The venture will combine the efforts of Europe and Japan
Science delegations to the European Space Agency (Esa) have approved a much more expensive mission to Mercury.
BepiColombo, due for launch in 2014, was supposed to have cost Esa about 665m euros (£595m; $995m).
The challenge, though, of building a probe able to survive the torrid heat and radiation at the innermost world has pushed this price up to 970m euros.
Many researchers feared Bepi might be cancelled, but Esa's Science Programme Committee has passed the bigger budget.
"Technically it has a clean bill of health to proceed, and financially it also has a clean bill of health to proceed," Professor David Southwood, the agency's director of science and robotics, told BBC News.
BepiColombo will be one of Europe's most sophisticated scientific missions to date.
It is designed to be a joint endeavour with the Japanese. Two probes will travel jointly to Mercury, and separate when they achieve orbit (in 2020).
Europe will produce a Mercury Planetary Orbiter (MPO) which will be equipped with 11 scientific instruments.
Flying in a polar orbit, it will study Mercury for at least a year, imaging the planet's surface, generating height profiles, and collecting data on Mercury's composition and wispy atmosphere.
The US Messenger probe is currently returning remarkable imagery
Japan will be responsible for the Mercury Magnetospheric Orbiter (MMO). It will investigate the planet's magnetic field with its five on-board instruments.
Working at Mercury, however, is an immense undertaking.
BepiColombo will be baked directly by the Sun, receiving some 14,000 watts per square metre; about 10 times what a spacecraft in orbit around Earth would receive.
Developing the systems that can manage this environment has led to the mission getting heavier. The original solar panel design, for example, was found to be incapable of dealing with a combination of high temperatures and extreme ultraviolet light.
The panels will now have a special coating, but they will also be bigger and hence more massive.
Overall, Bepi's launch mass has grown from some three tonnes to just over four tonnes. This has necessitated the use of a larger, more expensive rocket - an Ariane 5 instead of a Soyuz.
BEPI'S EUROPEAN ORBITER
Europe's main contribution is Mercury Planetary Orbiter
MPO will operate in tight 400 by 1,500km orbit; 2.3-hour period
Seeks comprehensive, high-resolution global coverage
Will study surface and internal composition of the planet
Systems will experience high temperatures and radiation
Time in orbit: One year nominal plus a year's mission extension
Some critics had queried whether the extra cost of Bepi could be justified given that the Americans are already at Mercury with a sophisticated orbiter called Messenger.
"That was one of the central issues," said Professor Southwood.
"We had information given to us not only by our own scientific advisers but by the Messenger team as well. I don't think there's any doubt about it. You'd expect two spacecraft to deliver more science than one; and Bepi is a much, much bigger mission than Messenger."
Dr David Rothery, the lead scientist on Bepi's Mercury Imaging X-ray Spectrometer (MIXS), said the science case for another Mercury mission was exceptional.
"The best way I heard it expressed, very kindly by a member of the Messenger team, was that Messenger is providing the 'hors d'oeuvre' and BepiColombo will be the 'feast'.
"BepiColombo has more instruments and more capable instruments than Messenger does.
"Messenger doesn't have imaging capacity in the X-ray part of the spectrum, which is the main UK contribution to Bepi. Messenger also lacks mid-infrared capacity spectroscopy which is very important for the most diagnostic minerals on Mercury's surface."
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