BY JUSTIN RAY

Follow the countdown and launch of the Orbital Sciences Minotaur rocket with the six COSMIC atmospheric research spacecraft. Reload this page for the latest on the mission.

1810 GMT (2:10 p.m. EDT)

T-minus 6 hours. Launch remains on schedule for now. But it is a wet, cloudy and cold day at Vandenberg due to a weather system impacting the area. Officials are expected to get an update on the weather in about an hour and decide whether to proceed with the countdown. Saturday's forecast is significantly better.

The Minotaur rocket is enclosed within its protective mobile service gantry at Space Launch Complex 8 on the southern end of Vandenberg. The tower will be retracted to expose the booster later in the countdown.

1445 GMT (10:45 a.m. EDT)

The Air Force launch weather team is now forecasting a 70 percent chance that conditions will prohibit liftoff of the Minotaur rocket later today.

The launch window extends from 5:10 to 8:10 p.m. PDT (8:10-11:10 p.m. EDT; 0010-0310 GMT).

The specific reasons for concern: Thick clouds 70%, disturbed weather 60%, cumulus cloud 60%, precipitation at/above 2,000 feet 60%, lightning 10%, debris clouds 10%, anvil clouds 10%.

For today's launch opportunity, the forecast calls for overcast skies at 1,500 feet, additional cloud decks at 12,000 and 21,000 feet, visibility of five miles, isolated rain showers, a temperature of 56 to 60 degrees F and southwesterly winds of 12 to 18 knots and gusts to 25 knots.

Should the launch be delayed to Saturday for any reason, weather is expected to be greatly improved.

WEDNESDAY, APRIL 12, 2006

Bad weather is threatening to delay Friday's scheduled launch of six small satellites that will study the atmosphere and climate change by monitoring Global Positioning System transmissions.

Liftoff of the Minotaur rocket from Vandenberg Air Force Base in California is scheduled for 5:10 p.m. PDT (8:10 p.m. EDT; 0010 GMT), the opening of a three-hour launch window. But meteorologists are predicting an 80 percent chance of violating the launch weather rules.

The forecast calls for overcast skies at 1,500 feet, plus higher cloud decks at 15,000 and 23,000 feet, visibility of five miles, isolated rain showers, a temperature of 56 to 60 degrees F and southwesterly winds of 15 knots and gusts between 20 and 25 knots.

The primary worries will be cloud thickness, cumulus-type clouds, precipitation and overall "disturbed weather" in the area.

The outlook for Saturday is greatly improved. Meteorologists predict only a 20 percent chance that clouds would pose a problem on the backup launch date.

The Orbital Sciences-managed Minotaur rocket uses decommissioned first and second stages from Minuteman 2 ICBM missiles and solid-propellant motors from the commercial Pegasus rocket program for its third and fourth stages. The vehicle was created to launch small satellites.

Friday's mission, the fifth for a Minotaur booster since 2000, will deploy six spacecraft for the joint Taiwan-U.S. project known as COSMIC, or Constellation Observing System for Meteorology, Ionosphere and Climate. The mission also carries the name FORMOSAT 3 by its Taiwanese participants.

The satellites will fly 500 miles above the planet and use a technique called radio occultation to examine how the Earth's atmosphere distorts signals emitted by the U.S. military network of Global Positioning System spacecraft. The COSMIC satellites will look for GPS transmissions just above Earth's horizon and measure the "bending" in the signals caused by the atmosphere. The extent of "bending" implies the underlying atmospheric conditions, scientists say.

"This is the first time the technique of radio occultation has been used on a large scale in real time to provide nearly continuous measurements of worldwide atmospheric conditions at all altitudes," says William Kuo, director of the COSMIC office at the University Corporation for Atmospheric Research.

The data will allow researchers to generate atmospheric temperature and water vapor profiles, and mission officials anticipate such information should improve weather forecasting and assist long-term monitoring of climate change.

"The satellites will convert GPS measurements into a precise worldwide set of weather, climate and space weather data," said Jay Fein, program director in the National Science Foundation's Division of Atmospheric Sciences that funded COSMIC. "The resulting new information will have a tremendous impact on geosciences research and weather prediction, and will be an important contribution to global Earth observations."

The COSMIC satellites will take about 2,500 measurements across the globe every day. Scientists are especially eager for data over the oceans where observations by weather balloon aren't possible.

"Centers around the world will have access to this new information for both research and operational forecasting," said Richard Anthes, president of the University Corporation for Atmospheric Research. "User-friendly versions of the data will enable those with less sophisticated systems to benefit as well."

Watch this page for live updates during Friday's launch.