By BBC News Online's Helen Briggs in Kourou, French Guiana

Europe's largest and most expensive satellite has begun its voyage to monitor the health of the planet, blasting off from the Kourou spaceport in French Guiana.

The environmental satellite, Envisat, soared into the night sky aboard a giant Ariane 5 rocket. It took off right on schedule at 22:07 local time (01:07 GMT Friday).

The rocket sped north out over the ocean

Mission controllers followed the progress of the rocket as it flew north over the ocean and then burst into applause when the signal was received that the precious cargo had been delivered safely into its correct orbit.

"Bravo Europe," said Jean-Marie Luton, the chairman and CEO of Arianespace, the company that runs the Ariane launcher programme. "Envisat is a potent symbol of the role a united Europe can play in the world."

He added: "Envisat is not only a technical success, it is a success for the co-operation of all the European industries that took part in the project."

Costly project

The 10 instruments on Envisat will provide vital information on how the Earth's land, oceans, ice caps and atmosphere are changing. The data it collects will be analysed by scientists and inform European policies on the environment.

Weighing more than 8,000 kg, and measuring 10 metres in length, the satellite is the largest payload to be launched on an Ariane vehicle.

Envisat facts

The size of a bus 8.2 tonnes in weight Circles the Earth every 100 minutes from 800 km up 10 instruments will scan the planet for environmental changes Built at cost of 2.3bn euros (�1.4bn)

The satellite will circle the planet every 100 minutes in a polar orbit, looking down from a height of 800 kilometres (500 miles).

The Envisat project has been put together by 13 European Space Agency (Esa) members and Canada. It has cost �1.4bn (2.3bn euros), with a contribution from the UK of �300m. Only France has spent more on Envisat.

"This has been a particularly exciting day for Esa and the European space community as a whole," said Jos� Achache, Esa's director of Earth Observation. "We will be able to trace the smallest changes to the Earth's surface anywhere on the globe. The importance of this mission has triggered great interest in the Earth-science community, both at a European level and worldwide."

'Large programmes'

Thursday's blast-off was the first mission of an Ariane 5 rocket since a flawed launch in July last year left two satellites in defective orbits.

Of its 10 previous flights, three had malfunctioned. The most spectacular failure was in 1996 - the maiden flight. The rocket blew up, destroying four identical spacecraft that were designed to investigate the Earth's magnetosphere.

Some commentators had questioned the wisdom of putting one large satellite on the vehicle.

Before Thursday's launch, Dr Craig Underwood, of the Surrey Space Centre at the University of Guildford, UK, said the space industry was divided over whether smaller, cheaper missions were a better alternative to flagship missions.

"If, God forbid, they lost Envisat, that's a huge investment down the drain," he told BBC News Online. "But many scientists are wedded to the idea that they can only get recognition by working on large programmes and therefore there's pressure to work on large programmes."

According to officials at Arianespace, modifications have been made to the engine blamed for July's problem to prevent a similar thing happening again.

Jean-Marie Luton said Thursday's perfect launch showed Ariane 5 was truly back on track.