CAPE CANAVERAL -- Twin inspector spacecraft and a microsatellite testbed will share a Delta 4 rocket ride into space Wednesday evening from Cape Canaveral.

Credit: ULA video

The 205-foot-tall United Launch Alliance booster rocket is scheduled for liftoff at 7:03 p.m. EDT (2303 GMT) from Complex 37.

The countdown begins with final preparations to retract the 33-story mobile service gantry away from the Delta 4 rocket. Once the tower is pulled back around 11 a.m. EDT, pad configuration steps will be completed before the site is cleared of all workers.

Fueling, which begins in the early afternoon, is a multi-hour process will load the cryogenic liquid hydrogen and liquid oxygen tanks on both stages of the rocket.

Once clocks enter the last phase of the countdown at T-minus 4 minutes, the vehicle will be armed, pressurized and placed on internal battery power.

In the final 15 seconds of the count, the radial outward-firing ignitors, or ROFIs, will be triggered beneath the main engine to burn away excess hydrogen and the terminal countdown sequencer rack, or TCSR, will assume control of the last 10 seconds.

The main engine will begin its ignition sequence start at T-minus 5 seconds. Flowing hydrogen through the engine for cooling, the fuel will strike the ROFIs and create the trademark Delta 4 fireball.

Photo credit: Walter Scriptunas II / Scriptunas Images

Once the engine is up and running at full thrust, it passes a pre-planned health check and the solid strap-on boosters are commanded to light at T-0 for liftoff on 1.2 million pounds of thrust.

The three swing arms will pull away as the rocket ascends from the launch pad, then executes pitch and yaw maneuvers to obtain the proper heading and minimize aerodynamic stresses on the vehicle.

Delta pushes through Mach 1 in 47 seconds and the region of maximum air pressure at 59 seconds as the RS-68 main engine consumes liquid hydrogen and supercold liquid oxygen. The twin solid motors burn out at 94 seconds and jettison at 100 seconds.

Approaching main engine cutoff four minutes into flight, the vehicle is burning propellants at a rate of 1,000 pounds per second. The first stage propels the Delta to 11,000 mph over 80 miles in altitude and 200 miles downrange of the launch pad before the Common Booster Core stage separates, the upper stage lights and the payload shroud jettisons.

The rocket then goes into a news blackout.

But the final destination: geosynchronous orbit 22,300 miles above the planet.

It will take several hours and work by the rocket's cryogenic upper stage to reach the high ground to deploy the two Geosynchronous Space Situational Awareness Program, or GSSAP, satellites and release ANGELS, a microsatellite riding a ring-like adapter below the primary passengers.

Together, the two different projects make up the Air Force Space Command mission No. 4, a once-classified flight until GSSAP was unveiled to the public by AFSPC commander William Shelton in February.

"The GSSAP will uniquely contribute to timely and accurate orbital predictions, enhancing our knowledge of the geosynchronous orbit environment and further enabling spaceflight safety to include satellite collision avoidance. ANGELS, a microsatellite, is designed to achieve a high level of safety and experimental flexibility," said Jim Sponnick, ULA's vice president for Atlas and Delta programs.

All three satellites were built by Orbital Sciences.

Credit: ULA

GSSAP will fly one satellite just below and its twin just above the geosynchronous satellite belt where spacecraft match the Earth's rotation and appear fixed above a certain spot along the equator on the globe.

"From that unique vantage point they will survey objects in the GEO belt and allow us both to track known objects and debris and to monitor potential threats that may be aimed at this critically important region," said Douglas Loverro, deputy assistant secretary of defense for space policy, at the Department of Defense.

"In short, threats can no longer hide in deep space."

It is where the U.S. military operates a number of critical communications, missile warning and reconnaissance platforms, making it key to tracking man-made objects for U.S. Strategic Command and characterizing threats in that orbit.

"Up in geosynchronous orbit, 22,300 miles up in space, some of our most precious satellites fly in that orbit. The reason they're there obviously is to be able to look down and surveil vast pieces of the Earth and also provide communications support," said Shelton. "It is capability in existential circumstances for the United States. The President will rely on these for information that he needs and to provide communications out to the forces he needs to get to.

"But because our assets in GEO are so valuable, we've been building additional space surveillance capability over that orbital regime in any number of ways," said Shelton.

"The electro-optical payload on GSSAP gives us very close-up neighborhood watch capability that helps prevent surprise, and that protects our assets in GEO."

In 2010, the Air Force launched the Space Based Space Surveillance satellite that flies in low-Earth orbit but can track some geosynchronous objects.

"This satellite brings persistence and timeliness to our GEO coverage that isn't available from ground-based sensors or from ground-based radars or optical sensors or from our low Earth orbiting satellites. We must continue the unique capability of SBSS and build the follow-on program to sustain the capability," said Shelton.

But adding the GSSAP will put inspectors at the geosynchronous belt itself.

Credit: USAF

"This neighborhood watch program will complement our ground based sensors and our SBSS by producing (space situation awareness) data from a unique perspective not achievable by sensors some 22,000 miles away," said Shelton.

"To bring all this data together we are building a new system at the Joint Space Operations Center at Vandenberg Air Force Base in California. And the JSpOC Mission System will give us a much better capability to be proactive versus reactive in Space.

"We are really data rich, but we don't have the system that can ingest disparate data from all kinds of sensors, fuse it together, bring all source intelligence into the picture; bring things like missile defense radars into the space surveillance realm, telescopes from around the world. The JSpOC Mission System will give us that capability to do this and to do it very well in a high performance computing environment."

Shelton added that the Air Force routinely tracks about 23,000 objects on a daily basis. "There are some 500,000 objects in space, so a big traffic management problem and a big threat to fragile spacecraft," he said.

Co-passenger ANGELS is a technology testbed from the Air Force Research Laboratory. Officially called the Automated Navigation and Guidance Experiment for Local Space, the microsatellite will demonstrate autopilot space situational awareness in geosynchronous orbit.

Credit: USAF

ANGELS, will fly above the GPS constellation but use those positioning signals to test maneuverability around the spent Delta 4 upper stage. It's also equipped with a space situational awareness sensor payload "to evaluate techniques for detection, tracking and characterizing of space objects, as well as, attribution of actions in space," according to the AFRL fact sheet.

The launch will be the 368th for a Delta rocket, the 27th Delta 4 and the 12th to fly in the Medium+ (4,2) configuration with a pair of strap-on solid motors. For United Launch Alliance, it is the company's 33rd flight for the Air Force and the 85th launch overall since 2006.

For tips on taking pictures of the launch, see our photography guide.

For details on where the best spots are to see the launch, see the viewing guide.

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About the author

Justin Ray has been a reporter with Spaceflight Now since the website's inception in November 1999. The online news service, based at Cape Canaveral, has documented U.S. and international space news with a specialty of live launch coverage.

Prior to that, Justin worked for two years as an aerospace reporter at the Florida Today newspaper and its pioneering Space Online website. He began his career as an intern at Patrick Air Force Base's public affairs office in 1996 and wrote for the Missileer base newspaper.

The Ohio native has covered 134 Delta rocket launches, 94 Atlas flights, 65 space shuttle missions, 46 Pegasus launches and the entire construction of the International Space Station, plus scientific spacecraft such as the Mars rovers and Cassini.

He attended college at the University of Central Florida and now resides in Viera, Florida.