From a distance of approximately 4 million kilometers (2.5 million miles), MESSENGER's Mercury Laser Altimeter (MLA) imaged the half-moon shape of Mercury one week after the January 14, 2008, flyby of the planet. This image was produced by recording the noise counts in the MLA detector as the spacecraft scanned slowly across the face of the planet. Higher counts are shown as red in this image and lower counts as blue. The scale of the image is given in milliradians (mrad), a unit of angular size, relative to the calculated center of Mercury. At a distance of 4 million kilometers, 1 mrad covers a size of about 4000 kilometers (2500 miles).
This activity was coordinated with scans conducted by the Mercury Dual Imaging System (MDIS) and the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instruments (shown as the wider patterns near the bottom of the image) in order to co-align the instruments and the spacecraft inertial reference frame. The crosshairs show the calculated position of Mercury, while the circle is a fit to the crescent shape of the planet as determined by the MLA measurements. The offset between the crosshairs and the center of the circle is only roughly 0.05 mrad, which is well within the 0.4-mrad field of view (FOV) of the MLA detector, shown in the top left corner of the image. The MLA detector FOV is designed to allow the altimeter receiver to cover the 0.08-mrad-diameter laser spot when the instrument is within a range to the planet of 1800 kilometers (1100 miles), so it produces a somewhat blurry image at the much greater distance of this image. Observations such as this one continue to improve the calibration of the MESSENGER spacecraft and its science payload.
Credit: NASA/Goddard Space Flight Center/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
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