In the dark of night, scientists expect to launch a satellite they hope will provide new insights into the energy the sun provides for Earth's climate in the light of day.

At 2:09 a.m. Pacific Standard Time Wednesday, NASA is launching Glory on a mission that will give the most accurate measurements yet of incoming sunlight, as well as highly accurate measurements of the size, distribution, and effects of tiny particles in the atmosphere known as aerosols, planners say.

The satellite's data should help atmospheric scientists improve climate models. Better models not only would increase scientists' knowledge of how the climate system operates, but also would help them make more accurate projections of the effects of global warming – even as atmospheric concentrations of greenhouse gases continue to rise, as a result of burning fossil fuels and land-use changes.

Direct and indirect effects of aerosols and black-carbon soot represent "the greatest uncertainty in our ability to predict climate," says Hal Maring, the Glory program scientist at NASA headquarters in Washington.

Aerosols reflect sunlight or absorb it, cooling or heating the atmosphere depending on the particles' size and composition.

Aerosols also can have indirect effects on climate by serving as tiny platforms on which atmospheric water vapor can condense to form cloud droplets.

These tiny particles can vary widely in size, composition, and sources. Aerosols in the atmosphere have natural sources, such as volcanic eruptions, or can result from human activities, such as burning fossils fuels or using wood for cooking and heating. Aerosols from different sources and with different compositions can occupy the same parcel of air.

Numerous field studies have monitored aerosols and black-carbon soot in individual regions. But until now, no project has provided a global view. As a result, the uncertainty in the estimates of aerosols' effects are thought to be as large as the effects being estimated.

"This tells us that we don't know much about aerosols at all," says Michael Mishchenko, the mission's project scientist and a researcher at the Goddard Institute for Space Studies in New York.

The human-caused change in Earth's climate, from the addition of greenhouse gases to the atmosphere, Dr. Mishchenko adds, "also has an uncertainty comparable to the estimate – and almost all of this uncertainty comes from the poor knowledge of aerosols."

Glory will provide real-world aerosol measurements to help reduce these uncertainties, during a mission slated to last up to five years, depending on how well the spacecraft and instruments hold up.

Glory is designed to make more than 10,000 measurements at 600,000 locations around the world, comparing incoming sunlight with the solar radiation reflected back into space over those locations. In addition, the satellite carries an instrument that can calculate aerosols' sizes and composition from the polarization of the light that they reflect back into space.

Of particular interest to some scientists will be the data on black-carbon soot. Within the past few years, researchers have conducted field measurements suggesting that black-carbon soot's warming effect on the atmosphere is as much as half that of carbon dioxide.

Yet climate models produce only one-third to half of the observed warming from black carbon, explains Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography in La Jolla, Calif., and a pioneer in the study of black-carbon soot's impact on climate.

"The only thing that beats my excitement for this mission is spending time with my grandchildren," he says. Over the next couple of years, two phenomena he studies – black-carbon soot's impact on cloud formation over the Arabian Sea, and the impact of dust from the western US and abroad on the timing of snow melt in the Rockies – would benefit greatly from having Glory in orbit and operating, says Dr. Ramanathan.

Indeed, the Glory science team notes that the satellite will become the sixth orbiter to join the so-called A-Train – a constellation of Earth-observing satellites launched during the past decade by the US, Canada, and France. The spacecraft focus on clouds, precipitation, ice and snow cover, and other climate-related phenomena.

All pass over the same spot within minutes of one another each afternoon, allowing scientists to track the interaction of different contributors to climate under virtually the same conditions.

"The value of Glory goes up substantially because it's in the A-Train," says NASA's Dr. Maring.