The first mission to the tiny sun-baked planet Mercury in more than three decades is all set for blast off on 2 August. NASA鈥檚 Messenger spacecraft will explore the unique geology, magnetic field and surface chemistry of the Solar System鈥檚 innermost planet.
The 500-kilogram craft will take a long and convoluted route to the planet to save on fuel. Six flybys 鈥� of Earth, Venus, and Mercury itself 鈥� will be used to manoeuvre the probe into orbit around Mercury in 2011.
During its one-year mission, the 120-centimetre-wide craft will orbit Mercury once every 12 hours, using seven science instruments to scrutinise the planet in detail.
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鈥淯nderstanding Mercury is essential for understanding the terrestrial planets,鈥� said Orlando Figueroa, director of NASA鈥檚 Solar System Exploration Division, at a press briefing on Thursday.
Sean Solomon, an astronomer at the Carnegie Institution of Washington and the mission鈥檚 principal investigator, agrees. Those planets 鈥� Mercury, Venus, Earth, and Mars 鈥� all have iron-rich cores under rocky mantles and formed from the same region of gas and dust around the Sun. 鈥淭hey鈥檙e all litter mates, but the siblings turned out very differently,鈥� Solomon said.
Giant impacts
Mercury boasts the highest density, with an enormous two-thirds of the planet鈥檚 mass made up of its metal core.
Three theories compete to explain this phenomenon. Dense particles may have been drawn closer to the Sun before the planet formed. Or the Sun might have vaporised part of the planet鈥檚 rocky exterior, early on in its life. Or, perhaps, giant impacts blasted away the planet鈥檚 outer layers.
Each scenario would have left behind a unique combination of surface rocks.
鈥淲e hope to distinguish between those competing ideas with chemical remote sensing,鈥� Solomon said.
Mercury鈥檚 magnetic field will be another target for study. The Mariner 10 spacecraft, which imaged 45 per cent of the planet鈥檚 surface during three flybys in the 1970s, discovered a dynamic global field on Mercury 鈥� the only terrestrial planet besides Earth to have one.
Earth鈥檚 magnetic field is thought to be generated by the movement of liquid metal around the planet鈥檚 solid iron core. But Mercury鈥檚 field puzzles scientists. It is only slightly larger than the Moon, suggesting its molten outer core would have cooled and solidified long ago.
Studies of how the planet wobbles on its axis as it rotates, along with measurements of the field itself, may determine how the field is created.
Polar craters
Another task the probe will undertake is to peer into Mercury鈥檚 polar craters. These never see the light of day because the planet rotates at a nearly perfect perpendicular angle to the plane of its orbit around the Sun.
The craters never get above 鈥�200掳C, and radar observations from Earth suggest they could be filled with water ice. A gamma-ray and neutron spectrometer will study the craters to determine if the source is water ice, sulphur ice, or just super-chilled rock.
Mercury experiences a 600掳C temperature swing every day, and these extremes of temperature 鈥� are 鈥渨hat made the mission very challenging and very exciting鈥�, said James Leary, the mission systems engineer at Johns Hopkins University鈥檚 Applied Physics Laboratory in Laurel, Maryland.
But this is also the main reason visiting Mercury has been put off for so long. 鈥淭he two big obstacles that have kept a Mercury orbit at bay for 30 years are temperature and the spacecraft鈥檚 mass,鈥� said Leary. The probe will be shielded from these temperature extremes by a ceramic-fibre sunshade similar in make-up to the space shuttle鈥檚 tiles.
Once its mission has been completed, the probe will remove itself from orbit around Mercury by crashing into the planet鈥檚 surface.