Introduction to Mars

Mars, the fourth planet from the Sun, revolves in an orbit lying between those of Earth, the third, and Jupiter, the fifth planet. From prehistoric times Mars has attracted interest; first, because of its unusual reddish color, then because of the difficulty of understanding its apparent motion in the sky, and later, after the invention of the telescope, because it is the only planet on which surface details and the changes they undergo can be distinguished.

The planet Mars has already played an important part in making a revolutionary contribution to knowledge, and it may well do so again during the 20th century. As will be seen in the next chapter, it was from a detailed and prolonged study of the motion of Mars that the German astronomer Johannes Kepler discovered, in the early 17th century, the basis of modern views on planetary orbits. This event was of outstanding significance because, once and for all, it resulted in the overthrow of the idea, which had dominated man's thinking for many centuries, that Earth was the center of the universe (fig. 1.1).

As for the future, Mars will undoubtedly be the first planet—other than Earth—upon whose surface instruments will be landed to transmit scientific information and pictures back to Earth for extended periods of time. Venus is usually the nearest planet to Earth, and instrumented spacecraft, such as the Soviet Venera 4 in October 1967, may have already reached that planet. But the high temperatures at the surface of Venus would soon render both instruments and radio transmitters inoperative. Furthermore, the continued exploration of space will inevitably lead to the landing of men on Mars, rather than on Venus. In addition to the high surface temperature, the high atmospheric pressure and perpetual cloud cover would make the latter planet an extremely difficult objective for manned exploration.

The temperatures on Mars, on the other hand, are not greatly different from those on Earth. Except for very occasional dust-storms, the visibility is expected to be good. It is true that Martian gravity is less than that of Earth, but it is more than twice as great as the gravity on the Moon. The days on Mars are of the familiar length, about 24 hours, and there are four seasons somewhat resembling those on Earth, but almost twice

FIGURE 1.1. Earth at the center of the universe. (From 15th Century manuscript in Pierpont Morgan Library; courtesy Time, Inc.)

as long. There are a few important differences between the two planets, such as the very low atmospheric pressure on Mars, the lack of oxygen, and the absence of bodies of water, but these would not make the manned exploration of Mars any more difficult than that of the Moon.

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