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WASHINGTON - A car-sized Nasa probe zoomed about 200km above the rocky, crater-scarred surface of Mercury, becoming the first spacecraft since 1975 to fly past the closest planet to the sun.
The US space agency's Messenger probe travelled at about 25,750 km/h as it passed over Mercury on a mission designed to resolve some of the mysteries about the solar system's innermost planet, officials said.
"So far, things look pretty good. The spacecraft was on the course we wanted it to be on," Michael Paul, a mission engineer, said in a telephone interview.
In addition to the rendezvous, Messenger is scheduled to pass Mercury again this October and in September 2009, using the pull of the planet's gravity to guide it into position to begin a planned yearlong orbit of the planet in March 2011.
It flew roughly along the equator and at a slightly higher altitude than originally planned, but the change had no negative effects, Paul said.
Mark Robinson of Arizona State University, a member of the mission's science team, said the closest approach was on the planet's "night side" - the one facing away from the sun.
The probe is due to begin transmitting data it collected during the fly-by back to Earth, Paul said. Nasa said it hopes to have the first scientific results available for the public later this month.
The probe's equipment is gathering data on the mineral and chemical composition of Mercury's surface, its magnetic field, its surface topography and its interactions with the solar wind, according to scientists working on the project.
By the time the mission is completed, scientists also hope to get answers on why Mercury is so dense, its geological history, the structure of its iron-rich core and other issues.
Messenger stands for 'Mercury Surface, Space Environment, Geochemistry and Ranging'. Launched in 2004, it already has flown past Venus twice and Earth once en route to Mercury.
"This was fantastic," Paul said. "We were closer to the surface of Mercury than the International Space Station is to the Earth."
The only previous times Mercury was visited by a spacecraft were in 1974 and 1975 when Nasa's Mariner 10 flew past it three times and mapped about 45 per cent of its surface.
With Pluto now considered a dwarf planet, Mercury is the solar system's smallest planet, with a diameter of 4879 km, about a third that of Earth.
Robinson said the probe's seven scientific instruments were turned on, although some may not be fully utilised until it goes into orbit three years from now.
A surface feature of great interest to scientists is the Caloris basin, an impact crater about 1300 km in diameter, one of the biggest such craters in our solar system. It likely was caused when an asteroid hit the planet long ago.
By studying material in the crater, scientists hope to learn about the subsurface of the planet.
Mercury experiences the largest swing in surface temperatures in our solar system. When its surface faces the sun, temperatures hit about 425 Celsius, but when its faces away from the sun they can plummet to minus-185 C.
- REUTERS
