Nasa launches first Mercury mission in three decades

5.00pm

CAPE CANAVERAL - Nasa launched its first mission to the planet Mercury in a generation early on Tuesday, one that scientists hope will strip away much of the mystery surrounding the tiny planet closest to the sun.

The MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft, riding a Boeing Co Delta 2 rocket, blazed across the nighttime sky above Florida's Cape Canaveral Air Force Station as the US$427 million mission got underway with lift off on Tuesday.

Among the questions scientists hope to answer is whether Mercury, just slightly larger than Earth's moon, was once Earth-sized itself but lost its rocky exterior either to some cataclysmic collision or to slow ablation by the solar winds.

Scientists also believe there may be frozen water there, trapped in shadowy craters at the planet's poles, never exposed to the sunlight that creates a 1,100 degree F difference between daytime and nighttime temperatures on the planet.

"The inner planets (Mercury, Venus, Earth and Mars) all formed from the disk of gas and dust, the solar nebula, that surrounded our young sun. They formed by the same processes, they formed at the same time, (but) their outcomes were extremely different. And Mercury is the most extreme of those four planets," said Sean Solomon, principle scientist for the US$426 million mission.

MESSENGER will reach Mercury after a seven-year sojourn through the solar system that will take it 15 times around the sun, making near passes of Earth once, Venus twice, and Mercury itself three times.

Each planetary pass will act as a gravitational tug to slow MESSENGER's speed so that it can eventually slip into Mercury's orbit for a year-long study.

The only other up-close look planetologists have had of Mercury came in the mid-1970s when Nasa's Mariner 10 spacecraft made three fly-bys, photographing about 45 per cent of the planet and discovering that it had a strong magnetic field, an indication, scientists say, that Mercury is about two-thirds iron.

Stormy weather associated with Tropical Storm Alex, the same system that caused a launch attempt on Monday to scrub, kept the Cape Canaveral launch area socked in for much of the day prior to launch.

But the storm system moved off to the north about an hour before launch, just as forecasters had predicted, Nasa said.

MESSENGER was developed by the Applied Physics Laboratory at John Hopkins University and is the seventh in Nasa's Discovery series of relatively low-cost solar system missions.

One of those, Genesis, launched in 2001, is due to return some samples of solar wind material to Earth in September.

- REUTERS