MESSENGER mission operators at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., have received the first eight optical navigation images from the spacecraft. "We're going to be taking these images every day, up until just before the flyby, to make sure that we are on target for our aim point above the surface of Mercury," said Louise Prockter of APL, the instrument scientist for the Mercury Dual Imaging System (MDIS).

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. Because MESSENGER will fly very close to the surface of Mercury during the January 14 flyby, optical navigation is used to provide an independent method for finding and correcting subtle errors in the trajectory. "This information will enable the navigation team to verify that the spacecraft is in the correct position for the flyby, or whether a last-minute maneuver will be needed to avoid either coming too close to the planet or missing it by so much that a large amount of fuel would be needed to return the probe to its optimum trajectory," Prockter noted.

To determine the position of the spacecraft, it is necessary to see the target body - in this case, Mercury - in the same field of view as the background star field, using MDIS. "The stars are far away, so their positions may be assumed to be fixed in space," Prockter said. "The position of Mercury along its orbit is also well known from hundreds of years of ground-based telescopic observations. Thus, by comparing where Mercury is in the field of view to the stars visible behind it, and by controlling where the camera is pointing, we can estimate the position of the spacecraft."

The MDIS instrument consists of two imagers, a Wide Angle Camera (WAC) with a 10.5o field of view, and a Narrow Angle Camera (NAC), with a 1.5o 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.

"Unfortunately, such long exposures tend to saturate bright objects, such as Mercury, making it difficult to image both the planet and the stars in the same WAC image," Prockter said. "The NAC is not sensitive enough to see stars, but it has a resolution ~7 times better than the WAC and is excellent for imaging the planet limb."

To carry out optical navigation with MESSENGER, the team uses a combination of the two imagers, taking a star image with the WAC, then quickly switching to the NAC and taking 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 team has obtained practice optical navigation images on previous flybys of Earth and Venus, but the Mercury encounter is the first time we have used this method 'for real' to determine the position of the MESSENGER spacecraft," Prockter said.

NASA Teleconference to Preview MESSENGER's Flyby of Mercury

NASA will host a media teleconference at 1 p.m. EST on Thursday, January 10, to preview MESSENGER's historic January 14 flight past Mercury. The briefing participants are:

• Marilyn Lindstrom, MESSENGER program scientist, NASA Headquarters, Washington
• Sean Solomon, MESSENGER principal investigator, Carnegie Institution of Washington
• Eric Finnegan, MESSENGER mission systems engineer, Johns Hopkins University Applied Physics Laboratory, Laurel, Md.
• Faith Vilas, MESSENGER participating scientist and director, MMT Observatory at Mt. Hopkins, Ariz.

To participate in the teleconference, reporters should call 1-888-398-6118 and use the pass code "Mercury." International journalists should call 1-210-234-0013. Audio of the teleconference also will be streamed live at: http://www.nasa.gov/newsaudio

As the flyby continues toward closest approach, additional information and features will be available online at http://messenger.jhuapl.edu/mer_flyby1.html, so check back frequently. Following the flyby, be sure to check back to see the latest released images and science results!

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus, and Mercury will start a yearlong study of its target planet in March 2011. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as principal investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.