Metop, Europe's most sophisticated weather and climate satellite, has launched successfully from Baikonur Cosmodrome in Kazakhstan.
Metop lifted off on a night launch from Baikonur
The satellite should improve weather forecasts, and give scientists the data they need to refine climate models.
A Russian Soyuz rocket lifted Metop off the launch pad at 1628 GMT.
The launch had been thwarted on five occasions due to technical hitches and bad weather - and it was even dropped while being loaded on to a train.
The spacecraft was set to go into orbit from Kazakhstan on Wednesday, but the launch had to be scrubbed due to strong winds at the Baikonur Cosmodrome.
On Tuesday, an error in commands sent to the modernised Soyuz-2 rocket forced controllers to postpone the launch.
The satellite will now slip into a 850-km (531-mile) orbit around the Earth's poles.
The platform is a joint project of the European Space Agency (Esa) and Eumetsat, the intergovernmental organisation charged by European member states with operating a series of orbiting weather observatories.
"The spacecraft's in very good shape; it's a big relief," Mikael Rattenborg, director of operations for Eumetsat, told BBC News.
The spacecraft will now spend the rest of the week deploying its solar wing and configuring its control systems.
Metop has eight instruments to gather a range of data about the planet's atmospheric and surface conditions.
It has a further three instruments that will be used to assess the space environment and relay data.
Day and night, Metop will monitor temperature, humidity, wind velocity and ozone cover across the whole globe.
THE METOP SATELLITE
Dimensions: 17.6m by 6.5m by 5.2m; Total mass: 4,093Kg
Full orbit every 101 minutes; Crosses day equator at 0930
Will take key temperature and humidity measurements
Monitors wind direction and speed, especially over oceans
Builds profiles of ozone and other trace atmospheric gases
Joint project for Eumetsat and the European Space Agency
"The instruments on board will provide numerical weather predictions to national meteorological services in Europe and around the world," said Mr Rattenborg.
"Some of the instruments have flown before, but there are innovative new ones that will provide data for weather predictions of unprecedented accuracy."
Dr Stephen Briggs, head of Esa's Earth Observation Science and Applications Department, said: "This is a huge step forward for Europe.
"Metop's technology will be the benchmark for future systems for the next 20 years. It continues a historical series of data collected by previous Esa satellites but takes brand new measurements with a new set of instruments.
"In particular, the French-built Iasi instrument - the Infrared Atmospheric Sounding Interferometer - will give us three-dimensional temperature, pressure and atmospheric chemistry soundings that are much better than anything we've had before."
Dick Francis, of the UK Met Office satellite team, commented: "This is easily the most significant satellite launch in my 25 years of working in the science. The advances this launch will bring to monitoring weather and climate will be enormous."
The satellite weighs more than four tonnes and measures almost 18m (60ft) with its solar wing unfurled.
Metop is a first for a European meteorological satellite in that it will circle the Earth via the poles.
The two follow-on missions will be launched over the next 10 years to ensure there is continuity of service. The programme is costing 2.4 bn euros (£1.6bn; $3bn).
Metop will work in tandem with polar orbiting platforms operated by the US National Oceanic and Atmospheric Administration (Noaa).