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The Mission




Rocket: Delta 2 (7920)
Payload: Gravity Probe-B
Date: April 20, 2004
Time: 1657:24 GMT (12:57:24 p.m. EDT)
Site: SLC-2W, Vandenberg Air Force Base, California
Satellite feed: AMC 9, Transponder 9, C-band

GP-B mission overview

Launch events timeline

Ground track map



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Gravity Probe-B flies
The Boeing Delta 2 rocket launches with NASA's Gravity Probe-B spacecraft from Vandenberg Air Force Base, California. (4min 16sec file)
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Umbilicals
Umbilicals connecting the Delta rocket to the launch pad yank away as the vehicle lifts off. (175sec file)
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Tower view
Liftoff as seen from a camera mounted on the mobile service tower next to the Delta 2 rocket. (26sec file)
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Back side
A camera positioned on the back side of the Space Launch Complex-2 West provides this angle of liftoff. (19sec file)
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Separation!
The Gravity Probe-B spacecraft is successfully deployed from the Delta rocket as seen by an onboard video camera. (2min 20sec file)
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Post-launch chat
An interview with NASA launch manager Chuck Dovale following the successful deployment of Gravity Probe-B. (2min 20sec file)
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Mission Status



Gravity Probe-B was launched at 1657:24 GMT today from the SLC-2W pad at Vandenberg Air Force Base, California.

The two-stage Boeing Delta 2 rocket successfully delivered its cargo into orbit 75 minutes later.




The Payload




NASA's Gravity Probe-B is an exotic satellite mission to test aspects from Albert Einstein's general theory of relativity.

GP-B spacecraft overview

Learn about the experiment

An intro to general relativity



The Launcher




Boeing's workhorse Delta 2 rocket has flown more than 100 times, launching military, scientific and commercial satellites.

Delta 2 fact sheet

Learn about launch site

Archived Delta coverage



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BY JUSTIN RAY

Follow the countdown and launch of the Boeing Delta 2 rocket with NASA's Gravity Probe-B spacecraft. Reload this page for the very latest on the mission.

Spaceflight Now Plus
Additional coverage for subscribers:
VIDEO: DELTA ROCKET LAUNCHES GRAVITY PROBE-B QT
VIDEO: LIFTOFF AS SEEN FROM MOBILE SERVICE TOWER QT
VIDEO: LAUNCH PAD UMBILICALS YANK AWAY AT LIFTOFF QT
VIDEO: LAUNCH REPLAY FROM CAMERA ON BACK SIDE OF PAD QT
VIDEO: ROCKET CAMERA SHOWS GRAVITY PROBE-B DEPLOYMENT QT
VIDEO: POST-FLIGHT INTERVIEW WITH NASA LAUNCH MANAGER QT

VIDEO: MOBILE SERVICE TOWER IS RETRACTED EARLY MONDAY QT
VIDEO: MONDAY'S LAUNCH COUNTDOWN IS SCRUBBED QT
VIDEO: SUNDAY'S PRE-LAUNCH NEWS CONFERENCE QT
VIDEO: SCIENTISTS PREVIEW THE GRAVITY PROBE-B SCIENCE QT
VIDEO: GRAVITY PROBE-B ARRIVES AT VANDENBERG LAST SUMMER QT
VIDEO: WORKERS UNLOAD THE SPACECRAFT FROM TRAILER QT
VIDEO: GRAVITY PROBE-B TURNED UPRIGHT ONTO WORKSTAND QT
VIDEO: INITIAL TESTING IS PERFORMED ON THE SATELLITE QT
VIDEO: DELTA 2 ROCKET'S FIRST STAGE IS ERECTED ON THE PAD QT
VIDEO: INTERSTAGE IS HOISTED INTO THE LAUNCH PAD TOWER QT
VIDEO: SECOND STAGE IS ATTACHED TO THE ROCKET QT
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TUESDAY, APRIL 20, 2004

A slender Boeing Delta 2 rocket boosted NASA's Gravity Probe B spacecraft into polar orbit today, kicking off a $700 million mission to precisely measure how gravity warps and twists the fabric of space and time. Read our launch story.

See our launch photo gallery here.

1839 GMT (2:39 p.m. EDT)

T+plus 102 minutes. The Delta rocket's second stage has fired for its fourth and final time. This burn depleted the remaining onboard fuel to safe the stage. It also moved the stage out of the GP-B satellite's orbital plane. The orbit reached is 338.87 nautical miles at apogee, 104.06 miles at perigee and inclined 94.45 degrees.

1831 GMT (2:31 p.m. EDT)

T+plus 94 minutes. The second stage has just completed another firing. This evasive maneuver was designed to move the spent stage away from the Gravity Probe-B spacecraft to prevent contamination. The 4.8-second burn was successful, putting the stage in an orbit with a high point of 342.78 nautical miles, perigee of 160.86 miles and inclination of 90.41 degrees.

1819 GMT (2:19 p.m. EDT)

The Boeing Delta 2 rocket has successfully delivered the Gravity Probe-B spacecraft into the correct orbit today.

"The orbit looked right on. We will still look at the numbers a little bit closely. But the initial predictions seem to be right on. The Delta 2 put us right where we needed to be," NASA launch manager Chuck Dovale says.

1813 GMT (2:13 p.m. EDT)

T+plus 76 minutes, 30 seconds. The GP-B satellite appears stable in the live video coming from the departing second stage.

1813 GMT (2:13 p.m. EDT)

T+plus 75 minutes, 40 seconds. The rocket stage is performing a retro maneuver to back away from the GP-B spacecraft.

1812 GMT (2:12 p.m. EDT)

T+plus 75 minutes, 10 seconds. SPACECRAFT SEPARATION! NASA's Gravity Probe-B spacecraft has been released from the Delta rocket's second stage!

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

T+plus 73 minutes. The second stage remains in a good coast mode.

1809 GMT (2:09 p.m. EDT)

T+plus 72 minutes, 30 seconds. Officials report that the solar arrays appear to have deployed based upon the onboard video.

1809 GMT (2:09 p.m. EDT)

T+plus 71 minutes, 50 seconds. The Swedish tracking station has acquired the rocket's telemetry.

1808 GMT (2:08 p.m. EDT)

T+plus 71 minutes. In about three minutes, the second stage is programmed to begin a slow 0.1 rpm roll maneuver to provide the necessary spin-up on the GP-B spacecraft for release.

1807 GMT (2:07 p.m. EDT)

T+plus 70 minutes. The four solar arrays should be deployed at this time.

1805 GMT (2:05 p.m. EDT)

T+plus 68 minutes. Since the vehicle is outside of the tracking station coverage, deployment of the arrays cannot be confirmed in real-time from Vandenberg. Onboard video cameras mounted to the second stage will begin transmitting video to the Sweden site in a few minutes, hopefully showing the spin-up and separation of GP-B from the Delta rocket.

1804 GMT (2:04 p.m. EDT)

T+plus 67 minutes. Gravity Probe-B's solar panels should be unfurling at this time.

Deployment of the arrays while the satellite is still riding on the launch vehicle is quite unusual. But the spacecraft's micro-thruster design is what requires this process.

"The ability of our control system is at the milli-Newton level, which...is about 1/50th of your breath," explained Rex Geveden, Gravity Probe-B program manager. "When you deploy the solar arrays, they put very significant forces on the spacecraft. We would like that happen while we are still stable on the upper stage."

1801 GMT (2:01 p.m. EDT)

T+plus 64 minutes, 30 seconds. The vehicle is now passing out of range from the Malindi tracking station. The next telemetry will come via the Kiruna, Sweden site in about seven minutes.

1800 GMT (2:00 p.m. EDT)

T+plus 63 minutes, 30 seconds. The 17.5-second burn of the second stage has injected the vehicle into the proper orbit. The apogee is 356.004 nautical miles, 343.849 miles at perigee and an inclination of 90.011 degrees.

1759 GMT (1:59 p.m. EDT)

T+plus 61 minutes, 12 seconds. SECO 2. The liquid-fueled second stage engine has completed its firing.

In about five minutes, the power-generating solar arrays on the Gravity Probe-B spacecraft will be deployed in a process that will last about three minutes.

1759 GMT (1:59 p.m. EDT)

T+plus 61 minutes, 38 seconds. The second stage engine has ignited, boosting the vehicle and attached GP-B spacecraft into a more circular orbit than the parking orbit achieved earlier.

1758 GMT (1:58 p.m. EDT)

T+plus 61 minutes. Standing by for ignition of the second stage.

1751 GMT (1:51 p.m. EDT)

T+plus 54 minutes, 20 seconds. The rocket is in a BBQ roll -- a thermal conditioning maneuver to prevent any one side of the vehicle from overheating.

In about four minutes, the rocket will maneuver itself into the engine re-start orientation.

1750 GMT (1:50 p.m. EDT)

T+plus 53 minutes. The Malindi, Kenya tracking station has picked up the second stage's signal.

1742 GMT (1:42 p.m. EDT)

T+plus 45 minutes. The Delta rocket remains in its quiet coast in the parking orbit. Re-start of the second stage motor is just over 16 minutes away.

1732 GMT (1:32 p.m. EDT)

T+plus 35 minutes. The track in which the rocket is following in today's launch can be seen here.

1726 GMT (1:26 p.m. EDT)

T+plus 29 minutes. A signal from the Gravity Probe-B spacecraft is being received via NASA's Tracking and Data Relay Satellite System. Mission officials report the craft's systems are "nominal."

1720 GMT (1:20 p.m. EDT)

T+plus 23 minutes. The rocket is coasting until the second stage restarts at about T+plus 62 minutes for a brief firing to circularize the orbit. The spacecraft's solar arrays will deploy, followed by a slow spin-up by the rocket stage. Release of Gravity Probe-B from the launch vehicle is expected 75 minutes after liftoff.

1715 GMT (1:15 p.m. EDT)

T+plus 18 minutes. The official Range liftoff time was 1657:23.734 GMT.

1711 GMT (1:11 p.m. EDT)

T+plus 14 minutes. The rocket has passed out of range from the P-3 aircraft. This creates a data blackout until the vehicle soars above a tracking station in Kenya about 45 minutes from now.

1710 GMT (1:10 p.m. EDT)

T+plus 13 minutes. The rocket has successfully achieved a good parking orbit with an apogee of 351.69 miles, perigee of 89.94 miles and inclination of 90.0139 degrees.

1708 GMT (1:08 p.m. EDT)

T+plus 11 minutes, 22 seconds. SECO 1. The second stage engine has shut down to complete the first of two firings to deliver GP-B into the proper polar orbit today.

1707 GMT (1:07 p.m. EDT)

T+plus 10 minutes, 50 seconds. The second stage engine continues to burn as expected.

1707 GMT (1:07 p.m. EDT)

T+plus 10 minutes, 20 seconds. The rocket is 1,244 miles downrange from the launch pad, traveling at 15,500 mph.

1705 GMT (1:05 p.m. EDT)

T+plus 8 minutes, 20 seconds. A P-3 aircraft stationed over the Pacific Ocean about 1,300 miles downrange has acquired the rocket's signal to receive live telemetry. The data relay plane will transmit the information back to Vandenberg.

1705 GMT (1:05 p.m. EDT)

T+plus 8 minutes. The vehicle is 100 miles in altitude, 822 miles downrange from the launch pad, traveling at 13,500 mph.

1703 GMT (1:03 p.m. EDT)

T+plus 6 minutes, 25 seconds. Second stage engine performance reported normal.

1702 GMT (1:02 p.m. EDT)

T+plus 5 minutes, 40 seconds. The Delta 2 rocket is 81 miles in altitude, 410 miles downrange from the launch pad, traveling at 12,000 mph.

1702 GMT (1:02 p.m. EDT)

T+plus 4 minutes, 55 seconds. The protective payload fairing enclosing the GP-B satellite atop the rocket has separated.

1702 GMT (1:02 p.m. EDT)

T+plus 4 minutes, 42 seconds. The Delta's second stage engine has ignited!

1701 GMT (1:01 p.m. EDT)

T+plus 4 minutes, 36 seconds. MECO. The first stage main engine cutoff confirmed, and the spent stage has been jettisoned.

1701 GMT (1:01 p.m. EDT)

T+plus 3 minutes, 25 seconds. The Delta 2 rocket is 45 miles in altitude, 97 miles downrange from the launch pad, traveling at 6,700 mph.

1700 GMT (1:00 p.m. EDT)

T+plus 2 minute, 50 seconds. The first stage is continues to burn normally. Stage systems are operating as expected.

1659 GMT (12:59 p.m. EDT)

T+plus 2 minutes, 18 seconds. The three air-start solid rocket boosters have burned out and separated. The rocket continues its trek to orbit on the power of the first stage liquid-fueled main engine.

1658 GMT (12:58 p.m. EDT)

T+plus 1 minute, 30 seconds. The ground-lit boosters have jettisoned from the first stage. They remained attached until the rocket cleared off-shore oil rigs.

1658 GMT (12:58 p.m. EDT)

T+plus 1 minute, 10 seconds. All six ground-start solid rocket boosters have burned out. The three remaining motors strapped to first stage have ignited to continue assisting the rocket's RS-27A main engine on the climb to space.

1658 GMT (12:58 p.m. EDT)

T+plus 56 seconds. Vehicle has passed maximum dynamic pressure.

1657 GMT (12:57 p.m. EDT)

T+plus 35 seconds. Vehicle is now traveling faster than the speed of sound.

1657 GMT (12:57 p.m. EDT)

The Boeing Delta 2 rocket has cleared the tower at Vandenberg Air Force Base's Space Launch Complex-2 West as the main engine and six ground-lit solid boosters propel the vehicle into the central California sky.

1657:24 GMT (12:57:24 p.m. EDT)

LIFTOFF! Liftoff of NASA's Gravity Probe-B spacecraft -- a machine to test Albert Einstein's theories of space and time.

1656:54 GMT (12:56:54 p.m. EDT)

T-minus 30 seconds. Hydraulics and electronics status checks are reported "go."

The launch ignition sequence will begin at T-minus 2 seconds when a Boeing engineer triggers the engine start switch. The process begins with ignition of the two vernier engines and first stage main engine start. The six ground-lit solid rocket motors then light at T-0 for liftoff.

1656:24 GMT (12:56:24 p.m. EDT)

T-minus 1 minute. The Range has given its final clear-to-launch.

The Delta 2 rocket's second stage hydraulic pump has gone to internal power after its pressures were verified acceptable.

1656 GMT (12:56 p.m. EDT)

T-minus 80 seconds. LOX topping to 100 percent is underway.

1655:24 GMT (12:55:24 p.m. EDT)

T-minus 2 minutes. The first stage liquid oxygen vents are now being closed so the LOX tank can be pressurized for launch. Puffs of vapor from a relief valve on the rocket will be seen in the remainder of the countdown as the tank pressure stabilizes.

1654:54 GMT (12:54:54 p.m. EDT)

T-minus 2 minutes, 30 seconds. The GP-B spacecraft has been declared "go" for launch.

1654:24 GMT (12:54:24 p.m. EDT)

T-minus 3 minutes and counting.

1653 GMT (12:53 p.m. EDT)

T-minus 3 minutes, 45 seconds and counting. The Delta 2 rocket's systems are now transferring to internal power for launch. And the launch pad water system is being enabled.

1653:24 GMT (12:53:24 p.m. EDT)

T-minus 4 minutes and counting. The final phase of the countdown is now underway for the launch of Delta 304 and the Gravity Probe-B satellite. Liftoff is set for 1657:24 from Vandenberg Air Force Base, California.

1651:24 GMT (12:51:24 p.m. EDT)

Countdown will resume in two minutes.

1649 GMT (12:49 p.m. EDT)

The launch team has confirmed there are no technical problems during the just-completed poll by the Boeing launch conductor. The team is now receiving final instructions on countdown procedures.

1648 GMT (12:48 p.m. EDT)

Now half-way through this built-in hold. The launch team readiness poll is beginning.

1647 GMT (12:47 p.m. EDT)

The flight profile based on the latest weather balloon wind data is being loaded aboard the Delta 2 rocket and verified.

1647 GMT (12:47 p.m. EDT)

NASA launch manager Chuck Dovale says all factors are "go" for liftoff across the board, including upper level winds!

1644 GMT (12:44 p.m. EDT)

Gravity Probe-B is now confirmed to be on internal battery power for flight.

1643:24 GMT (12:43:24 p.m. EDT)

T-minus 4 minutes and holding. Clocks have entered the final scheduled hold point for today's countdown. This is a planned 10-minute hold. During this time, officials will perform a series of readiness checks to ensure all systems are "go" for launch.

1638 GMT (12:38 p.m. EDT)

The first stage propellant tank is being pressurized for launch.

1637:24 GMT (12:37:24 p.m. EDT)

Launch is now 20 minutes away. Upper level winds continue to be examined.

1633 GMT (12:33 p.m. EDT)

Inhibited checks are now beginning 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.

1631 GMT (12:31 p.m. EDT)

The Gravity Probe-B spacecraft is switching to internal battery power for launch.

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

A reminder -- should launch not occur today for some reason an attempt is not possible Wednesday because the Gravity Probe-B spacecraft must be serviced. Additional tries would be available on Thursday and Friday at 1649 and 1645 GMT, respectively.

1627:24 GMT (12:27:24 p.m. EDT)

T-minus 20 minutes and counting. Clocks are ticking again after the planned 20-minute hold. The countdown will proceed to T-minus 4 minutes where the final hold is scheduled.

1623 GMT (12:23 p.m. EDT)

Readiness polls of the launch team have been performed to verify everyone is prepared to continue the countdown. Clocks will resume counting at 1627 GMT as scheduled.

1622 GMT (12:22 p.m. EDT)

The trickle charge of Gravity Probe-B's batteries is ending.

1621 GMT (12:21 p.m. EDT)

NASA launch manager Chuck Dovale says upper level winds are presently "go" for launch. But the winds in the maximum dynamic pressure area of flight are trending upward and will be watched closely as launch time nears.

There are no technical problems with the Delta 2 rocket, GP-B spacecraft or Range at this time. Today's instantaneous launch opportunity is 1657:24 GMT.

1617 GMT (12:17 p.m. EDT)

Now half-way through this built-in hold at T-minus 20 minutes.

Once the countdown resumes, clocks will tick down to the T-minus 4 minute mark where a 10-minute hold is planned.

1610 GMT (12:10 p.m. EDT)

The launch weather officer says the low-level stratus clouds will remain over the pad through liftoff time. Winds are from the northwest at 8 to 12 knots. The temperature is in the upper 50s F.

Meanwhile, data link tests between the Range and rocket are beginning.

1607:24 GMT (12:07:24 p.m. EDT)

T-minus 20 minutes and holding. The countdown has entered the 20-minute built-in hold. Liftoff is still slated for 1657:24 GMT (12:57:24 p.m. EDT; 9:57:24 a.m. local time).

The latest weather balloon has revealed acceptable conditions in the transonic region of flight that had been troublesome. However, winds in the maximum dynamic pressure area that occurs 20 seconds later in launch is trending unfavorably, NASA reports.

1604 GMT (12:04 p.m. EDT)

The first stage steering checks are complete.

1602 GMT (12:02 p.m. EDT)

T-minus 25 minutes and counting. The countdown is nearing a planned 20-minute built-in hold at the T-minus 20 minute mark. Another hold is scheduled at T-minus 4 minutes.

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

Second stage engine slews are complete. First stage tests have started.

1557 GMT (11:57 a.m. EDT)

The launch team is beginning 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.

1547 GMT (11:47 a.m. EDT)

There are no technical issues being addressed on the Delta 2 rocket or Gravity Probe-B spacecraft with 70 minutes remaining until liftoff time today.

However, upper level winds are a concern.

A series of weather balloons are being launched to gather data on winds aloft, giving engineers the information they need to generate a flight profile for the Boeing Delta 2 rocket to ascend through the atmosphere.

1546 GMT (11:46 a.m. EDT)

Pressurization of the second stage helium and nitrogen systems and fuel tanks has been completed.

1538 GMT (11:38 a.m. EDT)

LOX loading complete! Filling of the Delta 2 rocket's first stage liquid oxygen tank was completed at 1537:53 GMT. The operation took 24 minutes and 5 seconds today. The tank will be replenished through the countdown to replace the super-cold liquid oxygen that naturally boils away.

The rocket is now fully fueled for launch. The vehicle's first stage was successfully loaded with RP-1 kerosene fuel along with the liquid oxygen over the past hour. The second stage was filled with its storable nitrogen tetroxide and Aerozine 50 fuels last Friday. The nine strap-on booster rockets are solid-propellant.

1528 GMT (11:28 a.m. EDT)

Now 15 minutes into liquid oxygen loading. Once the first stage tank is 95 percent full, the "rapid load" valve will be closed and the slower "fine load" phase will continue to fill the rocket.

1527 GMT (11:27 a.m. EDT)

The countdown is entering the final 90 minutes to launch from Space Launch Complex-2 West at Vandenberg Air Force Base in California. The site is on the Pacific Coast, about 140 miles northwest of Los Angeles.

1523 GMT (11:23 a.m. EDT)

A plume of white vapor is streaming from the vent port of the first stage as the cryogenic tanking passes the 10-minute mark. And the normal layer of ice is beginning to form on the rocket's skin. No problems have been voiced by the launch team during fueling this morning.

1518 GMT (11:18 a.m. EDT)

The first stage liquid oxygen loading operation has been underway for five minutes. It will take approximately 25 minutes to fill the rocket's tank.

Meanwhile, the Rafety Safety beacon checks have been completed.

1513 GMT (11:13 a.m. EDT)

Chilled to Minus-298 degrees F, super-cold liquid oxygen is being pumped into the first stage of the Boeing Delta 2 rocket as the countdown continues for today's 1657 GMT launch.

The liquid oxygen is flowing from a storage tank at the launch pad, through plumbing and into the bottom of the rocket. The LOX and the RP-1 kerosene fuel -- loaded aboard the vehicle less than an hour ago -- will be consumed by the first stage main engine.

1509 GMT (11:09 a.m. EDT)

The launch team members have begun preparations to load the rocket's first stage liquid oxygen tank.

1508 GMT (11:08 a.m. EDT)

Boeing officials have confirmed their approval for liquid oxygen tanking.

1501 GMT (11:01 a.m. EDT)

NASA launch manager Chuck Dovale has polled his management team and the space agency is "go" for loading liquid oxygen into the first stage starting at approximately 1512 GMT. The only significant concern at this time is the upper level wind. The winds are currently five percent out of limits for the rocket's ability to control itself during the transonic region of flight.

1458 GMT (10:58 a.m. EDT)

Range Safety reports that modeling shows the air-lit solid rocket boosters will impact about one mile outside of the intended zone. This is being caused by upper level winds. Officials are working to ensure this is not a problem.

1456 GMT (10:56 a.m. EDT)

The launch weather officer Lt. Breea Lemm has given the management team a briefing in advance of loading liquid oxygen into the Delta 2 rocket's first stage.

Unlike yesterday's spectacularly clear, beautiful weather at Vandenberg, there is a low overcast hanging above the Central California launch site this morning.

All weather rules are currently "go" and there is a 100 percent chance of acceptable conditions today.

1447 GMT (10:47 a.m. EDT)

Yesterday's launch was affected by upper level winds. The latest weather balloon this morning shows the high-altitude winds are presently "no go." Additional balloons will be released as the countdown continues to monitor the situation.

1439 GMT (10:39 a.m. EDT)

Fuel loading complete! The Boeing Delta 2 rocket's first stage fuel tank has been filled for today's launch of NASA's Gravity Probe-B. The tank was loaded with a highly refined kerosene, called RP-1, during a 17-minute, 57-second process that concluded at 1439:34 GMT.

Up next in this morning's countdown will be loading super-cold cryogenic liquid oxygen into the first stage.

1434 GMT (10:34 a.m. EDT)

The launch team has computed that the full load for the first stage fuel tank is 9,938 gallons. Once the tank is filled to 98 percent, the "rapid load" valve will be closed and the slower "fine load" phase will continue top off the tank.

More than 7,000 gallons are already aboard.

1428 GMT (10:28 a.m. EDT)

About 4,000 gallons of kerosene propellant have been loaded into the rocket in the first seven minutes of fueling.

The fuel will be used with liquid oxygen by the first stage Rocketdyne RS-27A main engine and twin vernier steering thrusters during the initial four-and-a-half minutes of flight. The liquid oxygen will be pumped into the rocket later in the countdown.

1421 GMT (10:21 a.m. EDT)

Fueling of the Delta 2 rocket's first stage has begun for today's launch. About 10,000 gallons of a highly refined kerosene propellant, called RP-1, are being pumped into the rocket from a 15,000-gallon storage tank at the Space Launch Complex-2 West pad.

1418 GMT (10:18 a.m. EDT)

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

1417 GMT (10:17 a.m. EDT)

The first stage helium and nitrogen system pressurization has been completed. And officials have given approval for first stage fueling.

1357:24 GMT (9:57:24 a.m. EDT)

Terminal Countdown begins! The count has entered the final three hours to launch of Boeing's Delta 2 rocket with NASA's Gravity Probe-B spacecraft.

The launch team will spend the Terminal Count prepping the rocket, payload and ground support systems for the planned 1657:24 GMT liftoff from the SLC-2W pad at Vandenberg Air Force Base, California.

The countdown currently stands at T-minus 150 minutes. However, there are a pair of holds -- totaling 30 minutes in duration -- planned at T-minus 20 minutes and T-minus 4 minutes.

With the countdown underway, the activities planned over the next hour include verifying the hazard danger area is cleared, activating the rocket's Redundant Inertial Flight Control Assembly guidance computer, pressurizing the first and second stage helium and nitrogen systems and second stage fuel tanks and checking the C-band tracking beacon on the rocket.

The loading of RP-1 kerosene fuel into the rocket's first stage will begin in about 20 minutes. This operation will be followed by loading of super-cold liquid oxygen in about 75 minutes.

1349 GMT (9:49 a.m. EDT)

The team has been polled for a "ready" status to pick up the countdown. There were no concerns voiced. Terminal Count will begin as scheduled.

1342 GMT (9:42 a.m. EDT)

"Man stations for Terminal Count." The launch team members have been instructed to take their positions at consoles. A readiness poll will be performed in a few minutes to verify everyone is prepared to resume the countdown at the end of this scheduled hold.

Launch remains set for 1657 GMT today.

1315 GMT (9:15 a.m. EDT)

The second launch countdown for Gravity Probe-B is underway at Vandenberg Air Force Base this morning. The mobile service tower was been retracted again and crews are progressing through their checklists.

The Terminal Count is slated to commence at 1357 GMT. Clocks are currently pausing at the T-minus 150 minute mark for a planned 60-minute hold.

0115 GMT (9:15 p.m. EDT Mon.)

NASA's Gravity Probe-B spacecraft that will test Einstein's theories of the universe gets another shot at launch Tuesday aboard a Boeing Delta 2 rocket from Vandenberg Air Force Base, California.

Monday's countdown ended in a cliffhanger three minutes before liftoff time when officials ruled there wasn't sufficient time to ensure the correct high-altitude wind data had been loaded into the rocket's guidance computer.

Engineers were racing to develop a flight profile that the Delta could safely follow as it soared through the atmosphere based on weather balloon information. But with just one-second available to perform the launch each day, the clock expired before the work could be completed.

The rocket and satellite were safed, the mobile service tower returned to its position enclosing the vehicle and the countdown clock reset for another try.

The instantaneous launch time for Tuesday is 1657:24 GMT (12:57:24 p.m. EDT; 9:57:24 a.m. local time). The one-second launch opportunity is required to place Gravity Probe-B into the optimum orbit for its science mission, allowing the craft to focus on a star 300 light years away during the relativity experiment.

"To do the measurement with Gravity Probe-B depends upon being able to have an inertial reference throughout the entire mission," explained Rex Geveden, Gravity Probe-B program manager.

"That inertial reference in our case is a guide star called HR 8703. Essentially what you have to do on launch day is you have to launch the spacecraft so it is in-plane with the guide star. If we get in-plane with the guide star, then we can get perpendicular separation of the two relativity effects we are trying to measure.

"So basically you have to get into orbit when the star is directly overhead and that happens once a day, and that puts you into a very short launch window."

Although it may seem confusing, the upper level winds are not a weather constraint. The conditions aloft are judged as a rocket issue -- whether the vehicle can successfully withstand the winds without being pushed off course or being damaged.

For its part, the Air Force weather team is focused on criteria such as clouds, rain, lightning and surface winds. There is a 90 percent chance that the weather will cooperate on Tuesday, with ground winds at tower rollback time the chief concern.

Launch weather officer Lt. Breea Lemm explains the forecast for Tuesday:

"A strong ridge building to the west will significantly increase temperatures at all levels of the atmosphere. As a result, the trailing low-level moisture associated with a trough sliding down the backside of this ridge will be mostly stratus. A very slim chance of precipitation with this trailing moisture, but any precipitation will be extremely light and fall well before T-0, not posing an impact to launch. The strong Pacific high is still developing off the coast, SLC-2W remaining under the influence of northwest winds at launch."

The launch time prediction calls for stratus clouds at 1,500 feet with 7/8ths sky coverage and tops at 5,000 feet, cirrus clouds 30,000 feet with 3/8ths sky coverage and tops at 32,000 feet, visibility of 7 miles, northerly winds from 320 to 360 degrees at 12 to 18 knots and a temperature between 55 and 60 degrees F.

Upper level winds on launch day are forecast to be from the west-northwest, reaching a maximum of 65 knots between 35,000 and 40,000 feet.

Should launch not occur Tuesday, an attempt is not possible Wednesday because the Gravity Probe-B spacecraft must be serviced. Additional tries would be available on Thursday and Friday at 1649 and 1645 GMT, respectively.

Read our earlier status center coverage.

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