Liftoff occurred at 8:39am EDT (12:39 GMT) from the Baikonur Cosmodrome in Kazakhstan, oneday later than planned. A launch attempt Saturday was aborted at thelast minute because of a problem with a connection between the launch padand the rocket.

There were no apparent problemswith Sunday's launch, however, and European Space Agency (ESA) officialswere anxious to begin their science mission after learning the spacecrafthad safely arrived in orbit.

"We've been waiting fouryears to see them in space," Philippe Escoubet, project scientist for theCluster 2 mission, told SPACE.com during a phone interview fromBaikonur.

"Now during the next sevendays we will conduct a series of five major maneuvers to bring them intothe operatonal orbit. Then we just have to wait for the second pair tobe launched in the same way on August 9," Escoubet said.

That second pair of Clustersatellites will also ride into orbit atop a StarsemSoyuz rocket, a commercial version of the workhorse booster launched morethan 1,640 times since 1957 and offered by the joint French-Russian company.

After arriving in orbit laterthis summer, the Cluster satellites will work as a four-spacecraft constellationto study how particles ejected by the sun interact with Earth's magneticfield.

These phenomena can takethe appearance of auroras above Earth's poles, or have dramatic effectson human activities by disrupting electrical power and communications,as well as cause serious anomalies in the operation of orbiting satellites.

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Another perfect launch

Almost nine minutes aftera perfect launch from the Baikonur Cosmodrome in the desert steppes ofKazakhstan, the new Fregat upper stage of the Soyuz separated and ignitedits engine.

After coasting about onehour, the Fregat fired its engine again to place the Cluster probes inan orbit that ranges in altitude from 124 miles (200 kilometers) to 11,600miles (18,000 kilometers). The twin satellites then separated from theFregat, concluding the launch phase.

SoyuzReady to Fly

The Soyuz rocket that carriedthe first two Cluster probes into orbit for the European Space Agency awaitslaunch from the Baikonur Cosmodrome. ESA image.

At 10:14 a.m. EDT (14:14GMT), when telemetry through the ESA ground station of Kiruna in Swedenconfirmed the last critical step was nominal, about 100 controllers ofthe European Space Operations Center (ESOC), near Darmstadt in Germany,burst into applause to celebrate the success.

The two satellites launchedon Sunday, along with the spacecraft scheduled for launch on August 9,are replacements for the full flotilla of four Cluster satellites destroyedduring the June 1996 explosion of the first Ariane 5 launch.

Because the launch carriedno insurance, it was a costly loss of $508 million. Sunday's launch alsowas uninsured, but fortunately cleared the dangerous liftoff hurdle. Thesecond identical set was built under tight financial constraints for $315million.

The strict budget on theCluster 2 mission imposed the use of two Russian Soyuz-type rockets. Totallaunch costs for the four spacecraft are estimated at $35 million, halfwhat it would have cost using an Ariane launcher.

Now the two satellites willuse their own on-board propulsion systems to reach their final orbit.

It will take one week forthe five critical orbital maneuvers to be executed that will change eachsatellite's path to orbit Earth's poles with a high point of 73,780 miles(119,000 Km) and a low point of 11,780 miles (19,000 Km).

Cluster's space science

Once operational in polarorbit, the Cluster probes will observe in unprecedented detail the magneticand electric interactions between the Earth and the sun, by making directmeasurements of the field and particles trapped by Earth's magnetic field.

The ideal spacecraft flightformation for measuring the irregular solar plasma waves in three dimensionis a triangular pyramid with one Cluster spacecraft at each corner, thousandsof miles (kilometers) apart.

The four identical cylinder-shapedspacecraft -- named Samba, Tango, Salsa and Rumba by ESA-nation schoolchildrenin a contest -- each carry 11 instruments, making a record number of 44instruments for an pilotless space mission, ESA officials said.

Electrical power is suppliedby a solar array which covers the cylindrical surface. The upper part ofthe spinning cylinder accommodates the scientific instruments.

Magnetic-field sensors aremounted on two deployable booms, while four 164-foot (50-meter) wire boomscarry electrical field instruments.