Key technical milestones have yet to be achieved for the €1 billion ($1.46 billion) joint European Space Agency/Japan Aerospace Exploration Agency Mercury mission BepiColombo, while a €350 million contract is signed with its spacecraft's manufacturer EADS Astrium.

The spacecraft is to be launched by an Arianespace/Starsem Soyuz 2-1b in 2013. On reaching Mercury in 2019 it will separate into four parts: the Mercury transfer module (MTM), two orbiters, ESA's Planetary Orbiter (MPO) and JAXA's Magnetospheric Orbiter (MMO) with the sun shield that protects it during the journey. The orbiters will survey the planet for up to two years.

The key milestones include determining the separation method for the MPO from the MTM, completing technology progammes such as the solar arrays' rotation joints and ensuring the Soyuz 2-1b can be launched from French Guiana.

ESA's BepiColombo project manager Jan van Casteren says: "We have a guarantee that the 2-1b will launch from [its Sinnamary launch complex in] French Guiana by 2013."

While the first Sinnamary Soyuz 2-1a launch is expected by June 2009, the complex will not be able to handle the 2-1b, which uses a different upper stage engine fuel.

The 2-1b can be launched from Baikonur, Kazakhstan, but because of the payload and orbit required for BepiColombo, the booster has to fly from Sinnamary.

The MPO-MTM separation method is linked to use of the 2-1b as the rocket's launch environment is the deciding factor.

Sun shield separation from the MMO can use pyrotechnics because, at the top of the spacecraft stack, they are not load-bearing structures and will not endure the same launch forces as the MTM.

Due to Mercury's high radiation levels and temperatures, ESA started research on protecting the spacecraft four years ago. Solar cells that can operate in this harsh environment have been developed, but work on rotation joints that will keep the arrays at an oblique angle to the Sun to minimise heating is not completed.

If the spacecraft loses attitude control, direct exposure of the arrays and sensitive navigation instruments to the Sun for more than 3min will cause damage.

Source: Flight International