The most ambitious environmental monitoring project ever proposed by the federal government is under renewed assault by scientists who question the program's emphasis on concentrating its resources in a few large, expensive and vulnerable satellites.

As now planned, the National Aeronautics and Space Administration plans to launch six mammoth satellites, known as the Earth Observing System (EOS). Beginning in the late 1990s, the first platforms will be lofted into polar orbit, where they are to monitor the planet's oceans, forests and atmosphere, and send a flood of data streaming back to Earth where it will be used to understand the dynamics of global change in climate, land use and pollution.

But some scientists say they fear the enormous cost and complexity of the space-based EOS. The researchers are concerned that the $30 billion, 30-year EOS will absorb so much money and garner so much attention that less glamorous but more important environmental studies will be delayed or abandoned.

"If we spend all our money on a few large pieces of hardware, we may not be better off than when we started," said James Hansen, director of NASA's Goddard Institute for Space Studies and a leading climate modeler, who has criticized NASA's emphasis on large space platforms, especially if the emphasis comes at the expense of basic research on climate change and observations involving ground-based instruments and smaller satellites.

Indeed, a series of recent reports, including one released yesterday by the National Research Council, challenges the wisdom of relying exclusively on large satellites.

A panel of experts assembled by the National Research Council, which advises the federal government, agrees with NASA that it makes sense to deploy several jumbo-platforms, which would support a dozen instruments that could make simultaneous measurements of elusive targets such as fast-moving clouds and chemical reactions. But the group suggests NASA consider breaking up its second series of platforms and flying the instruments on smaller satellites that could be deployed sooner.

The program's reliance on a few big platforms has come under increasing scrutiny following the discovery of a major flaw that has crippled the Hubble Space Telescope. While the large EOS platforms would not be vulnerable to the same kind of error, the satellites could suffer from a major power or communications failure, or worse, an entire satellite could explode on launch or be dumped into the ocean.

"There's always going to be a lot of concern about all the eggs being in one basket," said Robert Dickinson of the National Center for Atmospheric Research, who participated in the National Research Council's report and is also a principal investigator in the EOS program.

NASA officials say they are considering breaking up the big platforms and flying the instruments aboard several smaller satellites, though agency managers say such an approach would cost more money.

Critics of EOS emphasize that they are not questioning the goals of the program, but the means. The researchers worry that sooner or later, the EOS program will begin to seriously erode other projects devoted to understanding the still mysterious dynamics of climate change.

In particular, the researchers want to protect a favored series of smaller satellites scheduled to be launched in the next few years to measure such things as the oceans' bottom topography, circulation and productivity, atmospheric ozone depletion, tropical rainfall, the upper atmosphere and changes in the energy coming in from the sun and going out into space. The scientists are also concerned that the focus on a few large space platforms will take money away from ground-based observing stations, which have been in decline over the last decade.

The National Research Council warns that if the currently expanding budget for global change research is reduced, NASA should give higher priority to its near-term missions and delay the EOS satellites.

Lennard Fisk, associate NASA administrator for space science and applications, defended the current plan. He said that both the big platforms and the smaller satellites and their ground-based counterparts are necessary.