A British-built spacecraft has blasted off to begin a seven year, 8.5 billion kilometre journey to Mercury, the planet closest to the sun.
BepiColombo was launched into space by an Ariane 5 heavy lift rocket from the European space port at Kourou, French Guiana, on Saturday.
The PS1.4 billion ($A2.6 billion) mission will send two orbiters, one European and the other Japanese, to circle the planet while mapping and probing its surface and enveloping magnetic field.
Scientists hope to unravel some of Mercury's mysteries, such as the reason for its over-large iron core, its spectacular volcanic vents, and why the planet's dark side emits X-rays.
The answers they get will shed new light on the origin and evolution of the solar system.
BepiColumbo is the first interplanetary spacecraft to be fitted with a futuristic electric ion drive.
Its four T6 engines, supplied by UK defence tech company QinetiQ, produce thrust using electrostatic forces to eject beams of positively charged, or "ionised", xenon gas.
BepiColumbo's ion thrusters will be firing for 4.5 years, more than half the journey time.
They will be used not to accelerate the craft but as a brake to help slow it down as it falls into the sun's gravitational "well".
A complex series of fly-bys past Earth, Venus and Mercury will further reduce the spacecraft's velocity and prevent it being caught by the sun's enormous gravity.
Another major challenge for mission planners was ensuring the spacecraft could withstand the searing temperatures of more than 350C so close to the sun.
Protective measures include a heat shield, novel ceramic and titanium insulation, ammonia-filled "heat pipes" and in the case of the Japanese orbiter, spinning.
The European Space Agency's Mercury Planet Orbiter (MPO) and Japanese space agency Jaxa's Mercury Magnetospheric Orbiter (MMO) will study Mercury for up to two years.
Only two spacecraft have previously visited Mercury. Nasa's Mariner 10 flew past the planet three times in 1974-75, and the American space agency's Messenger probe orbited Mercury from 2011 to 2015, taking photos of the surface.
BepiColombo was named after the late Guiseppe "Bepi" Colombo, an Italian scientist and engineer who played a leading role in the 1974 Mariner 10 mission to Mercury.
Australian Associated Press
