ALBERT

Description: Flash rates measured from the Space Shuttle range from 27.8 flashes per minute to 77 flashes per minute. The cloud is an optically thick medium which effectively scatters the energy from a lightning discharge and thereby broadens the risetime and duration of each lightning pulse. Because of the small size, spacecraft sensors with resolutions of 1 km or more are unlikely to detect the individual lightning channels. Instead, the energy from the lightning channel is scattered within the cloud, thereby broadening the apparent area. All of these measurements of lightning flash area and flash rate have involved manual manipulation and analysis of the video or film data. Only a small percentage of the Space Shuttle lightning video has been analyzed. An attempt is made to combine the use of real-time digital disk system and an automated analysis routine in order to overcome this limitation and make processing of a sequence of video frames a much less labor-intensive task.

Description: Using the payload bay television cameras from various Space Shuttle missions, a study was performed to investigate the relationship between the size and duration of lightning flashes, though separated by 1-100 km, that appeared to be flashing in synchronization. The area, duration, and rate of propagation of the lightning in these storms was also studied. By employing the analytical methods described, the use of deinterlaced video of lightning observed from STS-32 provides the highest sampling rate ever utilized for this purpose from space (1 image each 16.7 m/sec).

Description: The preliminary results from attempts to derive depth and diameter information from digitized stereo images of impact features on the LDEF are reported. Contrary to our prior assumption, we find that impact craters in the T6 A1 alloy are not paraboloid in cross section, but rather are better described by a 6th-order polynomial curve. We explore the implications of this discovery.

Description: The entire LDEF spacecraft was examined by M&D SIG for impact (i.e., craters greater than or = 0.5 mm and penetrations greater than or = 0.3 mm in diameter) and related features (e.g., debris, secondaries). During the various detailed surveys conducted at NASA Kennedy, approx. 5,000 impact related features were photodocumented, and their locations measured and recorded; an additional approx. 30,000 smaller features were counted. The equipment and techniques used by the M&D SIG permitted the determination and recording of the locations and diameters of the 5,000 imaged features. A variety of experimental and LDEF structural hardware was acquired by the M&D SIG and is presently being examined and curated at NASA Johnson.

Description: Since the return of the Long Duration Exposure Facility (LDEF) in January, 1990, members of the Meteoroid and Debris Special Investigation Group (M&D SIG) at the Johnson Space Center (JSC) in Houston, Texas have been examining LDEF hardware in an effort to expand the knowledge base regarding the low-Earth orbit (LEO) particulate environment. In addition to the various investigative activities, JSC is also the location of the general Meteoroid & Debris database. This publicly accessible database contains information obtained from the various M&D SIG investigations, as well as limited data obtained by individual LDEF Principal Investigators. LDEF exposed approximately 130 m(exp 2) of surface area to the LEO particulate environment, approximately 15.4 m(exp 2) of which was occupied by structural frame components (i.e., longerons and intercoastals) of the spacecraft. The data reported here was obtained as a result of detailed scans of LDEF intercoastals, 68 of which reside at JSC. The limited amount of data presently available on the A0178 thermal control blankets was reported last year and will not be reiterated here. The data presented here are limited to measurements of crater diameters and their frequency of occurrence (i.e., flux).

Description: During the 3 month deintegration of the LDEF, the M&D SIG generated approximately 5000 digital color stereo image pairs of impact related features from all space exposed surfaces. Currently, these images are being processed at JSC to yield more accurate feature information. Work is currently underway to determine the minimum number of data points necessary to parametrically define impact crater morphologies in order to minimize the man-hour intensive task of tie point selection. Initial attempts at deriving accurate crater depth and diameter measurements from binocular imagery were based on the assumption that the crater geometries were best defined by paraboloid. We made no assumptions regarding the crater depth/diameter ratios but instead allowed each crater to define its own coefficients by performing a least-squares fit based on user-selected tiepoints. Initial test cases resulted in larger errors than desired, so it was decided to test our basic assumptions that the crater geometries could be parametrically defined as paraboloids. The method for testing this assumption was to carefully slice test craters (experimentally produced in an appropriate aluminum alloy) vertically through the center resulting in a readily visible cross-section of the crater geometry. Initially, five separate craters were cross-sectioned in this fashion. A digital image of each cross-section was then created, and the 2-D crater geometry was then hand-digitized to create a table of XY position for each crater. A 2nd order polynomial (parabolic) was fitted to the data using a least-squares approach. The differences between the fit equation and the actual data were fairly significant, and easily large enough to account for the errors found in the 3-D fits. The differences between the curve fit and the actual data were consistent between the caters. This consistency suggested that the differences were due to the fact that a parabola did not sufficiently define the generic crater geometry. Fourth and 6th order equations were then fitted to each crater cross-section, and significantly better estimates of the crater geometry were obtained with each fit. Work is presently underway to determine the best way to make use of this new parametric crater definition.

Description: The Meteoroid & Debris Special Investigation Group has been studying the low-Earth orbit particulate environment by examining and documenting impact craters that occurred on the Long Duration Exposure Facility (LDEF) during its 5.7 year stay in orbit.