The Ariane 5 rocket is scheduled to deliver a pair of communications satellites into space during an evening launch tonight from its South American spaceport.

Liftoff of Europe's workhorse rocket from Kourou, French Guiana, is set for the opening of a 37-minute launch window at 2344 GMT (7:44 p.m. EDT). The mission will be the third for Arianespace so far this year.

A key step in launch preparations occurred Monday, when the Ariane 5 rocket was rolled on dual rail tracks from the final assembly building to the ELA-3 launch zone at the Guiana Space Center. Workers connected the 166-foot-tall to the pad late in the day, setting the stage for the final countdown to begin early Tuesday.

The heavy-lifting booster will carry two satellites into an egg-shaped geostationary transfer orbit. The mass of the two craft for the orbital delivery mission totals more than 17,700 pounds.

The launch will mark the 117th flight of the Ariane rocket family since its 1979 debut. It will also be the 33rd launch of the Ariane 5.

The Spaceway 3 spacecraft will fly into space on top of the Ariane 5's dual payload adapter. The smaller BSAT-3a satellite will be housed inside the Sylda adapter during the launch. Both satellites will use on-board propulsion systems to reach a circular geostationary orbit about 22,300 miles above Earth.

Spaceway 3 will begin a 12-year mission serving as a space-based Internet switchboard. The 13,400-pound satellite will be operated by Hughes Network Systems, a U.S.-based satellite broadband provider.

Built by Boeing Satellite Systems, the spacecraft will be the first satellite to route broadband traffic using on-board signal processors. Ka-band antennas aboard the satellite can be dynamically shaped and formed to meet broadband demands from business customers, government users and end consumers throughout the United States, according to Hughes.

Hughes originally awarded the launch contract for Spaceway 3 to Sea Launch, but officials decided to switch the innovative satellite to Arianespace after Sea Launch suffered an on-pad explosion of the Zenit 3SL rocket during an attempted launch in January.

BSAT-3a was manufactured by Lockheed Martin Commercial Space Systems for the Broadcasting Satellite System Corp. of Japan. The satellite carries 12 Ku-band channels for direct television broadcasting to homes and businesses throughout the Japanese island chain.

Weighing 4,365 pounds at launch, BSAT-3a will use on-board propellant to reach its final position in geostationary orbit along the equator at 110 degrees east longitude. The craft is designed for a mission lasting more than 13 years.

Today, the countdown will commence at 1214 GMT (8:14 a.m. EDT). The launch team will conduct a check of electrical systems at 1614 GMT (12:14 p.m. EDT).

Cryogenic liquid hydrogen and liquid oxygen will begin flowing into the Ariane 5 rocket at 1854 GMT (2:54 p.m. EDT), followed by the chilldown procedure to condition the first stage Vulcain 2 main engine at 2024 GMT (4:24 p.m. EDT). A final check of connections between the launcher and telemetry, tracking and command systems is set for 2234 GMT (6:34 p.m. EDT).

Computers will begin controlling the countdown about seven minutes prior to launch. The synchronized launch sequence governs a fast-paced series of automated events, including fuel tank pressurization, transitioning the rocket and payloads to internal power, and taking systems to flight mode.

The Vulcain 2 engine will roar to life when countdown clocks reach zero, followed seven seconds later by ignition of the two solid rocket boosters and liftoff.

The solid-fueled boosters will burn for 2 minutes, 20 seconds before dropping into the Atlantic Ocean. The payload fairing shielding the satellites during the initial phases of launch will be jettisoned 3 minutes, 9 seconds after liftoff. About 9 minutes into the flight, the first stage will exhaust its propellant and separate from the cryogenic upper stage, which will continue the push into orbit.

The upper stage's HM7B engine will fire for nearly 16 minutes to reach the targeted transfer orbit. Deployment of the Spaceway 3 satellite should occur 27 minutes, 38 seconds after launch. BSAT-3a will be released 34 minutes, 10 seconds into the mission.