To put a suite of instruments in orbit around Mercury, flight planners faced three major obstacles: Keeping MESSENGER's weight low enough to fly on a relatively inexpensive Delta 2 rocket, a critical consideration for a low-cost Discovery-class mission; figuring out a way to slow the probe enough to get into orbit; and keeping the spacecraft cool in the inferno of the inner solar system.

Artist's impression shows MESSENGER departing Earth. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

In theory, a spacecraft launched by an expensive heavy-lift rocket could make the trip from Earth to Mercury in just three-and-a-half months. But the spacecraft would have to carry an enormous amount of rocket fuel, so much that little room would be left for the scientific payload.

"Several people have asked me, can't we get there a lot faster? And yes, instead of six-and-a-half years, we could get there in there in three-and-a-half months," said Robert Farquhar, MESSENGER mission manager at Johns Hopkins University's Applied Physics Laboratory. "The only trouble is, we can't use the Delta 2 launch vehicle, we have to go to a much larger launch vehicle like an Atlas 5 or a Delta 4.

"And then, unfortunately, we have to use a lot more fuel. With the current spacecraft, it's about (1,323 pounds) of fuel and a (1,100-pound) dry weight spacecraft, which means it's about 55 percent fuel. Well, that's going pretty far as it is. But if you tried to do it in just one swoop where you come down in three-and-a-half months, then the spacecraft is going to be 85 percent fuel. And we haven't built anything like that recently. And if we did, there wouldn't be much room left for any instruments."

For years, Solomon said, scientists believed it was impossible to put a probe in orbit around Mercury using chemical propulsion rockets. Probes launched directly to Mercury would get there traveling around 25,000 mph relative to the planet, roughly the approach velocity of an Apollo moonship returning to Earth.

Then, in the mid 1980s, Chen-wan Yen at the Jet Propulsion Laboratory in Pasadena, Calif., figured out a way to use the gravity of the inner planets to reduce a spacecraft's inbound approach velocity to Mercury.

Mariner 10 used four planetary flybys for its mission and "we're going to try to one up them a little bit, we're going to be doing six planetary flybys," Farquhar said. "This is a very complicated flight path."

"We're flying by the Earth one time, Venus twice and Mercury three times and eventually going into Mercury orbit," he said. "Why is this such a complicated thing, why are we taking so long to get there? ... We have to gradually slow down the spacecraft relative to the planet Mercury so it doesn't cost us as much fuel to get into orbit."

Launch originally was targeted for March 10, getting MESSENGER to Mercury in April 2009, but the flight was delayed to May 11 because of late hardware deliveries and the discovery of delaminated circuit boards. Arrival at Mercury slipped to July 2009. NASA managers then ordered additional testing, focusing on the spacecraft's fault protection system, which delayed the May launch to Aug. 2 and pushed Mercury arrival to March 2011.

Here are the major milestones in MESSENGER's flight (assuming an Aug. 2 launch):

• 08/02/04: Launch from the Cape Canaveral Air Force Station
• 08/01/05: Earth flyby at 1,777 miles
  
• TBD: Deep Space Maneuver No. 1
  
• 10/24/06: Venus flyby No. 1 at 2,239 miles
• 06/06/07: Venus flyby No. 2 at 186 miles
  
• TBD: Deep Space Maneuver No. 2
  
• 01/15/08: Mercury flyby No. 1 at 124 miles or lower
  
• March 2008: Deep Space Maneuver No. 3
  
• 10/06/08: Mercury flyby No. 2 at 124 miles or lower
  
• December 2008: Deep Space Maneuver No. 4
  
• 09/30/09: Mercury flyby No. 3 at 124 miles or lower
  
• November 2009: Deep Space Maneuver No. 5
  
• 03/18/11: Mercury orbit insertion
• 03/18/12: End of primary mission

MESSENGER will follow a looping, seven-year trek from Earth to Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER's main engine will burn more than one-third of the craft's propellant during a 15-minute firing to put the probe into an elliptical orbit tilted 80 degrees to Mercury's equator. The low point of the 12-hour orbit - roughly 124 miles up - will be positioned at 60 degrees north latitude, allowing high-resolution imagery of the 840-mile-wide Caloris impact basin, the largest known feature on the planet.

MESSENGER's orbit must be adjusted with two rocket firings every 88 days - each mercurian year - to counteract the effects of the sun's radiation pressure. Without those firings, radiation pressure and other subtle forces would slowly raise the low-point of MESSENGER's orbit and change the orbit's tilt.

MESSENGER's primary mission is budgeted to last one Earth year, but if fuel is available and if NASA provides additional funding, the mission could be extended.

"We have a limited amount of fuel on the spacecraft and so we're going to try to budget the fuel very carefully throughout the mission so we'll have the opportunity for an extended mission if we get the extra funding," Farquhar said.

A mission extension, which Figueroa said was all but certain if the spacecraft remained healthy, would permit scientists to take additional risks in a bid to collect bonus data.

"If we do have this extra fuel and we complete all the objectives of our primary mission, then we could think about the possibility of taking a little extra risk and going in closer, say, getting down to like (18 miles) altitude and getting much higher resolution images," Farquhar said. "So that would be the one thing I would like to see at the end."

But at some point, MESSENGER will run out of fuel and it will be subject to natural forces that eventually will result in a crash landing on Mercury. Should any being ever stumble upon the wreckage, an American flag will show its origin.

MISSION PREVIEW
   PART 1: MESSENGER MISSION READY TO LAUNCH
   PART 2: THE LONG AND WINDING ROAD
   PART 3: THE 'HEAT' IS ON
   PART 4: A COMPLEX SCIENCE AGENDA