The two NASA satellites, to be launched on Friday morning from Vandenberg Air Force Base, Calif., aboard the same Boeing Delta II rocket, are to join three spacecraft already surveying the planet for a detailed study of the interlocking factors that affect Earth's climate.
This constellation of satellites in a string 4,400 miles long will loop around the poles at an altitude of 438 miles measuring the interactions of the air, water and surface with the Sun's energy as they drive near-term weather and longer-term climate changes.
The newest additions are the CloudSat, which will profile cloud formations with radar more than 1,000 times more sensitive than typical weather radar, and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations, or Calipso, spacecraft. The two orbiters are to operate for at least 22 months to provide a minimum of two seasons of data on climate dynamics, scientists said.
CloudSat and Calipso were to have flown last year, but the missions were delayed because of technical problems and a year-end strike by Boeing technicians preparing the rocket.
The CloudSat radar will take vertical surveys of clouds and layers of clouds and determine how much water they contain and its form. Graeme Stephens of Colorado State University, the principal investigator, told a news conference on Wednesday at the Jet Propulsion Laboratory in Pasadena, Calif., that although just 1 percent of Earth's water existed in the atmosphere, little was known about its overall role in climate.
"The new information from CloudSat will answer basic questions about how rain and snow are produced by clouds, how rain and snow are distributed worldwide and how clouds affect the Earth's climate," Mr. Stephens said.
The role of atmospheric water is a major inconsistency in producing good computer models for global warming and climate, he said.
"The water in clouds is absolutely tiny, but it is critical," Mr. Stephens said. "This is the renewable part of our fresh water."
David Winker of the NASA Langley Research Center in Hampton, Va., the principal scientist for the Calipso, said the satellite would send laser pulses into the atmosphere to determine the amount, type and distribution of aerosol particles. Aerosols are particles from human and natural sources that absorb or reflect solar energy based on their size, shape, color and chemical composition. Their effects on climate are largely unknown, experts said. They are also the bits that collect moisture in clouds to form raindrops.
The laser pulses and instruments aboard the Calipso that can determine the composition and size of detected aerosols will help measure how long the particles stay in the atmosphere and how far they travel. That will let scientists profile layers of aerosols and clouds and see their relationships, Mr. Winker said.
"We expect data gathered by Calipso to help improve forecasts of weather and climate," he said.
The two craft are to travel 15 seconds apart after they join the satellite constellation, allowing them to scan the same features almost simultaneously. The line of satellites is nicknamed the "A-Train" because it crosses the Equator in the afternoon and because the lead spacecraft is called the Aqua and the last is the Aura.
The Aqua, launched in 2002, studies water cycles, and the Aura, which went up in 2004, examines atmospheric chemistry. The remaining satellite in the group, flying ahead of the Aura, is called Parasol, a French craft that studies clouds.
Although on independent missions, each satellite will be positioned to pass over a region one after another, building up a database of complementary observations that should create a global picture of the changing Earth climate, researchers said.