ON Jan. 29, the Soviet Phobos 2 spacecraft will begin orbiting Mars to open what is scheduled to be a banner year for planetary science. But for American planetologists and their international partners, it is the launch of the Magellan Venus radar-mapper that should provide what Geoffrey A. Briggs, executive director of NASA's Solar System Exploration Committee, calls the ``psychologically ... critical event.''
These scientists have waited a decade for this mission. It's also the first United States planetary craft of any kind to leave Earth since the still-active Pioneer Venus satellite departed on Aug. 8, 1978. Successful launches of Magellan during its April 28-May 23 launch window and of the also much delayed Galileo Jupiter mission between Oct. 12 and Nov. 22 will reactivate the dormant US planetary exploration program.
As if to celebrate that revival, long-lived Voyager 2 will complete its planetary ``Grand Tour'' Aug. 25, giving humanity its first close look at the farthest planet on its itinerary - Neptune.
There are several ``ifs'' in this prospectus. The Magellan and Galileo launch windows are critical. They depend on the relative positions of the relevant planets. If the space shuttle Atlantis, assigned to these launches, cannot keep to this schedule, the missions will again have to wait, for up to two years. That's why they have the highest priority among the seven shuttle missions set for this year by the National Aeronautics and Space Administration.
Also, Voyager 2 has been in space since Aug. 20, 1977. It has lost backup for crucial components. Nevertheless, it should still be able to make a scientifically valuable inspection of Neptune, if its remaining components continue to function well.
Finally, there is some doubt about the capability of the Soviet two-craft Phobos mission. Controllers lost contact with Phobos 1 a couple of months after it was launched last July 7. Phobos 2, launched July 12, duplicates many of its companion's functions. If it works well, most mission goals will be accomplished. The Soviets report the loss of some of the Phobos 2's TV channels. They also say some instruments now ``need a little push'' to give good data. But at this writing, the mission was still a ``go.''
This Soviet project, with scientists from 13 nations and the European Space Agency participating, is one of the most ambitious Mars probes yet. It's as complex as the US Viking program, which put two orbiters around Mars and landed two robot research stations on its surface in 1976.
This is the first effort to study the Martian moons, especially Phobos, close-up. In April, the 6,200-kilogram (13,700-pound) craft is to arrive at Phobos. It will land a robot station there for a year's research, including penetrating beneath the surface.
A second instrumented lander can hop like a frog, making up to 10 leaps about 20 to 40 meters (65 to 130 feet) each. On tiny Phobos, a man would weigh only a few ounces.
Meanwhile, on-board instruments will ``sniff'' the composition of surface material while the spacecraft hovers 30 to 80 meters (100 to 260 feet) over Phobos for up to 20 minutes. Ten-billionths-of-a-second bursts of a one-millimeter-wide laser beam will blast samples from the moon.
The mission's data could settle the question of whether the tiny Martian moons were formed in orbit or are captured asteroids. As project scientist Alexander V. Zakharov of the Soviet Academy of Sciences Space Research Institute explained in reviewing the mission in the magazine Sky & Telescope: ``If Phobos and Deimos were indeed captured, then the spacecraft will do more than study the moons of Mars - they will make the first close-up study of ... asteroids.''
Soviet scientists have waited a long time for this mission. Their last Mars project, in 1984, failed. But with 18 missions in 25 years, they have put their brand on Venus. These missions have flown by the planet, orbited it, tested its atmosphere with balloon-borne instruments and other probes, and landed on its surface. Even with ground temperatures hot enough to melt lead and an atmosphere 90 times that on Earth, those landers survived long enough to return pictures of that rugged environment.
THE US Pioneer 12 mission did send five probes from the top to the bottom of the Venusian atmosphere a decade ago. And with nearly 3,700 orbits of the planet since then, the Pioneer 12 satellite has extensively photographed cloud patterns and radar-mapped 90 percent of the surface in broad detail. But the Soviets' more-active program has overshadowed the American effort. Launching Magellan this April would open an opportunity for an American-sponsored mission to take the lead in Venusian research.
Even with an on-time launch April 28, Magellan will not orbit Venus until August 1990. So its work will not be part of this year's planetary achievements. When it does arrive, though, its radar should significantly sharpen America's view of the Venusian surface, even though the instrument is less advanced than planned because of budget cuts.
Pioneer 12 could resolve features no smaller than about 95 kilometers (60 miles) across. Five years ago, the Soviets' Venera 15 and 16 mapped the northern third of Venus with a resolution of 1 to 2 kilometers (about a mile). Magellan should map 90 percent of Venus with details as small as 140 meters (150 yards) across.
Galileo, likewise, is a `90s mission. The craft carrying an orbiter and atmospheric probe will not arrive at Jupiter until December 1995. First it must go to Venus. As a result of repeated delays, new safety-related constraints of launching on the shuttle, and budget tightening, NASA can no longer afford the rocket power to sent the mission directly to Jupiter. Controllers must use the gravitational assistance of both Venus and Earth to get Galileo there. After flying once past Venus and twice past Earth, the spacecraft will be on course for the giant planet.
What planetary scientists can look forward to in terms of payoff this year is Voyager 2's Neptune flyby Aug. 29. The spacecraft will pass within about 29,200 kilometers (18,100 miles) of the planet's center and then come within 40,000 kilometers (25,000 miles) of the major moon Triton.
For the first time, scientists will have on-site data from every planet in the Solar System except Pluto. As these are digested over the coming decade, more than one chapter in astronomical texts will have to be rewritten.