Liftoff from Cape Canaveral's pad 17B is retargeted for Friday morning at 8 a.m. EDT.

"Following Wednesday morning's weather launch scrub, teams detected a small leak in a flange in the fuel transfer system under the launch pad," a NASA spokesman says.

"Teams plan to assess the fuel leak and determine what steps to take to support the next launch attempt."

NASA says the Delta 2 rocket and the STSS Demo spacecraft remain in good condition with no technical issues.

The official forecast now puts the odds of acceptable weather on Thursday at 60 percent. Clouds and showers from the Atlantic will be the concerns once again.

The launch window will extend from 8:00 to 9:00 a.m. EDT. We'll provide complete live coverage throughout the countdown. And the streaming video plus the interactive chat will resume at 6 a.m. EDT and continue throughout the ascent.

Additional coverage for subscribers:
VIDEO: NARRATED REVIEW OF STSS DEMO PREPS PLAY | HI-DEF
VIDEO: NARRATED REVIEW OF DELTA 2'S PREPARATIONS PLAY | HI-DEF
VIDEO: PAD TOWER ROLLS BACK AROUND DELTA 2 AFTER SCRUB PLAY
SUBSCRIBE NOW

The launch time outlook for tomorrow includes scattered cumulus clouds at 3,000 feet, isolated coastal showers, good visibility, easterly winds of 10 knots and a temperature around 76-78 degrees F.

The rocket now stands fully fueled for liftoff. The vehicle's first stage was successfully loaded with RP-1 kerosene fuel along with the liquid oxygen this morning. The second stage was filled with its storable nitrogen tetroxide and Aerozine 50 fuels on Sunday. The nine strap-on booster rockets use solid propellants.

The anvil and cumulus clouds around remain "no go" for launch. But forecasters say those conditions should clear in the next hour.

The countdown clocks are going to be reset for the revised time.

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.

Once the tank is filled to the 98 percent level, or 9,700 gallons, the "rapid load" valve will be closed and the slower "fine load" phase will continue to top off the tank.

First stage propellant loading has passed the 3,000-gallon mark. This process to load the kerosene fuel takes about 20 minutes.

And the launch team is starting the steps to pressurize the first and second stage helium and nitrogen systems and the second stage fuel and oxidizer tanks.

The next few hours will be spent fueling the rocket, activating systems and performing final testing before liftoff at 8:00 a.m. EDT 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 later hold -- at the T-minus 4 minute point -- will give the launch team some time to catch up on any work running behind and verify all is in readiness for liftoff. The hold will last 10 minutes in duration.

Current observations have cleared the earlier lightning and rain issues. But the cumulus and anvil cloud rules are "no go" at present.

The launch time outlook includes scattered cumulus clouds at 3,000 feet, showers in the area, good visibility, southeasterly winds of 10 gusting to 15 knots and a temperature around 76-78 degrees F.

The Terminal Countdown commences at 5:20 a.m. EDT, followed shortly thereafter by loading kerosene fuel and then super-cold liquid oxygen into the first stage. Liftoff remains scheduled for 8:00 a.m. EDT.

Our live play-by-play call will begin with the Terminal Count. And then live streaming video with an interactive chat starts at 6 a.m. EDT on the right-hand side of this page.

Work has been underway this evening at Complex 17, as ground crews retracted access platforms and stowed gear in the tower in advance of the structure wheeling away from the rocket.

The STSS spacecraft launching in the morning were delivered to the pad on August 27 and hoisted into the cleanroom and bolted to its Delta for the ride to orbit. The rocket's two-piece nose cone to shroud the satellite during ascent through the atmosphere was installed around the craft a couple of weeks later.

A few more access platforms still have to be folded up, then the tower will be free to roll back later tonight.

A reminder that if you will be away from your computer but would like to receive occasional countdown updates, sign up for our Twitter feed to get text message updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

The two-stage rocket is aiming for a 730-nautical-mile circular orbit inclined 58 degrees to the either side of the equator where it will deploy a pair of demonstration satellites for the Space Tracking and Surveillance System, a $1.5 billion project known as STSS Demo.

The satellites will be used by the U.S. Missile Defense Agency to test the "birth to death" tracking of missiles from launch to re-entry. NASA is lending its expertise to the oversee the Delta rocket flight and getting the satellites into space on behalf of the MDA.

Liftoff from pad 17B is targeted for 8 a.m. EDT (1200 GMT) at the opening of an hour-long window. Officials gave approval to proceed with the countdown at their Launch Readiness Review held Monday following several days of delays.

Ground crews loaded the Delta 2 rocket's second stage with a storable hydrazine propellant mixture and nitrogen tetroxide oxidizer on Sunday. The fuels will power the stage's main engine during the two firings needed to place the satellites into the planned orbit around the planet.

The pad gantry should be retracted to reveal the 12-story rocket just before midnight Tuesday. The overnight hours will be spent finishing preps ahead of starting the Terminal Countdown at 5:20 a.m. EDT launch morning.

The main question mark going into the launch attempt is the weather. Air Force meteorologists say there's a 50-50 chance clouds and coastal rain showers will interfere with the schedule.

"A tropical wave bearing the remnants of Hurricane Fred is currently over the Bahamas. The timing of this feature over the peninsula is the key forecast issue. Unfortunately, the wave has slowed its westward motion and is now forecast over our area Tuesday through Wednesday afternoon, increasing rain probability during the launch window," forecasters say.

"The main weather concern on launch day will be cumulus clouds and precipitation associated with this system.

"In the event of a launch delay, we will see gradually improving conditions as the tropical wave continues to move westward, away from the launch area."

The odds of acceptable weather on Thursday and Friday increase to 60 and 70 percent, respectively.

The STSS Demo mission has been in the works for years. Northrop Grumman built the spacecraft and was responsible for overall satellite integration, plus the ground control system. The satellites' sensors were made by Raytheon.

"The STSS Demo program represents an investment of approximately $1.5 billion in cutting edge technologies, spacecraft assembly and test, and spacecraft operations center capability," the MDA told Spaceflight Now.

The two craft, weighing nearly 5,000 pounds at launch and each having pairs of power-generating solar wings, are equipped with infrared and visible sensors to spot missile launches, track the vehicles through space and observe the entry back into the atmosphere. By working together from separate vantage points in low-Earth orbit, their imagery will combine to provide 3-D tracking of objects.

"The STSS satellites will demonstrate the unique value of a space-based sensor for the Ballistic Missile Defense System. Space-based systems have inherently long range, are not limited by geographic constraints, and collect infrared tracking and discrimination data that complements conventional ground and sea-based radars," the MDA says.

The U.S. government has long considered fielding constellations of missile-tracking satellites like STSS. Whether such a system is constructed could hinge on how these two demonstration craft perform.

For decades, the Defense Support Program spacecraft have flown in geosynchronous orbits to warn of enemy missile launches. Those satellites use infrared telescopes to spot the superhot plumes of boosters during initial ascent. But STSS is designed to identify and follow missiles throughout all phases of flight.

"STSS will demonstrate the key functions of a space-based sensor, passing missile tracking data to missile defense interceptors with the accuracy and timeliness necessary to enable them to successfully intercept missile targets," says Northrop Grumman.

"STSS is the U.S. Missile Defense Agency's only space-based midcourse tracking system. Using onboard sensors capable of detecting infrared and visible light, STSS will become part of land-, sea-, air- and space-based Ballistic Missile Defense System sensors."

The MDA has no funding or firm plans for developing any satellites beyond the two demonstration craft launching Wednesday. The real-life results from space using MDA's own missile test-launches, including two specifically configured for STSS viewing, could give the political boost needed for Congress to fund a future operational program.

"The greatest hedge against missile defense threats of all ranges remains a highly available early missile tracking capability from space. Decisions on any follow-on programs will be made based on the performance of the STSS satellites," MDA told Spaceflight Now.

The STSS Demo mission is expected to last two to four years.

These two craft follow a quasi-classified research and development testbed satellite launched in May aboard a Delta 2 rocket from Vandenberg Air Force Base in California. Valued at $400 million, that mission is known as the Advanced Technology Risk Reduction, or STSS-ATRR.

An earlier research craft called the Near Field Infrared Experiment, or NFIRE, was launched for the MDA in 2007 aboard a Minotaur rocket from Wallops Island, Virginia. It tested sensor technology to provide high resolution imagery of missiles and their plumes.

The Missile Defense Space Experimentation Center at Schriever Air Force Base in Colorado Springs is the control site for the four satellites under the MDA umbrella.

"Collections by all four satellites provide a unique opportunity to validate technology and sensor performance from different platforms," the MDA says.

Watch this page for complete live coverage of Wednesday morning's countdown and launch, including play-by-play updates and streaming video.