Right around noon today, if all goes as planned, a spacecraft called Messenger will swoop past the planet Mercury and begin two days of unprecedented picture-taking and data-collecting.
The flyby, the first visit to Mercury in more than 33 years by an emissary from Earth, will mark a key moment in a NASA mission that will ultimately place the first satellite into orbit around the tiny planet that sits closest to the sun.
The planetary science community is eagerly awaiting images and information that should shed light on some of the enduring mysteries about the planet -- such as where in the solar system it was formed and why its hard metal core is so large and its outer rock crust so scant, compared with those of Earth and the other rocky planets.
"Mercury is a difficult place to get to, and it's taken a long time to get back," said principal investigator Sean Solomon, who has worked on the mission for more than 11 years. "But now we're in place to learn things about one of our few sister rocky planets, and we're ready for some real surprises."
The desk-size spacecraft was launched in 2004 and has taken a circuitous path to Mercury, swinging twice by Venus and once by Earth for gravity assists. Messenger will make two more passes by Mercury to let the planet's gravity slow it down enough for it to swing into orbit in 2011.
Still, today's whisker-close flyby will be, NASA officials say, a high point of the mission. Not only will the spacecraft pass within a record 124 miles of Mercury's surface at a relative speed of more than 16,000 mph, but it also will quickly begin sending back its first observations of the physical and magnetic makeup of the planet, to be made by instruments that could answer some of the most basic questions about Mercury's character and history. It will be the closest pass by Messenger in the entire mission, and the nearest to the planet's equator.
"The biggest mystery of Mercury is why it has so much heavy metal -- a core very different in size from other planets," Solomon said. "We think we can begin to unravel the mystery once we know the chemical makeup of the planet's surface."
There are several competing theories on how Mercury came to be what and where it is. One is that the searing heat of the sun stripped the crust off a once-larger planet and left primarily the core. Another is that the planet collided with another celestial body during a time when the early solar system was cluttered with them. Under this theory, Mercury's outer crust and mantle were smashed away and the planet was knocked into its close-in orbit.
Because Mercury is so close to the sun, designing a spacecraft that could stand the heat and calculating a trajectory that would place Messenger into orbit -- rather than plunging into the sun -- were daunting tasks. Temperatures on the ceramic-cloth sunscreen that protects its instruments will reach 600 degrees Fahrenheit.
The spacecraft's instruments were designed never to face the sun because they would otherwise quickly overheat and be destroyed, and even pointing them at Mercury will be done for very limited periods. On the side facing the sun, the planet reaches 1,100 degrees at the equator, and on the dark side, it drops to as low as 300 degrees below zero.
The name of the probe, Messenger, is an the acronym for Mercury Surface, Space Environment, Geochemistry and Ranging. It is the first spacecraft to visit Mercury since Mariner 10 in 1974, at a cost of $446 million for the life of its mission. Reflecting the new technologies and logistical knowledge that made the mission and its unprecedented orbiting possible, the European and Japanese space agencies will also be sending spacecraft to Mercury soon.
The planet they will scrutinize has craters, volcanoes, and many unusual and unexplained features, including what might be frozen water in polar crevasses protected from the sun. Its mass is only 5 percent that of Earth's, but its metal core accounts for 60 percent of that mass (compared with about 30 percent for Earth and Venus, and 20 percent for Mars). Mercury also has an active magnetic field in its thin atmosphere, the only rocky planet other than Earth with that feature. And it contains one of the largest impact craters in the solar system -- the Caloris Basin, which is 800 miles in diameter, or about a quarter of the planet's diameter.
Messenger's approach to Mercury is chronicled in this image taken by the Wide Angle Camera on Messenger. 
Today, NASA's spacecraft , or the '''ME'''rcury '''S'''urface, '''S'''pace '''EN'''vironment, '''GE'''ochemistry, and '''R'''anging spacecraft, is expected to begin its two day mission at about noon (), of data collecting and photographing of the planet Mercury. It is the first spacecraft to visit the planet in 34 years, since 's visit to the planet in 1974.

"This is raw scientific exploration and the suspense is building by the day. What will MESSENGER see? Monday will tell the tale," said Alan Stern, associate administrator for NASA's Science Mission Directorate in Washington, D.C..

This encounter will provide a critical gravity assist needed to keep the spacecraft on track for its March 2011 orbit insertion, beginning an unprecedented yearlong study of Mercury. The flyby also will gather essential data for mission planning. It will flyby an impact crater called the  which is almost 800 miles (1,287 kilometers) in diameter. The basin is one of the largest impact craters in our solar system.

"Caloris is huge, about a quarter of the diameter of Mercury, with rings of mountains within it that are up to two miles high. Mariner 10 saw a little less than half of the basin. During this first flyby, we will image the other side," said Louise Prockter, the instrument scientist for the Mercury Dual Imaging System at the  in , Maryland.

It also will study the global magnetic field and improve our knowledge of the gravity field from the Mariner 10 flyby. The long-wavelength components of the gravity field provide key information about the planet's internal structure, particularly the size of Mercury's core. The flyby also will map Mercury's tenuous atmosphere with ultraviolet observations and document the energetic particle and plasma of Mercury's magnetosphere. In addition, the flyby trajectory will enable unique particle and plasma measurements of the magnetic tail that sweeps behind Mercury.

MESSENGER was launched on August 3, 2004 and will travel just under five billion miles in total. It already has flown past Earth once and Venus twice. The spacecraft will use the pull of Mercury's gravity during this month's pass and others in October 2008 and September 2009 to guide it progressively closer to the planet's orbit. Insertion will be accomplished with a fourth Mercury encounter in 2011.