Weather watch for Red Planet

Until now, the former Soviet Union and the United States have been the only countries to mount unmanned missions to Mars - a planet whose potential for hosting life has tantalized astronomers for more than 200 years.

This exclusive "Mars Explorers' Club" may soon admit its third member. Later this month, flight controllers at the Institute for Space and Astronautical Science here will beam commands to the institute's Planet-B spacecraft, sending it out of Earth's orbit and on to the Red Planet.

When it arrives next October, the orbiter, also called Nozomi (Hope), will begin a two-year mission to study Mars' upper atmosphere and ionosphere, and how they are affected by the solar wind - the relentless outpouring of charged particles from the sun. In addition, the craft will take close-up images of Mars' two moons, Phobos and Deimos.

The craft is aptly named. As an engineering project, Japan's first planetary mission is laying the foundation for future efforts, including two missions to the moon and an asteroid sample-return mission, according to project manager Koichiro Tsuruda. One possible benefit of this scientific endeavor is that it may help change Japan's reputation as a nation that exploits for its own economic interests the basic research and technological developments nurtured elsewhere.

If all goes as planned, Nozomi also will give Japan the distinction of being the only country other than the US to pull off a successful Mars mission this decade. Russia's Mars 96 spacecraft - that country's first Mars mission in eight years - lifted off in November 1996. But it burned up in Earth's atmosphere after its booster's fourth stage misfired.

Weighing in at slightly more than half a ton fully fueled, Nozomi carries a suite of 14 instruments built by Japan, Sweden, Germany, and the US. In addition, the National Aeronautics and Space Administration's Deep Space Network, run by Cal Tech's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., will anchor communications to and from Nozomi.

"We do not know very much about the Martian upper atmosphere," says Dr. Tsuruda as he describes the mission's objectives. "No instruments have ever measured its composition, dynamics, or vertical structure" and how they may change with time. The US Viking missions of the mid 1970s returned some information, he acknowledges, but the data come from two locations. "We have to make more synoptic observations" to unravel the mysteries of the solar wind's impact on the Martian atmosphere, he says.

That interaction represents an important link in understanding the Martian environment, how it changed over time, and whether the planet could once have harbored primitive forms of life, notes Daniel McCleese, chief scientist for NASA's decade-long Mars exploration effort at JPL. He notes that at one time, NASA was thinking about a Nozomi-like mission and gave it a high priority.

In tandem with NASA's Mars 98 missions - the Mars Climate Observer, scheduled for launch today, and next month's launch of NASA's Mars Polar Lander, which will look for evidence of water near Mars' South Pole - Nozomi will round out the most comprehensive effort yet to study the Martian environment.

Key to the solar-wind/atmosphere interaction is Mars' lack of a global magnetic field, Tsuruda explains. Mars is thought to have once had a magnetic field as strong as Earth's. But for reasons still murky, the planet's dynamo shut down long ago. As it did, the Martian atmosphere lost its solar-windbreak.

Meanwhile, atmospheric heating increased the movement and collision rates of gas molecules, adds Dr. McCleese. The Martian atmosphere may have lost much of its water vapor as collisions tore the hydrogen from oxygen atoms in water molecules. Hydrogen, the lightest element, gained enough energy from heating and collisions to escape the planet's weak gravity. Once the hydrogen atoms escaped, they were carried away by the solar wind.

Over two years, Nozomi will be able to test this idea as it watches that interaction not only through a complete Martian year, but during the most active part of the sun's 11-year cycle.