Mercury, the planet with the fastest orbit around the sun, was named after the fleet-footed messenger to the Roman gods. But NASA's spacecraft to Mercury will be a little less than mercurial. It will take nearly seven years from the scheduled launch of the Messenger spacecraft Monday until it enters Mercury's orbit.

Scientists say the unmanned, compact-car-size spacecraft -- designed and built at the Johns Hopkins University Applied Physics Laboratory in Laurel -- will provide important hints about how Mercury was formed and how it interacts with the sun. Messenger is short for MErcury Surface, Space ENvironment, GEochemistry and Ranging.

At its peak last year, the $426 million Messenger project involved 800 people working at the lab, supervising engineer James Leary said. In addition, APL engineers and scientists supervised 50 contractors from 24 states and six countries. For the past few weeks, a smaller staff has been putting the finishing touches on the project at APL's mission operations center and at Cape Canaveral Air Force Station in Florida, where Messenger is scheduled to lift off at 2:16 a.m. Monday atop a Delta II rocket.

"The guys are really anxious to get on with it and get it launched," said mission program manager David Grant of Glenelg, who is responsible for the day-to-day management of the mission. "We've gone through a lot of training, spent a lot of hours, and we're ready."

A second-by-second countdown can be viewed at messenger.jhuapl.edu. If there are delays, the launch could occur any night until Aug. 14.

Mercury remains a mystery, and Messenger will be the first spacecraft to orbit the planet closest to the sun. Messenger will seek to answer questions that have lingered since Mariner 10 flew by the planet three times in 1974 and 1975. The Mariner photos captured only half the planet, said Deborah Domingue, Messenger's deputy project scientist. Only features a kilometer (0.6 mile) or larger could be seen. Messenger will be able to see things as small as a volleyball court.

The heat on Mercury presents one of the greatest challenges for the spacecraft. The planet is just 36 million miles from the sun; Earth is 93 million miles from the sun. During tests, Messenger's components had to withstand temperatures up to 800 degrees. Many orbiting spacecraft, such as the Hubble Space Telescope, never turn their instruments toward Mercury because of the risk that they would burn up.

"We've been trying to do this for about 30 years, and the technological challenges are so great that it's taken about that long for somebody to come up with a plan in order to get to the planet," Leary said. "First of all, you have to do it in an efficient way, and then you need to have the technology in order to survive and protect yourself when you get there."

Messenger is the 61st spacecraft built at APL since the lab constructed the earliest satellite navigation system, known as Transit, in the late 1950s. The space program accounts for about 20 percent of the lab's $600 million budget, most of the budget comprising contracts with the Navy and other government agencies, said lab spokeswoman Dee Reese.

With 3,350 employees, the lab is Howard County's largest employer. In addition to Grant, several other key players in the Mercury mission live in Howard, including mission manager Robert Farquhar of Columbia, payload manager Robert Gold of Columbia and project scientist Ralph McNutt of Ellicott City.

From the time it leaves the launchpad in Florida, Messenger will be controlled at APL. The operations team there will communicate with Messenger through NASA's Deep Space Network of antenna stations, located in California's Mojave Desert, near Madrid and near Canberra, Australia. The control center will receive scientific data and information about the spacecraft's status and will, in turn, send commands to Messenger.

If things go as planned, the 1.2-ton spacecraft will orbit the sun 15 times, come by Earth in August 2005, by Venus in 2006 and 2007, and past Mercury three times in 2008 and 2009, before finally reaching orbit there in March 2011 after a journey of 4.9 billion miles.

The Messenger team faced the challenge of ultimately slowing the spiraling spacecraft enough to get it into orbit around Mercury. Instead of slowing down the craft with an expensive, fuel-consuming rocket, APL planners decided to use the gravity of three planets. That will extend the trip to 61/2 years. The 13-day launch window was chosen because it takes advantage of the proper planetary alignment to allow the spacecraft to enter into orbit using minimum fuel.

During the flybys of the other planets, scientists will gain important information that will help them prepare for the year-long, egg-shaped orbit of Mercury, Domingue said. The spacecraft ultimately will travel as low as 125 miles above Mercury's surface.

NASA had originally intended to launch Messenger in March, but the launch was delayed because of technical problems and the need for additional testing, Leary said. If it had launched during the March flight window, Messenger would have required fewer flybys and only five years to reach Mercury, he said. To minimize fuel consumption, Messenger is the first spacecraft made entirely of graphite composite.

Messenger will use a new heat shield to protect its instruments from the sun, estimated to be about 11 times stronger on Mercury than it is on the Earth. The shield is made of the same substance as the heat tiles on the space shuttle, except that they are stretched into a fiber and woven into a cloth, minimizing weight and cost. The shield will allow Messenger to use standard spacecraft instruments, eliminating the need for a more active onboard cooling system for the spacecraft.

Messenger's 12 months in orbit will cover just two of Mercury's solar days, that is, two sunrises and sunsets. It will take 12 hours for Messenger to orbit Mercury once. The most detailed data will be collected during the half-hour that the spacecraft is closest to the surface, Leary said. When the spacecraft returns to the farthest parts of its orbit, its instruments will have time to cool off, he said.

Mercury is the least-explored of the four terrestrial planets (the others are Venus, Earth and Mars). Scientists think that these planets started as barren, asteroid-like "planetesimals," Leary said. Studying Mercury will help test scientists' theories about how these planets evolved.

Messenger will reveal new data on Mercury's topography and on the patterns of its gravitational spin. It will also reveal which gases and other elements compose Mercury's surface and its exosphere -- the thin envelope of gas that surrounds the planet. Scientists are particularly interested by what's inside the large craters found on the planet's poles. Some think they could contain ice.

The Mariner 10 mission discovered that Mercury is the only planet other than Earth to have a global magnetic field. Earth's magnetic field is constantly changing because of interaction with solar flares and gas emissions from the sun, known as the solar wind. These interactions cause electrical blackouts and interference with radios and telephones. Understanding Mercury's field might help scientists better understand the interaction of the sun with Earth's field, Domingue said.

"In the end, we study other planets to better understand our own world. . . . To understand how life began, that's the ultimate question," she said.

This Delta II rocket will carry Messenger into space during the scheduled liftoff Monday morning.