Nasa heading back to Mercury after 30-year break

11.45am

WASHINGTON - Nasa is set to launch a spacecraft to Mercury in August for the first time in 30 years, with a probe meant to orbit the planet while it maps its surface and looks for frozen water in shady polar craters.

The Messenger spacecraft is set for launch August 2 on a seven-year journey that will loop around Earth, Venus and Mercury, getting a boost from the gravity of these three planets to guide it to a Mercury orbit for a one-year mission, the project's scientists said on Thursday.

Messenger won't make the first Mercury fly-by until 2008 and will not begin its main mission until 2011, after flying some 7.9 billion kilometres, the scientists said at a briefing. The roundabout route is geared to saving the cost and weight of the extra fuel required for a quicker trip.

The last time the National Aeronautics and Space Administration sent a spacecraft to this planet nearest to the sun was in 1974 and 1975, when the Mariner 10 probe made three close passes, returning detailed data on less than half of Mercury's surface.

By contrast, Messenger will carry seven scientific instruments, including cameras, spectrometers to figure out what chemicals are present, a magnetometer to learn more about Mercury's magnetic field and an altimeter to measure its topography.

Sean Solomon of the Carnegie Institution of Washington, the principal investigator on the project, characterised Mercury as unusual among the inner rocky planets, which include Earth, Venus and Mars.

It is so dense that scientists believe at least two-thirds of it must be made up of iron. The temperature between day and night on Mercury varies up to 600 degC, with a daytime temperature around 450degC, Solomon said. So at night it's several hundred degrees below zero Fahrenheit.

The US$426 million ($663.2m) Messenger mission will carry its own sunshade to protect the craft from the planet's high daytime temperatures, which are comparable to a pizza oven. Still, astronomers wonder if there is frozen water somewhere on the surface despite the heat.

"One of the most bizarre questions that faced students of the planet Mercury for the last decade ... is whether the planet closest to the sun, with these extreme variations in temperature, might really have ice lurking in shadowed regions at the poles," Solomon said.

Unlike Earth, Mercury does not spin on a tilted axis, which means a crater at its north or south pole would be in permanent shadow. And Mercury's ultra-thin atmosphere does not transport heat from the equator to the poles, as Earth's does.

The floor of a shadowed crater would never see the sun and would be cold enough -- minus 180 degC or colder -- to freeze water for the lifetime of the planet, Solomon said.

- REUTERS