Washington

Scientists and engineers will hold their breath next week as NASA prepares to shoot a spacecraft towards one of the last unexplored planets of our Solar System — Mercury.

The Mercury mission had significant challenges to overcome before reaching the launch pad. For example, it had to be designed to withstand temperatures of more than 400 °C and to perform a tricky orbital insertion. At the time it was approved in 2001, NASA officials called it the most complex Discovery-class planetary mission they had ever attempted. Discovery missions are designed to have the relatively low cost of less than US$400 million.

The Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft is set to leave Cape Canaveral, Florida, on 2 August, with launch windows every day until 14 August. It should arrive at Mercury in March 2011.

The only other visitor to Mercury has been Mariner 10, which mapped 45% of the planet's rocky, cratered surface in 1974 and 1975. Mariner left key questions unanswered, such as whether the surface rocks are volcanic in origin, and why Earth and Mercury have global magnetic fields, but Mars and Venus do not. A suite of seven instruments onboard MESSENGER, including X-ray, γ-ray and infrared spectrometers, will try to come up with answers, while cameras will map the entire surface at high resolution.

Getting MESSENGER into orbit around a planet so small and so close to the Sun will not be easy. In fact, it was considered impractical using current rockets until a mission designer in the 1980s found a way to slow down a spacecraft using a complicated series of loops around Venus and Mercury.

Even then the project nearly never took off. Last year NASA threatened to terminate it for running $26 million over its agreed cost limit. The agency's Space Science Advisory Committee saved the project by advising NASA to spend the extra money, even if it meant delaying other planetary missions.

If successful, it will be a relief for mission designers at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. The lab's last Discovery spacecraft built for NASA — the Comet Nucleus Tour (CONTOUR) — failed shortly after leaving Earth in 2002.

→ http://messenger.jhuapl.edu