Sorry, teach, but NASA delayed my homework.

Sounds like just another excuse, but when NASA delays a mission, the postponements affect not just agency employees and contractors, but graduate students whose academic careers can depend on that research.

The long-delayed launch of the Earth-observing satellite CALIPSO was postponed 48 seconds from liftoff Friday because of a communications problem. It was postponed again yesterday because of a refueling problem with a reconnaissance plane. NASA plans to launch today from Vandenberg Air Force Base in California. Work on the mission began in 1999.

Earlier delays came because of defective batteries and a strike last fall by employees at Boeing, whose rocket was to carry CALIPSO and its twin satellite, CloudSat, into a polar orbit 438 miles above Earth. The pair is to reveal the inner workings of clouds and their effects on Earth's atmosphere.

CALIPSO is managed by NASA Langley Research Center, along with its partner, Hampton University, one of several historically black colleges at which NASA is helping to develop space science programs. Several Hampton professors, including co-principal investigator Pat McCormick, and about a half-dozen graduate students are involved in the mission.

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After more than a year of canceled launches, "one day is no sweat," graduate student Charles Hill said after Friday's postponement. He spent what was to have been launch day putting together equipment for installation at Hampton's observatory that will shoot a laser into the sky as CALIPSO -- someday -- passes overhead. This is part of Hampton's work to ensure that CALIPSO's cloud measurements are accurate.

One of assistant research professor Tom Kovacs' students graduated before CALIPSO could launch. Another student, Emine Bay Koklu, had to ditch her plans to use some of CALIPSO's data, Kovacs said. Instead, she had to continue to use data garnered by another NASA satellite already in orbit.

"Launch delays are typically expected," Kovacs said, adding that he ensures students have an alternative. "You always want to have a Plan B."

Kovacs said that one of the delays' biggest ripple effects has been on the series of experiments he's coordinating to corroborate CALIPSO's findings.

These experiments will validate CALIPSO's measurements as the satellite snoops into the clouds that drift through Earth's atmosphere. Dozens of scientists from more than 400 sites, on every continent including Antarctica, plan to take measurements from the ground. Some projects scheduled for earlier this year were scrapped when CALIPSO's launch last fall was delayed. Some researchers whose funding was tied to the project have had to make other arrangements, Kovacs said.

Hampton graduate student Sydney Paul will assist the validation project.

"This CALIPSO mission was not how I got drawn to Hampton, but once I heard about the project, it became something that I wanted to study and learn more about while I was here," she said.

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Once CALIPSO and CloudSat are in orbit, they will join a team of NASA observational satellites to get a 3-D look inside clouds to see how water droplets and airborne particles called aerosols are distributed around the globe. The $400 million-plus combo package will start gathering data about a month after launch.

Clouds are one of the least-understood components of the Earth's energy budget, atmospheric scientists say. The largest uncertainty in current climate-prediction models is how to correctly account for their effects at trapping or reflecting the sun's energy.

CALIPSO, the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation satellite, will use its laser and visual and infrared cameras to spot where the aerosols in clouds occur in the atmosphere. The joint project includes France's space agency.

The observations should improve understanding of the role of aerosols and clouds in Earth's climate.

Aerosols are minute particles suspended in the atmosphere. Aerosols can either scatter sunlight back into space to cool the Earth's atmosphere or absorb sunlight and warm the atmosphere.

As a first-year graduate student, Chris Spells so far is not affected by the delays. His research will focus on the role of aerosols in hurricane formation, particularly those that began as Saharan desert dust and end up in the tropical storms that form off the western coast of African before heading into the Atlantic.

CALIPSO's companion, CloudSat, is managed by NASA's Jet Propulsion Laboratory. It will use radar to study cloud properties.

Hampton University received $8 million from NASA for its CALIPSO participation, which includes creating an outreach program to interest schools and teachers worldwide in the mission.