By Helen Briggs
BBC News Online
Europe's first solo mission to another planet, Mars Express, is nearing the fourth planet after a six-month voyage from Earth.
Mars landings are a risky business
On 19 December, the orbiter released the British spacecraft Beagle 2.
The tiny lander, less than a metre across, will glide through space like a Frisbee for six days, entering the Martian atmosphere in the opening moments of Christmas Day.
About the same time that Beagle 2 is touching down on the dusty red soil, the European Space Agency's Mars Express will go into orbit around the fourth planet.
Mission controllers will command the spacecraft from more than 100 million kilometres away at the European Space Operations Centre (Esoc) in Darmstadt, Germany.
They will "fly" the spacecraft by sending signals to its onboard computer via a ground station near Perth in Australia.
If all goes well, the mission will unfold as follows:
The position of Mars Express was fine-tuned. It needed to be positioned precisely to eject Beagle 2 on the correct trajectory for landing on Mars.
The cruise, descent and landing phases of Beagle are controlled by integrated software. The final parameters needed for the landing were uploaded today.
19 December 0650 GMT
Mission controllers at the European Space Agency made the final decision to jettison Beagle 2 from its "mothership", Mars Express.
19 December 0831 GMT
A command was sent to Mars Express to release Beagle 2 by what is known as a Spin-Up and Eject Mechanism. The lander glided towards the planet, spinning gently as it went. Beagle 2 has no propulsion system of its own; it is "in the hands of Mr Newton" for its last descent, as an astronaut once put it.
19 December 1031 GMT
A team at Logica in Surrey built the software for cruise, descent and landing
Mission controllers concluded that ejection has been successful by analysing telemetry data from Mars Express.
An onboard camera captured an image of Beagle 2 gliding away from the spacecraft.
20 December 0800 GMT
Mars Express was directed to rapidly fire its thrusters. It veered away from Mars to avoid crashing into the planet.
A series of minute adjustments were made to get the spacecraft ready for going into orbit around Mars.
25 December 0000 GMT
A timer switch on Beagle 2 activates its software ready for entry, descent and landing.
25 December 0248 GMT
Beagle 2 enters the thin atmosphere around Mars. The spacecraft's heat shield protects it as it plunges towards Isidis Planitia, a huge flat basin just north of the equator.
Contact with the gases in the upper atmosphere slows Beagle 2 down. The deceleration is detected by sensors. Once its speed has fallen to about 1,600 km/h, two parachutes deploy to slow it further.
A radar signals that Beagle 2 is about 200 metres above the ground. Large gas-filled airbags inflate to protect the precious spacecraft as it hits the surface at about 60 km/h. At the same time, the parachutes are cut to stop them getting tangled up.
25 December 0254 GMT
D-day for Beagle 2. It all goes to plan, it will touch down on the red soil, bouncing along on its airbags before coming to a halt somewhere on Isidis Planitia.
25 December 0300 GMT
Beagle only has enough battery power to survive a day
Mars Express goes into orbit around Mars. Its speed is reduced to about 9 km/s, which is slow enough for the planet's gravitational field to "capture" the spacecraft.
25 December 0630 GMT
The first chance to find out whether Beagle 2 has landed in one piece. The US Space Agency (Nasa's) Mars Odyssey spacecraft passes over Isidis Planitia. If Beagle 2 is in working order, the orbiter should pick up its call sign.
25 December 0700 GMT
The first chance to find out whether Mars Express is in the correct orbit.
25 December 2245 GMT - 0045 GMT
The giant radio telescope at the Jodrell Bank Observatory in Cheshire, UK, scans the sky for signals from Beagle 2's transmitter. The lander does not have enough battery power to last the night; it must unfurl its solar panels before the sunlight filtering through the Martian atmosphere ebbs away.
26, 27, 28 December
Further corrections are made to the orbit of Mars Express. It needs to get into a highly elliptical polar orbit, passing as close as 260 km to the Martian surface before swinging back out to a distance of 11,000 km away.