Exploring the Planets

Astronauts will not return to space until the joint U.S.-Russian venture in 1975. But last week unmanned robot craft were ranging far and wide across the solar system on missions of planetary exploration. NASA announced that Pioneer 11, already three-quarters of the way to Jupiter, will proceed to Saturn and provide the first close-up look at the ringed planet. From Mars, an orbiting Soviet spacecraft sent back new, detailed views of the Martian surface. At week's end, fresh from its reconnaissance of cloud-shrouded Venus, Mariner 10, now nearing Mercury, began transmitting its first pictures of the small planet that is closest to the sun.

The space agency's approval of a mission to Saturn will require a course change that will send the craft whipping by Jupiter at a distance of only 26,000 miles. Accelerated by Jovian gravity, Pioneer 11 will then be flung across the solar system for a rendezvous with Saturn in September 1979. The new flight plan was decided upon only after careful analysis of Pioneer 10's performance last December; it came within 81,000 miles of Jupiter's cloudtops and was subjected to an intense bombardment of charged particles from Jupiter's radiation belts. Pioneer's instruments behaved splendidly, sending back pictures and a wealth of data. When the ship finally reaches the vicinity of Saturn, where it will shoot close-up color pictures, it faces still another challenge; it may be sent close enough to Saturn's outermost ring to run the risk of being damaged or destroyed by its debris.

The Soviets, too, had something to show from their massive, but generally disappointing assault on Mars. Last week they released pictures of Mars taken by one of the four spacecraft they launched toward the red planet late last summer. The shots compared favorably with the spectacular photos taken by Mariner 9 in 1972. But the other Soviet Mars probes did not fare so well. Another intended orbiter went shooting by the planet, apparently because of trouble with its braking rocket. A third ejected a landing capsule that missed the planet completely. Another lander, fired from the fourth spacecraft, entered the Martian atmosphere but mysteriously stopped sending signals as it descended. (One theory: it may have been destroyed by the planet's high winds.) Before the signals ceased, Soviet scientists determined that the Martian atmosphere contains "several times" more water than expected. That finding was particularly interesting to NASA, which plans to send off two Viking landers, with life-detecting gear, to Mars next year.

More Secrets. Even as Mariner 10 approached Mercury (it was only 3.3 million miles away at week's end) U.S. scientists were examining the 3,500 pictures of Venus transmitted to earth by the spacecraft in February. Among other things, the computer-clarified photographs showed that 1) the thick Venusian clouds move 60 times as fast as the rotational velocity of the planet; 2) the Venusian poles are ringed by bright, most likely cooler regions; and 3) a huge "eye"--a break in the thick cloud cover --seems to have opened in the equatorial region, probably because of circulation effects from the sun's heat.

Mariner 10 may uncover even more secrets. The 1,108-lb., bug-shaped spacecraft will make its closest approach to Mercury on March 29--when it will come within 620 miles of the surface. Before it leaves the vicinity of Mercury, it should produce 2,000 photographs of the planet's surface and a flood of other data.

Following Mercury's path around the sun, Mariner will again pass by its target next Sept. 22. If the spacecraft's systems are still working, it will send back even more information.

Lopsided Orbit. Usually obscured by the bright glare of the sun, Mercury remains almost as much a mystery as the most distant planet, Pluto. Half again as large as the moon, Mercury may be almost twice as dense. Traveling in a lopsided orbit, it comes as close to the sun as 29 million miles, then sweeps as far away as 43 million miles. To anyone standing on Mercury's surface, the sun would seem to stand still at times, then move backward briefly, in the Mercurian sky. Another oddity: Mercury's trip around the sun takes 88 earth days; yet it rotates on its axis only once every 176 earth days--a fact first discovered in 1965 by earth-based radar. Thus Mercury's "day" is twice as long as its year.

Like the moon, Mercury apparently has no appreciable atmosphere or magnetic field. Its surface, bombarded by intense solar radiation, may be quite dusty. Recently, radar astronomers suggested that Mercury has mountains as high as 4,000 ft., rolling hills and valleys, and some lunar-like craters, some of them perhaps of volcanic origin. Surface temperatures are far more extreme than those on either the moon or Mars. At the height of the Mercurian day, they may reach 940DEG F., more than enough to melt lead. At night they plunge to -- 350DEG F. No living things could be expected to endure such a harsh climate; yet scientists do not entirely dismiss the possibility of some day finding evidence that water--or even life--once existed on Mercury.

This file is automatically generated by a robot program, so viewer discretion is required.