The US space agency is set to launch a spacecraft to Mercury on 2 August, its first visit to the planet in 30 years.
Messenger will conduct an in-depth study of the whole planet
The plan is to put the probe in an orbit around the heavy-metal planet to map its entire surface, study its geology and look for frozen water.
The Messenger spacecraft will make a seven-year journey that loops around Earth and Venus to get to Mercury.
The last Nasa craft sent to the planet was Mariner 10 which sailed past the world three times in 1974 and 1975.
Messenger will not make its first fly-by of the planet closest to the Sun until 2008, and will not begin its main mission until 2011.
Its 7.9-billion-km (4.9 billion miles) trip will see it loop the Sun 15 times, fly-by Earth once, Venus twice and Mercury three times.
During the Mariner 10 passes, detailed data was only collected for half of the planet's surface.
Messenger (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) will conduct an in-depth study of Mercury in its entirety. Carrying seven scientific instruments, it will provide the first images of the whole planet.
It will also collect information on the composition and structure of Mercury's crust, its geological history, the nature of its thin atmosphere and magnetic field, and the make-up of its core and polar materials.
Sean Solomon, of the Carnegie Institution of Washington, the lead investigator on the project, described 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 by up to 600C (1,100F), with a daytime temperature around 450C (840F)," he explained.
The $426m 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.
Even so, astronomers wonder if there is frozen water somewhere on the surface despite the heat.
"One of the most bizarre questions that has 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," Dr Solomon said.
THE PLANET MERCURY
Closest planet to the Sun
Mercurian year: 88 days
Has global magnetic field
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.
Dr Solomon said: "The floor of a shadowed crater would never see the Sun and would be cold enough - minus 180C (300F) or colder - to freeze water for the lifetime of the planet."
Messenger should eventually have company in orbit around Mercury. BepiColombo, a collaboration between Europe and Japan, is a twin spacecraft set for launch to the planet early in the next decade.