Destination Mars Read Online Free PDF Page B

attention. Its distance to Earth varies with two-year cycles, and its brightness waxes and wanes correspondingly to the intersection of these orbits. At the far end of this ever-changing distance it is over 250 million miles from our planet and a dim red star, distinguished only by its very un-starlike motions in the night sky (all visible planets move at rates different from the starry backdrop). At its closest, about 35–62 million miles from our world (depending on the year), it is the third-brightest object in the evening sky after the moon and Venus, Earth's other close planetary neighbor. Add to this that Mars has the most elliptical orbit of any planet (only Pluto's is more elongated, but that small body was recently demoted from the roster of planets in our solar system).
    To further stand out from its astral competition, Mars did the remarkable: it moved backward from time to time. Called retrograde motion , when the Earth (which orbits inside the ellipse traveled by Mars) closes on Mars and passes it, Mars appears to go from a forward motion to a backward one (“retrograde”) when viewed against the backdrop of stars. 2 Ptolemy came up with an explanation for this back in the Mars-worshipping days, but it assumed the Earth as the center of the universe and was mechanically flawed. Copernicus, as refined by Kepler, redesigned this mechanical explanation with the sun at its proper place at the center of the solar system, between 1510 and 1514 (the ancient Greek Aristarchus of Samos had posited this in the third centuryBCE, but it was traded off for the Earth-centric model). Kepler also spent an additional eight years working out the elliptical nature of Mars orbit, with the sun at one focus-point of the ellipse.
    In 1659, Christian Huygens, working in Holland, observed and drew Syrtis Major on a sketch of Mars. It was the first such recorded observation. Seven years later, in 1666, Giovanni Cassini measured the rotational period of Mars (i.e., its day) at 24 hours, 40 minutes (he was off by less than three minutes). Then, between 1777 and 1783, English astronomer William Herschel noted the axial tilt of Mars and deduced that it should have seasons not so different perhaps than Earth's.
    No matter how rational the great thinkers were regarding Mars, a planet (especially the color of blood) which occasionally traveled backward was sure to gain notoriety. Remember that in a time when electric lights were unknown and the entire world fell under a carpet of darkness except for the flickering flames in individual dwellings, a red star above stood out much more markedly than it does in modern times.
    But for our purposes it is the age of modern science, beginning in earnest in the 1800s, that is important. While men like Kepler toiled to develop concrete notions about the true nature of nearby space and the planets (including his laws of planetary motion), the pace accelerated dramatically with the advent of the large optical telescope.

    What was known about Mars at that time can be summarized thus:
     
Fourth planet from the sun
Smaller than Earth—about half of our planet's diameter
Larger orbit than Earth's
Thought to have no moons
Thought to have oceans and continents
Perhaps a thin, yet Earth-like, atmosphere
     
    And, in a colossal misinterpretation of telescopic observations, one English astronomer stated in 1860, “There is no portion of the planet Mars that cannot be reached by ship.” 3
    Mapping Mars came into vogue in the late 1800s as well, with the advent of improved optics, larger telescopes, and better tracking mechanisms. Astronomers like Camille Flammarion, an influential French scientist and spiritualist; Asaph Hall, an American in government employ; Giovanni Schiaparelli, who assiduously mapped Mars's surface through his telescope; and Percival Lowell, an American amateur astronomer, began to understand Mars as a planet. It was, however, a somewhat flawed vision.
    Flammarion (1842-1926) was perhaps