Europe's first mission to another planet is on its way.
A signal sent by the space craft to ground control confirmed what scientists had hoped - the launch had been successful and the probe was on the correct path for Mars.
A perfect launch from Soyuz
The Mars Express orbiter, carrying the British-built lander, Beagle 2, left Earth on a Russian Soyuz-Fregat launcher.
The rocket blasted off from Baikonur in Kazakhstan at 1845 BST (1745 GMT) on Monday.
It will take six months for the orbiter and its lander passenger to reach the Red Planet.
Beagle is expected to be on the Martian surface at 0254 GMT on 25 December. Mars Express will go into a highly elliptical polar orbit of the planet on the same day.
"It's done; it's been a brilliant job," said Professor David Southwood, the director of science at the European Space Agency (Esa). "I'd like to thank everyone involved for a very successful start to the mission."
The scientists and engineers who had travelled to Baikonur to watch the blast-off jumped up and down with delight and hugged each other as the Soyuz rocket headed into space.
Each successful stage separation was cheered. By 2015 BST (1915 GMT), it was confirmed Mars Express had successfully detached from the Fregat booster following a one-lap trip around the Earth.
Shortly after, mission managers at the European Space Operations Centre at Darmstadt, Germany, reported that the first telemetry from Mars Express had been received.
So-called because of speed and low cost of development
Will map Martian surface and analyse atmosphere
Radar instrument can detect water several km below surface
Data told the flight controllers that the probe's solar arrays had opened, its batteries were working and its thermal systems were functioning properly.
John Reddy, one of the principal engineers on the mission, said: "The news seems to be very good. When the signal acquisition came through, we were able to evaluate the performance of Mars Express very quickly.
"We have a spacecraft in excellent condition and we're on our way to Mars."
In London, scientists and engineers working on the British-built Beagle lander and Mars Express gathered for a launch party.
UK science minister Lord Sainsbury told them: "It's a great evening for British science and British engineering. It's a fantastic project which will capture people's imagination."
British Prime Minister Tony Blair sent a message to the party. He said Beagle 2 was an exciting project which offered fascinating potential to learn more about the fundamental origins of life.
European flight controllers track every moment of the launch procedure
"I wish it good luck in a difficult and challenging mission," he said.
The whole project has cost around £30m, most of it raised through private sponsorship.
European scientists and engineers know they still have testing times ahead.
Three days after lift-off, Beagle 2 has to blow the bolts that kept it securely attached to the orbiter during blast-off.
Its extra clamps must be removed remotely so that the lander can be ejected when Mars Express finally arrives at the Red Planet.
Trio of launches
The main scientific goal of Mars Express is to detect vast reservoirs of water thought to be trapped under the Martian surface using a ground-penetrating radar.
The probe will also take images of Mars and conduct a geological survey of the planet.
As Mars Express nears its destination, it will drop Beagle 2 into a basin that could once have contained water and life.
A delighted Colin Pillinger at the London launch party
The small robotic lander is about the size of a bicycle wheel. It will dig into Martian rocks and soil to search for the chemical signatures of life.
Between them, Mars Express and Beagle 2 could answer one of the biggest questions in science.
"It's designed to answer the age-old question - is there or has there ever been life on Mars?" Professor Colin Pillinger, lead scientist on Beagle, said.
"It's a small laboratory which will do it in situ and send the information back to us."
Monday's launch marks something a race to find life on another planet.
The US space agency (Nasa) is sending two robotic probes to Mars this June. And Japan's Nozomi craft should reach Mars early in 2004.