Galileo Galilei made the first celestial observations with a telescope. The next year, after making observations of the Moon, Jupiter, and Venus, Galileo turned his telescope toward Mars. Due to the use of a relatively crude instrument, Galileos observations of Mars where not particularly informative—other than to suggest that the planet was not a perfect sphere. In 1659, Dutch astronomer Christian Huygens, using a considerably more advanced telescope, was able to detect the first surface feature on Mars. The dark triangular area that he observed over a period of months, which is today called Syrtis Major, allowed him to conclude that Mars rotated on its axis like the Earth. Seven years later, in 1666, Italian astronomer Giovanni Cassini began a series of observations and discovered the planets white polar caps.[13]

In 1783, astronomer William Herschel, who two years earlier had discovered the planet Uranus, made a series of observations of Mars and found that the planet was tilted at an angle of almost 24 degrees on its axis of rotation. This finding showed that like Earth, Mars had seasons; however, considering that a Martian year is almost double that of Earth, its seasons are nearly twice as long. Herschel also confirmed the existence of Mars’s polar caps, and postulated correctly that they were composed of ice. Finally, Herschel found that the planet had “a considerable but moderate atmosphere.”[14]