The MESSENGER spacecraft, recently injected into orbit about the Solar Systems' inner-most planet is already yielding important new information about Mercury. This spacecraft carries a laser altimeter and, in its first two months of operation, has already built up a grid of ground tracks that span most of Mercury's surface in its northern hemisphere.

This grid provides a very good measure of the surface shape, and such contours offer clues to the interior dynamical and geological processes that have taken place in past eons.

Since MESSENGER is in a highly elliptical orbit, the altimeter is useful only when the spacecraft is near periapsis at about 1,800 km from the surface. Other techniques can be employed to measure the shape of Mercury's southern hemisphere. For example, the MESSENGER team is creating three-dimensional terrain models from stereo images obtained from other instruments and modeling techniques.

Stereo imaging is accomplished by photographing regions of Mercury's surface at various viewing geometries. The first step in the conversion of stereo images to topographic models is the identification of common points in a given area. For each image a line of sight is constructed from the camera location to each common point.

The intersection of the lines of sight to a given common point from two viewing geometries constrains the position of the point on the surface of a topographic model. As the number of common points increases the shape of the area can be imaged with higher and higher resolution.

The use of radio occultation techniques provides an independent means of measuring the shape of the planet. Occultation occurs when the planet blocks radio waves sent from MESSENGER to Earth.

By carefully measuring the times of disappearance or appearance of the radio signal at the beginning or end of an occultation, the science team determines the local radius of the planet. Another technique for measuring the planet's shape involves capturing images of the outer edge of the sunlit planet.

Combining images from various sources allows increased accuracy. Ultimately, the MESSENGER science team will accurately determine the global shape of Mercury for the first time.

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