Mercury in focus

German researchers are developing a new diode laser to provide astronomers with a 3D map of the most mysterious planet. Siobhan Wagner reports

The geology of Mercury, the innermost and smallest planet in our solar system, remains a mystery to astronomers. But a diode laser module designed by German researchers may one day map the hills and valleys of the Earth-like planet.

The Fraunhofer Institute for Laser Technology (ILT) in Aachen was asked by German satellite parts manufacturer Tesat Spacecom to build a prototype diode pump to excite light energy into a laser altimeter, for measuring altitudes on Mercury. If it wins the full contract, the lasers will be launched in 2013 as part of ESA’s BepiColombo Mercury mission.

The knowledge of the planet’s landscape is limited to a single NASA spacecraft flyby from 1974-1975, which mapped only 40-45 per cent of its surface. The space probe showed the surface of Mercury to be similar in appearance to the Moon, with extensive, volcanic rock plains and deep craters, indicating that it has been geologically inactive for billions of years.

In the new mission, diode lasers will orbit Mercury and map its geology by directing a beam of light to a point on the planet’s surface. The time it takes for the beam to reflect back determines the distance. By scanning the landscape in this way, the researchers believe it will be possible to form an accurate 3D map of the planet, detailing the size and depth of craters and mountains.

A similar laser altimeter was used to measure the height of surface features on Mars in the mid-1990s. The Mars Orbiter Laser Altimeter (Mola) calculated these by surveying the terrain from distances of around 30m.

However, the altimeter for Mercury will need to be built differently to withstand the long trip to the planet closest to the Sun. While the laser module prototype has yet to be put through environmental thermocycling and radiation tests, it has passed an important initial design evaluation.

Martin Traub, who led the development of the prototype at the ILT, said one of the biggest challenges for researchers was to make the module light and compact but also powerful.

The module measures only 15 x 5 x 5cm and weighs 650g (1.5lbs) but produces an impressive 530W. Most lasers of this power would be the size of a shoebox and weigh about 5kg. The Mola, for example, weighed almost 26kg.

While there are numerous earthbound applications for laser diodes — from light sources for fibre-optic communication to supermarket barcode readers — designing one for a space mission presented bigger challenges.

For example, the first hurdle the researchers had to overcome was to design a way of cooling the laser diode because lasers of such power usually produce a lot of excess heat.

‘A diode laser on Earth can be cooled by water cooling or air fans, but this is impossible in space because of the vacuum environment,’ said Traub.

So the team created a conductor to carry the heat away to the surface of the satellite, where it is dissipated by radiation.

‘We arranged the laser on a comparatively large surface so it can be easily cooled by conduction cooling,’ he said.

The researchers were also pressed to develop a laser that could reliably work in a vacuum environment. For that, they designed its outer sheath to be airtight so that when the laser meets the harsh environment of Mercury’s orbit it will be unaffected. According to Traub, the module can be filled with air or other gases to create a sustainable artificial atmosphere inside the laser that remains for several years.