NASA will make a final decision in April whether to launch the Hermes Global Orbiter spacecraft to map the planet Mercury as the fifth mission in the Discovery series. The Hermes has been proposed by the Jet Propulsion Laboratory (JPL), California and spacecraft-builder Spectrum Astro in Arizona.

The US spacecraft Mariner 10 flew past Mercury in 1974 but images of only 45% of the planet were returned. Other than that, no other spacecraft has yet been used to explore the planet. If selected, the Hermes would "-fill a gaping hole in planetary exploration", says Robert Nelson, a JPL project scientist.

The Hermes will be used to map the entire planet, with images with a resolution down to 1km. It will also measure the planet's chemical composition and very thin atmosphere. The craft would be launched in 2001-2 by a Delta booster and will take advantage of new space technologies being developed and demonstrated by the New Millennium programme.

The first New Millennium craft, called Deep Space 1, also built by Spectrum Astro, will use solar-electric xenon-ion propulsion and this will be applied to the Hermes Global Orbiter.

The Hermes will cruise past the planets Venus and Mercury itself twice en route, to enable it to be naturally manoeuvred by the planet's gravitational forces into a path which will take it into the correct orbit around Mercury. Its elliptical 12h orbit will range from 200-14,880km altitude.

The craft will be equipped with a magnetometer, laser-radar remote-sensing equipment, an ultra-violet spectrometer and a plasma experiment.

Source: Flight International