Topped with the heftiest commercial communications satellite to ever fly from Cape Canaveral, a tailor-made Lockheed Martin Atlas 5 rocket like none before it launches Thursday, bound for an exceptionally high orbit around Earth.

The first-of-its-kind rocket will be the most powerful Atlas in history, generating 38 million horsepower when it blasts off from Complex 41 at 4:42 p.m. EST (2142 GMT). The day's window in which the launch can occur extends 15 minutes to 4:57 p.m. EST (2157 GMT).

Producing the blazing thrust will be the Russian-designed RD-180 main engine making its 11th flight and a trio of Aerojet-built strap-on solid rocket boosters. The solid-propellant motors have flown in pairs twice before on Atlas 5, but this launch marks the first time three have been mounted to the first stage to give the added kick at liftoff.

Given the weight of the satellite cargo bolted atop the rocket, that extra boost is needed. At 13,100 pounds, the Inmarsat 4-F1 mobile communications spacecraft is a couple thousand pounds heavier than any commercial broadcasting satellite to launch from the Cape.

This particular version of the next-generation rocket, called an Atlas 5-431 vehicle in Lockheed Martin's naming matrix, has never flown before. The 431 designation illustrates a four-meter diameter nose cone shrouding the payload, three solid rocket boosters and a single engine on the Centaur upper stage.

Two of the previous four Atlas 5 missions have flown in the 400-series configuration with the four-meter payload fairing, but both lacked any solids. Thus, they were known as 401-model vehicles. The other two earlier Atlas 5's were 521-version rockets with the voluminous five-meter fairings, two solid motors and single-engine Centaurs.

In addition to the three solid rocket boosters, the other noticeable difference for Thursday's 400-series rocket compared to the previous two is the vehicle's taller height. The rocket's nose cone has been stretched three feet in length to accommodate the Inmarsat spacecraft, giving this Atlas 5 an overall height of 194 feet.

Lockheed Martin designed the Atlas 5 family to incorporate various configurations, allowing each to rocket and its specific payload to be paired together with just the right amount of power. An Atlas 5-400 series vehicle can support as many as three strap-on solids and the 500 family up to five. Several different nose cone sizes and lengths are available for picking, too. The Extra Extended Payload Fairing is the one selected for this launch.

Today, the rocket and Inmarsat 4-F1 stand fully assembled inside Lockheed Martin's Vertical Integration Facility building at Complex 41. The payload was mounted atop the Centaur upper stage last Tuesday. A mobile launching platform will wheel the vehicle to the launch pad Thursday morning, emerging from the 30-story VIF just after 6:30 a.m. EST (1130 GMT) for the 1,800-foot, half-hour trek.

Once positioned on the pad, crews will power up the rocket, pump a highly-refined kerosene propellant into the first stage and carry out a prescribed test schedule on various equipment aboard the Atlas 5.

Around 2:30 p.m. EST (1930 GMT), the loading of super-cold liquid oxygen into the Centaur upper stage begins. That will be followed by filling the first stage with liquid oxygen and Centaur with cryogenic liquid hydrogen fuel.

The countdown enters a planned 25-minute hold at the T-minus 4 minute mark at 4:13 p.m. EST (2113 GMT). The pause permits the launch team to catch up on late work and address any problems. The hold period has been extended by 15 minutes from previous missions given the short launch window and little margin for deal with issues late in the countdown.

Veteran launch conductor Ed Christiansen, seated in the Atlas Spaceflight Operations Center four miles from the pad, will poll the various console operators to ensure all systems are functioning properly. He passes the 'go' status to launch director Adrian Laffitte located above and behind the launch team in the management room. Laffitte will have the final say, giving the ultimate approval to resume the countdown.

During the computer sequencer-controlled last four minutes to blastoff, the fuel systems will be secured, the fuel tanks inside the rocket pressurized, onboard systems switched from ground-fed power to internal batteries and the final arming performed. All of the activities culminate with ignition of the main engine in the count's last three seconds.

The RD-180 surges to life, building up thrust as the powerplant's vital signs are checked before the three solid rocket boosters are commanded to light. Explosive bolts then blow, releasing the one-million-pound rocket from three posts on the mobile launching platform upon which the vehicle had rested.

The white, 67-foot long solids burn for about 90 seconds, expending their fuel to propel the vehicle skyward with the combined power from the main engine. Two of the spent SRB casings will be jettisoned at T+plus 2 minutes, 10 seconds, followed one-and-a-half seconds later by the third booster.

The RD-180 continues to fire on the mixture of kerosene and liquid oxygen until its fuel supply is exhausted four-and-a-half minutes into flight. The first stage then drops away, leaving the cryogenic Centaur upper stage to ignite its Pratt & Whitney RL10 engine at T+plus 4 minutes, 43 seconds. Less than 10 seconds later, the rocket's nose cone that protected the Inmarsat craft during atmospheric ascent is no longer needed and separates.

Centaur burns its engine for about 10 minutes, reaching an elliptical parking orbit at T+plus 14 minutes, 24 seconds with a high point of 1,305 statute miles, low point of 115 miles and inclination of 27.39 degrees to the equator.

The vehicle coasts in this orbit for nearly 10 minutes before the upper stage re-ignites the RL10 for a second firing to substantially increase the orbital altitude. The burn lasts five-and-a-half minutes, concluding at T+plus 29 minutes, 28 seconds.

Centaur reorients itself and at T+plus 32 minutes, 17 seconds deploys the satellite cargo into super-synchronous transfer orbit with a high point of 56,232 statute miles, low point of 273 miles and inclination of 20.83 degrees.

This high point, or apogee, is much higher than the typical 22,000-mile altitude for a geosynchronous transfer orbit. Performance from the Atlas 5 allows the added height, which is designed to aid Inmarsat's onboard maneuvers to reach a circular geostationary orbit and reduce inclination to the equator where it can match Earth's rotation and appear parked above the Indian Ocean at 65 degrees East longitude.

This spacecraft is the first of Inmarsat's newest generation of satellites that come with the tag line "broadband for a mobile planet." Built in Europe by EADS Astrium, the Inmarsat 4-series spacecraft will provide office-like services such as Internet, email and other broadband capacity to laptop and palm-sized terminals on land, at sea or in the air.

The sophisticated satellites transmit over 200 spot beams to deliver strong signals to the small receivers. Each craft has a giant unfurlable antenna reflector stretching 30 by 39 feet.

The satellite going up Thursday, called Inmarsat 4-F1, will cover Europe, Africa, the Middle East and Asia, as well as the Indian Ocean. A sister satellite -- Inmarsat 4-F2 -- is supposed to fly later this year atop a Sea Launch rocket for South America, most of North America, the Atlantic Ocean and part of the Pacific Ocean. A third craft is under construction.

In addition to the new broadband features, the new spacecraft will offer continuity of existing Inmarsat services for maritime, air and emergency services, officials said, and enhance navigation signals for air traffic uses.

"I applaud the industrial performance of EADS Astrium who met the challenge of designing and building such a complex and flexible spacecraft featuring the most advanced technologies. The extraordinary I-4 fleet of satellites will allow Inmarsat to provide mobile users with high speed Internet connectivity virtually anywhere in the world," said Andrew Sukawaty, CEO of Inmarsat.

"Inmarsat 4 is certainly one of the most sophisticated communications satellites ever built, and tangible proof of our ability to build advanced and flexible communications payloads. We are proud of this achievement, and thank Inmarsat for the confidence they had in EADS Astrium on this innovative and ambitious program," added Antoine Bouvier, CEO of EADS Astrium.