As the Baikonur Cosmodrome reached its 45th anniversary on Friday, processing crews at the Russian space center in Kazakhstan rolled out a Proton rocket set for launch on Tuesday, June 6.

This is not simply another Proton launch. This rocket will be the first Proton powered by modified engines, known as Phase 2, on its second and third stages.

The Gorizont 45 communications satellite will be the first payload to ride a Proton outfitted with Phase 2 engines. At the same time, the launch will mark the end of an era for the Gorizont- (Horizon-) series spacecraft, which is being replaced by more powerful and sophisticated communications satellites.

The 4,850-pound (2,200-kilogram) Gorizont 45 features eight transponders: one Ku-band, one L-band and six C-band. In comparison, Russia's newest Express A communications satellite carries 17 transponders.

Since the end of the 1970s, the NPO PM design and production center in the city of Zheleznogorsk built 35 Gorizont spacecraft. Twenty-nine were successfully launched and deployed in orbit. Three spacecraft were lost in launch mishaps and one failed due to a loss of pressure in its cooling system.

Along with modified engines, the June 6 Proton launch will also test the Breeze M upper stage, which promises to improve the performance of the Proton in future high-orbit launches. The Breeze M previously was launched on a Proton in July 1999, but the lower-stage booster exploded before the new upper stage had a chance to fire its engines.

Breeze M features an exotic design, allowing overall improvement in the Proton rocket performance without a considerable change to the launcher's dimensions. The Salyut Design Bureau in Moscow -- the development arm of the Khrunichev enterprise -- essentially "recycled" its smaller Breeze K upper stage by changing it to a thyroid-shaped external tank. The result is the older upper stage, which normally carries around 12,345 pounds (5,600 kilograms) of propellant, can store about an additional 15 tons of propellant in the external tank. (The Breeze K so far has completed four successful launches as part of the Rockot booster.)

Unlike the space shuttle, where the propellant from the external tank flows directly to the main engines, Breeze M's external tank just replenishes the propellant reservoirs inside its core stage. "By the time Breeze M sheds its external tank, its internal tanks are filled up to the cork," Vladimir Karrask, deputy designer general at the Salyut bureau, told SPACE.com.

Depending on the mission, Breeze M will be able to re-ignite its engines two to four times, Karrask said. The new upper stage will allow the Proton booster to deliver 3 to 3.3 tons of payload into the geostationary orbit used by most commercial satellites. Currently, Proton can launch about 2.5 tons of payload into that orbit.

In such an orbit, 22,300 miles (35,885 kilometers) above the equator, the spacecraft makes one revolution around the planet in 24 hours. This allows it to "hang" over a single point on Earth's surface, providing a stable communications link for users on the ground.

Proton's rollout on Friday coincides with the 45th anniversary of Baikonur, Russia's primary launch facility. To commemorate the occasion, the chief-commander of Russia's Strategic Missile Forces (RVSN), General Colonel Vladimir Yakovlev and his deputy for space systems Lt. General Valeri Grin will meet with veterans of the launch center, an RVSN spokesman told SPACE.com.

Baikonur was founded as a test site for the first Soviet intercontinental ballistic missile -- the R 7. The missile's chief designer, Sergei Korolev, convinced the Soviet leadership at the time to use the R 7 for space exploration. On October 4, 1957, an R 7-derived launch vehicle lifted off from Baikonur and delivered Sputnik, the world's first artificial satellite, into orbit.