by Isaac Asimov
There have been no moon landings of any kind since December 1971; and none are, at the moment, planned. There is no question, however, that human technology is capable of placing human beings and their machines on the lunar surface at any time that seems desirable, and the space program continues in other ways.
Venus and Mercury
Of the planets that circle the sun, two—Venus and Mercury—are closer to it than Earth is. Whereas Earth’s average distance from the sun is 92,900,000 miles, the figure for Venus is 67,200,000 miles, and that for Mercury, 36,000,000 miles.
The result is that we never see Venus or Mercury very far from the sun. Venus can never be more than 47 degrees, from the sun as seen from Earth, and Mercury can never be more than 28 degrees from the sun. When east of the sun, Venus or Mercury shows up in the evening in the western sky after sunset and sets soon after, becoming then the evening star.
When Venus or Mercury is on the other side of its orbit and is west of the sun, it shows up before dawn, rising in the east not long before sunrise and then disappearing in the solar blaze when the sun rises not long after—and becoming then the morning star.
At first, it seemed natural to believe that the two evening stars and two morning stars were four different bodies. Gradually it was borne in on observ ers that when one of the evening stars was in the sky, the corresponding morning star never was; and vice versa. It began to seem that there were two planets, each of which shuttled from side to side of the sun, serving as evening star and morning star alternately. The first Greek to express this idea was Pythagoras in the sixth century B.C.—and he may have learned it from the Babylonians.
Of the two planets, Venus is by far the easier to observe. In the first place, it is closer to Earth. When Earth and Venus are on the same side of the sun, the two can be separated by a distance of as little as 25 million miles. Venus is then just about 100 times as far from us as the moon is. No sizable body (except the moon) approaches us more closely than Venus does. Mercury’s average distance from Earth, when both are on the same side of the sun, is 57 million miles.
Not only is Venus closer to Earth (at least, when both planets are on the same side of the sun), but it is the larger body and catches more light. Venus has a diameter of 7,526 miles, while Mercury’s diameter is only 3,014 miles. Finally, Venus has clouds and reflects a far larger fraction of the sunlight that falls upon it than Mercury does. Mercury has no atmosphere and (like the moon) has only bare rock to reflect light.
The result is that Venus, at its brightest, has a magnitude of −4.22. It is then 12.6 times as bright as Sirius, the brightest star, and is indeed the brightest object in the sky except for the sun and the moon. Venus is so bright that, on a dark, moonless night, it can cast a detectable shadow. At its brightest, Mercury has a magnitude of only −1.2, which makes it nearly as bright as Sirius but, still, only one-seventeenth as bright as Venus at its brightest.
Mercury’s closeness to the sun means that it is visible only near the horizon and at times when the sky is still bright with twilight or dawn. Hence, despite its brightness, the planet is hard to observe. It is frequently said that Coper nicus himself never observed Mercury.
The fact that Venus and Mercury are always found close to the sun, and oscillate from side to side of that body, would naturally make some people suppose that the two planets circle the sun rather than Earth. This notion was first suggested by the Greek astronomer Heracleides about 350 B.C. but was not accepted, until Copernicus raised the idea again, not only for Mercury and Venus but for all the planets, nineteen centuries later.
If Copernicus were correct, and if Venus were an opaque body shining by the reflected light of the sun (as the moon did), then, as observed from Earth, Venus ought to show phases like the moon. On II December 1610, Galileo, observing Venus through his telescope, saw that its sphere was only partly lit. He observed it from time to time and found that it did show phases like the moon. That was just about the last nail in the coffin of the older geocentric picture of the planetary system, which could not explain the phases of Venus as those were actually observed. Mercury, too, was eventually observed to show phases.
MEASURING THE PLANETS
Both planets were difficult to observe telescopically. Mercury was so close to the sun, so small and so distant, that very little could be made out in the way of markings on its surface. The Italian astronomer Giovanni Virginio Schiaparelli studied those markings carefully from time to time, however, and, on the basis of the way they changed with time, announced in 1889 that Mercury rotated on its axis in 88 days.
This statement seemed to make sense, for Mercury revolved about the sun in 88 days, too. It was close enough to the sun to be gravitationally locked by it, as the moon by Earth, so that Mercury’s period of rotation and revolution should be the same.
Venus, though larger and nearer, was even more difficult to observe because it was perpetually obscured by a thick and unbroken cloud layer and presented a featureless white expanse to all viewers. No one knew anything about its rotation period, although some thought that Venus, too, might be gravitationally locked to the sun, with a rotation period equal to its period of revolution of 224.7 days.
What changed the situation was the development of techniques for handling radar, for emitting beams of microwaves, which could be reflected from objects, and then detecting those reflected beams. During the Second World War, radar could be used to detect airplanes, but beams of microwaves could be bounced off heavenly bodies as well.
In 1946, for instance, a Hungarian scientist, Zoltan Lajos Bay, bounced a microwave beam off the moon and received the echoes.
The moon, however, was a comparatively easy target. In 1961, three different American groups, one British group, and one Soviet group all succeeded in sending microwave beams to Venus and back. Those beams traveled at the speed of light, which was then precisely known. From the time taken by the beam to reach Venus and return, it was possible to calculate the distance of Venus at that time with greater accuracy than had hitherto been possible. From that determination, all the other solar-system distances could be recalculated, since the relative configuration of the planets was well known.
In addition, all objects that are not actually at absolute zero (and no object is) continually emit beams of microwaves. From the wavelength spread of the beam; it is possible to calculate the temperature of the emitting body.
In 1962, microwaves were detected being emitted by the night side of Mercury, the portion of the visible sphere that was not in sunlight. If Mercury’s period of rotation was really 88 days, one face of the planet would be forever facing the sun and would be very hot, while the opposite face would be forever away from the sun and would be very cold. From the nature of the emitted microwaves, though, the night side had a temperature considerably higher than one would expect, and thus must at some time or other get sunlight.
When a beam of microwaves is bounced off a rotating body, the beam undergoes certain changes in reflection because of the motion of the surface; and the nature of the changes allows one to calculate the speed of the moving surface. In 1965, two American electrical engineers, Rolf Buchanan Dyce and Gordon H. Pettengill, working with microwave beam reflection, discovered that Mercury’s surface was turning faster than expected: Mercury was rotating on its axis in 59 days, so that every bit of its surface was in sunlight at one time or another.
The exact figure for the rotation proved to be 58.65 days—just two-thirds of the revolution period of 88 days. This, too, indicates a gravitational lock, though one less extreme than when rotation and revolution are equal.
THE VENUS PROBES
Venus offered even more startling surprises. Because it was nearly the same size as Earth (with a diameter of 7,526 miles, compared with Earth’s 7,927 miles), it was often viewed as Earth’s “twin sister.” Venus was closer to the sun but had a shielding layer of clouds that might keep it from becoming too hot. It was assumed the clouds were composed of wate
r droplets, and that Venus itself therefore had an ocean, perhaps an even more extensive one than Earth did, and might therefore be rich in sea life. Many science-fiction stories were written (including some by me) concerning such a water-rich, life-rich planet.
In 1956 came the first shock. A team of American astronomers, headed by Cornell H. Mayer, studied the microwaves emitted by Venus’s dark side and came to the conclusion that that side had to be at a temperature far above the boiling point of water. Venus had to be very hot and, therefore, very high in radiation.
This conclusion was almost incredible. Something more impressive than a feeble beam of microwaves seemed to be required. Once rockets could be sent successfully to the neighborhood of the moon, it seemed logical to try for similar probes to various planets.
On 27 August 1962, the first successful Venus probe, Mariner 2, was launched by the United States. It bore instruments capable of detecting and analyzing microwaves being emitted by Venus, and forwarding the results across tens of millions of miles of vacuum to Earth.
On 14 December 1962, Mariner 2 passed within 22,000 miles of Venus’s cloud layer, and there could be no further doubt. Venus was hellishly hot all over its surface, near the poles as well as at the equator, and on the night side as well as on the day side. The surface temperature is something like 475 degrees C—more than hot enough to melt tin and lead and to boil mercury.
That was not all for 1962. Microwaves can penetrate clouds. Microwaves that were beamed at Venus went right through the clouds to Venus’s solid surface and bounced off it. These waves could “see” the surface as human beings, dependent on light-waves, cannot. In 1962, from the distortion of the reflected beam, Roland L. Carpenter and Richard M. Goldstein found that Venus was rotating in a period of something like 250 earth-days. Later analysis by the American physicist Irwin Ira Shapiro showed it to be 243.09 days. This slow rotation was not the result of a gravitational lock on the sun, for the period of revolution was 224.7. Venus rotates on its axis more slowly than it revolves about the sun.
What is more, Venus rotates on its axis in the “wrong direction.” Whereas the general direction of spin, when viewed (in imagination) from a point high above Earth’s north pole, is counterclockwise, Venus rotates on its axis in a clockwise direction. There is no good explanation so far for this retrograde rotation.
Every time Venus is at its closest to us, it has spun on its axis, the wrong way, exactly five times since its previous approach and thus always has the same face in the direction of Earth at closest approach. Apparently, Venus is in gravitational lock with Earth, but the latter would seem far too small to influence Venus across the distance between the two.
After Mariner 2, other Venus probes were launched by both the United States and the Soviet Union. Those of the Soviet Union were so designed as to penetrate Venus’s atmosphere and then parachute to a soft landing. Conditions were so extreme that none of the Soviet’s Venera probes lasted long after entry, but they did gain certain information about the atmosphere.
In the first place, the atmosphere was surprisingly dense, about 90 times as dense as that of Earth, and consisted chiefly of carbon dioxide (a gas present in Earth’s atmosphere only in a very small amount). Venus’s atmosphere is 96.6 percent carbon dioxide and 3.2 percent nitrogen. (Still, with Venus’s atmosphere as dense as it is, the total quantity of nitrogen in it is about three times that in Earth’s.)
On 20 May 1978, the United States launched Pioneer Venus which arrived at Venus on 4 December 1978 and went into orbit about it. Pioneer Venus passed very nearly over Venus’s poles. Several probes left Pioneer Venus and entered Venus’s atmosphere, confirming and extending Soviet data.
The main cloud layer on Venus is about 2 miles thick and is about 30 miles above the surface. The cloud layer consists of water containing a quantity of sulfur; and above the main cloud layer is a mist of corrosive sulfuric acid.
Below the cloud layer is a haze down to a height of 20 miles above the surface; and below that, Venus’s atmosphere seems completely clear. The lower atmosphere seems stable, without storms or weather changes—just incredibly steady heat everywhere. There are only gentle winds; but considering the density of the air, even a gentle wind would have the force of an earthly hurricane. All in all, one can scarcely think of a more unpleasant world than Earth’s “twin sister.”
Of the sunlight striking Venus, almost all is either reflected or absorbed by the clouds, but 3 percent penetrates to the clear lower reaches, and perhaps 2.5 percent reaches the ground. Allowing for the fact that Venus is closer to the sun and gets brighter sunlight to begin with, Venus’s surface receives about one-sixth the light that Earth’s does, despite the former’s thick and permanent cloud layer. Venus may be dim compared with Earth; but, if we could somehow survive there, we could see perfectly well on Venus’s surface.
Indeed, after landing, one of the Soviet probes was able to take photographs of Venus’s surface. These showed a scattering of rocks, which had sharp edges, indicating that not much erosion had taken place.
Microwaves striking Venus’s surface and reflecting back can be used to “see” the surface, just as light-waves can, if the reflected beams can be detected and analyzed by instruments as light-waves are by eye or photograph. Microwaves, which are much longer than light-waves, “see” more fuzzily but are better than nothing. Pioneer Venus was able to map Venus’s surface by microwaves.
Most of Venus’s surface seems to be the kind we associate with continents, rather than with sea bottoms. Whereas Earth has a vast sea bottom (waterfilled) making up seven-tenths of the planetary surface, Venus has a huge supercontinent that covers about five-sixths of the total surface, with small regions of lowland (no water) making up the remaining sixth.
The supercontinent that covers Venus seems to be level, with some indications of craters, but not many. The thick atmosphere may have eroded them away. There are, however, raised portions on the supercontinent, two of them being of huge size.
In what on Earth would be the arctic region, on Venus is a large plateau, which is named Ishtar Terra and is about as large in area as the United States. On the eastern portion of Ishtar Terra is the mountain range Maxwell Montes, with some peaks reaching a height of 7.3 miles above the general level outside the plateau. Such peaks are distinctly higher than any of Earth’s mountain peaks.
In the equatorial region of Venus, there is another and even larger plateau called Aphrodite Terra. Its highest peaks are not quite as high as those on Ishtar Terra.
It is hard to tell whether any of the mountains of Venus are actually volcanoes. Two may be—at least extinct ones; and of them, Rhea Mons, spreads out across an area as large as New Mexico.
THE MERCURY PROBES
Mercury’s surface does not present the problems Venus’s does. There is no atmosphere on Mercury, no cloud layer. It is only necessary to send out a Mercury probe.
On 3 November 1973, Mariner 10 was launched. It passed close by Venus on 5 February 1974, from which neighborhood it sent back useful data, and then moved on toward Mercury.
On 29 March 1974, Mariner 10 passed within 435 miles of Mercury’s surface. It then moved into orbit about the sun in such a way as to make one revolution in 176 days, or just twice Mercury’s year. That brought it back to Mercury in the same spot as before, because for each of Mariner 10’s circuits of the sun, Mercury completed two. On 21 September 1974, Mariner 10 passed Mercury a second time; and on 16 March 1975, it passed a third time, coming within 203 miles of Mercury’s surface. By then, Mariner 10 had consumed the gas that kept it in a stable position, and was thereafter useless for further study of the planet.
In the three passes, Mariner 10 photographed about three-eighths of the surface of Mercury and showed a landscape that looked much like the surface of the moon. There were craters everywhere, with the largest about 125 miles in diameter. Mercury has very few “seas,” however. The largest region that is relatively crater-free is about 870 miles across. It
is called Caloris (“heat”) because it is almost directly under the sun when Mercury is at its closest approach (perihelion) to that body.
Mercury also possesses long cliffs, 100 miles or more long and about 1.5 miles high.
Mars
Mars is the fourth planet from the sun, the one just beyond Earth. Its average distance from the sun is 141,600,000 miles. When Earth and Mars are on the same side of the sun, the two planets can approach within 50,000,000 miles of each other on the average. Because Mars’s orbit is rather elliptical, there are times when Mars and Earth are separated by only 30,000,000 miles. Such close approaches take place every thirty-two years.
Whereas the sun and the moon change their positions more or less steadily moving from west to east, against the background of the stars, the planets have a more complicated motion. Most of the time, they do move west to east, relative to the stars, from night to night. At some points the movement of each planet slows; it comes to a complete halt and then starts moving “backward,” from east to west. This retrograde motion is never as great as the forward motion, so that, on the whole, each planet moves from west to east and eventually makes a complete circuit of the sky. The retrograde motion is largest and most prominent in the case of Mars.
Why does this happen? The older picture of the planetary system with Earth at the center had great trouble explaining this retrograde motion. The Copernican system, with the sun at the center, explained it easily. Earth, moving in an orbit closer to the sun than that of Mars, has a shorter distance to cover in completing its revolution. When Earth is on the same side of the Sun as Mars is, it overtakes Mars so that Mars seems to move backward. Comparison of Earth’s orbital motion with that of any other planet can explain all the retrograde appearances—a great factor in forcing the acceptance of the sun-centered planetary system.