Hubble Presentations
Meteoroid and Debris Impacts on the WF/PC I Radiator
Donald H. Humes / NASA Langley Research Center
William H. Kinard / NASA Langley Research Center

We spent two days in the clean room at Goddard examining the WF/PC-I radiator with a microscope to measure the damage done by meteoroids and man-made orbital debris during its 3.6 years in orbit.

It was a difficult job - moving the microscopes around on a heavy stand, positioning it close to the WF/PC I radiator while never touching it, and looking through the microscopes while in awkward positions on the steps of a ladder.

I (Don Humes) looked at every impact site examined, but Mark Kulick, a Lockheed meteoroid and debris researcher, made all the measurements. Only a few photographs were taken because of the difficulty in taking them in the short time we had.

We were aided immensely by the survey done at Goddard using theodolites to obtain the location of possible impact sites. Goddard provided the coordinates of 100 possible impact sites, assigned a number to each, and rated them by size on an arbitrary scale of 1 to 10 (10 being the largest).

HST WF/PC-I

In GSFC Clean Room - Building 29

Outline

I will talk about (1) the crater flux based on the number of craters of various sizes found in the aluminum radiator plate, (2) the spallation of the ZOT paint in an area around the craters, (3) rings seen in paint on the LDEF but not seen in the ZOT paint and (4) the shape of the craters in the aluminum.

Comparison of the damage seen on the WF/PC I radiator to damage seen on the LDEF will be made throughout the talk.

Crater Flux

The theodolite survey data was given to me by Henry Sampler and also had the name of Jerry Gay on it. They did a great job - finding impact sites with craters as small as 270 microns, that is .011 inches.

We examined 72 of the 100 possible impact sites and found 53 to be true impacts and 18 to be deposits of gooey particles or to be scrapes in the paint. At one site we found nothing. We examined all the size 4 to size 10 impact sites, and half of the size 1 to size 3 sites.

Impact Sites Found During GSFC Theodolite Survey

Arbitrary Size Scale

The WF/PC-I radiator was an aluminum plate, 0.160 inch thick, that was painted with ZOT paint. ZOT paint is a ceramic thermal control paint that gives a <0.18 and e>0.85 in the 0.28 to 2.50 micron range. The pigment is zinc orthotitanate and the binder is potassium silicate. The specs call for the ZOT to be 3 - 6 mils thick. A pre-flight inspection report lists the thickness as 6.8 mils. We measured a thickness of 4.3 mils at one place.

Radiator Surface

The large craters in the aluminum were slightly to moderately irregular in shape and had pits (or sub-craters) of different depths inside the primary cavity.

The lips were not well formed like those in unpainted aluminum - having broken off or never developed.

There was a dark area around the crater where the raised lips would have been.

Condensed molten droplets, apparently of aluminum, were common near the top of the craters and at all depths.

The ZOT paint spalled off the aluminum plate in a large area around the craters.



Large Crater

The spall area was irregular in shape too.

The 14 impact sites that had craters with a diamter greater than 450 microns (measured at the aluminum plate surface) were of the type classified here as large craters. They had a single crater that, while not usually round, was not extended in any direction.

The very largest craters had nearly round rims, but with irregular bottoms.

Large Impact Craters

This is the largest crater found in the WF/PC I radiator - 900 microns in diameter. It is nearly round and resembles craters found in unpainted aluminum on the LDEF, except that the lips are poorly developed.

Largest Crater In WF/PC-I Radiator

900 micron diameter.

Craters smaller than about 450 microns (there is no clear cutoff size) were highly irregular in shape with a number of cavities that were sometimes connected and sometimes not.

It appeared as if the ZOT paint acted like a meteoroid bumper, shattering the impacting particle before it hit the aluminum plate, and allowing the fragments to disperse somewhat. The surprising thing is that the fragments could disperse so much in such a short distance - the thickness of the paint.

Perhaps this shows that many meteoroids are a loose aggregate, a porous and fragile assemblage of grains, as Brownlee has suggested in a recent study of the densities of captured stratospheric meteoroids.

And I do suspect that most of the impacts on the WF/PC I radiator were caused by meteoroids and not by man-made debris, as I will discuss later.

Crater fields like this were seen on the LDEF plates also, but only very rarely (less than 1 percent of the impacts). Those crater fields were usually linear and showed evidence of the impact direction and seemed to be caused by highly oblique impact angles (greater than 80 degrees from the normal). Those impacts also suggested that many meteoroids are loose aggregates. The ZOT paint seemed to enhance the breakup and dispersion. The painted aluminum plates on the LDEF did not exhibit the type of cratering seen on the WF/PC-I radiator.

But there are other possibilities.

Secondary particles created when meteoroids and debris were fragmented while penetrating the solar panels could have struck the radiator. That would explain the dispersion of the fragments. But I think it is unlikely that the ejecta would have created small clusters of craters so widely separated from each other.

There was a highly irregular spall area around the crater field.

Small Impact Damage Sites

The measurements we made were (1) the diameter of the crater at the aluminum plate surface, (2) the diameter at the top of the raised lips, when they existed in any form, (3) the depth from the plate surface to the deepest point and (4) the diameter of the spell area around the crater.

Because the large craters and their spall areas were irregular in shape, the diameters measured are crude measurements but did not require much judgement in assigning a representative diameter.

Large Crater

On the other hand, the measurement of the crater diameter at the small impact sites did require a judgement.

The depth was straightforward, and the spall diameter was straight-forward, but the crater diameter was not.

We chose to imagine a circle surrounding most of the crater field and call that the crater diameter. This may overestimate the size of the crater that would have produced in an unpainted plate - but that is what we measured.

In some cases the crater field was extended in one direction and the width and length of the crater field was measured.

SMALL IMPACT DAMGE SITES

There is a correlation between the Goddard size estimate and our measured crater diameter, although it is not perfect. A few entries are out of place.

GSFC Size Estimate Versus Measured Crater Diameter

We can expect that all the large craters were found and measured and that, in fact, essentially all the craters in the aluminum were found, although some were not examined.

Impacts that only damaged the paint and did not produce spallation would not have been found during the Goddard theodolite survey and hence were not examined by us.

Many size 1,2, and 3 possible impact sites were not examined. We can estimate from the statistics of those that were examined how many probably are impact sites and what size they probably are. Having done that we can estimate the flux of various size craters in the radiator.

Impact Sites Found During GSFC Theodolite Survey

Crater Diameter at Plate Surface, microns

Here then is the cumulative crater flux as a function of crater diameter, i.e. the number of craters larger than some limiting size per unit area per unit time.

The error bars are the 90 percent confidence limits based on the number of craters included in the flux measurement. The previous discussion about the Goddard size estimate and the correlation to our measured crater diameters was included to show that we have not made the best flux measurements that we could have made. We guessed at what size craters we would have found at some sites because we did not have time to look at them.

The production of small craters in the aluminum, those with a diameter less than the paint thickness, was probably inhibited by the presence of the paint.

But we expect that the diameters of the largest craters were not affected much by the paint and that the flux measurements for the three largest threshold crater sizes are close to what the crater flux in an unpainted aluminum plate would have been. We have evidence to support that from a painted aluminum plate from the LDEF.

Crater Flux on Radiator

Here is the data for a painted aluminum plate on the LDEF.

The painted plate was on Row 12 of the LDEF. The cumulative flux of various threshold crater sizes on the that plate are shown along with the 90 percent confidence limits.