THE Mars Observer spacecraft launched by the National Aeronautics and Space Administration (NASA) on Sept. 25 is "the next step along the way to the Red Planet."
That's the large perspective in which NASA administrator Daniel S. Goldin sees the new phase of Mars exploration now opening. Like his counterparts in other space-faring nations, he feels the lure of a new frontier for human exploration.
The date for that epic travel adventure is still uncertain. However, American, European, and Russian unmanned survey missions now authorized or being planned should gain the scientific knowledge needed to prepare a human expedition.
Given the fast-growing cooperation between these programs, the 1990s could be a decade in which a major international effort makes Mars one of the best known planets in the solar system.
Bringing back samples will be a milestone. "It is necessary to have a long-term program for Mars, the final phase of which is a sample return," Alexander A. Sukhanov of the Space Research Institute in Moscow told a session of the recent World Space Congress here.
The United States Mars Observer was designed to begin such a study by providing a complete map of the planet's surface and studying its climate through all four seasons of the 687-day Martian year.
Two Russian/European craft, to be launched, respectively, in 1994 and 1996, are to take detailed surface pictures in stereo and study the characteristics of the surface and atmosphere. These spacecraft will deploy instrumented landing craft, a roving robot explorer, and a low-flying balloon.
Meanwhile, NASA and the European Space Agency (ESA) are each studying programs to establish unmanned robot observatories on Mars.
Finally, there are preliminary discussions of how to return a little bit of Mars to Earth. Mr. Sukhanov says he hopes there will be a decision on the Russian version of a sample return mission by the end of the decade. It could well become a fully international venture.
Scientists want to understand Mars so they can know Earth better. As Mr. Goldin explains: "Mars appears to have been a warmer, wetter planet with a dense atmosphere like prehistoric Earth. What happened to the water that once flowed there? What happened to the dense atmosphere? And what can that tell us about the past and future of Earth's own climate?"
While Mars has many Earth-like characteristics, it is farther from the Sun than our planet. It also is smaller than Earth. Its 6,794-kilometer (4,219 mile) equatorial diameter is only 53 percent that of Earth. Therefore, its surface gravity is weaker. Much of its original atmosphere and much - but not all - of its water has escaped to space.
Learning how such differences in these basic factors have shaped Mars's development as compared with that of Earth will help scientists understand planetary evolution in general.
Because there is so much basic knowledge to be gained there, planetary scientists have focused on Mars as their "highest priority" target, according to Mars Observer's chief scientist Arden L. Albee of the California Institute of Technology.
Russia's Mars-94/96 craft are the next probes likely to head for that target. Mission planning, which started under the former Soviet Union, has been clouded by Russia's financial troubles.
These missions are rapidly becoming internationalized. France and Germany have bought into the projects to an extent that may run as high as 10 percent of the Mars-94 cost. The Russian team now refers to the missions as a "Russian/European" endeavor. NASA is studying the possibility of buying one of the Mars-94 landers to carry American instruments.
This mission should break new ground in Mars exploration, both literally and figuratively. As Sukhanov points out, "It involves quite a new element ... the use of penetrators."
Mars-94 is to send two of these probes into the Martian surface. It also will drop two instrument-carrying landers. Meanwhile, an orbiter circling the planet will take color pictures in stereo.
With a resolution fine enough to pick out objects as small as 10 to 15 meters (33 to 50 feet) across, such views should enable scientists to estimate the volume of Martian volcanoes, according to G. Neukum of the DLR Planetary Remote Sensing Section at Oberpfaffennhofen, Germany. Britain, France, Israel, and the US are joining Germany and Russia in this study.
The Mars-94 orbiter will also probe the Martian ground with radar. Erling Nielsen of Germany's Max Planck Institute for Aeronomy at Katlenburg-Lindau told a World Space Congress session this is the first time low-frequency radar - in the range 3.2 to 5 megahertz - will have been used for Mars.
He said it should penetrate several hundred meters looking for permafrost and ice mixed with rock near the surface and wet rock farther down. "If there is water, it will probably be a strong ... reflection," Dr. Nielsen said.
In 1996, the second Russian/European probe is scheduled to head for Mars. It will carry a small roving robot explorer to scoot over the Martian surface and a low-altitude balloon. This French-supplied balloon is to drift with Martian winds for at least 10 days.
Jacques Blamont of the National Space Center (CNES) in Paris says a major objective is to try to learn more about ground ice. Instruments and cameras in the balloon gondola will look at mineral composition of rocks and image the surface with resolution fine enough to show objects 50 centimeters (20 inches) across.
Balloons are tricky to handle. This one will float by day but sink during nighttime cooling. Designers plan to use a high-tech guide rope to keep the balloon off the ground where rocks would tear it up.
This titanium rope acts as a variable ballast. During the day, its weight keeps the balloon from floating too high. When the balloon sinks down at night, some of the rope settles on the ground. This lightens the weight supported by the balloon. Instruments inside the rope's metal structure will analyze the soil as the balloon drags the rope along. Its flexible structure is designed to keep the rope from snagging on rocks, crevices, and ledges.
LOOKING beyond such limited duration missions as Mars Observer or Mars-94/96, NASA and ESA are studying ways to lay down a permanent network of instrumented stations. These would monitor Martian weather and seismic activity. They would analyze the surface in many places that the other types of missions would not go - such as the ice-covered poles.
If approved, NASA's Mars Environmental Survey (MESUR) would send 16 such stations plus a communications orbiter in a series of launches. ESA's Marsnet plan would send a smaller number of similar stations.
As Heinrich Wanke of the Max Planck Institute for Chemistry at Mainz told the space congress, however, setting up a full-scale Marsnet system is "beyond the financial capability of ESA."
Mr. Wanke said that this program would be an ideal project for international cooperation - a point with which NASA officials have said they agree.
For scientists, this new phase of Martian exploration is a leap into the unknown.
NASA administrator Goldin points out that the probes are designed to answer "the questions we are smart enough to ask." However, he adds: "There's so much waiting to be discovered, we don't even know what we don't know. But there's one thing I'm absolutely certain we'll find: the unexpected."