A NASA space probe is its final days on Earth before rocketing on a mission to understand Mercury, the solar system's innermost planet, early Monday morning.

Sitting atop a Boeing-built Delta 2 rocket, the MESSENGER spacecraft is slated to launch Aug. 2 at 2:16:11 a.m. EDT (0716:11 GMT) from its staging grounds at Pad 17B at Florida's Cape Canaveral Air Force Station. Though Monday space shot has a 12-second window for that day, the mission itself has 12 days to loft MESSENGER in order to reach Mercury.

MESSENGER's principal investigator Sean Solomon told reporters Saturday that the spacecraft represents a boon for astronomers hoping to understand the solar system.

"The family of the four inner planets...share a common origin, they were formed by the same processes but came out different," said Solomon, of the Carnegie Institution of Washington, during a prelaunch briefing Saturday. "Mercury is the most extreme of those planets."

MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission, will spend seven years in transit and make a number of flybys around all three inner planets before settling into orbit around Mercury in March 2011. Researchers hope the $427 million probe's seven science instruments will answer a number of questions about Mercury's surface, interior and magnetic field, as well as provide clues to the formation of the solar system during its one year orbiting the planet.

The mission is also NASA's first Mercury probe since Mariner 10, which flew by the planet three times between 1974 and 1975. While that mission gave astronomers their first glimpse of Mercury, it only imaged half of the surface and raised more questions about the planet's density and magnetic field than it answered.

"It has been 30 years since we visited Mercury last," said Orlando Figueroa, NASA director of solar system exploration, during the briefing. "I was we're long overdue for a visit."

During its lifetime, MESSENGER will be subject to temperatures greater than 800 degrees Fahrenheit (426 degrees Celsius) in an environment where the Sun's intensity can be 11 times greater than it is on Earth. The mission is a cooperative effort between NASA, Johns Hopkins University Applied Physics Laboratory and the Carnegie Institution of Washington.

"It's going to be flying in a very difficult environment," said Robert Strom, a MESSENGER science team member from the University of Arizona who worked on the Mariner 10 mission. Strom said that while an orbiter mission to Mercury was always planned as a follow-up to Mariner 10's flybys, he was especially excited about the robust and hardy nature of MESSENGER.

"Never in my wildest imagination did I think we would have a spacecraft like this to Mercury," Strom said.

The Mercury path

After launch, the spacecraft MESSENGER should take 56 minutes and 43.7 seconds to reach Earth escape velocity and start its multi-year journey to Mercury, said Kris Walsh, director of NASA programs for Boeing's Expendable Launch Systems, during the briefing.

MESSENGER mission engineers plan to spend the next few days after launch performing checks to ensure the health of the spacecraft.

"We want to make sure everything is up and running, leading into our first flyby one year later," said James Leary, MESSENGER mission systems engineer.

That flyby, a swing by Earth in August 2005, is the first of many to help guide MESSENGER on its inward course to the heart of the solar system. Following that first Earth pass, the spacecraft will also swing by Venus, not once but twice - in October 2006 and June 2007. The spacecraft is expected to take its first look at Mercury in January 2008, on the first of three flybys of the inner planet - the second is set for October of that year, with the final pass in September 2009 - before reaching orbit in 2011.

"A lot of people have worked very hard to get to this point, and so far we're good to go," Figueroa said. "Launching is only the beginning."