RESULTS OF THE EXAMINATION OF THE / WINDOWS FOR MICRO- METEOROID IMPACTS. Burton Cour-Palais, NASA Johnson Space Center, Houston, TX 77058 Introduction: The examination of the external surfaces of the Command Module windows conducted duri ng the Apol lo Program (1 ,2,3) continued duri ng the Skylab Program. In this program, an Orbital Workshop (Skylab 1) was placed in an orbit at 435 Km (235 nm) and was visited by Apollo space- craft on three separate occasions. These were known as 2, 3, and 4. However, the docked assembly was commonly referred to as "the Sky1 ab. " This report covers the results of the window examinations performed at JSC after the Skylab 3 and 4 spacecraft returned to Earth. The durations of the two rnissions were approximately 59.5 and 84 days, respectively, and the orientation of the spacecraft allowed the solar panels to be pointed at the sun throughout the orbit. This resulted in windows #I and 3 being always sun- lit, but a1 ternately pointed towards and away from the Earth. Window #5 was always across the spacecraft sunlight terminator in shadow. The average shielding by the Earth of each window was calculated to be 34% for the orbit chosen. The Skylab/Apollo Command Module windows used as the detection surface for m?crometeoroid impact craters were identical to those used in the Apollo Win- dow Meteoroid Experiment, S-176, (1 ). The windows were of 99% pure fused si3 - ica and indented approximately 1 inch from the exter a1 surface of the Cornmand Modu:e heat-shield. The total surface area of 0.26~) examined for each space- craft consisted of the hatch and two side windows; the rendezvous windows were not included as they were deeply inset into the heat-shield. Expevience gained during the Apol 1o window examinations for micrometeoroid impacts showed that these events were easily distinguishable from surface polishing defects and flaws in the glass, (1). A.s a result it was decided that a preflight scan of the windcw surfaces was not necessary. The high surface temperatures gen- erated during the lunar returr, entry through the Earth's for the Apol lo experiment (about t 180°K on the side windows) did not noticeably affect crater morphology. Hence the lower temperatures expected for each window in the Skylcb/Apollo re-entry from Earth orbit was not considered to present a problem. The window surfaces were optically examined using a Bausch and Lomb stereo-zoom microscope mounted on an extendable radial arm, due to the large size of each window (approximately 0.3 x 0.3 m). The instrument was fitted with a measuring eyepiece and had provision for a Polaroid camera attachment. ti0 depth measurement capability was available; therefore, only pit and spa1 1 diameters were measured, A total exposed window area of 0.26m2 for each spacecraft was examined at 5x and followed by a 20x scan of a total of 0.022m2 for the three Skylab 4 windows only. At the 5x magnification level, the spa11 diameters threshold was 80@, and at the 20x level, surface defects as small as 20,m were measured. A total of 18 micrometeoroid impact events was ob- served at the 5x level, most of which were photographed and a further six were identified at the 20x magnification. The distribution of impacts relative to spacecraft and window are given in Table lltogether with the areas scanned and the calculated and experimentally determined structural shielding factors to

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MICROMETEOROID IMPACTS Cour-Palais, B. --et al, be applied. To date no scanning electron microscopy has been performed on either craters or deposits as this would have required the windows to have been cored. The cumulative 5x and 20x pit and spa11 diameter density per square centi- meter per year,for the combined Skylab 3 and 4 missions,is shown in Figure 1. The data has been corrected for Earth and structural shielding of the window surfaces. Discussion of Results: It is evident from Table 1 that the difference in the number of impacts observed for the two spacecraft cannot be completely at- tributed to their mission durations. Furthermore, there are significant dif- ferences in the 5x impact distribution. The five impacts on Skylab 3 were on windows 1 and 3, most shielded by the solar panels, with none on 5; for Skylab 4, there were four on 1, two on 3, and seven on 5. These variations are con- sidered to be statistical in nature and were previously noted for the Earth- orbiting Apollo 7 and 9 spacecraft, (1). It is also felt that the sharp in- crease in the flux of 20x craters is significant, expecially when the differ- ence in structural shielding of window #I to 5 is considered. This evidence of a superimposed additional flux of the micrometeoroid population emanating from the solar direction is graphically illustrated in Figure 1. The pit and spa1 1 diameter distributions shown in the figure are reasonably consistent, considering that in several instances pit and spallation geometries were poor- ly defined. References : 1 . Cour-Palais , B. G. et a1 ., NASA SP-289, 1972 2. Cour-Palai s , B. G. et a1 . , NASA SP-315, 1972 3. Cour-Pal ai s , B. G., NASA SP-330, 1973

SKYLAB 3 (Duration 59.5 Days)

5x Scan 20x Scan Window Shielding ~rea(n2) Inlpacts Shielding ~rea(rn~) Impacts

SKYLAB 4 (Duration 84 Days)

5x Scan 20x Scan -Window Shielding Area(m2) Impacts Shieldirq Area (m2) Impacts

1 50% 0.094 4 50f 0.012 ' 6 3 46% 0.074 2 46% O.OC39 0 5 14% 0.094 7 14% 0.0065 0

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MICROMETEOROID IMPACTS Cour-Palais, B. --et al.

PIT AND SPPLL DIAMETERS, Dp AND Ds (ran)

FiGURE 1: Cuaulative crater dertsity distributiorl as a fu~ctionof pit an< spallation diarttcr fcr 5x an4 20x scans of the Skylab 3 and 4 Apollo windows.

0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System