Evaluation of Space Enviroment
and Effects on Materials
(ESEM) Archive System
NASA Langley Research Center
Hampton, Virginia

Final Report
United States Developed ESEM Experiments
Evaluation of Space Environment and Effects on Materials

Appendix A

Results of the Cosmic Dust
Collection Experiment on STS-85

Donald H. Humes and William H. Kinard

INTRODUCTION

The Cosmic Dust Collection Experiment (CDCE) exposed 0.0685 m² of aerogel to the space environment for 11.6 days in August 1997 to collect meteoroids and orbital debris.

It was flown on the Space Transportation System (STS) mission STS-85.

The CDCE was part of the Evaluation of Space Environment and Effects on Materials (ESEM) program, a co-operative effort between the NASA Langley Research Center and the National Space Development Agency of Japan (NASDA), which culminated in a flight experiment on STS-85.

Aerogel is an open cell glass foam with chains of silica particles, 2 nm to 5 nm in diameter, surrounding 10 nm to 100 nm diameter pores, giving a very low density material, in this case, three to four percent of the density of silica. Aerogel tends to decelerate and capture hypervelocity particles without inducing pressures or temperatures that would alter the physical or chemical properties of the material. Particles do tend to fracture during capture but the fragments, if large enough, can be picked from the aerogel, examined, and analyzed.

Five tracks from hypervelocity or high-velocity impacts were found, one of which had a visible particle at the end of the track. In addition, particles were found partially embedded in the aerogel, obviously the result of low-speed impacts. Shallow circular depressions with collapsed aerogel bottoms, apparently the result of low speed liquid droplets striking the surface, were found. Many particles were found resting on the surface that may have been collected during manufacturing or pre-flight handling.


DESCRIPTION OF EXPERIMENT

The aerogel was mounted in an aluminum frame as nine individual blocks, see Figure 1. The blocks were about 9.5 cm by 9.5 cm and were 1.2 cm thick. They were retained in the frame by an aluminum lip plate that covered the edges of the aerogel blocks so that the exposed area was 0.0685 m².

Seven of the aerogel blocks had a density in the 0.070 g/cm³ to 0.078 g/cm³ range, and two blocks had a density of 0.090 g/cm³.

STS-85 was launched at 10:41 am EDT on August 7, 1997. The cargo bay doors were opened at 12:16 pm EDT on August 7, 1997 exposing the CDCE aerogel to the space environment. The cargo bay doors were closed at 3:32 am EDT on August 19, 1997 ending the Cosmic Dust Collection Experiment after an exposure time of 11 days 15 hours 16 minutes. The orbiter touched down at 7:08 am EDT on August 19, 1997.

The aerogel was not covered at any time during the mission, so it was exposed to the cargo bay environment during ascent and descent and could have been struck by particles flying around in the cargo bay.

The orbiter was initially in a slightly elliptical orbit where the altitude was between

287 km and 296 km and the inclination was 57 degrees. The CDCE was exposed to the space environment (cargo bay doors open) for 9 days 20 hours 39 minutes while in this orbit. The orbiter dropped to a circular orbit with an altitude of 256 km at 8:55 am on August 17, 1997, remaining at an inclination of 57 degrees. The cargo bay doors remained open for another 1 day 18 hours 37 minutes while in this lower orbit.

The CDCE faced the port side, from an elevated stand, giving the experiment a nearly unobstructed view of space, see Figure 2.

The STS-85 orbiter made a large number of maneuvers, many of which moved the CDCE surface normal vector to or from alignment with the orbiter velocity vector. The CDCE faced the ram direction for a total of 77.00 hours, 52.48 hours while at an altitude of 287 km to 296 km, and 24.52 hours while at an altitude of 256 km. There was some exposure to the ram direction every day of the mission. The normal vector to the CDCE surface was perpendicular to the velocity vector for a large portion of the mission, see Table I.

The number of meteoroids and debris particles encountered depends not only on the angle between the velocity vector and the normal to the experiment surface, but also on the angle between the zenith direction and the normal to the experiment surface. Seventy-four percent of the mission was spent in one of ten specific orientations with respect to these two angles, see Table II.


RESULTS

Examination of the Aerogel
The aerogel blocks were scanned with a stereo microscope at 50X using a ring light around the objective lens. The blocks were in the aluminum frame during scanning. The aluminum lip had been removed. The frame was mounted horizontally, with the microscope overhead, so that the aerogel blocks could not fall out of the frame. A glass cover over the aerogel kept laboratory debris from falling onto the aerogel.

Particles Resting on the Surface
Many microscopic particles were found resting on the surface of the aerogel that appeared black in the microscope. It is not known if the particles were there prior to the flight because no microscopic examination of the aerogel was made prior to the flight.

Energy Dispersive Spectroscopy (EDS) performed on dozens of particles on one of the 0.090 g/cm³ aerogel blocks showed that almost all of the particles were glass, presumably aerogel itself. Some external contamination particles were also found, some with high calcium and sulfur content, a few with iron, and one of the 20 micron spherical particles found was examined and found to contain large amounts of iron, titanium, potassium, aluminum, magnesium, and silicon.

The glass particles appeared black in the microscope because most of the light entering the microscope was light reflected from the aluminum back of the frame, and the particles on the surface were therefore seen in backlighting. An aerogel block with no reflecting surface behind it does not reflect enough light to be seen in the microscope.

The presence of the black particles on the surface of the aerogel, so numerous that there were almost always particles in the field of view of the microscope, aided in the microscopic examination of the surface of the aerogel. At the rare sites where there were no particles on the surface it was nearly impossible to tell whether the microscope was focused on the transparent surface of the aerogel. Small impact tracks, which are clearly visible when the microscope is in focus, could be missed when the microscope is slightly out of focus. And the focus must be constantly adjusted during scanning because the surface of the aerogel was not flat.

Particles Partially Embedded in the Aerogel
Several glass fibers, about 10 microns in diameter and several hundred microns long, were found embedded from one-third to two-thirds of their length in the aerogel, see Figures 3 and 4. The fibers are the same diameter as the fibers used in the STS cargo bay liner material and have nearly the same chemical composition, see EDS data in Figure 5. EDS data on 10 micron diameter fibers from the scrim cloth used in the MIR solar panel returned to Earth on STS-89 are also shown in Figure 5. In addition to the silicon peak, prominent calcium and aluminum peaks appear in all three spectra. Traces of magnesium in the cargo bay liner material and the MIR solar panel scrim cloth did not appear in the fibers removed from the aerogel.

There were about twenty fibers found in the aerogel. Three of those fibers were examined in the scanning electron microscope (SEM) and EDS showed them all to have the same chemical composition.

The impact speed of the fibers was probably around 20 m/s to 40 m/s. Impact tests conducted with a 1.05 mm diameter aluminum rod that was 31.8 mm long showed that that impact speed was needed for the rod to embed itself one-third to two-thirds of its length in the flight aerogel, see Figure 6. Aluminum has about the same density as glass. For these tests three blocks of the flight aerogel were stacked, one behind the other, to give a total thickness of 36 mm. To a first approximation, one would expect the penetration results in Figure 6 to scale directly to 10 micron diameter fibers.

Given their size, chemical composition, and impact speed, the fibers probably came from the STS cargo bay liner.

Also shown in Figure 6 is data for the aluminum rod striking NASDA aerogel which has a density of 0.028 g/cm³. The rod buried itself completely in the aerogel when the impact speed exceeded 18 m/s. Fibers that would be partially embedded in the CDCE aerogel would be buried deep inside that NASDA aerogel.

Several microscopic glass sheets were also found partially embedded in the aerogel. The sheets were 100 microns to 200 microns long and wide, and with a thickness of about 10 microns. Perhaps the sheets are remnants of the manufacturing process.

Shallow Circular Depressions
The largest features found in the aerogel were shallow, circular depressions with concave bottoms that look like crushed or collapsed aerogel, see Figures 7 and 8. It appears as if a drop of some liquid that causes aerogel to collapse struck the surface at low speed and sheared out a circular section before coming to rest and causing the aerogel it touched to collapse. There were six of these depressions with a diameter of 1 mm or larger, the largest being 1.6 mm in diameter. There were many smaller depressions of this type. The depths of the depressions were about 10 percent to 30 percent of the diameter. One of the large depressions was examined in the SEM and small spots with high carbon content were discovered while doing EDS.

It is not known if these features were on the aerogel prior to flight because the aerogel was not scanned prior to flight, but they were not noticed in the last pre-flight visual inspection which occurred prior to delivery of the aerogel to the Kennedy Space Center. The six depressions with a diameter of 1 mm or greater are easily seen with the unaided eye.

Water droplets sprayed from an atomizer onto the flight aerogel produced this type feature, see Figure 9. The depths of the depressions made by the water droplets were about 15 percent of the diameter. Alcohol droplets did not produce this type feature.

The shallow circular depressions probably were produced by a water-based liquid sprayed over the aerogel some time after the experiment was installed on the STS orbiter.

The CDCE aerogel may be useful as a detector for high-water-content liquid droplets, giving the flux and the size distribution, as long as the flux is not too great.

High-Velocity or Hypervelocity Tracks
Five high-velocity or hypervelocity tracks were found in the aerogel. Sketches of the five tracks are shown in Figure 10. The largest track had a diameter of 100 microns at the entrance hole and was 700 microns deep. The other four had entrance holes of 10 microns, 12 microns, 45 microns, and 68 microns. It is very difficult to photograph such small features through a microscope because they are extended, three-dimensional features and the depth of focus of a microscope is very small at the high magnification needed to show these features. No good photographs of these features were obtained.

The only track with an obvious particle at the end was the one shown in Figure 10(d). A gray metallic-looking particle about 4 microns in diameter and 14 microns long was captured, but unfortunately it was lost while trying to remove it from the aerogel.

The two tracks with a uniformly decreasing diameter, those shown in Figure 10(c)> and 10(d), look like they were caused by particles having a modest impact speed, not by hypervelocity particles. They almost certainly were not meteoroids. They may have been particles that are not what is usually considered orbital debris either, i.e. particles produced from some past space mission hardware. Such particles would tend to strike a spacecraft at hypervelocities. The particles may be associated with the STS-85 orbiter itself. There may be a significant particulate environment in space that is not now being addressed in the models, not meteoroids, not orbital debris in the usual sense, but co-orbiting debris that is of substantial size and that somehow obtains enough relative speed to cause damage comparable to meteoroids and orbital debris. Large, low-speed fragments ejected from craters produced by meteoroids or orbital debris on other parts of the spacecraft (secondary particles) also fall into this category but are not necessarily the predominant particles.

Two particles were recovered in multi-layer insulation (MLI) on the Long Duration Exposure Facility (LDEF) that fit into this category, a 5.7 mg aluminum disc and a 7.8 mg irregularly shaped flat silver particle, both of which penetrated many layers of aluminized mylar and dented an aluminum plate underneath before coming to rest, intact, in the MLI. Photographs and more discussion of these two particles can be found in Part III of this document.

The particles causing the tracks seen in Figure10(c) and 10(d) were probably about the same size as the entrance holes in the aerogel, 45 microns and 12 microns.

The three tracks shown in Figure 10(a), 10(b), 10(e) look like they were caused by hypervelocity particles, meteoroids or orbital debris. The sizes of the particles that created these tracks are not known. Hypervelocity impact data on the relationship between particle size and the track entrance hole size have not yet been obtained for the CDCE aerogel.

Models of the meteoroid environment and the orbital debris environment were used to calculate the number and the size distribution of the particles that should have struck the CDCE aerogel, see Figure 11. One particle larger than seven microns is predicted. The chance of a particle larger than 28 microns (either a meteoroid or debris) striking the aerogel is calculated to be about 10 percent. Until hypervelocity impact data on the CDCE aerogel is obtained, the CDCE track data cannot be compared to the models of the meteoroid and orbital debris environments. There is, however, no obvious disagreement between the data and the models.


CONCLUSIONS

There was no evidence of a captured particle in any of the three tracks suspected to have been produced by hypervelocity impacts. The aerogel did perform well as a collector of high speed particles, capturing one of the two suspected high speed particles encountered. It performed well as a collector of low speed fibers and sheet-like fragments. The CDCE aerogel also serves as a good detector of low speed, water-based, liquid droplets, reflecting the number and size distribution of the small droplets encountered in the easily seen white circular depressions left in the aerogel.

Fibers from the STS cargo bay liner material became embedded in the CDCE aerogel, perhaps during ascent or descent.

Another particulate environment, not meteoroids, not orbital debris in the usual sense, but co-orbiting debris, may be a significant hazard that needs to be addressed.

The number and size of the tracks in the aerogel suspected to be from hypervelocity particles, meteoroids or orbital debris, do not show any obvious disagreement with models of those environments.

Final Report | Appendix A | Appendix B | Appendix C | Appendix D | Appendix E

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