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Space Topics: MESSENGERMercury Dual Imaging SystemOptical Remote Sensing Instrument for the MESSENGER MissionScientific Objectives - How It Works - Saturn Exploration Context - Camera Facts - Data Release Schedule Scientific ObjectivesThe Mercury Dual Imaging System (MDIS) is designed to produce the first global maps of Mercury. A global monochrome map will have 500 meter-per-pixel resolution (with about 90% of the map having resolution better than 250 meters per pixel), and a global color map will have 2,000-meter-per-pixel resolution (with about 60% of the map having resolution better than 1,000 meters per pixel). The majority of the color data will be obtained during the three flybys. Once in orbit, imaging is challenged by the fact that the orbit is highly elliptical, with periapsis in the high northern latitudes. As a result, the spacecraft approaches northern latitudes much more closely than southern latitudes. The MDIS team plans to acquire images for the global map using the narrow-angle camera for the southern hemisphere and the wide-angle camera for the northern hemisphere. In addition, the narrow-angle camera will be used to obtain very high resolution swaths in the northern hemisphere, with scales down to 20 meters per pixel. In addition to mapping, the cameras are used for optical navigation: images of planetary targets are taken against background star fields and compared to predicted images to ensure that the spacecraft is on course. Typically, an optical navigation observation consists of two images, one taken through the narrow-angle camera with an exposure set for the planetary target, and one taken through the clear filter on the wide-angle camera with a longer exposure to permit imaging of faint stars; typically, the planetary target is saturated in the wide-angle shot. How It WorksMDIS consists of two cameras, a Wide-Angle Camera (WAC) and a Narrow-Angle Camera (NAC). Both cameras produce images 1024 by 1024 pixels in size. The NAC is monochrome, but the WAC is equipped with a 12-position filter wheel. Because of the extreme thermal environment at Mercury orbit, where solar radiation is 4.6-10.6 times stronger than in Earth orbit, MESSENGER's spacecraft bus and instruments must be shielded from the Sun behind the spacecraft sunshade at all times. This strongly limits the freedom with which the spacecraft can point body-mounted instruments. So MDIS includes a pivot that allows the camera to swing through 90 degrees fore and aft. The resolution of the cameras depends upon their distance from the target. The WAC has a field of view of 10.5 degrees, while the NAC has a field of view one-seventh that size, or 1.5 degrees. The WAC achieves a resolution of 1 kilometer per pixel at a distance of 5,590 kilometers; the NAC achieves that resolution at 39,100 kilometers. How Does MDIS Fit in the Context of Mercury Exploration?More than thirty years separates the development of MESSENGER from the development of the only previous mission to Mercury, Mariner 10. MESSENGER's images will revolutionize the study of Mercury, being sharper, more detailed, and in more colors than Mariner 10's images. In addition, MESSENGER will achieve global coverage of the planet at a variety of different sun angles; Mariner 10 imaged only about 45% of Mercury, and because the local time of day was the same during all three Mariner 10 flybys, most of Mercury was imaged with high sun, so that topography was disguised. Camera FactsMDIS consists of two framing cameras, a Wide-Angle Camera (WAC) and a Narrow-Angle Camera (NAC).
Filters
Image Compression Because MESSENGER is not equipped with a high-gain antenna, data transfer to Earth is slow, so images must usually be compressed before they are transferred. Each pixel is digitized to 12 bits, so an uncompressed, 1024-pixel-square image would occupy 12 Megabytes on the solid-state recorder. MDIS can perform on-chip binning of images, reducing their size by a factor of 2 (to 512 pixels square) or 4 (to 256 pixels square), or can compress the images from 12 bits down to 8 bits using lookup tables or differencing of adjacent pixels. Also, once the data has been passed to the spacecraft's solid-state recorder, the spacecraft computer can read MDIS images and perform additional binning, extract subframes, and use more sophisticated compression algorithms to reduce the images' size. Data Release ScheduleUnlike most other current NASA missions, the MESSENGER mission has elected not to make raw images available via the Internet. Some images are being released, but for the bulk of the data, the public must wait until the mission's required releases to NASA's Planetary Data System (PDS). Scheduled release dates are:
Sources"The
MESSENGER Mercury Dual Imaging System (MDIS): Imaging Strategy at Mercury,"
Louise Prockter et al., 2003.
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