BY JUSTIN RAY

Follow the countdown and launch of the Boeing Delta 2 rocket with NASA's STEREO solar observatories. Reload this page for the very latest on the mission.

2038 GMT (4:38 p.m. EDT)

T-minus 150 minutes and holding. Clocks are entering the first of three planned holds in today's the countdown timeline. This pause lasts 60 minutes, and during the next hour all launch team members and management officials will be seated at their consoles. A series of polls will be conducted to verify everyone is ready to enter Terminal Count at end of the built-in hold.

Liftoff of the Delta 2 rocket remains scheduled for 8:38 p.m. EDT from Cape Canaveral, Florida.

1938 GMT (3:38 p.m. EDT)

Launch is now five hours away. Technicians are finishing up their work at the pad. The launch team and mission managers are gathering at the control centers. Activities are proceeding toward the start of Terminal Count at 5:38 p.m. EDT.

1830 GMT (2:30 p.m. EDT)

Today's launch will be the 319th for a Delta rocket since the program began in 1960, the sixth of 2006 and the 124th for the Delta 2 fleet since debuting in 1989. It is the 60th flight of the 7925 three-stage configuration for Delta 2.

Covering the STEREO satellites during launch is the first "long" version of Boeing's 10-foot diameter composite nose cone to fly from the Cape. However, the "10L" shroud has flown before from Vandenberg Air Force Base, California. But STEREO takes the distinction as being the first Delta 2 with a 10L and also a third stage -- the West Coast missions were two-stage configurations.

The Delta 2 rocket is riding a string of 68 consecutive successful launches dating back to 1997. The workhorse launcher has 121 successes in its history.

1717 GMT (1:17 p.m. EDT)

The tower has reached its parked position for launch. The fully assembled Delta rocket is being seen for the first time now that the gantry has been pulled back.

1650 GMT (12:50 p.m. EDT)

At pad B of Cape Canaveral's Launch Complex 17, retraction of the mobile service tower from around the Boeing Delta 2 rocket has begun in preparation for tonight's 8:38 p.m. EDT liftoff of the STEREO mission to observe the sun in 3-D.

The gantry was used to stack the multi-stage vehicle atop the pad's launch mount, attach the nine strap-on solid motors and hoist the payload aboard the rocket. The tower also provided the primary weather protection and worker access to the rocket during its stay on the oceanside complex at Cape Canaveral Air Force Station.

The next few hours will be spent configuring launch pad equipment and securing the site.

The Terminal Countdown remains scheduled to start at 5:38 p.m. EDT, followed about 20 minutes later by loading the first stage with kerosene propellant. Liquid oxygen should begin flowing into the first stage shortly before 7 p.m.

1630 GMT (12:30 p.m. EDT)

A swarm of workers just arrived at the pad for moving the service tower into the launch position.

1600 GMT (12:00 p.m. EDT)

Rollback of the pad 17B service tower to reveal the Boeing Delta 2 rocket for today's launch has not yet started. Reporters and photographers have gathered at the pad under beautiful blue skies to watch the milestone event. We'll let you know when the tower is moved.

1340 GMT (9:40 a.m. EDT)

This morning's edition of the launch weather forecast is available here.

TUESDAY, OCTOBER 24, 2006

Twin space observatories designed to change the way we view the sun are packaged aboard a Boeing Delta 2 rocket for liftoff Wednesday night from Cape Canaveral, Florida.

NASA's $550 million Solar Terrestrial Relations Observatory (STEREO) mission is scheduled for launch at 8:38 p.m. EDT (0038 GMT). The available launch window extends 15 minutes.

"We're getting really excited about launching," NASA launch manager Omar Baez told reporters during an afternoon news briefing Tuesday.

Two relatively minor issues were dealt with Tuesday involving missing washers in a piece of equipment on the rocket's nose cone and an unexpected computer reset with the Mercury-bound MESSENGER spacecraft. Technicians at launch pad 17B were expected to put adhesive or even tape in place of the retention washers, while STEREO controllers were reviewing similar boards used in their spacecraft computers and MESSENGER. An evening management meeting put both concerns to rest.

The weather forecast continues to predict a 95 percent chance of favorable conditions at launch time. The outlook calls for a few low and high level clouds, good visibility, light winds and temperatures in the low 60s.

Launch day activities will begin at 7 a.m. EDT with a weather briefing for mission managers in preparation for final pad configuring work and rollback of the mobile service gantry. The tower is scheduled to be retracted around noontime. The afternoon will be spent securing Complex 17 and putting the final touches on the rocket.

The Terminal Countdown begins at 5:38 p.m. EDT from the T-minus 150 minute mark. Twenty minutes later, loading of the Delta rocket's first stage with 10,000 gallons of kerosene propellant will start. Filling the stage with 14,700 gallons of liquid oxygen is expected around 6:50 p.m.

Tests of the rocket's range safety system commences at 7:30, followed by first and second stage engine steering checks. The countdown will enter a 20-minute planned built-in hold at 7:53 p.m. as clocks reach T-minus 15 minutes.

A final hold is scheduled at the T-minus 4 minute point starting at 8:24 p.m. Managers and the launch team will spend the pause going through readiness polls to ensure all systems are "go" to proceed with the count. Inside the last four minutes, the rocket will be switched to internal power and armed for liftoff at 8:38 p.m. EDT.

"I hope we can hit that on time and on the money," Baez said.

The three-stage Delta vehicle will need 25 minutes to propel the STEREO satellites into a looping orbit that extends just past lunar distance. The craft then await a close encounter with the moon December 15 that will catapult one observatory into an orbit around the sun that flies ahead Earth. The second observatory makes another lunar swingby January 21 to be flung into an orbit that lags behind Earth.

The STEREO craft, each equipped with 16 instruments, will use their vantage points to generate the first 3-D images of the sun.

The unique mission aims to study solar storms that erupt from the sun and streak through space. Such disturbances hitting Earth are known to harm orbiting satellites and ground-based power grids, and can pose a threat to astronauts.

Scientists hope to better understand what triggers these so-called coronal mass ejections and their impacts to the inner solar system.

"In terms of space-weather forecasting, we're where weather forecasters were in the 1950s," said Michael Kaiser, STEREO project scientist at NASA's Goddard Space Flight Center. "They didn't see hurricanes until the rain clouds were right above them. In our case, we can see storms leaving the sun, but we have to make guesses and use models to figure out if and when they will impact Earth."

SUNDAY, OCTOBER 22, 2006

A launch weather forecast doesn't get any better than this. Air Force meteorologists now predict a 95 percent chance of favorable conditions for Wednesday's liftoff. See the latest forecast here.

SATURDAY, OCTOBER 21, 2006

Twin solar observatories designed to create 3-D images of the sun are scheduled for launch Wednesday evening from Cape Canaveral aboard a Boeing Delta 2 rocket. The early weather outlook predicts excellent conditions for the 8:38 p.m. EDT liftoff.

See the full forecast here.