The two spacecraft, named Rumba and Tango, were taken aloft at 1213 GMT aboard a Russian-built Soyuz rocket from the Baikonur space centre in Kazakhstan, the European Space Agency (ESA) told AFP.

Rumba and Tanga were to be placed in orbit after a flight of approximately 90 minutes.

They will later team up with two identical satellites, Samba and Salsa, which were launched on July 16.

The four will then fly in tetrahedral formation -- a shape like a pyramid with a triangular base -- in an egg-shaped orbit, ranging from 119,000 kilometers (74,000 miles) at its farthest point from the Earth to 19,000 kms (11,800 miles) at its closest.

The so-called Cluster mission will measure the magnetic field that invisibly cloaks the Earth and protects it from the solar wind, the buffeting flow of electrically-charged particles spewed out by the Sun.

Electrical currents generated by magnetosphere also cause the famous northern and southern lights at the Earth's poles.

The ESA is a 25-year-old consortium gathering 14 European countries. The launch was carried out by STARSEM, a Franco-Russian joint venture that markets Soyuz launches.

Topped by a giant fairing emblazoned with a rectangular blue Cluster logo, the white, dart-shaped launch vehicle lifted off from pad 6 exactly on time.

Within a few minutes, the ground team was able to confirm that the first three stages of the Soyuz had operated perfectly and the protective fairng had successfully separated from the Soyuz.

12 minutes after lift-off, confirmation came that the first engine firing of the Fregat upper stage had also been successfully completed.

The Fregat and its Cluster satellites, Rumba and Tango are now safely installed in their initial preliminary orbit, some 200 km above the Earth.