09-APR-98 3:30 PM PDT
Mars Global Surveyor has completed the first of three sets (clusters) of specially targeted imaging opportunities during its current hiatus in aerobraking. While it was estimated that probability of successfully accomplishing this imaging would be on the order of 30-50% for each of the images, owing to navigation, spacecraft attitude control and map location uncertainties, it appears that we have done somewhat better.
On April 3, 1998, at 0958 UTC, MGS pointed the Mars Orbiter Camera toward the Viking Lander 1 site in Chryse Planitia. The area was covered with a thin cloud layer and patchy thick clouds, reducing but not eliminating surface visibility. The narrow angle image had relatively low contrast but the contrast was sufficient for adequate feature identification. This analysis showed that the targeted spot for the Viking Lander 1 sites was approximately 40 pixels or 150 m off the western edge of the image, although it was well centered in the north to south direction. We came very close, and the pointing performance was well within the expected variations of the spacecraft’s attitude control system, and we’ll count this as a miss. Because the Lander was not in the picture, we won’t release the image at this time.
Viking Lander 2 is located in Utopia Planitia, further north and on the other side from Mars from Viking Lander 1. When MGS imaged this area on April 3, at 2137 UTC, on the orbit following the Viking Lander 1 observation, it found the area in heavy overcast, with clouds and haze severely reducing the surface visibility by over 70-80%. These clouds and possible surface frost led to a scene substantially brighter than anticipated, and thus much of the image data was saturated bright. Aggressive application of image processing techniques enhanced faint brightness variations, rendering a small number of surface features visible. These features were used to attempt to determine the success of the targeting. It is believed that the Viking-era landing location may be with the extreme south-west portion of the image and the western-most of three new estimated positions, more recently determined, may be in the lower south-eastern portion of the image. The location of Viking Lander 2 is the least well known of the sites being imaged. So, we’ll count this attempt as a hit, however, because the image is mostly clouds, we won’t release it at this time.
Mars Pathfinder, the Sagan Memorial Station, and the Sojourner rover are located in Ares Vallis. On April 4th, at 0916 UTC, the spacecraft was successful in targeting the point that it was directed to, however, a controversy has arisen as to the true coordinates of the real landing site. It appears that, after some reevaluation of the targeting information used with the Mars Pathfinder project, we have used the wrong coordinate references for our target point. Better information will be used in the next opportunity. Because it appears that Mars Pathfinder is not in the image, we won’t release the image at this time.
The feature known as the "Face on Mars" in the Cydonia region was imaged quite successfully on April 5th and its image was released in raw form and in an enhanced form on April 6th. The picture was acquired 375 seconds after the spacecraft's 220th closest approach to Mars. At that time, the "Face", located at approximately 40.8° N, 9.6° W, was 275 miles (444 km) from the spacecraft. The "morning" sun (about 10 AM local solar time) was 25° above the horizon. The picture has a resolution of 14.1 feet (4.3 meters) per pixel, making it ten times higher resolution than the best previous image of the feature, which was taken by the Viking Orbiter 1 in 1976. The full image covers an area 2.7 miles (4.4 km) wide and 25.7 miles (41.5 km) long.
Since winter clouds cover much of the northern hemisphere of Mars above 40° N latitude at this time of the Martian year, the raw image of the "Face" was of very low contrast, that is, variations in brightness of the picture elements cover a very small range compared to the ultimate capability of the camera. Thus, the "raw" image appears very dark and flat. This very washed out appearance of the northern hemisphere of Mars can be readily seen in the wide angle image taken at the same time as the narrow angle picture that contains the "Face" (see http://mars.jpl.nasa.gov/mgs/msss/camera/images/4_6_face_release/index.html). The enhanced version of the raw data has made the feature visible. The enhanced version also flipped the image left to right to make it appear in the same orientation as the familiar Viking image.
The targeting was very good for this attempt and is clearly a hit. The "Face" was nearly exactly in the center of the image. A portion of another feature in the area, the "D&M Pyramid", is the bottom left-hand corner of the image.
The Mars Surveyor Operations Project assesses the results of the first cluster of targeted imaging to have been quite successful. The results have provided information useful in fine tuning the processes for the second cluster of images that will be taken starting on April 12th.