A British-built spacecraft is set to blast off from the Earth bound for Mercury, the mysterious planet closest to the sun.
BepiColombo, one of the most ambitious missions ever undertaken by the European Space Agency (ESA), will send two orbiters to explore the hellish world where surface temperatures reach 450C.
One probe was built by satellite makers Airbus Space and Defence at its assembly centre in Hertfordshire. The other was constructed in Japan. The mission, launching on Saturday, has cost an estimated £1.4bn.
Unlike any other interplanetary spacecraft in history, BepiColombo carries a futuristic ion electric propulsion drive, also designed and built in the UK.
Four ion engines on the Mercury Transfer Module transporting the orbiters will provide thrust by electrically ejecting a "plasma" of charged xenon gas.
Although the force an ion motor produces is very small, it can be kept firing for a long period of time. The four-tonne spacecraft will be launched into an "escape trajectory" orbit by Esa's most powerful rocket, the Ariane 5, from the European spaceport at Kourou, French Guiana.
It will then set off on a seven-year, 5.2 billion mile journey.