NASA inches toward smaller satellites.

NASA is slowly backing away from its controversial plan to launch an expensive set of giant satellites in a project designed to monitor Earth's climate and resolve questions about global warming. Several scientific advisory panels, including one this week, have recommended that the agency fly its monitoring instruments on smaller, less expensive satellites that could go up several years before the larger crafts would be ready for launch.

Original plans for the Earth Observing System (EOS) called for a pair of orbiting observatories, EOS-A and EOS-B, each carrying a dozen sensors aimed at Earth's surface and atmosphere. NASA hoped to launch the first observatory in 1998 and the second one several years later.

But a key review panel of outside experts, assembled by NASA at the White House's urging, now concludes that the agency should redesign EOS in favor of smaller spacecraft. The use of small and intermediate-sized satellites could reduce costs, get instruments into orbit earlier and allow greater resiliency in case problems arise, says panel chairman Edward Frieman, director of the Scripps Institution of Oceanography in La Jolla, Calif. The panel released its report this week.

NASA officials say they are reconsidering the grand plan for EOS in light of the new report and important events that occurred this summer. "We're in the process of developing different alternatives to the implementation of EOS," says Shelby G. Tilford, director of NASA's earth sciences division in Washington, D.C.

Budget constraints are one factor forcing NASA to rethink the program's design. In recent months, both the Senate and House voted to make significant cuts in the proposed EOS budget for fiscal year 1992. And the Senate called on the agency to chop $5 billion off the $16 billion planned for EOS between now and the turn of the century. These cuts would make it impossible for NASA to meet the scope and timing of the project as originally conceived, the panel concludes.

Other recent developments offer NASA more flexibility in exploring alternatives. The Air Force told NASA this summer that it plans to adapt a facility at Vandenberg Air Force Base to launch Atlas rockets, which can carry intermediate-sized satellites. Launches from the California facility could send satellites into the polar orbits needed by EOS instruments. The Air Force decision thus provides an opportunity for distributing the instruments among six intermediate-sized satellites rather than grouping them on two large space platforms.

NASA had not considered the Atlas option in the past, Tilford says, because "it's hard to plan on something that isn't there." EOS managers says they learned of the Air Force's Atlas intentions early this summer. However, a description of the Air Force plans appeared last February in public documents issued as part of the President's budget request.

NASA officials had long maintained that EOS' goals required orbiting the instruments on big platforms because certain sensors must monitor the same part of the Earth at the same time. But the review panel asserts that other options could also work. A smaller platform could hold instruments that truly need to make simultaneous measurements, while other sensors could make nearly simultaneous observations from small satellites orbiting in close formation.

Splitting up the instruments would allow NASA to launch the most important sensors as soon as possible and leave the less critical ones for later. Some might fly as early as the mid-1990s, says Frieman. He points out that government leaders need EOS data to help resolve pressing questions about climate change.

Smaller craft would make the program more resilient, he adds, because NASA could change the instruments on a later satellite to answer questions raised by data from early missions. Moreover, distributing the instruments among several satellites would provide better protection against mishaps such as the Challenger accident or the equipment glitches now hobbling the Hubble Space Telescope and the Galileo probe.

"People [on the review panel] was mindful of the fact that, when carrying an enormous number of instruments, if there's any difficulty, the whole mission can be destroyed," Frieman says.

The panel also calls on NASA to speed up its satellite development process. Recent NASA projects have typically taken nine years to move from chalkboard to launched complex space experiments in less than two years, the panel notes.

NASA has considered splitting the single EOS-A payload onto two Titan rockets, but the review panel recommends going even smaller by using three Atlas rockets or a mixture of Atlas and even smaller rockets.

The idea of separating the instruments upsets some scientists, especially those who research projects might not survive a program redesign. At the same time, many researchers and policymakers criticize NASA for holding on too long to the concept of flying large EOS satellites. While agency officials say they are considering new options, they appear to be "somewhat reluctant," says one member of an independent advisory committee charged with evaluating the future of the U.S. space program.