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Interpretation of the Apollo 14 Thermal Degradation Sample experiment
Icarus 221 (2012) 167–173
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Interpretation of the Apollo 14 Thermal Degradation Sample experiment James R. Gaier National Aeronautics and Space Administration, Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, United States
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Article history: Received 20 March 2012 Revised 28 June 2012 Accepted 3 July 2012 Available online 20 July 2012 Keywords: Moon, Surface Moon Solar wind
a b s t r a c t The Thermal Degradation Sample (TDS) experiment was one of the many investigations performed on the lunar surface during Apollo 14. Remarkably, the results of this 40 year old experiment were never fully interpreted, perhaps in part because the hardware vanished after its return. Mission records, high resolution photographs returned from the mission, and recent laboratory investigations have been used to glean important results from this experiment. It is most likely that the dust adhesion to the TDS was less than anticipated because of atomic-level contamination of its surfaces. These contaminants were probably removed from most equipment surfaces on the Moon by sputter cleaning by the solar wind, but the TDS experiments were not exposed to the solar wind long enough to affect the cleaning. Published by Elsevier Inc.
1. Introduction On February 5, 1971 the Apollo 14 Lunar Module, Antares, landed on the Fra Mauro formation of the Moon, and astronauts Alan Shepard and Edgar Mitchell became the third pair of humans to walk the lunar surface. They would remain on the Moon for 33 h before returning to their colleague Stuart Roosa in the Command Module Kitty Hawk and returning to Earth. During their brief stay they ventured onto the surface for two extravehicular activities (EVAs) for a total of 9 h and 22 min. Much of the first EVA was spent setting up camp, activating the radioisotope thermoelectric generator (RTG), emplacing the Apollo Lunar Science Experiment Package (ALSEP) and several other stand-alone experiments which characterized the lunar environment. The second EVA was a long range traverse with a focus on the collection of interesting geological samples. Their time was tightly managed, and nearly every action was choreographed down to the minute. The result was a flood of new data characterizing the geology and environment of the Moon. During the second EVA, at the first geological site visited, 8 min of Alan Shepard’s time was scheduled for an activity designated the Thermal Degradation Sample (TDS) experiment (Zedekar, 1970). The origin of this experiment is unclear. In Sullivan’s comprehensive Catalog of Apollo Experiment Operations, the Principal Investigator is listed as ‘‘Unknown’’ (Sullivan, 1994). Perhaps the best clue lies in the original print for the fabrication of the TDS sample holder, which was ordered by R.S. Harris, and the NASA Manned Spacecraft Center. In addition, a paper by Jacobs, Durkee, and Harris describes the experiment and plans to analyze it (Jacobs et al., 1971). Since this is the only contemporary reference in the E-mail address: [email protected] 0019-1035/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.icarus.2012.07.002
scholarly literature (i.e., outside of NASA documents) to mention the experiment, and since the authors were from the NASA Manned Spacecraft Center, it is likely that one of them was the Principal Investigator. The TDS experiment was a simple test ‘‘To evaluate the effect of lunar dust on the optical properties (absorptivity and emissivity) of 12 candidate thermal coatings. Two duplicate arrays each containing 12 coatings were taken to the Moon. After covering them with dust, one was tapped to remove the dust and the other was cleaned with a nylon bristle brush’’ (Sullivan, 1994). Photographs of the samples were to be taken with the Apollo Lunar Closeup stereographic Camera (ALCC) in their pristine state, after they were dusted and shook, and after the one was brushed (Zedekar, 1970). The TDS samples were then to be returned to Earth so that their solar absorptance and thermal emittance could be measured. The experiment was ‘‘expected to yield material data to aid in the selection of radiator surfaces on the Lunar Roving Vehicle (LRV) and other advanced lunar operational equipment’’ (Zedekar, 1970). After completing the experiment there was a well choreographed plan to load the TDS into the EVA-2 Equipment Transfer Bag which was loaded onto the Lunar Module (LM) Ascent Stage at the end of the EVA. Here it was stowed in the interim stowage assembly over the ascent stage engine cover. It was then transferred to the Command Module in the ISA Decontamination Bag and stowed on top of Command Module (CM) Vol (A1). Presumably this occurred as planned because there is no mention otherwise in the mission records and Jacobs, Durkee, and Harris report that at the time of writing their manuscript it was in quarantine. However, there is no further mention of the TDS in the record. Jacobs, Durkee, and Harris never published a follow-on paper describing the results of the TDS. No NASA reports of the results have been found. A search through the archives at the NASA
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Johnson Space Center finds no reports, no data, no TDS hardware. The Smithsonian Institution likewise has no record of it being transferred to them. The principals and their managers are either deceased or have no memory of the TDS or any post-flight tests. Except for the photographs and these few details, the Apollo 14 TDS is lost to history. The purpose of this report is to inspect the TDS record in detail, and to interpret those results in light of recent laboratory experiments on the effects of dust on thermal control surfaces in order that some insight might be gained from this rare experiment conducted in the lunar environment. 2. Results from Apollo 14 The TDS hardware was very simple and straightforward. There were two identical metal sample holders, designated 1001 and 1002. Each sample holder was made up of two trays connected by a hinge. Each tray held six samples in a 2 3 array. The sample holders were stowed with the trays closed together, protecting the samples, and then opened to apply the dust. After the experiment was completed, the trays were again folded together and placed in a Teflon Equipment Transfer Bag (ETB). According to the original design prints, each 6 sample carrier of the TDS sample holders was 3.25 in. wide and 3.00 in. deep. It also had a 3.2 in. long handle grip to make it easier for the astronauts to handle. The samples were 1 in. squares with 0.75 0.75 in. exposed to the lunar environment, and the remaining under the rim to lock it in place. The 12 samples were candidate thermal control materials in 1971, and are listed in Table 1 (Gold, 1971). The events surrounding the TDS experiment can be reconstructed through the nominal mission plan, corrected by the recorded transcripts (Jones, 1995). At the beginning of EVA-2, astronauts Shepard and Mitchell loaded up the Modular Equipment Transporter (MET). At a mission time of 131:36:21 Shepard confirms to Mitchell that he has loaded the TDS onto the MET. They made the traverse to Geological Site A, and at 131:55:13 Shepard said he will start the TDS experiment. His cuff checklist (Zedekar, 1970) said he was to first mate the scoop with the extension handle, and then ready the Modularized Equipment Stowage Assembly (MESA) brush. This was a 5 in. wide nylon bristle brush that was used to dust off the EVA suits before they entered the LM in order to try to keep the dust transferred to the LM at a minimum (Jones, 1997). It had already been used at least once, at the end of EVA-2. He was to unstow the TDS and unbag it. At 131:56:39 Shepard read out the serial number 1002. The checklist calls for him to take a close-up photo of one side. This first stereo pair photos was labelled AS14-77-10361A and AS14-77-10361B. The B photo is shown in Fig. 1. Although only four samples and the hinge area are shown, it provides confirmation that the TDS was free of lunar soil at the beginning of the experiment.
Table 1 Candidate thermal control materials tested in the TDS experiment (after Gold, 1971). Sample #
Thermal control material
1 2 3 4 5 6 7 8 9 10 11 12
S-13G (white paint) Z-93 (ZnO/potassium silicate white paint) Goddard MS-74 (white paint) Ag-FEP (Inconel back film, FEP side exposed) Ag-Quartz (quartz side exposed) Dow Corning 92-007 (TiO2/silicone white paint) Cat-a-lac White (TiO2/epoxy) 3 M white velvet (400 series TiO2/epoxy polyester) Dacron on Al-Mylar fabric laminate Oxidized SiO–Al-Kapton with SiO side exposed Al–Kapton with Kapton side exposed Anodized 6061 Al MIL-A-8625, type II, class I
Fig. 1. Photograph AS14-77-10361B showing the condition of part of the 1002 TDS plate before scooping dust onto it.
At 131:57:28, 49 s after Shepard read out the serial number he says, ‘‘And I’m now dusting that sample. (Pause) Remark before I start, that number 3 block on this sample appears to have a smudge on it, before I start. A very light, black smudge.’’ So the clean sample was exposed to the lunar environment, most importantly to the solar wind, for no more than a minute before he dusted it. At this point the cuff checklist says he is to shake the dust off of it, though in other documents the instructions are to tap the dust off of it. It is not clear which motion was used. He next took close-up photos of both halves of 1002. These two stereo photo pairs were labelled AS14-77-10362A and B, and AS14-77-10363A and B, and are shown in Fig. 2. It is notable that much of the dust is found in clumps on the surface of the experiment, particularly on the tray that houses samples 7–12 (right). Sample 6 (92-007, in the lower left hand corner) seems to have enhanced affinity for the dust, it being the only sample that has markedly more dust on it than the surrounding sample holder. The rest of the samples appear to have a coating of dust that is similar to that of the sample holder around them. It is noted that samples 2 (Z-93) and 4 (Ag-FEP) are the cleanest of the samples, though the surrounding sample holder is also clean. Although Shepard called attention to a smudge on sample 3, that smudge is not obvious in the photograph and did not appear to affect the adhesion of dust to it. He then brushed off the samples with the MESA brush, took close-up photos of both sides, and re-bagged it. These two stereo photo pairs were labelled AS14-77-10364A and B, and AS14-7710365A and B, and are shown in Fig. 3. The MESA brush had been used at the end of EVA-1 to brush off the spacesuits before they re-entered the Antares in an effort to minimize the dust tracked into the spacecraft. So it is probable that the brush was not entirely clean when it was used to brush the samples. Nevertheless, brushing appears to have removed most of the dust from the surface of the samples. Shepard was directed to brush the TDS until it was clean, and since he made no remark, it is unclear how many brush strokes it took to clean the samples to this extent. Inspection reveals that the surfaces of samples 4 (Ag-FEP) and 11 (Al-Kapton) were scratched during the brushing process. This was probably due to the dust particles being drug across the surfaces by the bristles. The fact that sample 4 was scratched even though it had little if any dust on it may indicate that the MESA brush was dirty at the outset. In recent laboratory tests five configurations of nylon bristle brushes only scratched AZ93 and FEP surfaces when dust was present (Gaier et al.,
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Fig. 2. Photograph AS14-77-10362B (left) and AS14-77-10363A (right) showing the condition of the 1002 TDS plate after scooping dust onto it and shaking it off. Sample numbers shown in photograph correspond to those in Table 1.
Fig. 3. Photograph AS14-77-10364A (left) and AS14-77-10365A (right) showing the condition of the 1002 TDS plate after scooping dust onto it, shaking it off, and then brushing it with a nylon bristle brush.
2011). The noticable scratching of these two samples may be due to their having polymeric surfaces, whereas the surfaces of the other samples were mineral-like. The exception is sample 9 (Al-Mylar). This may have scratched as well, but since it is a woven fabric the scratches may not have been as obvious. The Ag-quartz appeared to also sustain some scratching, though not to the same extent. At 132:01:25, Shepard verified that the second TDS was serial number 1001. So it took him less than 4 min to dust the TDS, shake off, photograph both halves of it, brush it, photograph the two halves again, stow the 1002 sample, and unstow the 1001 sample. The checklist did not call for him to photograph the second sample as it was before the test, but just to dust the sample, and shake it off. He took close-up photos of both sides, folded the TDS and rebagged it. He reported having completed the experiment at 132:03:54, just 2.5 min after he started it. At 132:04:26 he reported getting the TDS bagged up to be stowed. The two stereo photo pairs were labelled AS14-77-10366A and B, and AS14-7710367A and B, and are shown in Fig. 4. This concluded the experiment. Shepard had little to say about the TDS experiment in the Technical Crew Debriefing. ‘‘I did what I was supposed to do and put it back in the bag. I was surprised that there was little adherence of the surface dust. I expected a little bit more. It didn’t adhere very much’’ (Hargenrader, 2005). Fig. 4 doubles the data on the interaction of lunar soil with these surfaces in the lunar environment. Unfortunately, there is almost no view of samples 7 and 8. But there is a hint that sample 3 (MS-74), 6 (92-007), 8 (white velvet), and 12 (6061 Al) might have
a higher affinity for the dust than does the frame. But taken together with the same data from the 1002 sample leads to the conclusion that probably only the 92-007 sample actually exhibits any enhanced adhesion. Consideration of Figs. 2 and 4 raises the question of exactly what motion was used to ‘‘shake’’ the dust off. It is unknown whether the sample was tipped or perhaps even inverted, or how difficult it was to reproduce the exact shaking motion between the two samples. There are such large clumps of soil remaining on the plates that it is difficult to believe that turning the plates on end and wrapping them a time or two would not dislodge much of that dust. It seems highly likely that when the bag holding the TDS 1001 plate was opened on Earth that a fair amount of dust would be left in the bag or would fall off the plates nearly immediately under terrestrial conditions. The most intriguing feature of these photographs does not involve the TDS samples but the dust itself. It is clear that whatever shaking or tapping motion Shepard used dislodged dust that was trapped in the crevasses of the stamped labels. This is most apparent with the number ‘‘5’’ located near the center of AS14-7710367B which has apparently been dislodged en masse a few mm from the identification number SEB39015737-301 S/N 1001 and now sets outside of it. Although this is the most striking case, nearly every digit in the stamp has some part of its soil out of the stamp intact. This dramatically shows that the cohesion of the dust is much larger than the adhesion to the frame. It is suspected that the cohesion is also greater than the adhesion to any of the thermal control materials as well, given the size of the clumps of soil all over the samples.
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Fig. 4. Photograph AS14-77-10366A (left) and AS14-77-10367B (right) showing the condition of the 1001 TDS plate after dusting it and shaking it off.
The Apollo 14 TDS experiment yields some conclusions that appear to conflict with the rest of the Apollo experience with lunar dust. The first is that lunar dust is not going to easily fall off on its own accord. Although the exact motion used to remove the dust was unclear, it was not very effective. But this must be tempered by Shepard’s remark in the technical debriefing that he was ‘‘surprised that there was little adherence of the surface dust. I expected a little bit more. It didn’t adhere very much.’’ (Hargenrader, 2005). Second, it appears that the nylon bristle brush removed most of the dust from the surfaces, though it scratched the soft polymer surfaces. But again this must be tempered with the poor performance of the LRV radiators in Apollo 15, 16, and 17 and the failure of the nylon bristle brush to clean them (Gaier and Jaworske, 2007). And last, what is to be made of the very strong soil cohesion compared to the adhesion to the thermal control materials? 3. Preliminary Science Report The only contemporary account of the results of the TDS experiment is a bit over a page of text with two tables and six of the seven photos within a photography report by Gold in the Apollo 14 Preliminary Science Report (NASA-SP-272, 1971). In Gold’s analysis he attributes the distribution of the dust particles solely to the differences in the materials’ properties, rather than any variation in the initial application of the dust. For example, in the discussion of Array 1002 he comments that ‘‘the screw above panel 3 appears to have attracted more dust than the neighboring surface’’. Since that screw is not visible in the photograph, and the screws that are visible in AS14-77-10362 are clean, he is probably referring to AS14-77-10366, the corresponding photo taken of sample 1001. But Figs. 2 and 4 should be nearly identical if the distribution of the dust was controlled principally by material properties of the TDS. After all, the 1001 sample should have been treated identically to the 1002 sample at the time the photos were taken. So it is not unreasonable to conclude either that the 1001 sample had more dust on it to begin with, or that it was not shaken/tapped with the same force, or both. Given that the experiment was carried out quickly by a suited astronaut wearing stiff gloves, this is not difficult to imagine. 4. Laboratory investigations 4.1. Adhesion measurements Recently, Berkebile investigated the adhesion of a synthetic volcanic glass with a composition similar to glassy lunar dust particles to spacecraft materials (Berkebile et al., 2001). One of the major
conclusions from this work was that the adhesion was enhanced by one or two orders of magnitude when the surfaces were cleaned by sputtering with argon ions for 5 min or more. Under the conditions in the vacuum chamber this corresponds to a fluence of about 4 1015 Ar+/cm2. FEP, the same perfluoropolymer used in samples 4, AgFEP, showed enhanced adhesion when sputtered only 15 s, corresponding to a fluence of 8 1014 Ar+/cm2. The regolith on the lunar surface has also been bombarded by the solar wind and micrometeoroids for literally millions of years, leaving them free of adsorbed organics, water, and gases which lower Van der Waals, Lewis acid/base, and electrostatic adhesion. But the same could not be said of the TDS sample plates. They were exposed for a long period of time to the Earth’s atmosphere, and then the spacecraft atmosphere. As is well known in the surface science community, all materials such exposed are covered by a film of adsorbed water, and organic compounds. Because the Equipment Transfer Bag in which the TDS were stored was not sealed, the TDS was exposed to at least a partial lunar vacuum for about 45 min before the experiment was started (Jones, 1995). In addition, the temperature as measured on the Lunar Portable Magnetometer was reported by Mitchell to be 125 F (52 °C) at the start of the EVA (Jones, 1995). This may have served to remove volatile gases and perhaps even some of the water vapor from the surface, but would not have removed the organic contamination. This could only have been removed by a more energetic process, such as sputtering by the solar wind. As noted above, sample 1002 was exposed to the solar wind for no more than 49 s before the dust was applied to it. The dust would have blocked much of the solar wind from reaching the surface and cleaning it. According to Wilson, the solar wind proton flux ranges 109–1012 particles/cm2 s with energy ranging from 0.2 to 3 keV (Wilson et al., 1991). This means that the fluence of protons striking the 1002 sample probably ranged from 1010 to 1013 protons/ cm2 s. The proton energy would have been similar to that of the Ar+ used to sputter clean Berkebile’s samples (2 keV). It depends on the material being sputtered, but Ar+ sputters roughly 1000 times more efficiently than protons (National Physics Laboratory, 2011). Proton sputtering is so inefficient that sputtering by He2+ actually is more important. Even though He2+ only comprises about 4% of the solar wind (Gaiss et al., 1972), it sputters about 35 times more efficiently than protons and so accounts for about 60% of the sputtering. So the total fluence of the solar wind ranged from 1011 to 1014 ions/cm2. But in terms of sputtering efficiency, it is the equivalent of 109–1012 Ar+ ions/cm2 sputtering. So, assuming the shortest cleaning time in the Berkebile experiment, in order to clean the TDS samples they would have had to have been exposed to the solar wind for at least 11 min, and perhaps as long as several days.
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The tenacity with which the dust stuck to other surfaces during Apollo leads to the conclusion that this cleaning occurs in minutes or hours rather than days. But it seems unlikely that 49 s was long enough. Although the record of how long the 1001 sample was exposed to the solar wind is less certain, since the entire second experiment was completed in 2.5 min, the fluence range for this sample was probably similar, and not sufficient to clean the samples. Thus, the conditions under which the TDS experiments were conducted were not comparable to the conditions under which general problems with the dust were noted. Except for the first minute of exposure of spacesuits, experiments, and the Lunar Roving Vehicle to the external lunar environment, all of these components were better cleaned by the solar wind than the TDS. It is therefore not surprising that the dust adhesion to the TDS was low by comparison. O’Brien has suggested that the adhesion of the dust to the TDS was also affected by the fact that the experiment was hand held,
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and so happened entirely within the aura of gas that diffused into the environment from Alan Shepard’s space suit (O’Brien, 2011). The aura is not well characterized, but Cold Cathode Gage (CCG) measurements on Apollo 12 saturated when an astronaut passed within several meters of it (Johnson et al., 1970). O’Brien suggests that the pressure might have been as high as 10 8 Torr or more. Although this is plausible, the adhesion measurements of Berkebile suggest that there is not a large drop in the adhesion forces due to adsorbed gases on the surfaces until the pressure exceeds about 10 6 Torr. However, if the spacesuit aura contained organic contaminants, and it likely did, the aura could have replenished the contamination layer that the solar wind was sputtering off. 4.2. Cohesion measurements Measurements of the surface regolith cohesion by the Surveyor spacecraft vernier and attitude control thruster firings found it to
Fig. 5. The top two rows of photographs (a) showing the condition of the TDS AgFEP samples 1001 and 1002 after being dusted and shaken/tapped, and 1002 after being brushed and corresponding samples from ground test simulations. The bottom two rows (b) show the same for the Z-93 samples. Also listed is the fractional increase in the a of the ground test samples.
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be in the range of 0.05–0.12 N/cm2 (Winn, 1969). Subsequent analysis of Apollo data generally bore out the Surveyor data finding a range of 0.01–0.10 N/cm2 (Mitchell et al., 1974). Laboratory measurements of silicates cleaved in ultrahigh vacuum lead to the conclusion that the cohesion is dominated by electrostatic forces, which may persist for months and maintain itself at pressures as high as 10 2 Torr (Grossman and Ryan, 1970). So the cohesion of the soil particles, which have been subjected to the solar wind for literally millions of years, would be little affected by contamination that lowered their adhesion to the TDS samples. The cohesion demonstrated by the transposition of dust in the label of the 1001 plate, shown in Fig. 4, demonstrates that the adhesive forces are much lower than the cohesive forces. From the Apollo report it follows that the adhesive forces are much less than 0.01 N/cm2. The large cohesive forces also explain why the dust remaining after the plates were shaken did not spread out in an even layer but formed clumps. 4.3. Absorptance measurements In a series of recent articles, Gaier has investigated the relationship between the amount of occluding dust on the performance of two thermal control surface materials included in the TDS, Z-93 white paint (sample 2) and Ag-FEP (sample 4) (Gaier, 2010). It was found that even a sub-monolayer of dust on the thermal control surface can dramatically increase its integrated solar absorptance (a). A full monolayer of a dark dust can increase the a by more than a factor of 3, and thicker layers can be expected to increase a by nearly a factor of 8. Unfortunately, a measurements were never reported for the TDS experiment. Given that the TDS were simply closed together by their hinges and placed in an unsealed bag, it is doubtful that the dust would have remained in the photographed configuration during the journey back to the LM, the loading of the samples into the LM, the ascent from the lunar surface, the transfer to the CM, the re-entry back to Earth, the transfer to quarantine, and the transfer to the analysis lab. But measuring the a of the samples containing whatever residual lunar dust would have remained stuck to the surface would have been valuable. Some estimate of the degradation can be made by comparison to the laboratory-based dust degradation measured by Gaier. Fig. 5 shows photographs of the TDS AgFEP samples (a) and the TDS Z-93 samples (b), along with photos of corresponding samples from ground-based laboratory simulations using lunar simulants. Also reported in the figure is the fractional increase in the a of the samples shown. It is not claimed that these are the exact increases in a experienced by the TDS samples, but they are likely indicative of the magnitude of the increase. 4.4. Brushing measurements Gaier also reported the results of in situ brushing of lunar simulants from AgFEP and Z-93 thermal control surfaces in a simulated lunar environment (Gaier et al., 2011). Although no brush with the same configuration as used on the TDS was tested, five variations of nylon bristle brushes were. The Apollo 14 brush was 5 in. wide and 1.5 in. deep with 3 in. long white nylon bristles. A nylon strip brush with a 1 in. long, 300 lm diameter bristles removed about 90% of the dust from the surface under laboratory conditions, but only restored about half of the a under simulated lunar conditions. This is probably due to the extra adhesion at the high vacuum (10 8 Torr) and plasma cleaning the dust. The best performing nylon bristle brush was a much softer fan brush and this was able to restore over 90% of the a under simulated lunar conditions. While there is little doubt that the Apollo brush removed most of the dust from the TDS, these results indicate that it
might not have returned the surfaces to full functionality, or even to the level of the fan brush. This was confirmed by the difficulty encountered during Apollo 15, 16, and 17 when they tried unsuccessfully to use the same brush to remove dust from the LRV radiator (Gaier and Jaworske, 2007). 5. Conclusions The results of the Apollo 14 TDS have been interpreted in light of both contemporary mission records and modern ground-based experiments. Of the 12 materials tested, only the Dow Corning 92-007 (TiO2/silicone white paint) showed enhanced dust adhesion. The AgFEP and Al-Kapton were both noticeably scratched by brushing. The Al-quartz appeared to sustain some scratching as well, though not to the same extent. The surprisingly low adhesion of soil to the TDS reported by Shepard and confirmed by the photography was probably due to residual organic contamination that was not removed by the short exposure to the solar wind. This may well have been exacerbated by contamination by the out-gassing of Shepard’s space suit. This line of reasoning is supported by the results of ground-based ultra-high vacuum adhesion studies which show that a thin contamination layer can reduce adhesion by orders of magnitude. In contrast to the low adhesion, the lunar soil exhibited surprising high cohesion. This was dramatized by dust that was jarred out of the label on the TDS 1001 plate and retained the shape of the character it was in. Taken with cohesive force measurements made by Surveyor and Apollo, it implied that the adhesive forces between the TDS and the soil must be much less than 0.01 N/cm2. Photographic comparisons suggest that the dust layer that was not removed by shaking may have increased the a of the AgFEP samples by 3–10% and the Z-93 samples by 10–65%. Brushing efficiency experiments reveal that even when a surface looks clean to the eye, or in a photograph, there may be enough small particles occluding the surface to significantly increase the a of the thermal control surface. This analysis suggests ways that a future experiment with the same objectives might be better carried out. Most importantly, surface contamination should be minimized. This could be accomplished by exposing the experiment to the solar wind for sufficient time to sputter clean the surface. Having the experiment carried out robotically would eliminate contamination from spacesuit out-gassing. It is also important to have a more systematic procedure for dusting, shaking, and brushing the samples. Once again, robotic control over the process would facilitate this. Each step should be verified by photography. Since no photos were taken after the samples were dusted but not yet shaken, it is not known whether the initial dust layer was comparable over all 24 samples. And finally, the a should be measured in situ, so that the photography and the a of each sample can be correlated. The TDS was an important experiment when it was carried out, and it is an important experiment going forward. In situ characterization of thermal control surfaces and even of dust mitigation technologies is important to validate mathematical models and ground test simulations. The Apollo 14 TDS has provided critical data on which to build effective, dust tolerant thermal control for human and robotic systems to explore the Moon, asteroids, and other airless bodies in the Solar System. Acknowledgments The author would like to thank B.J. O’Brien for many helpful conversations about the fate of the Apollo 14 TDS. S. Berkebile kindly provided the adhesion experiment sputter fluences. J. Alton, C. George, and many others at the NASA Johnson Space Center spent
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hours looking for TDS reports and hardware, and A. Young of the Smithsonian Institute confirmed that the TDS was not in their hands. References Berkebile, S., Street Jr., K.W., Gaier, J.R., 2001. Adhesion between volcanic glass and spacecraft materials in an airless body environment. In: AIAA 3rd Atmospheric and Space Environments Conference, 2011. AIAA-2011-3675. Gaier, J.R., 2010. Effect of lunar simulant type on the absorptivity and emissivity of dusted thermal control surfaces in a simulated lunar environment. In: International Conference on Environmental Systems 2010, AIAA-2010-6111 (and references contained therein). Gaier, J.R., Jaworske, D.A., 2007. Lunar Dust on Heat Rejection System Surfaces: Problems and Prospects. Space Technologies and Applications International Forum 2007. Gaier, J.R., Journey, K., Christopher, S., Davis, S., 2011. Evaluation of brushing as a lunar dust mitigation strategy for thermal control surfaces. In: International Conference on Environmental Systems 2011, AIAA-2011-5182. Gaiss, J. et al., 1972. NASA-SP-289, Apollo 15 Preliminary Science Report. SolarWind Composition Experiment, pp. 15.1–15.7. Gold, T., 1971. Lunar-Surface Close-up Stereoscopic Photography. Apollo 14 Preliminary Science Report, NASA SP-272, pp. 239–247. Grossman, J.J., Ryan, J.A., 1970. Comments on lunar surface adhesion. In: Proc. of the 7th Annual Working Group on Extraterrestrial Resources, pp. 113–115 (and references therein).
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Hargenrader, J. (Ed.), 2005. Apollo 14 Technical Crew Debriefing. Manned Spaceflight Center, February 17, 1971, pp. 10–53. Jacobs, S., Durkee, R.E., Harris Jr., R.S., 1971. Lunar dust deposition effects on the solar absorptance of thermal control materials. In: AIAA 6th Thermophysics Conference, Tullahoma, TN, April 26–28, 1971. Johnson F.E., Evans, D.E., Carroll, J.M., 1970. Cold Cathode Gage (Lunar Atmosphere Detector). Apollo 12 Preliminary Science Report SP-235, pp. 93–97. Jones, E.M. (Ed.), 1995. Apollo 14 Lunar Surface Journal Transcripts.. Mitchell, J.K. et al., 1974. Apollo Soil Mechanics Experiment S-200. Final Report, NASA Contract NAS9-11266. NASA-SP-272, 1971. Apollo 14 Preliminary Science Report, pp. 239–247. National Physics Laboratory, 2011. Sputter Yield Values. . O’Brien, B.J., 2011. Review of measurements of dust movements on the Moon during Apollo. Planet. Space Sci. 59 (14), 1708–1726. Sullivan, T.A. 1994. Catalog of Apollo Experiment Operations. NASA Reference Publication, 1317, p. 112. Wilson, J.W. et al., 1991. NASA Ref Pub 1257. Winn, F.B., 1969. Surveyor Project Final Report, Part II: Science Results, Technical Report 32-1265, JPL, p. 17. Zedekar, R.G., 1970. Apollo 14 Final Lunar Surface Procedures. Lunar Surface Operations Office, Manned Spacecraft Center, Houston, TX, December 31, p. 24.
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Icarus journal homepage: www.elsevier.com/locate/icarus
Interpretation of the Apollo 14 Thermal Degradation Sample experiment James R. Gaier National Aeronautics and Space Administration, Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135, United States
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Article history: Received 20 March 2012 Revised 28 June 2012 Accepted 3 July 2012 Available online 20 July 2012 Keywords: Moon, Surface Moon Solar wind
a b s t r a c t The Thermal Degradation Sample (TDS) experiment was one of the many investigations performed on the lunar surface during Apollo 14. Remarkably, the results of this 40 year old experiment were never fully interpreted, perhaps in part because the hardware vanished after its return. Mission records, high resolution photographs returned from the mission, and recent laboratory investigations have been used to glean important results from this experiment. It is most likely that the dust adhesion to the TDS was less than anticipated because of atomic-level contamination of its surfaces. These contaminants were probably removed from most equipment surfaces on the Moon by sputter cleaning by the solar wind, but the TDS experiments were not exposed to the solar wind long enough to affect the cleaning. Published by Elsevier Inc.
1. Introduction On February 5, 1971 the Apollo 14 Lunar Module, Antares, landed on the Fra Mauro formation of the Moon, and astronauts Alan Shepard and Edgar Mitchell became the third pair of humans to walk the lunar surface. They would remain on the Moon for 33 h before returning to their colleague Stuart Roosa in the Command Module Kitty Hawk and returning to Earth. During their brief stay they ventured onto the surface for two extravehicular activities (EVAs) for a total of 9 h and 22 min. Much of the first EVA was spent setting up camp, activating the radioisotope thermoelectric generator (RTG), emplacing the Apollo Lunar Science Experiment Package (ALSEP) and several other stand-alone experiments which characterized the lunar environment. The second EVA was a long range traverse with a focus on the collection of interesting geological samples. Their time was tightly managed, and nearly every action was choreographed down to the minute. The result was a flood of new data characterizing the geology and environment of the Moon. During the second EVA, at the first geological site visited, 8 min of Alan Shepard’s time was scheduled for an activity designated the Thermal Degradation Sample (TDS) experiment (Zedekar, 1970). The origin of this experiment is unclear. In Sullivan’s comprehensive Catalog of Apollo Experiment Operations, the Principal Investigator is listed as ‘‘Unknown’’ (Sullivan, 1994). Perhaps the best clue lies in the original print for the fabrication of the TDS sample holder, which was ordered by R.S. Harris, and the NASA Manned Spacecraft Center. In addition, a paper by Jacobs, Durkee, and Harris describes the experiment and plans to analyze it (Jacobs et al., 1971). Since this is the only contemporary reference in the E-mail address: [email protected] 0019-1035/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.icarus.2012.07.002
scholarly literature (i.e., outside of NASA documents) to mention the experiment, and since the authors were from the NASA Manned Spacecraft Center, it is likely that one of them was the Principal Investigator. The TDS experiment was a simple test ‘‘To evaluate the effect of lunar dust on the optical properties (absorptivity and emissivity) of 12 candidate thermal coatings. Two duplicate arrays each containing 12 coatings were taken to the Moon. After covering them with dust, one was tapped to remove the dust and the other was cleaned with a nylon bristle brush’’ (Sullivan, 1994). Photographs of the samples were to be taken with the Apollo Lunar Closeup stereographic Camera (ALCC) in their pristine state, after they were dusted and shook, and after the one was brushed (Zedekar, 1970). The TDS samples were then to be returned to Earth so that their solar absorptance and thermal emittance could be measured. The experiment was ‘‘expected to yield material data to aid in the selection of radiator surfaces on the Lunar Roving Vehicle (LRV) and other advanced lunar operational equipment’’ (Zedekar, 1970). After completing the experiment there was a well choreographed plan to load the TDS into the EVA-2 Equipment Transfer Bag which was loaded onto the Lunar Module (LM) Ascent Stage at the end of the EVA. Here it was stowed in the interim stowage assembly over the ascent stage engine cover. It was then transferred to the Command Module in the ISA Decontamination Bag and stowed on top of Command Module (CM) Vol (A1). Presumably this occurred as planned because there is no mention otherwise in the mission records and Jacobs, Durkee, and Harris report that at the time of writing their manuscript it was in quarantine. However, there is no further mention of the TDS in the record. Jacobs, Durkee, and Harris never published a follow-on paper describing the results of the TDS. No NASA reports of the results have been found. A search through the archives at the NASA
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Johnson Space Center finds no reports, no data, no TDS hardware. The Smithsonian Institution likewise has no record of it being transferred to them. The principals and their managers are either deceased or have no memory of the TDS or any post-flight tests. Except for the photographs and these few details, the Apollo 14 TDS is lost to history. The purpose of this report is to inspect the TDS record in detail, and to interpret those results in light of recent laboratory experiments on the effects of dust on thermal control surfaces in order that some insight might be gained from this rare experiment conducted in the lunar environment. 2. Results from Apollo 14 The TDS hardware was very simple and straightforward. There were two identical metal sample holders, designated 1001 and 1002. Each sample holder was made up of two trays connected by a hinge. Each tray held six samples in a 2 3 array. The sample holders were stowed with the trays closed together, protecting the samples, and then opened to apply the dust. After the experiment was completed, the trays were again folded together and placed in a Teflon Equipment Transfer Bag (ETB). According to the original design prints, each 6 sample carrier of the TDS sample holders was 3.25 in. wide and 3.00 in. deep. It also had a 3.2 in. long handle grip to make it easier for the astronauts to handle. The samples were 1 in. squares with 0.75 0.75 in. exposed to the lunar environment, and the remaining under the rim to lock it in place. The 12 samples were candidate thermal control materials in 1971, and are listed in Table 1 (Gold, 1971). The events surrounding the TDS experiment can be reconstructed through the nominal mission plan, corrected by the recorded transcripts (Jones, 1995). At the beginning of EVA-2, astronauts Shepard and Mitchell loaded up the Modular Equipment Transporter (MET). At a mission time of 131:36:21 Shepard confirms to Mitchell that he has loaded the TDS onto the MET. They made the traverse to Geological Site A, and at 131:55:13 Shepard said he will start the TDS experiment. His cuff checklist (Zedekar, 1970) said he was to first mate the scoop with the extension handle, and then ready the Modularized Equipment Stowage Assembly (MESA) brush. This was a 5 in. wide nylon bristle brush that was used to dust off the EVA suits before they entered the LM in order to try to keep the dust transferred to the LM at a minimum (Jones, 1997). It had already been used at least once, at the end of EVA-2. He was to unstow the TDS and unbag it. At 131:56:39 Shepard read out the serial number 1002. The checklist calls for him to take a close-up photo of one side. This first stereo pair photos was labelled AS14-77-10361A and AS14-77-10361B. The B photo is shown in Fig. 1. Although only four samples and the hinge area are shown, it provides confirmation that the TDS was free of lunar soil at the beginning of the experiment.
Table 1 Candidate thermal control materials tested in the TDS experiment (after Gold, 1971). Sample #
Thermal control material
1 2 3 4 5 6 7 8 9 10 11 12
S-13G (white paint) Z-93 (ZnO/potassium silicate white paint) Goddard MS-74 (white paint) Ag-FEP (Inconel back film, FEP side exposed) Ag-Quartz (quartz side exposed) Dow Corning 92-007 (TiO2/silicone white paint) Cat-a-lac White (TiO2/epoxy) 3 M white velvet (400 series TiO2/epoxy polyester) Dacron on Al-Mylar fabric laminate Oxidized SiO–Al-Kapton with SiO side exposed Al–Kapton with Kapton side exposed Anodized 6061 Al MIL-A-8625, type II, class I
Fig. 1. Photograph AS14-77-10361B showing the condition of part of the 1002 TDS plate before scooping dust onto it.
At 131:57:28, 49 s after Shepard read out the serial number he says, ‘‘And I’m now dusting that sample. (Pause) Remark before I start, that number 3 block on this sample appears to have a smudge on it, before I start. A very light, black smudge.’’ So the clean sample was exposed to the lunar environment, most importantly to the solar wind, for no more than a minute before he dusted it. At this point the cuff checklist says he is to shake the dust off of it, though in other documents the instructions are to tap the dust off of it. It is not clear which motion was used. He next took close-up photos of both halves of 1002. These two stereo photo pairs were labelled AS14-77-10362A and B, and AS14-77-10363A and B, and are shown in Fig. 2. It is notable that much of the dust is found in clumps on the surface of the experiment, particularly on the tray that houses samples 7–12 (right). Sample 6 (92-007, in the lower left hand corner) seems to have enhanced affinity for the dust, it being the only sample that has markedly more dust on it than the surrounding sample holder. The rest of the samples appear to have a coating of dust that is similar to that of the sample holder around them. It is noted that samples 2 (Z-93) and 4 (Ag-FEP) are the cleanest of the samples, though the surrounding sample holder is also clean. Although Shepard called attention to a smudge on sample 3, that smudge is not obvious in the photograph and did not appear to affect the adhesion of dust to it. He then brushed off the samples with the MESA brush, took close-up photos of both sides, and re-bagged it. These two stereo photo pairs were labelled AS14-77-10364A and B, and AS14-7710365A and B, and are shown in Fig. 3. The MESA brush had been used at the end of EVA-1 to brush off the spacesuits before they re-entered the Antares in an effort to minimize the dust tracked into the spacecraft. So it is probable that the brush was not entirely clean when it was used to brush the samples. Nevertheless, brushing appears to have removed most of the dust from the surface of the samples. Shepard was directed to brush the TDS until it was clean, and since he made no remark, it is unclear how many brush strokes it took to clean the samples to this extent. Inspection reveals that the surfaces of samples 4 (Ag-FEP) and 11 (Al-Kapton) were scratched during the brushing process. This was probably due to the dust particles being drug across the surfaces by the bristles. The fact that sample 4 was scratched even though it had little if any dust on it may indicate that the MESA brush was dirty at the outset. In recent laboratory tests five configurations of nylon bristle brushes only scratched AZ93 and FEP surfaces when dust was present (Gaier et al.,
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Fig. 2. Photograph AS14-77-10362B (left) and AS14-77-10363A (right) showing the condition of the 1002 TDS plate after scooping dust onto it and shaking it off. Sample numbers shown in photograph correspond to those in Table 1.
Fig. 3. Photograph AS14-77-10364A (left) and AS14-77-10365A (right) showing the condition of the 1002 TDS plate after scooping dust onto it, shaking it off, and then brushing it with a nylon bristle brush.
2011). The noticable scratching of these two samples may be due to their having polymeric surfaces, whereas the surfaces of the other samples were mineral-like. The exception is sample 9 (Al-Mylar). This may have scratched as well, but since it is a woven fabric the scratches may not have been as obvious. The Ag-quartz appeared to also sustain some scratching, though not to the same extent. At 132:01:25, Shepard verified that the second TDS was serial number 1001. So it took him less than 4 min to dust the TDS, shake off, photograph both halves of it, brush it, photograph the two halves again, stow the 1002 sample, and unstow the 1001 sample. The checklist did not call for him to photograph the second sample as it was before the test, but just to dust the sample, and shake it off. He took close-up photos of both sides, folded the TDS and rebagged it. He reported having completed the experiment at 132:03:54, just 2.5 min after he started it. At 132:04:26 he reported getting the TDS bagged up to be stowed. The two stereo photo pairs were labelled AS14-77-10366A and B, and AS14-7710367A and B, and are shown in Fig. 4. This concluded the experiment. Shepard had little to say about the TDS experiment in the Technical Crew Debriefing. ‘‘I did what I was supposed to do and put it back in the bag. I was surprised that there was little adherence of the surface dust. I expected a little bit more. It didn’t adhere very much’’ (Hargenrader, 2005). Fig. 4 doubles the data on the interaction of lunar soil with these surfaces in the lunar environment. Unfortunately, there is almost no view of samples 7 and 8. But there is a hint that sample 3 (MS-74), 6 (92-007), 8 (white velvet), and 12 (6061 Al) might have
a higher affinity for the dust than does the frame. But taken together with the same data from the 1002 sample leads to the conclusion that probably only the 92-007 sample actually exhibits any enhanced adhesion. Consideration of Figs. 2 and 4 raises the question of exactly what motion was used to ‘‘shake’’ the dust off. It is unknown whether the sample was tipped or perhaps even inverted, or how difficult it was to reproduce the exact shaking motion between the two samples. There are such large clumps of soil remaining on the plates that it is difficult to believe that turning the plates on end and wrapping them a time or two would not dislodge much of that dust. It seems highly likely that when the bag holding the TDS 1001 plate was opened on Earth that a fair amount of dust would be left in the bag or would fall off the plates nearly immediately under terrestrial conditions. The most intriguing feature of these photographs does not involve the TDS samples but the dust itself. It is clear that whatever shaking or tapping motion Shepard used dislodged dust that was trapped in the crevasses of the stamped labels. This is most apparent with the number ‘‘5’’ located near the center of AS14-7710367B which has apparently been dislodged en masse a few mm from the identification number SEB39015737-301 S/N 1001 and now sets outside of it. Although this is the most striking case, nearly every digit in the stamp has some part of its soil out of the stamp intact. This dramatically shows that the cohesion of the dust is much larger than the adhesion to the frame. It is suspected that the cohesion is also greater than the adhesion to any of the thermal control materials as well, given the size of the clumps of soil all over the samples.
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Fig. 4. Photograph AS14-77-10366A (left) and AS14-77-10367B (right) showing the condition of the 1001 TDS plate after dusting it and shaking it off.
The Apollo 14 TDS experiment yields some conclusions that appear to conflict with the rest of the Apollo experience with lunar dust. The first is that lunar dust is not going to easily fall off on its own accord. Although the exact motion used to remove the dust was unclear, it was not very effective. But this must be tempered by Shepard’s remark in the technical debriefing that he was ‘‘surprised that there was little adherence of the surface dust. I expected a little bit more. It didn’t adhere very much.’’ (Hargenrader, 2005). Second, it appears that the nylon bristle brush removed most of the dust from the surfaces, though it scratched the soft polymer surfaces. But again this must be tempered with the poor performance of the LRV radiators in Apollo 15, 16, and 17 and the failure of the nylon bristle brush to clean them (Gaier and Jaworske, 2007). And last, what is to be made of the very strong soil cohesion compared to the adhesion to the thermal control materials? 3. Preliminary Science Report The only contemporary account of the results of the TDS experiment is a bit over a page of text with two tables and six of the seven photos within a photography report by Gold in the Apollo 14 Preliminary Science Report (NASA-SP-272, 1971). In Gold’s analysis he attributes the distribution of the dust particles solely to the differences in the materials’ properties, rather than any variation in the initial application of the dust. For example, in the discussion of Array 1002 he comments that ‘‘the screw above panel 3 appears to have attracted more dust than the neighboring surface’’. Since that screw is not visible in the photograph, and the screws that are visible in AS14-77-10362 are clean, he is probably referring to AS14-77-10366, the corresponding photo taken of sample 1001. But Figs. 2 and 4 should be nearly identical if the distribution of the dust was controlled principally by material properties of the TDS. After all, the 1001 sample should have been treated identically to the 1002 sample at the time the photos were taken. So it is not unreasonable to conclude either that the 1001 sample had more dust on it to begin with, or that it was not shaken/tapped with the same force, or both. Given that the experiment was carried out quickly by a suited astronaut wearing stiff gloves, this is not difficult to imagine. 4. Laboratory investigations 4.1. Adhesion measurements Recently, Berkebile investigated the adhesion of a synthetic volcanic glass with a composition similar to glassy lunar dust particles to spacecraft materials (Berkebile et al., 2001). One of the major
conclusions from this work was that the adhesion was enhanced by one or two orders of magnitude when the surfaces were cleaned by sputtering with argon ions for 5 min or more. Under the conditions in the vacuum chamber this corresponds to a fluence of about 4 1015 Ar+/cm2. FEP, the same perfluoropolymer used in samples 4, AgFEP, showed enhanced adhesion when sputtered only 15 s, corresponding to a fluence of 8 1014 Ar+/cm2. The regolith on the lunar surface has also been bombarded by the solar wind and micrometeoroids for literally millions of years, leaving them free of adsorbed organics, water, and gases which lower Van der Waals, Lewis acid/base, and electrostatic adhesion. But the same could not be said of the TDS sample plates. They were exposed for a long period of time to the Earth’s atmosphere, and then the spacecraft atmosphere. As is well known in the surface science community, all materials such exposed are covered by a film of adsorbed water, and organic compounds. Because the Equipment Transfer Bag in which the TDS were stored was not sealed, the TDS was exposed to at least a partial lunar vacuum for about 45 min before the experiment was started (Jones, 1995). In addition, the temperature as measured on the Lunar Portable Magnetometer was reported by Mitchell to be 125 F (52 °C) at the start of the EVA (Jones, 1995). This may have served to remove volatile gases and perhaps even some of the water vapor from the surface, but would not have removed the organic contamination. This could only have been removed by a more energetic process, such as sputtering by the solar wind. As noted above, sample 1002 was exposed to the solar wind for no more than 49 s before the dust was applied to it. The dust would have blocked much of the solar wind from reaching the surface and cleaning it. According to Wilson, the solar wind proton flux ranges 109–1012 particles/cm2 s with energy ranging from 0.2 to 3 keV (Wilson et al., 1991). This means that the fluence of protons striking the 1002 sample probably ranged from 1010 to 1013 protons/ cm2 s. The proton energy would have been similar to that of the Ar+ used to sputter clean Berkebile’s samples (2 keV). It depends on the material being sputtered, but Ar+ sputters roughly 1000 times more efficiently than protons (National Physics Laboratory, 2011). Proton sputtering is so inefficient that sputtering by He2+ actually is more important. Even though He2+ only comprises about 4% of the solar wind (Gaiss et al., 1972), it sputters about 35 times more efficiently than protons and so accounts for about 60% of the sputtering. So the total fluence of the solar wind ranged from 1011 to 1014 ions/cm2. But in terms of sputtering efficiency, it is the equivalent of 109–1012 Ar+ ions/cm2 sputtering. So, assuming the shortest cleaning time in the Berkebile experiment, in order to clean the TDS samples they would have had to have been exposed to the solar wind for at least 11 min, and perhaps as long as several days.
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The tenacity with which the dust stuck to other surfaces during Apollo leads to the conclusion that this cleaning occurs in minutes or hours rather than days. But it seems unlikely that 49 s was long enough. Although the record of how long the 1001 sample was exposed to the solar wind is less certain, since the entire second experiment was completed in 2.5 min, the fluence range for this sample was probably similar, and not sufficient to clean the samples. Thus, the conditions under which the TDS experiments were conducted were not comparable to the conditions under which general problems with the dust were noted. Except for the first minute of exposure of spacesuits, experiments, and the Lunar Roving Vehicle to the external lunar environment, all of these components were better cleaned by the solar wind than the TDS. It is therefore not surprising that the dust adhesion to the TDS was low by comparison. O’Brien has suggested that the adhesion of the dust to the TDS was also affected by the fact that the experiment was hand held,
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and so happened entirely within the aura of gas that diffused into the environment from Alan Shepard’s space suit (O’Brien, 2011). The aura is not well characterized, but Cold Cathode Gage (CCG) measurements on Apollo 12 saturated when an astronaut passed within several meters of it (Johnson et al., 1970). O’Brien suggests that the pressure might have been as high as 10 8 Torr or more. Although this is plausible, the adhesion measurements of Berkebile suggest that there is not a large drop in the adhesion forces due to adsorbed gases on the surfaces until the pressure exceeds about 10 6 Torr. However, if the spacesuit aura contained organic contaminants, and it likely did, the aura could have replenished the contamination layer that the solar wind was sputtering off. 4.2. Cohesion measurements Measurements of the surface regolith cohesion by the Surveyor spacecraft vernier and attitude control thruster firings found it to
Fig. 5. The top two rows of photographs (a) showing the condition of the TDS AgFEP samples 1001 and 1002 after being dusted and shaken/tapped, and 1002 after being brushed and corresponding samples from ground test simulations. The bottom two rows (b) show the same for the Z-93 samples. Also listed is the fractional increase in the a of the ground test samples.
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be in the range of 0.05–0.12 N/cm2 (Winn, 1969). Subsequent analysis of Apollo data generally bore out the Surveyor data finding a range of 0.01–0.10 N/cm2 (Mitchell et al., 1974). Laboratory measurements of silicates cleaved in ultrahigh vacuum lead to the conclusion that the cohesion is dominated by electrostatic forces, which may persist for months and maintain itself at pressures as high as 10 2 Torr (Grossman and Ryan, 1970). So the cohesion of the soil particles, which have been subjected to the solar wind for literally millions of years, would be little affected by contamination that lowered their adhesion to the TDS samples. The cohesion demonstrated by the transposition of dust in the label of the 1001 plate, shown in Fig. 4, demonstrates that the adhesive forces are much lower than the cohesive forces. From the Apollo report it follows that the adhesive forces are much less than 0.01 N/cm2. The large cohesive forces also explain why the dust remaining after the plates were shaken did not spread out in an even layer but formed clumps. 4.3. Absorptance measurements In a series of recent articles, Gaier has investigated the relationship between the amount of occluding dust on the performance of two thermal control surface materials included in the TDS, Z-93 white paint (sample 2) and Ag-FEP (sample 4) (Gaier, 2010). It was found that even a sub-monolayer of dust on the thermal control surface can dramatically increase its integrated solar absorptance (a). A full monolayer of a dark dust can increase the a by more than a factor of 3, and thicker layers can be expected to increase a by nearly a factor of 8. Unfortunately, a measurements were never reported for the TDS experiment. Given that the TDS were simply closed together by their hinges and placed in an unsealed bag, it is doubtful that the dust would have remained in the photographed configuration during the journey back to the LM, the loading of the samples into the LM, the ascent from the lunar surface, the transfer to the CM, the re-entry back to Earth, the transfer to quarantine, and the transfer to the analysis lab. But measuring the a of the samples containing whatever residual lunar dust would have remained stuck to the surface would have been valuable. Some estimate of the degradation can be made by comparison to the laboratory-based dust degradation measured by Gaier. Fig. 5 shows photographs of the TDS AgFEP samples (a) and the TDS Z-93 samples (b), along with photos of corresponding samples from ground-based laboratory simulations using lunar simulants. Also reported in the figure is the fractional increase in the a of the samples shown. It is not claimed that these are the exact increases in a experienced by the TDS samples, but they are likely indicative of the magnitude of the increase. 4.4. Brushing measurements Gaier also reported the results of in situ brushing of lunar simulants from AgFEP and Z-93 thermal control surfaces in a simulated lunar environment (Gaier et al., 2011). Although no brush with the same configuration as used on the TDS was tested, five variations of nylon bristle brushes were. The Apollo 14 brush was 5 in. wide and 1.5 in. deep with 3 in. long white nylon bristles. A nylon strip brush with a 1 in. long, 300 lm diameter bristles removed about 90% of the dust from the surface under laboratory conditions, but only restored about half of the a under simulated lunar conditions. This is probably due to the extra adhesion at the high vacuum (10 8 Torr) and plasma cleaning the dust. The best performing nylon bristle brush was a much softer fan brush and this was able to restore over 90% of the a under simulated lunar conditions. While there is little doubt that the Apollo brush removed most of the dust from the TDS, these results indicate that it
might not have returned the surfaces to full functionality, or even to the level of the fan brush. This was confirmed by the difficulty encountered during Apollo 15, 16, and 17 when they tried unsuccessfully to use the same brush to remove dust from the LRV radiator (Gaier and Jaworske, 2007). 5. Conclusions The results of the Apollo 14 TDS have been interpreted in light of both contemporary mission records and modern ground-based experiments. Of the 12 materials tested, only the Dow Corning 92-007 (TiO2/silicone white paint) showed enhanced dust adhesion. The AgFEP and Al-Kapton were both noticeably scratched by brushing. The Al-quartz appeared to sustain some scratching as well, though not to the same extent. The surprisingly low adhesion of soil to the TDS reported by Shepard and confirmed by the photography was probably due to residual organic contamination that was not removed by the short exposure to the solar wind. This may well have been exacerbated by contamination by the out-gassing of Shepard’s space suit. This line of reasoning is supported by the results of ground-based ultra-high vacuum adhesion studies which show that a thin contamination layer can reduce adhesion by orders of magnitude. In contrast to the low adhesion, the lunar soil exhibited surprising high cohesion. This was dramatized by dust that was jarred out of the label on the TDS 1001 plate and retained the shape of the character it was in. Taken with cohesive force measurements made by Surveyor and Apollo, it implied that the adhesive forces between the TDS and the soil must be much less than 0.01 N/cm2. Photographic comparisons suggest that the dust layer that was not removed by shaking may have increased the a of the AgFEP samples by 3–10% and the Z-93 samples by 10–65%. Brushing efficiency experiments reveal that even when a surface looks clean to the eye, or in a photograph, there may be enough small particles occluding the surface to significantly increase the a of the thermal control surface. This analysis suggests ways that a future experiment with the same objectives might be better carried out. Most importantly, surface contamination should be minimized. This could be accomplished by exposing the experiment to the solar wind for sufficient time to sputter clean the surface. Having the experiment carried out robotically would eliminate contamination from spacesuit out-gassing. It is also important to have a more systematic procedure for dusting, shaking, and brushing the samples. Once again, robotic control over the process would facilitate this. Each step should be verified by photography. Since no photos were taken after the samples were dusted but not yet shaken, it is not known whether the initial dust layer was comparable over all 24 samples. And finally, the a should be measured in situ, so that the photography and the a of each sample can be correlated. The TDS was an important experiment when it was carried out, and it is an important experiment going forward. In situ characterization of thermal control surfaces and even of dust mitigation technologies is important to validate mathematical models and ground test simulations. The Apollo 14 TDS has provided critical data on which to build effective, dust tolerant thermal control for human and robotic systems to explore the Moon, asteroids, and other airless bodies in the Solar System. Acknowledgments The author would like to thank B.J. O’Brien for many helpful conversations about the fate of the Apollo 14 TDS. S. Berkebile kindly provided the adhesion experiment sputter fluences. J. Alton, C. George, and many others at the NASA Johnson Space Center spent
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hours looking for TDS reports and hardware, and A. Young of the Smithsonian Institute confirmed that the TDS was not in their hands. References Berkebile, S., Street Jr., K.W., Gaier, J.R., 2001. Adhesion between volcanic glass and spacecraft materials in an airless body environment. In: AIAA 3rd Atmospheric and Space Environments Conference, 2011. AIAA-2011-3675. Gaier, J.R., 2010. Effect of lunar simulant type on the absorptivity and emissivity of dusted thermal control surfaces in a simulated lunar environment. In: International Conference on Environmental Systems 2010, AIAA-2010-6111 (and references contained therein). Gaier, J.R., Jaworske, D.A., 2007. Lunar Dust on Heat Rejection System Surfaces: Problems and Prospects. Space Technologies and Applications International Forum 2007. Gaier, J.R., Journey, K., Christopher, S., Davis, S., 2011. Evaluation of brushing as a lunar dust mitigation strategy for thermal control surfaces. In: International Conference on Environmental Systems 2011, AIAA-2011-5182. Gaiss, J. et al., 1972. NASA-SP-289, Apollo 15 Preliminary Science Report. SolarWind Composition Experiment, pp. 15.1–15.7. Gold, T., 1971. Lunar-Surface Close-up Stereoscopic Photography. Apollo 14 Preliminary Science Report, NASA SP-272, pp. 239–247. Grossman, J.J., Ryan, J.A., 1970. Comments on lunar surface adhesion. In: Proc. of the 7th Annual Working Group on Extraterrestrial Resources, pp. 113–115 (and references therein).
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Hargenrader, J. (Ed.), 2005. Apollo 14 Technical Crew Debriefing. Manned Spaceflight Center, February 17, 1971, pp. 10–53. Jacobs, S., Durkee, R.E., Harris Jr., R.S., 1971. Lunar dust deposition effects on the solar absorptance of thermal control materials. In: AIAA 6th Thermophysics Conference, Tullahoma, TN, April 26–28, 1971. Johnson F.E., Evans, D.E., Carroll, J.M., 1970. Cold Cathode Gage (Lunar Atmosphere Detector). Apollo 12 Preliminary Science Report SP-235, pp. 93–97. Jones, E.M. (Ed.), 1995. Apollo 14 Lunar Surface Journal Transcripts.