BAIKONUR COSMODROME, Kazakstan -- A European science probe designed to study the interaction of the sun and Earth was attached to its Russian booster on Tuesday in prepatation for launch on Saturday.

The Cluster spacecraft are mated to the top of a Starsem Soyuz booster in preparation for a scheduled launch on Friday. SPACE.com photo by Anatoly Zak.

Engineers in Area 32 of the launch site here mated a pair of Cluster spacecraft with the first two stagesof the Soyuz rocket, which will lift them into space. The next step will be to roll out the rocket and its cargo to the launch pad on Wednesday.

Altogether, four Cluster spacecraft are scheduled for launch, one pair this week and another pair in August.

Starsem, the joint Russian-French company, is responsible for the launch, which includes the use of a new upper stage called Fregat.

Once in orbit, the Cluster probes will watch how plasma ejected by our sun interacts with Earth's magnetic field.

The four Cluster spacecraft to be launched this summer are replacements for another four lost in 1996 during the inaugural launch of the Ariane 5 rocket, which tumbled out of control and was destroyed shortly after launch from the Guiana Space Center in South America.

Starsem conducted two successful test launches of the Soyuz rocket with the new Fregat upper stage earlier this year. This week's launch will be the first commercial use of the new launch vehicle.

Meanwhile, preparations for launching the second pair of Cluster spacecraft Baikonur is continuing on schedule, according to representatives of the European Space Agency.