The launch of the Mercury Messenger probe, which will conduct a detailed investigation of the first planet from the Sun, has been delayed.
The spacecraft will blast off from Kennedy Space Center in Florida
The spacecraft was due to launch at 0616 GMT on Monday from Cape Canaveral in Florida, US, aboard a Boeing Delta 2 rocket for a seven-year journey.
The rocket's lift-off has been delayed due to bad weather.
After arriving in 2011 the probe will orbit Mercury for a year to explore its atmosphere, composition and structure.
The launch will now stand down for 24 hours. US space agency officials said ground controllers had decided to scrub the launch due to thick clouds.
The poor weather conditions are understood to be residual effects of tropical storm Alex. Nasa said there was a 30% probability that the same conditions would be encountered on Tuesday.
The rocket carrying Mercury Messenger has a 13-day window in which to launch.
It is the first US mission to Mercury for more than 30 years.
During its 7.9bn km voyage it will also make one flyby of Earth, two of Venus and three of Mercury before slipping into orbit around the Solar System's innermost planet.
The probe will gather information on the composition and structure of Mercury's crust, its geological history, polar regions, atmosphere and magnetic environment as well as the make-up of its core.
Mission scientists hope the $427m mission will help solve some of the mysteries surrounding this enigmatic world.
Mercury is probably the least explored of the so-called rocky planets that also include Venus, Earth and Mars.
In 1973, Nasa launched the Mariner 10 spacecraft which flew by the planet three times, but was only able to photograph 45% of its surface.
Astronomers already know that Mercury - which has a comparable denisty to Earth even though it is much smaller - is about two-thirds iron. But why it became so much richer in this metal than the other inner planets is unknown.
Some scientists think the planet may once have been rocky but lost its exterior, perhaps through exposure to solar winds or through some giant impact.
Sean Solomon, principal scientist for the mission, said he also wanted to understand why the Solar System's inner planets (Mercury, Venus, Earth and Mars) are so different from its outer planets like Jupiter and Saturn.
For this it was important "to study the most extreme of those outcomes, and that's Mercury," Dr Solomon explained.
Mercury is also the only inner planet besides Earth with a global magnetic field. Scientists hope to learn how this formed.
Fire and ice
One of the most intriguing mysteries about Mercury, however, is the apparent presence of ice on its surface.
On a planet where surface temperatures at the equator can reach 450C (about 840F), any ice must stay out of the Sun, inside the shadowy interior of craters at the poles.
Radio telescopes on Earth have detected the signature of ice in craters at some shaded, high latitude regions, where the temperature could remain below -184C (-300F).
But some scientists caution that the "ice" could actually be super-frozen silica or something else.
Mercury Messenger carries seven scientific instruments to characterise the properties of its target planet. It will operate at room temperature behind a screen of heat-resistant ceramic fabric.
THE PLANET MERCURY
Closest planet to the Sun
Mercurian year: 88 days
Has global magnetic field
The 1.2-tonne spacecraft also carries a heat radiation system and will pass only briefly over the hottest regions of the planet, so limiting exposure to the intense heat bouncing back from Mercury's broiling surface.