The onboard cameras were equipped with 1500-mm focal length lenses to enable high-resolution pictures to be taken during both the approach and post encounter phases. During the first flyby (Figure 2-3), the closest approach of Mariner 10 to Mercury occurred when the cameras could not photograph its sunlit surface. The imaging sequence was initiated 7 days before the encounter with Mercury when about half of the illuminated disk was visible and the resolution was better than that achievable with Earth-based telescopes. Photography of the planet continued until some 30 min before closest approach, thereby providing a smoothly varying sequence of pictures of increasing resolution. Pictures with resolutions on the order of 2 to 4 km were obtained for both quadratures during M1. Resolution varied greatly, ranging from several hundred kilometers to approximately 100 m. Large-scale features observed at high resolution were used to extrapolate coverage over broad areas photographed at lower resolution. The highest resolution photographs were obtained approximately 30 min prior to and following the darkside periapsis during the first and third encounters. Pictures were taken in a number of spectral bands enabling the determination of regional color differences.
The second (bright side) Mercury encounter provided a more favorable viewing geometry than the first. In order to permit a third encounter it was necessary to target M2 along a south polar trajectory. This allowed unforeshortened views of the south polar region, an area which had not
Figure 2-3. Mariner 10 incoming view during the first encounter. (NASA Atlas of Mercury SP432.)
previously been accessible for study. Images from this region provide a geological and cartographic link between the two sides of Mercury photographed during M1. Stereoscopic coverage of the southern hemisphere was also achieved. Because of the small field of view resulting from the long focal length optics employed, it was necessary to increase the periapsis altitude to about 48,000 km to ensure sufficient overlapping coverage between consecutive images. The resolution of the photographs taken during closest approach ranged from 1 to 3 km.
The third Mercury encounter (Figure 2-4) was targeted to optimize the acquisition of magnetic and solar wind data, so that the viewing geometry and hemispheric coverage employed were very similar to those utilized during the first encounter. However, M3 presented an opportunity to provide high-resolution coverage of areas of interest that were previously seen only at relatively low resolution. Because of ground communication problems, the latter pictures were acquired as quarter frames.
Overall, Mariner 10 photographed about 45% of Mercury's surface with a resolution that varied from about 2 km to 100 m (the latter in extremely limited areas).
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