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A NASA probe next week will become the first spacecraft in 33 years to fly past Mercury, a sojourn that scientists hope will unlock secrets of the small sun-baked planet.

NASA's car-sized Messenger spacecraft is scheduled to zip about 200 kilometres above the cratered, rocky surface of the closest planet to the sun on Monday, part of a mission designed to place it into orbit around Mercury in 2011.

"I think we're in for some big surprises," Faith Vilas, one of the scientists involved in the mission, told reporters on Thursday.

"You know that we can't get cocky about this and say, 'Oh, it's going to look like this.' Every solar system body looks very different from every other solar system body," she added.

Messenger is expected to get the first spacecraft measurements of the mineral and chemical makeup of Mercury's surface, and also to retrieve key data about the planet's internal structure.

The probe will study Mercury's global magnetic field, which scientists hope will add to the understanding of the planet's gravity field.

Messenger's instruments will collect more than 1,200 images and make other observations during this initial fly-by. It will be the first up-close measurements since the Mariner 10 spacecraft's third and final fly-by in 1975.

Messenger, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, was launched in 2004 and has meandered through the inner solar system on the way to Mercury, already having flown past Earth once and Venus twice.

The other side

Scientists said the probe is expected to retrieve far more data about Mercury than did Mariner 10, which surveyed only one hemisphere. During next week's fly-by, scientists will begin to collect images of the other hemisphere.

"Mercury really has been long ignored – too long," NASA's planetary science chief James Green said. "With Messenger, we believe that many of Mercury's secrets will now be revealed."

With Pluto demoted from planetary status, Mercury is now recognized as the smallest planet in the solar system. Its diameter is about 4,880 kilometres, about one-third the size of Earth and only a bit larger than our moon. Mercury's surface is a mix of craters, plains and long, winding cliffs.

The planet's average distance from the sun is 58 million kilometres, about one-third of Earth's distance away. It orbits the sun once every 88 Earth days.

The spacecraft is due to make its closest approach to Mercury at 2:04 p.m. EST on Monday.

Mercury, along with Venus, Earth and Mars, is part of the solar system's family of four rocky inner planets.

"But Mercury, as a family member, is a real oddball," said Sean Solomon of the Carnegie Institution of Washington, the mission's principal scientific investigator.

He cited its extreme density and wide surface temperature swings from day to night due to its proximity to the sun.

Surprisingly considering its proximity to the sun, there may be water ice on the permanently shadowed crater floors at Mercury's poles, scientists think. A huge iron core is believed to make up more than 60 per cent of Mercury's total mass.

Messenger is due to make three passes of Mercury – next week, in October and again in September, 2009 – before finally starting a year-long orbit during a fourth encounter in March, 2011. The mission also includes fly-bys of Earth and Venus as the probe positions itself for orbiting Mercury.