The MESSENGER spacecraft has crossed the four-billion-mile mark since its launch. The probe has completed about 81 percent of its journey toward its destination to be the first spacecraft to orbit the planet Mercury.

That MESSENGER's odometer reading has reached another major milestone reminds us of the long and complex route that our spacecraft must follow. 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 that uses 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 trek, MESSENGER has flown by Earth once, Venus twice, and Mercury three times.

"Four billion miles, more than 43 times Earth's distance from the Sun, is an impressive figure," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington.

"But MESSENGER is a well-built vehicle, with many more miles of productive work ahead. The Mercury orbital phase of our mission is barely one year away, and the team is hard at work to ensure that we are ready for the intensive activity that awaits."

Throughout its long journey toward the orbital phase of the mission, MESSENGER has remained quite busy. In addition to completing six planetary flybys and five deep-space maneuvers, a wide variety of tests have been conducted to characterize the performance of the science payload and the spacecraft subsystems.

On February 22, the team conducted the first in a series of short solar-array-offset-characterization tests. "These exercises are designed to improve our model of solar-array performance prior to orbit, and each is a simple test that can be executed in a few minutes without load management," explained MESSENGER Mission Project Manager Peter Bedini, of The Johns Hopkins University Applied Physics Laboratory.

The test measures the output of each solar array wing individually by placing it at a 72 degrees Sun-offset angle. During the 5-minute measurement, the other wing is rotated off of the Sun at 95 degrees so that it contributes no power.

The measurement will be repeated at several solar distances to allow for correlation of results with those from earlier tests at ~0.5 astronomical units (AU), as well as additional insight into orbit performance at ~0.3 to 0.45 AU.

MESSENGER will enter orbit about Mercury on March 18, 2011.