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MESSENGER Mission News
September 29, 2008

MLA Ready to Range to Mercury's Surface
One week from today, the MESSENGER spacecraft will fly by Mercury for the second time this year. As part of the final preparations for this encounter, the Mercury Laser Altimeter (MLA) has been powered on after having been off since shortly after the first flyby at the beginning of the year. The entire MESSENGER science payload is now powered and configured to collect data during next week’s encounter.

“Right after the January flyby, the MLA completed passive observations of Mercury, without the laser firing, as a calibration,” explained MLA Instrument Scientist Olivier Barnouin-Jha of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “At that point it was switched off, and it has remained off since that time.”

During MESSENGER’s first Mercury encounter, the MLA provided the first direct measurements of the topography of Mercury from spacecraft. The results provide evidence for a complex geologic history and indicate that Mercury’s craters are shallower than those on the Moon at a given crater diameter, as expected because of the higher surface gravity.

“Unlike the topographic data obtained during the first flyby, which were of terrain for which we have no space-based imaging, some of the area to which MLA will range during this second encounter was imaged by the Mercury Dual Imaging System (MDIS) during the first Mercury flyby,” Barnouin-Jha said. Moreover, terrain sampled by MLA during the first flyby will in turn be imaged by MDIS during this visit.

“So this second flyby will allow the first inter-comparison between the topographic observations and high-resolution spacecraft images,” he added.

Imaging Plans for MESSENGER’s Second Mercury Flyby

Mariner 10, the only spacecraft to visit Mercury prior to the MESSENGER mission, imaged about 45% of the planet’s surface. In January, MESSENGER’s Mercury Dual Imaging System (MDIS) captured an additional 21% of Mercury’s surface. During its upcoming encounter with Mercury, the 1,287 planned MDIS images will cover much of the remaining portion of Mercury’s surface not yet seen by spacecraft. A map of Mercury’s surface with images from Mariner 10 overlaid by mosaics by the MDIS narrow-angle camera (NAC) acquired during MESSENGER’s first Mercury flyby is available online at http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=205.

MESSENGER’s first flyby of Mercury covered two general areas of Mercury surface: the crescent view of Mercury seen as the spacecraft approached the planet and the fuller view of Mercury acquired as the spacecraft departed. Similarly, Mercury will appear as a thin crescent during the inbound portion of MESSENGER’s second Mercury flyby and as a nearly full disk during the outbound portion of the encounter. The areas of the surface that will be imaged by the NAC are shaded in purple in the figure. One week from today, as may be seen from the figure, spacecraft imaging of Mercury’s surface will be nearly global in coverage for the first time.

Experience MESSENGER’s Second Mercury Flyby Virtually

See Mercury through the “eyes” of MESSENGER’s imagers with the Mercury Flyby Visualization Tool, now available at http://messenger.jhuapl.edu/encountersm2/. This updated Web feature offers a unique opportunity to see simulated views of Mercury from MESSENGER’s perspective, during approach, flyby, and departure, or in real-time (as the observations actually occur).

This tool combines the best available image map of Mercury’s surface with observation sequences for the Mercury Dual Imaging System (MDIS), Mercury Atmospheric and Surface Composition Spectrometer (MASCS), and Mercury Laser Altimeter (MLA). The map of Mercury’s surface combines Earth-based low-resolution radar images from the Arecibo Observatory in Puerto Rico, high-resolution image mosaics from the Mariner 10 spacecraft flybys of Mercury in 1974 and 1975, and images from MESSENGER's first flyby of Mercury on January 14, 2008.

There are many helpful tips available on the pages of this visualization tool. Pointing and clicking on any color bar will display the projection of each completed image mosaic on Mercury or show the end of the active MLA or MASCS observation. Information accompanying each simulated image includes the latitude and longitude of the point at the center of each image, the resolution in meters (or kilometers when farther from the planet) per pixel (picture element) at the image center, the altitude (how far the spacecraft is above Mercury's surface), and the time relative to closest approach.

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.

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