"A lot of people understand we go out and explore planets," says Mark Pestana, a mission manager at NASA's Dryden Flight Research Center. "One of the ways we do that is by exploring the Earth itself."

Pestana is one of dozens of NASA experts traveling this month to Kwajalein, a remote Pacific island, where they will conduct weather measurements by airplane, ship and balloon. The two-month project, called KWAJEX, highlights a side of the space program that's often little noticed: the use of terrestrial instruments to double-check and improve satellite data.

Working in a DC-8 "Flying Laboratory," Pestana will gather data similar to that collected by a U.S.-Japanese scientific satellite called TRMM, or Tropical Rainfall Measuring Mission. The aircraft will simulate the satellite's overpasses by flying above the clouds and using the same type of radar and radiometers that are aboard TRMM. The DC-8 also will take samples and images within clouds, something no satellite can do.

Both the space-based and aircraft-based observations aim to get a better picture of how the sun's energy, which is concentrated in the tropics, is transferred from the ocean to the atmosphere. Such research is important for weather forecasting and climate modeling.

The TRMM satellite was launched by NASA and Japan's National Space Development Agency. The KWAJEX project is the last of a series of experiments conducted under the TRMM program. More than 200 experts from NASA, the Japanese space agency, and other research institutions will conduct measurements around the Kwajalein atoll, which is part of the Republic of the Marshall Islands.