MESSENGER mission operators at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., have received the first optical navigation images from the spacecraft.

"We will be taking seven additional sets over the next three days as the spacecraft approaches the planet,' said APL's Eric Finnegan, the Mission Systems Engineer.

Optical navigation is commonly used to tie the position of a spacecraft to the position of a target body to ensure a safe and well-positioned flyby, particularly when the position of the target body is uncertain or if the navigation process has not been validated in flight.

"During the first encounter with Mercury, both of these issues were of concern to mission planners," Finnegan explained. "However, following the highly accurate flyby in January, the necessity of these images for critical trajectory planning was removed."

"For successful optical navigation, we need to see the target body in the same image sequence as the background star field,' said MESSENGER's Navigation Team Chief Ken Williams of KinetX, Inc. "Stars are far away, so to us, it appears that their positions are fixed in space. By comparing where Mercury is in the field-of-view with the stars visible behind it, and by controlling where the camera is pointing, we can estimate the position of the spacecraft."

The Mercury Dual Imaging System (MDIS) instrument consists of two imagers, a wide-angle camera (WAC) with a 10.5 degrees field of view, and a narrow-angle camera (NAC), with a 1.5 degrees field of view. These imagers are always pointed at the same place, and the NAC footprint falls in the center of the WAC footprint.

The WAC has a filter specially designed for imaging stars, most of which are so faint that long (up to 10-second) exposures are required.

The MESSENGER team employs both cameras for optical navigation, taking a star image with the WAC, and then quickly switching to the NAC for an image of the planet limb. Because the images are taken within seconds of each other, they can be used to see where the planet is compared with the star field.

The navigation images snapped during this flyby will also help the team plot MESSENGER's yearlong orbital survey of Mercury, which begins in March 2011. MESSENGER will fly very close to the surface of Mercury - within 200 kilometers (124 miles) - during the October 6 flyby, as it did in January. However, during this encounter, the navigation team will rely only on radiometric tracking data during closest approach.