BY JUSTIN RAY

Follow the countdown and launch of the ULA Delta 2 rocket with NASA's THEMIS spacecraft. Reload this page for the very latest on the mission.

2300 GMT (6:00 p.m. EST)

HOLD EXTENDED. The countdown will not resume as planned.

2258 GMT (5:58 p.m. EST)

Three minutes remain in the built-in hold. Liftoff remains set for 6:05 p.m., pending the final OK on upper level winds.

2256 GMT (5:56 p.m. EST)

The launch team has been polled to give a "go" for liftoff. No constraints were voiced.

2255 GMT (5:55 p.m. EST)

The THEMIS spacecraft cargo atop the Delta 2 rocket are switching to internal power for launch.

2254 GMT (5:54 p.m. EST)

NASA launch manager Chuck Dovale says "go" upper level winds are expected from the latest weather balloon. The final readiness poll of the space agency's management team indicated no technical problems are being worked.

2252 GMT (5:52 p.m. EST)

The upper level winds have just switched to "go" status. Balloon data is being crunched to verify conditions are now safe for ascent today.

2251 GMT (5:51 p.m. EST)

T-minus 4 minutes and holding. The countdown has entered the final planned hold point for this afternoon's launch attempt. During this 10-minute hold, officials will poll the various team members in the soft blockhouse, Range Operations Control Center and Mission Directors Center.

Upper level winds have been reported by launch management as "no go." Officials are assessing the game plan.

2240 GMT (5:40 p.m. EST)

T-minus 15 minutes and counting. The countdown has resumed following the planned hold. Clocks will tick down to T-minus 4 minutes where the final hold is scheduled.

The countdown is proceeding while officials continue to evaluate the upper level wind conditions. Weather balloons have been showing the winds aloft to be unfavorable for launch.

2236 GMT (5:36 p.m. EST)

The launch team has been polled for a "ready" status to resume the countdown at the end of the hold as scheduled.

2234 GMT (5:34 p.m. EST)

Upper level winds remain "no go" for launch. The computer modeling is showing that winds aloft are unacceptable in transonic region of flight due to structural loads concerns.

2230 GMT (5:30 p.m. EST)

We're half-way through this built-in hold. Launch team and management polls will be coming up shortly. Once the countdown resumes, clocks will tick down to the T-minus 4 minute mark where a 10-minute hold is planned.

Upper level winds are the only concern at the time. A series of weather balloons are released throughout the countdown to measure the speeds and directions of winds that the rocket would encounter during flight. That information is fed into computer programs to determine if the conditions aloft are safe for launch. A final "go" or "no go" on the winds could come very late in the countdown.

2220 GMT (5:20 p.m. EST)

T-minus 15 minutes and holding. The countdown has entered a planned 20-minute built-in hold. The pause is designed to give the launch team a chance to work any problems or catch up on activities that might be running behind schedule. Engineers will also have time to examine all the data from the just-completed steering tests.

2215 GMT (5:15 p.m. EST)

The first stage steering checks just finished.

2211 GMT (5:11 p.m. EST)

The second stage engine slews have been accomplished.

2210 GMT (5:10 p.m. EST)

Technicians are starting the "slew" or steering checks of the first and second stage engines. These are gimbal tests of the nozzles on the first stage main engine and twin vernier engines and second stage engine to ensure the rocket will be able to steer itself during launch.

2205 GMT (5:05 p.m. EST)

Now 60 minutes from the scheduled launch time. Ground winds remain just fine. However, upper level winds are a concern this afternoon.

2201 GMT (5:01 p.m. EST)

Inhibited checks are underway for the Range Safety command destruct receivers that would be used in destroying the Delta rocket should the vehicle veer off course or malfunction during the launch.

2155 GMT (4:55 p.m. EST)

Next up in the countdown will be Range Safety checks.

2147 GMT (4:47 p.m. EST)

Loading of the Delta 2 rocket's first stage liquid oxygen tank has been completed. The operation took 24 minutes and 58 seconds today. The tank will be replenished through the countdown to replace the supercold liquid oxygen that naturally boils away.

The rocket is now fully fueled for liftoff at 6:05 p.m. EST. The vehicle's first stage was successfully loaded with RP-1 kerosene fuel along with the liquid oxygen over the past hour-and-a-half. The second stage was filled with its storable nitrogen tetroxide and Aerozine 50 fuels earlier this week. The nine strap-on booster rockets and third stage use solid propellants.

2140 GMT (4:40 p.m. EST)

Once the first stage liquid oxygen tank reaches the 95 percent full level, the "rapid load" valve will be closed and the slower "fine load" phase will continue to fill the tank.

2135 GMT (4:35 p.m. EST)

Now 90 minutes to launch, if the upper level winds cooperate.

2131 GMT (4:31 p.m. EST)

Liquid oxygen loading is passing the 10-minute mark. The process should take about 25 minutes in total.

2121 GMT (4:21 p.m. EST)

Cryogenic liquid oxygen, chilled to Minus-298 degrees F, has started flowing from the storage reservoir at Complex 17, through plumbing and into the bottom of the ULA Delta 2 rocket. The LOX will be consumed by the first stage main engine during the first four-and-a-half minutes of flight along with the 10,000 gallons of RP-1 kerosene already loaded aboard the vehicle.

2117 GMT (4:17 p.m. EST)

The official "go" has been given to start filling the rocket's first stage with liquid oxygen.

2116 GMT (4:16 p.m. EST)

Early preparations are beginning for first stage liquid oxygen loading.

2110 GMT (4:10 p.m. EST)

NASA launch manager Chuck Dovale says upper level winds are "no go" right now. Several more weather balloons will be dispatched during the rest of the countdown to continue watching the winds aloft in hopes of launching the Delta 2 this evening. Today's window extends from 6:05 to 6:23 p.m. EST.

Dovale just polled the NASA management team to verify there are no constraints with proceeding into liquid oxygen loading. No technical issues with rocket systems, the THEMIS spacecraft or Range are being worked. Upper level winds are the only problem being tracked.

2106 GMT (4:06 p.m. EST)

The launch team has completed work to turn on and configure the Delta's onboard guidance computer.

2104 GMT (4:04 p.m. EST)

A weather briefing was just given to launch management in advance of the decision to begin liquid oxygen loading into the first stage. Ground winds remain within limits for launch. Upper level winds are the strongest between 35,000 and 40,000 feet, with balloons measuring 140 knots. The area of greatest shear is between 5,000 and 10,000 feet.

2050 GMT (3:50 p.m. EST)

The first stage fuel tank of the Delta 2 rocket has been fully loaded for today's planned 6:05 p.m. launch. The tank was filled with a highly refined kerosene, called RP-1, during a 19-minute, 42-second process that concluded at 3:49 p.m. EST.

The next major task in the count will be loading super-cold cryogenic liquid oxygen into the first stage starting around 4:20 p.m.

The kerosene and liquid oxygen will be consumed by the stage's Pratt & Whitney Rocketdyne RS-27A main engine and twin vernier steering thrusters during the initial four-and-a-half minutes of flight.

2048 GMT (3:48 p.m. EST)

Rapid-loading of the RP-1 tank has concluded with nearly 9,900 gallons already aboard the rocket. Fine load is continuing to finish filling the tank.

2042 GMT (3:42 p.m. EST)

The launch team has computed that today's full load for the first stage fuel tank is 10,081 gallons.

Once the tank is filled to 98 percent or 9,890 gallons, the "rapid load" valve will be closed and the slower "fine load" phase will continue to top off the tank.

2035 GMT (3:35 p.m. EST)

Launch of the Delta rocket with the THEMIS spacecraft is two-and-a-half hours away. First stage RP-1 fuel loading is passing the 3,000-gallon mark.

The weather outlook has improved based on the latest update. Forecasters report there is a 90 percent chance of acceptable conditions at launch time; that's up from 80 percent. The forecast calls for scattered clouds at 12,000 and 22,000 feet, northwesterly winds of 12 gusting to 20 knots and a temperature between 50 and 52 degrees F.

Upper level winds continue to be watched.

Should liftoff slip to Saturday for some reason, the forecast is 90 percent "go" for backup launch opportunity. Scattered clouds at 3,000 feet, westerly winds of 10 gusting to 18 knots and a temperature of 54 to 56 degrees is predicted for the 6:01 to 6:19 p.m. launch window.

2030 GMT (3:30 p.m. EST)

About 10,000 gallons of the kerosene propellant are pumping into the base of the rocket from storage tanks at pad 17B as fueling of the Delta 2's first stage begins for today's launch.

2025 GMT (3:25 p.m. EST)

Preparations for loading the Delta 2 rocket's first stage RP-1 fuel tank are starting. After verifying valves, sensors, flow meters and equipment are ready, the highly-refined kerosene fuel will start flowing into the vehicle a few minutes from now.

2023 GMT (3:23 p.m. EST)

First stage helium and nitrogen pressurization is complete. Fueling should begin momentarily.

2010 GMT (3:10 p.m. EST)

Activation of the rocket's Redundant Inertial Flight Control Assembly (RIFCA) guidance computer is beginning now. And the launch team is starting pressurization steps for the first and second stage helium and nitrogen systems and second stage fuel tanks.

2005 GMT (3:05 p.m. EST)

T-minus 150 minutes and counting. The Terminal Countdown has commenced for today's launch of NASA's THEMIS mission aboard the Delta 2 rocket.

The next three hours will be spent fueling the rocket, activating systems and performing final testing before liftoff at 6:05 p.m. EST (2305 GMT) from pad 17B at Cape Canaveral Air Force Station in Florida.

Workers have cleared the Complex 17 area in advance of the hazardous portion in today's launch operation. But a warning horn is now being sounded three times at the seaside complex as a precaution to alert any remaining personnel in the vicinity that they should leave immediately.

The pad clear status will allow the start of activities such as pressurizing the helium and nitrogen storage tanks inside the rocket's first and second stages, along with the second stage fuel and oxidizer tanks.

The countdown clocks currently stand at T-minus 150 minutes. Two holds -- totaling 30 minutes in duration -- are planned at T-minus 15 minutes and T-minus 4 minutes that will lead toward a T-0 and liftoff at the opening of today's 18-minute launch window.

FRIDAY, FEBRUARY 16, 2007
1935 GMT (2:35 p.m. EST)

Here's some stats on this launch:

• The 323rd Delta rocket launch since 1960
• The 127th Delta 2 rocket mission since 1989
• The 1st of 2007
• The 62nd Delta 2 to fly in the 7925 configuration
• And United Launch Alliance's second rocket flight

1905 GMT (2:05 p.m. EST)

T-minus 150 minutes and holding. Clocks are entering a planned 60-minute built-in hold in the countdown. Holds are scheduled into the count to give workers a chance to catch up on any activities that may be running behind.

Over 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 6:05 p.m. EST from Cape Canaveral, Florida.

1820 GMT (1:20 p.m. EST)

Activities are proceeding at the pad for this evening's launch. Crews have moved the exhaust ducts into position next to the launch deck following the service tower rollback. Pad 17B has a unique flame ducting system, originally put in place for the larger Delta 3 vehicle.

Skies have cleared and the winds are staying below the 25-knot limit. Upper level winds remain strong, but whether they are acceptable won't be determined until later in the count when weather balloon data is used to verify the conditions aloft are safe for the rocket to ascend to space.

1456 GMT (9:56 a.m. EST)

Retraction of the service gantry from around the ULA Delta 2 rocket has begun at Cape Canaveral's pad 17B in advance of today's 6:05 p.m. EST liftoff of the THEMIS mission.

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 3:05 p.m. EST, followed about 20 minutes later by loading the first stage with kerosene propellant. Liquid oxygen should begin flowing into the first stage around 4:20 p.m.

1415 GMT (9:15 a.m. EST)

Preparations are underway for this morning's rollback of the mobile service tower that has enclosed the Delta 2 rocket during its pre-flight stay on pad 17B. It has been an overcast and rainy start to the day along the Space Coast. But the showers have moved through and the outlook still predicts partly cloudy skies at launch time.

THURSDAY, FEBRUARY 15, 2007

Ground and high-altitude winds are the primary concerns entering Friday's attempt to launch a quintet of NASA probes that seek to understand the physics behind Earth's magnificent auroral displays.

The $200 million THEMIS mission will begin with liftoff aboard a United Launch Alliance Delta 2 rocket from pad 17B at Cape Canaveral Air Force Station, Florida. The day's 18-minute launch window runs from 6:05 to 6:23 p.m. EST (2305-2323 GMT).

"We're complete through all of our readiness reviews. The team is poised to start the countdown tomorrow, and we're ready to go," NASA launch manager Chuck Dovale said Thursday afternoon.

"I certainly don't want to jinx it because any time you say you haven't had any problems sure enough you'll have one. But the THEMIS mission has actually gone smoother than average."

Weather forecasters are calling for scattered high level clouds, good visibility and chilly temperatures in the mid-40s. But northwesterly winds of 12 gusting to 20 knots will be monitored carefully by launch weather officer Joel Tumbiolo, particularly in the final segment of the countdown. There is a 20 percent chance that the winds could gust above the 25-knot limit imposed for launch.

"In simple terms, that's a one-out-of-five chance during the last four minutes I'll have to call a hold for ground winds. If that does happen, we'll have to recycle and try again," Tumbiolo said.

The T-minus 4 minute point is the mark when the launch team commences the final phase of preparations. If the count is stopped inside the last four minutes for any reason, the rocket is safed and clocks reset to T-minus 4 minutes before another try is made. Friday's window is long enough for two attempts.

Tumbiolo will be making his "go" or "no go" call based on data from a wind sensor positioned near the pad, about 90 feet above the ground.

"If we have a one-time peak during that four-minute period of 26 knots or higher, I'll call a hold."

The wind limit is dependent on the direction. Should the winds shift into a more westerly direction, the constraint relaxes to 34 knots.

Forecasters do not expect any wind worries during the morning rollback of the pad's mobile service gantry or while the rocket stands exposed during the day. The only ground wind concern involves liftoff, as the vehicle flies away from the pad.

Meanwhile, exceptionally strong winds aloft will be a topic watched during the countdown as well. With the jet stream currently over Central Florida, winds around 35,000 feet are about 150 knots. The issue of concern is wind shear -- the varying of wind speeds and directions at different altitudes that the rocket would experience during its climb toward space.

The upper level winds aren't part of the launch weather forecast and aren't taken into account when meteorologists predict an 80 percent chance of acceptable conditions for Friday. Those odds are strictly ground winds and range safety-related weather rules.

The winds aloft are considered a rocket constraint. Weather balloons will be dispatched throughout the count to gauge the winds, with that information fed to Delta engineers in California who will determine whether the conditions are too severe for the rocket's structural integrity as well as the vehicle's ability to remain under control during ascent.

"I can say what the wind speeds are going to be like and I can say what the wind shears are going to be like; how that affects the vehicle I'm not really sure right now. They will have to crunch the numbers and see what happens," Tumbiolo said.

"As the data comes back from those balloons, it is sent back to Huntington Beach (in California) and the Delta analysts then go and actually analytically fly through those winds," said Kris Walsh, ULA's director of NASA and commercial programs for the Delta rocket.

"As long as we don't have large shears, we should be OK."

Walsh said engineers have three different flight profiles available and they will select the one best suited given the winds Friday evening.

The flight will be ULA's first with NASA as the customer since opening for business in December. The joint venture between Boeing and Lockheed Martin combined the Delta and Atlas rocket fleets under the ULA organization.

"The transition to ULA has been fine. We've seen no repercussions from that," Dovale said. "They assured us that near-term we wouldn't see any changes because we've got quite a busy launch schedule for this year. We wanted to make sure that remained our focus and the changes were going to minimal. We've seen just that."

ULA has carried out one launch in its brief history, successfully flying a Delta 2 rocket from California's Vandenberg Air Force Base two months ago with a classified spy satellite payload.

The THEMIS launch will take 73 minutes from liftoff until deployment of the five spacecraft. The probes will fly in highly elliptical orbits around Earth to find the origin of explosive events, called substorms, that trigger auroral sky shows in the form of the Northern and Southern Lights.

"For over 30 years, the source location of these explosive energy releases has been sought after with great fervor. It is a question almost as old as space physics itself," said THEMIS principal investigator Vassilis Angelopoulos. "A substorm starts from a single point in space and progresses past the moon's orbit within minutes, so a single satellite cannot identify the substorm origin. The five-satellite constellation of THEMIS will finally identify the trigger location and the physics involved in substorms."

"Substorms are what make the aurorae interesting," said John Bonnell, a plasma physicist and aurora specialist with THEMIS. "Without them, the greenish white sheets are static, like a cloud. Substorms make the sheets ripple and create different colors, such as red borders and colorful edge effects."

"THEMIS is so important because the same fundamental physical process is seen around all planets, it happens on the sun in solar flares, and in astrophysical systems such as black holes," Angelopoulos said. "It's amazing that being so close to us, here at Earth, it is not understood yet."

The mission will get underway just before sunset Friday evening, if the winds permit.

THURSDAY, FEBRUARY 15, 2007
1340 GMT (8:40 a.m. EST)

This morning's update to the launch weather forecast has removed the concern for thick clouds violating the liftoff rules. Gusty winds are the only worry going into Friday's countdown, the weather team reports. See the outlook here.

WEDNESDAY, FEBRUARY 14, 2007

The second stage was fueled today, putting preparations back on track for launch Friday evening.

Senior managers will gather for the Launch Readiness Review on Thursday. That meeting will give the final "go" to proceed with the countdown.

1550 GMT (10:50 a.m. EST)

Loading of the second stage with its hydrazine propellant mixture is underway at launch pad 17B this morning. Bad weather forced the fueling to be delayed from yesterday, which in turn postponed the launch a day.

Once the fueling is complete later today, the rocket's orndance will be installed by technicians.

1400 GMT (9:00 a.m. EST)

The odds of acceptable launch weather on Friday has improved a bit. Forecasters are giving an 80 percent chance of good conditions. See the outlook here.

TUESDAY, FEBRUARY 13, 2007
2100 GMT (4:00 p.m. EST)

DELAY. Stormy Florida weather that will preclude the completion of second stage fueling this afternoon has forced a 24-hour postponement in launching NASA's THEMIS aurora-studying mission aboard a United Launch Alliance Delta 2 rocket.

The inclement weather impacting the Cape Canaveral area means technicians won't have time today to pump the storable Aerozine 50 fuel into the booster as planned. Hazardous fueling operations are prevented when there is a threat of severe thunderstorms and lightning. Officials met this afternoon and decided to slip the launch one day as a result.

Liftoff is now targeted for Friday during an 18-minute launch window extending from 6:05 to 6:23 p.m. EST. The launch time weather outlook predicts a 70 percent chance of acceptable conditions. Thick cloud cover, rain showers and gusty winds are the primary worries.

The fueling activities got behind schedule earlier today when difficulties were encountered loading the nitrogen tetroxide aboard the rocket. The oxidizer was six degrees too warm, a spokesperson said, prompting technicians to spend time chilling it to the desired temperature. That part of the fueling was successfully completed before the weather hold occurred.

Plans call for the second stage to receive its Aerozine 50 supply on Wednesday, followed by resumption of the standard pre-launch activities, NASA officials said.

The second stage features a single Aerojet AJ10-118K engine that consumes nitrogen tetroxide and Aerozine 50, which is a hydrazine propellant mixture. The stage ignites for the first time at T+plus 4 minutes, 37 seconds into the flight to propel the vehicle into a parking orbit around Earth. That burn will last until T+plus 9 minutes, 54 seconds to achieve a 304 by 100 nautical mile orbit inclined 28.5 degrees.

The rocket then coasts to a point northeast of Australia where the second stage ignites again. That 54-second firing begins at T+plus 64 minutes, 19 seconds to increase the orbit to 825 by 280 nautical miles with an inclination of 26.58 degrees. With its role completed in the THEMIS ascent, the motor is jettisoned to allow the third stage to fire.

THEMIS is a constellation of five tiny probes that will attempt to locate where explosive energy releases occur high above Earth that power auroral eruptions.

1935 GMT (2:35 p.m. EST)

Launch of the THEMIS mission on a Delta 2 rocket is facing a possible 24-hour delay. However, no decision has been made yet.

The concern stems from the slow progress filling of the second stage with storable propellants at launch pad 17B. While the nitrogen tetroxide oxidizer is now aboard on the rocket after earlier difficulties, the second phase of filling -- loading the Aerozine 50 fuel -- hasn't started. Stormy weather is moving across Central Florida this afternoon, making it increasingly iffy whether the hazardous fueling operation can find a window of acceptable weather to be accomplished today.

Officials are meeting this afternoon to assess the situation before deciding if launch will remain targeted for Thursday or have to be delayed until Friday.

The launch window on Friday would be 6:05 to 6:23 p.m. EST.

MONDAY, FEBRUARY 12, 2007

Weather forecasters are predicting a 70 percent chance of acceptable conditions for Thursday evening's liftoff of the United Launch Alliance Delta 2 rocket carrying NASA's THEMIS mission. Thick clouds and showers are the two concerns for the 6:08 p.m. launch time from Cape Canaveral. See the outlook here.

The Flight Readiness Review was conducted at Kennedy Space Center this morning. Launch preparations are proceeding on schedule without any significant technical issues, officials said.

The Delta rocket's second stage will be loaded with its storable propellants Tuesday. A countdown rehearsal is planned, too.

Wednesday's timeline includes buttoning up the rocket and mission management conducting the Launch Readiness Review, which will conclude with approval to enter into the countdown.