BELTSVILLE, MD.: NASA plans to launch a satellite next year that will help scientists fill in a gap in their understanding of global warming: the role of clouds and airborne particles.

The satellite Glory, targeted for launch next June, will give scientists a much better tool to measure particles than any satellite so far. The particles, known as aerosols, are bits of things such as dust and smog.

''Undoubtedly, greenhouse gases cause the biggest climatic effect. But the uncertainty in the aerosol effect is the biggest uncertainty in climate at the present,'' said Michael Mishchenko, the Glory project scientist at the Goddard Institute for Space Studies.

Mishchenko and other scientists, in an article describing the Glory mission, wrote that accurately explaining the human-caused contributions to global warming is essential to establishing sound policies to combat it.

Scientific observations have led scientists to rule out the sun and volcanic eruptions as explanations for why the Earth's average temperature has been increasing steadily, said Robert Cahalan, a NASA physicist who heads the climate and radiation branch at NASA's Goddard Space Flight Center outside Washington in Beltsville, Md.

Cahalan said that while the best explanation of the warming trend is the accumulation of heat-trapping gases, the role of aerosols is also important, but less well understood.

''Aerosols aren't producing a discernable trend,'' Cahalan said. Scientists are beginning to understand how aerosols can increase or decrease cloud cover, but they need more information about how all this affects global warming, he said.

The tiny particles are difficult to track. Gases such as carbon dioxide disperse and mix evenly in the atmosphere and remain there for decades, so they can be measured anywhere. However, aerosols are short-lived and aren't evenly mixed.

Some particles are produced from burning fossil fuels. In the air, they're dangerous for human health and the environment.

Mishchenko said it was very difficult to estimate the net effect of the particles on climate.

Other satellites have measured aerosols, but without the accuracy of the aerosol polarimetry sensor that Glory will carry.

Mishchenko published a report in Science magazine that said aerosols had declined slightly in the past 30 years, allowing more sunlight to reach the Earth's surface. But he said in an interview that the data he used came from instruments that produced widely differing findings.

Glory also will carry a cloud camera and a total irradiance monitor to measure the sun's energy.

Cahalan said the sun was a ''superstable star.'' Its energy goes up and down only by small amounts, but scientists must keep watching it to see whether there are important changes in the future, he said.

Instruments in space have been measuring the sun's energy for nearly 30 years. Glory will extend that long-term record.

''People don't always realize that NASA is the agency that does this sort of thing, but we've been doing it from the beginning because we need to understand the planet Earth,'' Cahalan said. ''We have many satellites going around the planet now, and we think of it as taking the pulse of the planet.''