On December 26, the Messenger spacecraft will have traveled three billion miles since its launch, marking somewhat more than 60 percent of the probe's journey toward its destination to be inserted into orbit about Mercury.

"That Messenger's odometer reading has reached another major milestone reminds us of the long and complex route that our spacecraft must follow," offers Principal Investigator Sean Solomon, of the Carnegie Institution of Washington.

"The year now ending has seen the first two spacecraft flybys of the innermost planet in more than three decades, encounters that have yielded a rich lode of new observations. The journey is far from over, but Messenger has a skilled team to guide it the rest of the way."

Mercury orbits deep within the Sun's gravity well. So, even though the planet can be as close as 82 million kilometers (51 million miles) from Earth, getting the probe into orbit around Mercury depends on an innovative trajectory using the gravity of Earth, Venus, and Mercury itself to slow and shape the probe's descent into the inner solar system.

On its 4.9 billion-mile journey to becoming the first spacecraft to orbit the planet Mercury, Messenger has flown by Earth once, Venus twice, and Mercury twice. Still to come is one more flyby of Mercury in late September 2009.

Today the spacecraft entered its fourth superior solar conjunction of the mission, placing it on the opposite side of the Sun from Earth. The Sun-Earth-probe angle will be between 2 degrees and 3 degrees until January 6, 2009, so during the next two weeks there will be no communication with the spacecraft.

To support the conjunction period, the Messenger team performed several activities to prepare the spacecraft and keep it safe, explains Messenger Mission Operations Manager Andy Calloway.

Examples include extension of the onboard command detection timer and inclusion of attitude alternations to avoid an autonomous propulsive burn to unload spacecraft angular momentum.

In addition, all instruments have been turned off except for the Gamma-Ray Spectrometer sensor, which will remain in a maintenance mode to control closely the temperature of its cryogenic cooler.

"The team will gather for a spacecraft health assessment on the first contact after the conjunction, and then the payload will be powered on again, timers will be restored, and nominal operations will resume," says Calloway, of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

"The next superior solar conjunction lasts only five days - from June 6 to June 10 - and will therefore not require such extensive preparations." The next long conjunction spans about two weeks beginning on November 2, 2009.

The Messenger spacecraft is a little more than two years from reaching its final destination, but the mission Science Team has been collecting data and sharing it with the larger scientific community. Those plans and results are available online.