An artist's concept of the Fregat upper stage firing with "Dumsat" along for the ride. Photo: Starsem

A crucial test launch is planned today for a Russian rocket that will carry a quartet of European satellites into space this summer to study how the solar wind affects Earth.

The venerable Soyuz rocket is poised for liftoff sporting the new Fregat upper stage, which will be put through its paces during a 90-minute dress rehearsal for two upcoming launches carrying the European Space Agency's Cluster 2 satellites.

The mission is scheduled to get underway at 1828 GMT (1:28 p.m. EST) from complex 6 at Baikonur Cosmodrome, Kazakhstan.

The Soyuz-Fregat rocket was rolled from its preparation building to the launch pad on March 16 by rail car. Once at the pad the rocket was hoisted vertically and positioned for launch.

Mounted atop the Fregat is a metallic cylindrical structure, nicknamed "Dumsat". The payload mock-up was built to accurately simulate two Cluster 2 satellites. "Dumsat" also carries a suite of sensors and meters to tell engineers what the launch was really like.

Two Cluster 2 spacecraft will be launched on June 15, the other pair on July 13 in flights that should mimic today's mission.

The "Dumsat" mock-up payload undergoes final pre-launch processing at Baikonur. Photo: Starsem

The Cluster 2 satellites are replacements for those lost aboard the failed inaugural launch of Ariane 5. Scientists will use the spacecraft to probe the interaction between the supersonic stream of charged particles from the sun, called the solar wind, and Earth's magnetosphere. Flare-ups between the two can disrupt communications and power on the planet and harm satellites in space.

Within minutes of liftoff today, the Soyuz's first three stages will each complete their firings and drop away. After the third stage separates from the Fregat, the upper stage's S5.92 engine will ignite to achieve a near-circular 125 by 140-mile parking orbit around Earth.

Once arriving in the parking orbit, Fregat and attached "Dumsat" begin a quiet coast period through space. The stage will provide attitude control to keep the duo stable.

A second Fregat firing will then occur to deliver the dummy payload into a highly elliptical orbit like the Cluster 2 satellites will require. The orbit will range from a low point of 150 miles and high point of 11,185 miles.

Fregat later will begin spinning like a top for the simulated separation of its mock-up cargo. Two satellite deployment commands will be issued about 90 minutes into flight just as it will happen this summer for the Cluster 2 spacecraft. However, the deployments will be simulated and "Dumsat" will remain attached to the stage.

Officials say success today will be judged on the achieving the highly elliptical orbit planned for the Cluster 2 launches.

The first Soyuz-Fregat rocket roars to life on February 9. Photo: Starsem

The joint Russian and French venture Starsem, founded in 1996 to commercially market the Soyuz rocket, is responsible for the "Dumsat" payload and designing today's mission. Aerospatiale Matra Lanceurs of France developed and built "Dumsat."

Starsem was created by the Russian Aviation and Space Agency and Russia's Samara Space Center, Europe's Arianespace and Aerospatiale Matra of France.

Fregat has flown once before, successfully completing its first qualification flight on February 9. However, that flight proved the Fregat could be fired more than once in space and did not practice the Cluster 2 launches.

The new upper stage is being developed not only for use by the Cluster 2 spacecraft but also for ESA's Mars Express probe in 2003. In addition, the stage has commercial potential for lofting communications satellites.

The stage was built using flight-proven technologies and propulsion systems inspired by historic Russian lunar and deep space missions.