Mars Express on its Fregat booster

The Soyuz-Fregat launcher carrying the Mars Express orbiter, and its lander, Beagle 2, is expected to leave the launch pad at 2345 local time (1745 GMT).

The launch date and time were set following a successful review of all the spacecraft's systems last week.

Mars Express will be leaving Earth slightly later than originally planned because engineers needed extra time to fix a problem in one of the probe's electronic modules.

"Of course, it was the most difficult box to remove from the spacecraft," Rudi Schmidt, the Mars Express Project Manager, said.

But the work was completed quickly and the revised launch date of "no earlier than 6 June" was able to be brought forward by a few days.

The blast-off is still well within the four-week launch window.

Six-month trip

Mars Express is currently being fuelled, an operation that takes about a week.

It will then be attached to the Fregat upper-stage rocket booster - which will give the spacecraft its final big shove towards the Red Planet - and mated with the main Soyuz vehicle.

The whole system will be rolled out to the pad four days before launch.

The journey to Mars will take six months - a relatively short period thanks to a close alignment between Earth and the Red Planet - and the spacecraft should enter its Martian orbit on 26 December, 2003.

The US space agency (Nasa) should despatch the first of its Mars rovers to the fourth planet just days after the European probe blasts off - on 5 June, with the second rover heading off later in the month.

Europe's first mission to the Moon, the Smart 1 spacecraft, will launch in late August.

All images are courtesy of the European Space Agency