Seven years ago, on August 3, 2004, MESSENGER left Earth aboard a three-stage Boeing Delta II rocket to begin a journey that would take it more than 15 laps through the solar system, through six planetary flybys, and ultimately into orbit around Mercury.

The spacecraft has travelled 5.247 billion miles (8.445 billion kilometers) relative to the Sun, and the team is one-third of the way through the one-year science campaign to understand the innermost planet.

"As exciting as it was at the time, MESSENGER's launch seems a long time ago," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "Our spacecraft is now in the prime of its life, enjoying a year of sustained excitement as spectacular new observations of Mercury are acquired and sent to Earth every day."

As of July 30, the team has been taking observations of the planet's surface for two Mercury sidereal days. Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky. One full rotation of Mercury about its spin axis relative to the distant stars is a sidereal day, about 59 Earth days.

"This is certainly a great milestone for us, since the spacecraft has now experienced every thermal and solar eclipse season at least once," says Mission Operations Manager Andy Calloway.

"But we're really looking forward to one complete Mercury solar day. This milestone will be achieved after three full Mercury rotations relative to the stars, but more importantly it will reset the clock for Mercury's rotation relative to the Sun and is one-half of our primary mission phase."

MESSENGER's primary mission is divided into two Mercury solar days, so that lighting conditions can be repeated for filling gaps in imaging, targeted observations can be optimized, and stereo imaging can be accomplished.

Mercury is the only planet in the solar system with three rotations for every two trips around the Sun. Mercury's day and year were once thought to have the same length, which would mean that Mercury would keep the same face to the Sun much as the Moon does to Earth, until radar observations in 1965 confirmed the 3:2 resonance.