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.
These images are from MESSENGER, a NASA Discovery mission to conduct the
first orbital study of the innermost planet, Mercury. For information
regarding the use of images, see the MESSENGER image use policy.