Accelerated space environmental testing and analysis of ultra-thin polymer films for Gossamer space structures like solar sails

C.O.A. Semprimoschnig (1) P. A. Gray (2), M. K. Nehls (2), D. L. Edwards (3)

(1) ESA/European Space Research and Technology Centre (ESTEC), Materials Physics and Chemistry Section, PO 299, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands, email: [email protected] (2) NASA-Marshall Space Flight Center, Environmental Effects Branch, Mailcode EM50, Huntsville, AL 35812, USA email: [email protected], [email protected] (3) NASA- Marshall Space Flight Center, Natural Environments Branch, Mailcode: EV13, AL 35812, USA [email protected]

Keywords: Space environmental effects, materials, Gossamer structures, Ultra-thin polymer films, Electron radiation effects, Materials Degradation

ABSTRACT deployable space structures such as shields, large antennas, deployable solar arrays etc. All Ultra-thin materials for Gossamer Structures like these concepts are based on ultra-low mass solar sails will enable those missions but pose structures which rely on the use of (hybrid) various challenges for space environmental testing membrane materials that give those structures an and materials analysis. engineering functionality.

In this work absolute dose and dose rate effect were Another well known application of hybrid Gossamer assessed on ultra-thin polymer films (1.87 µm thick type structures are solar sails. In this case incident aluminised PET, 2.1 µm thick aluminised PEN and give rise to a “propellant-less” form of 7.5 µm thick aluminised and bare Polyimide films. propulsion which works via the transfer or exchange of impulses. Solar (in some concepts even laser) Mechanical and thermo-optical properties were photons can be used to propel spacecraft based on assessed by exposing materials to electron ultra-thin materials that capture photons’ impulses. radiation. Most work was done on the 2.1 µm thick This “way of travel” was first described in the 1920s PEN films. For that material up to a medium rate of [1, 2, 3]. damage, 23.3 MGy no dose rate dependence was found. For higher absorbed damage a small trend It is well known that the impulse of photons in the indicating an inverse dependence of dose rate was earth orbit (1 Astronomical Unit = 1 AU) is extremely 2 noted, i.e. lower dose rates could lead to more small [4], i.e. about 4.5 µN/m can be exploited as damage. The dominant degradation mechanism for thrust. If that thrust is perfectly reflected that thrust that material is chain scission and damage is can be doubled, so ideally a total thrust of about 9 2 increasing with increasing absolute dose. µN/m can be achieved at 1 AU. Such very small numbers require that solar sail areas need to be In terms of thermo-optical stability, i.e. solar large and of the Gossamer type. Engineering absorptance degradation, a saturating effect from a parameters that are important to consider are the 2 threshold of 12 MGy was noted for PEN and more sail loading in units of [g/m ] and the characteristic 2 degradation, though mostly recoverable, was achievable acceleration in units of [mm/s ]. observed on the tested Polyimide film. The recovery Naturally, the lighter the sail, the lower the sail of the latter is light sensitive. loading and the higher the possible acceleration. Even though the numbers of the impulse are small the principle has already been demonstrated on a 1. INTRODUCTION small laboratory scale where the source was realised with a laser [5, 6]. In the past thrust by solar sailing has also been used on macroscopic Gossamer type structures have attracted and are spacecraft as reported by [7] on the flyby of Venus attracting considerable interest due to the fact that of Mariner 10. However, the use of that method as a they could enable new space missions with better or main form of propulsion requires dedicated unprecedented performance advantages. Examples materials and so far no solar sail space mission has of such Gossamer structures are inflatable/ been achieved.

______Proc. of the 10th ISMSE & the 8th ICPMSE, Collioure, France, 19-23 June 2006 (SP-616, September 2006) Another challenge for solar sailing is the Coated ultra-thin polymer films will be an enabling development and engineering of the solar sail materials technology as they hold great promise for materials and the assessment of the materials such future Gossamer type solar sails. Such resistance to the space environment. This includes polymer films are generally aluminised on the sun materials processing, coatings, folding & packaging, facing side, i.e. the side which shall reflect the anti-static properties, handling aspects, outgassing impinging photons. The thickness of these Al layers and venting etc. The real resistance of such ultra- is in the range of 50-100 nm. The polymer film is thin materials to the space environment will be used as a backing structure which needs to fulfil unknown until the first functional solar sail mission various ground requirements such as flexibility for will be in orbit. In preparation for such a mission, packaging, ease of handling, able to deploy in space environmental ground testing is the best space, tear and impact resistance (ground & space) method to assess and analyse candidate materials. and being able survive the various components of This is done by simulating the space environment the space environment. It shall also have a high with dedicated space radiation chambers. In this thermal emittance to minimise thermal loads as for field, NASA’s Space Environmental Effects Branch some mission concepts close approaches to the at the Marshall Space Flight Centre has been sun are favourable because naturally the photon actively involved. In the past, several papers were pressure scales with the inverse square distance to presented showing results of the activities the sun. A successful approach to the sun to about performed [9, 10, 11]. This work also showed the 0.10 - 0.20 AU could theoretically propel a solar sail great many difficulties that are encountered when to velocities in the percent range of the speed of assessing ultra-thin materials. The availability light. This could enable for instance missions to the however of suitable ultra lightweight and space solar Heliopause or even interstellar missions in a radiation resistant materials will determine the future reasonable duration [8]. It should be also noted that of solar sailing. depending on the impinging angle of the solar photons with the sail material a solar sail spacecraft The space environment with its various components can spiral closer to or further away relatively to the like high vacuum, electromagnetic radiation (UV, sun. Another promising mission concept is based on VUV, X-ray etc.), particle radiation, temperature so called non-Keplerian orbits. These are orbits extremes poses quite some challenges to ultra-thin which do not follow the traditional laws of Kepler materials. For solar sail materials, the main because by compensating gravitational attraction difference to “macroscopic” materials like polymeric with the thrust of the solar pressure “static” orbits foils commonly used on spacecraft Multi-Layer- could be described. One can easily visualise a Insulations (MLI) is the reduced thickness. The concept for such a mission to be a solar sail thickness of external MLI materials is commonly in “floating” above the sun’s pole. Such a position in the 25 µm to 125 µm range, whereas candidate the solar system is rather unique and very difficult to solar sail materials are significantly, about one to reach by traditional orbital mechanics. two orders of magnitude thinner. This implies that thickness effects can play a role, i.e. properties In the past several deployment tests have been obtained on thicker materials may deviate from conducted around the world in the last years and properties obtained on those ultra-thin materials. It even deployments have been demonstrated in also implies that the ultra-thin materials need to be space. For instance, in 1999 a large (20 x 20 m) somewhat differently assessed versus radiation. ground based deployment was done in Europe Especially low energy particle interaction might within an ESA/DLR (Deutsche Luft und become important as these low energies may lead Raumfahrtagentur) funded project. In 2004 an in- to a highly absorbed dose. In addition, the effect of space deployment test has been reported on-board very low energies might also become important as a Japanese rocket. In 2005 also a 20 x 20 m ground there could be quite some gradients of differently deployment test has been performed in the USA damaged zones caused in the material. within a NASA funded programme. Another activity worth mentioning was the in-space deployment 2. SPACE SIMULATION FACILITY attempt of the so called COSMOS 1 solar sail spacecraft from in June 2005. It was unfortunately unsuccessful due to a failure of In this study, space simulation was done with the the Russian rocket. All of the mentioned activities test chamber shown in Fig.1. It is a 75 cm diameter focussed on one of the critical points, namely the cylindrical chamber with a length of 135 cm. An deployment. array of four 10 inch Conflat (CF) ports was located on one end at 10 degrees off axis. They converged at the sample plane. This plane had also an array of LABVIEW program was established to control the four 10 inch CF ports located radially around it. A electron beam. This enabled to perform constant door was located on the sample end of the chamber dose rate exposures with a feedback control. to allow easy access to the sample holder assembly. Two Kimball Physics high energy An important point for any space simulation is to electron flood guns were used as particle sources. ascertain that the source provides a uniform beam A Kimball Physics Model EGPS-9B, 50keV electron and/irradiation area. To do that a suitable phosphor gun was mounted on the furthest right port (looking screen was used and the homogeneity of the from the sample position to the outside port where electron source was optimized by finding the most the particle sources are located). The other source, homogeneous image of the beam(s) on the a 100 keV source of type EGH-8104A/EGPS-8104 phosphor screen. The homogeneity was visualized from Kimball Physics was mounted on the top most by a sensitive TV camera. A good axial symmetry 10 degree port. The chamber was pumped with a was observed and from the measured intensity a Varian Tri-Scroll Roughing Pump backing a Varian beam homogeneity with a slight radial decay of dry lubed model V1000 Turbo-Pump. This pumping better than +/- 20% was found. The guns were system was used to achieve a vacuum of 1 x10-6 operated without rastering. It was found that mbar or better, and a Varian 1000 l/s ion pump was rastering did not improve the homogeneity of the used to maintain the vacuum after which the turbo- used conditions and guns. The advantage of that pump was turned off. The pressure during testing was that more current was available to expose was in the high 1 x 10-8 mbar range. samples and so higher dose rates could be achieved. The operating conditions are shown below: 100 kV Electron Gun Table 1

Facility Gun 1 Model EGPS- Anode: Focus : 1 (50kV) 9B 200V 200V Facility Gun 2 Model EGH- Anode: Focus: 1 (100kV) 8104A/EGPS- 200 V 121 V 8104

3. TEST MATERIALS

The following candidate materials were assessed in Fig.1: Test chamber with 100 kV electron source this study, Al coated 1.87 µm thick PET films (Mylar on the left (opposite door). ™), Al-coated 2.1 µm thick PEN films (Teonex™) and Al-coated and uncoated 7.5 µm thick Polyimide A dedicated circular sample holder was constructed (PI) films (Upilex-S™). The first two are a trademark which allowed performing all sample preparation of DuPont, USA and were provided by NASA, the outside the facility. The samples, after mounting on latter is a trademark of UBE Ind., Japan and was the sample holder, only needed to be fixed on a provided by ESA. Two set of samples were used. vertical beam located inside the facility. This One set was used to mechanically test samples. approach reduced sample handling inside the These were all 125 mm long and 25 mm wide for facility to a minimum. The samples were positioned the PET/PEN films and 15 mm wide for the PI films. over radially symmetric cut out areas inside the The second set of samples was about 15 mm wide circular sample holder and where fixed with and 40 mm long. This smaller set was used for adhesive tape. For the sample preparation own determining the thermo-optical properties. procedures were followed which included preparation, inspection (& rejection if necessary) 4. SPACE ENVIRONMENTAL SIMULATION and mounting. In total up to ten tensile samples and nine smaller samples for thermo-optical The detailed study of the interaction of candidate measurement could be exposed in one test run. solar sail materials with the space (radiation) Aluminised samples were exposed such that the Al- environment requires the definition of an exact orbit. side was facing the electron source. With that, space environmental models can be derived which provide data for the radiation Exposure current was measured with a custom built environment (e.g. particle). With that input so called Faraday cup. The measured current was at first only dose/depth profiles for the materials for that specific recorded and later-on a feedback control with a orbit can be derived. The approach of this generic study was however different. We wanted to assess accepting very high acceleration factors for particle the materials’ stability independent of any orbit and testing and rather low acceleration factors for therefore test conditions were searched for that UV/VUV testing. What has been shown in the past enabled to create “quasi” flat doses in the test is that dose rate can have clearly an effect in materials. After some initial assessment it was oxidising atmospheres but under inert atmospheres found that 50 keV provided a reasonable flat dose no dose rate effects are known [12-17]. profile for the investigated materials. Two examples are shown below. Therefore part of this study was used to look into more depth into the following points: In Fig. 2 the dose/depth profile of 2.1 µm thick Al/PEN films is shown, in Fig. 3 the one of bare 7.5 • What is the validity of accelerated testing? µm thick Upilex S film. As can be seen the dose for • Can damage be affected by dose rate? the Teonex material is rather flat though a small • Are we able to get reliable data with ultra- effect of the Al coating can be seen, the dose profile thin polymer films? for the Upilex S material is slightly increasing. The • What about statistical significance of data calculated dose variation between center and assessing accelerated testing? front/back side is about +/- 25%. A flatter dose for that material would have required increasing the The last two points are especially important for energy of the electron source. By that however the assessing solar sail materials. As space absorbed absolute dose of the thinner materials environmental testing is time and cost intensive would have been reduced. So eventually this testing is often carried out simultaneously on a variation was deemed acceptable for that material. variety of materials. This implies that a limited number of samples is available and that the Dose / Depth Profile for Al/TEonex statistical significance is rather doubtful. This point 1.40E-07 becomes especially important for ultra-thin samples where materials homogeneity, defects, sample 1.20E-07 preparation, sample failure due to handling etc. will 1.00E-07 play an important role [18, 19]. 8.00E-08 5. Test Programme 6.00E-08

4.00E-08 Space Simulation conditions were used within the 2.00E-08 constraints of the facility and were set such as to

D ose / particle0. (rad·cm²/electron)00E+00 avoid unwanted effects due to e.g. overheating of the samples. As the absorbed dose in solar sail 00.511.522.5 Fig. 2: Dose/ Depth Profiles Al/PEN) materials for low energetic particles can be very high, especially when close approaches to the sun are considered, emphasis was placed to reach Dose / Depth Profile for Upilex-S Materials reasonable high doses so that effects on materials 1.80E-07 Aluminized Upilex-S can be analysed. 1.60E-07 Upilex-S 1.40E-07

ctron) Three systematic test campaigns have been 1.20E-07 performed. The first one was a screening test 1.00E-07

8.00E-08 campaign and aimed for a total dose of 12 MGy with

6.00E-08 PEN as the reference material. In that test

4.00E-08 campaign nine individual test runs were carried out. Dose / particle (rad·cm²/ele / particle Dose 2.00E-08 The test conditions are shown in table Table 2. One

0.00E+00 test (8) was discarded due to a failure of the 0.00000 0.00010 0.00020 0.00030 0.00040 0.00050 0.00060 0.00070 0.00080 Depth (cm) electron gun control; the other eight tests were performed with stable dose rates as shown in the Fig. 3: Dose/Depth profile of bare Upilex S following table.

Another motivation of this study was to investigate an often neglected part of space environmental testing and analysis, namely the investigation of dose rate effects. Very little information is available in literature about that topic which confirms the commonly accepted engineering approach of Table 2 Test Conditions of Test Campaign 1 quality as mentioned above and exposure current was varied between 2 nA to 176 nA. This results in dose rates between tens of Gy/s to above a thousand Gy/s. The exact conversion can be deduced from Table 2.

In the following four figures the results of the mechanical testing for these test campaigns are 6. Mechanical Testing Results shown. In Fig. 4 and Fig. 5 the change in elongation and the change of E-modulus of test campaign two In test campaign one, ten tensile samples were is shown. The vertical lines indicate ± the standard exposed with four samples each of the PET and deviation of the measured values, the data point PEN films and two samples of the PI film. The plotted at 0 nA refers to the unexposed reference mechanical tests were carried out with a custom material. As can be seen a drastic decrease in built tensile tester. Samples were gripped with elongation at break is noted for the exposed rubber pads to avoid damaging the ultra thin material. No effect of the different exposure materials, alignment was checked and testing was conditions (nA or dose rates see Table 2) is noted. performed with a crosshead speed of 0.64 mm/min. This implies that damage is not affected by dose Stress/Strain curves were recorded with a Labview rates for this campaign. Regarding the E-Modulus programme. The load cell was calibrated for each (Fig. 5) better indications for a chain scission test and the movement of the shaft was translated dominant radiation interaction are noted. The trend into strain by a special resolver. versus dose rate is however not so clear.

Results of that campaign showed a clear For test campaign 3 more embrittlement is noted, embrittlement for all the polymers. As the tensile the elongation at failure drops even further, as tests were recorded on video the ultimate failure shown in Fig. 6. The E-Modulus also drops was particularly spectacular on the PET films which somewhat more which implies more chain scission. often “pulverised” at fracture. Regarding the dose What is however interesting to note is that the data rate dependence no obvious and clear trend was from Fig. 6 suggest some dose rate dependency. It found although small trends may be interpreted can be argued that the lower dose rates lead to from the data of the PEN and PET films. Due to the more damage. This could be explained by the limited number of samples and the related scatter it various time constants between the cross linking was also difficult to clearly establish which of the and (in this case) the dominant chain scission two radiation degradation mechanisms, i.e. chain events. As lower dose rates require more time for scission or crosslinking dominates. Again a weak testing there is also more time available for radical chain scission trend is observed though for the PET reactions to occur which could lead to more and PEN films. damage. Higher dose rates provide less time for the reactions and in addition will necessarily lead to The results of the first screening test campaign lead higher temperatures which could change the to the definition of a second and third dose rate kinetics/time constants of the degradation dependency test campaign. Due to the results of the mechanisms. It has to be stated though that there is screening test campaign we decided to focus for the some spread in the data and more results would be mechanical testing on the 2.1 µm PEN film. This needed to ascertain the observed trend. allowed testing ten tensile samples simultaneously for each test. It was hoped to generate statistically Another analysis result is presented in Table 3. significant data because the small number of Here the data were averaged – in the traditional samples of the same type and the challenge of way assuming no dose rate dependence - for all testing ultra-thin polymer films gives naturally some three campaigns for each total dose exposure. The scattered data. Therefore sometimes effects might dose rate dependence as mentioned above was not be covered within that scatter and more samples taken into account. As mentioned earlier for test are needed to ascertain trends. For the thermo- campaign one only four samples were exposed. It optical measurements the mix between the three has to be stated that data were normalized to each different materials was continued as here no reference data set, i.e. to unexposed samples. As “weakest link” problem exists. For both the second two different batches of materials were used (one and third campaign the total dose was increased, for campaign one and one for campaign two and the second exposed up to 23.3 MGy and the third three) somewhat different absolute values were up to 46.6 MGy. For both campaigns emphasis was found as starting values. What was found here is placed for a good preparation, handling and testing that a clear trend versus the absolute dose was noted. This is obvious by looking at the elongation at failure which is the most sensitive parameter. Reduction of about 59%, 69% and 87% were measured for 12.3 MGy, 23.3 MGy and 46.6 MGy respectively. Therefore this absolute dose rate assessment showed that the “half value dose” (50 % elongation) for the radiation damage assessment of the 2.1 µm thick foil is below 12 MGy.

Fig. 7: E-Modulus for 2.1 µm Al/PEN film, (Test campaign 3)

Table 3 Relative changes of 2.1 µm Al/PEN film for 12 MGy, 23.3 MGy and 46.6 MGy exposures

Fig. 4: Elongation at break for 2.1 µm Al/PEN film, (Test campaign 2)

In addition to the three dose rate campaigns a high dose exposure was done to assess the resistance of aluminised Upilex S. This test reached a total dose of 1.02 GGy and was performed with a dose rate of 1154 Gy/s. Encouraging properties have been reported in the past [20-23] and this material has been extensively investigated at ESTEC for inner planetary missions [24-27]. Due to its high temperature resistance it also has a potential for solar sail missions, especially for “hot” (close to the sun) approaches.

Fig. 5: E-Modulus for 2.1 µm Al/PEN film, (Test The mechanical results are shown in the following campaign 2) three Figures which show average values of five samples. As can be seen (Fig. 8) the elongation at break decreases drastically from about 30% to about 3 % but it was found remarkable that handling of this material was still possible and that the material showed that resistance. The failure stress (Fig. 9 ) was also reduced but by a smaller amount. The evaluation of the E - modulus (Fig. 10) indicates cross linking of the material at these extreme conditions. However, this indication is somewhat blurred by the increased scatter of the results.

Fig. 6: Elongation at break for 2.1 µm Al/PEN film, (Test campaign 3) an integrated reflectance measurement between the 0.35 3 µm to 35 µm wavelength band. 0.30

0.25 Both sample sides were measured for all aluminised materials. When uncoated PI samples were 0.20 assessed then that sample was backed by an 0.15 aluminised sample. By that relative deviations could

0.10 be determined. A strict procedure was followed to

Elongation at Break [%] atElongation Break minimise exposure to air and samples were 0.05 measured immediately after testing. With that the 0.00 first measurements were done a few minutes after BOL 1.02 GGy sample recovery.

Fig. 8 Elongation at Break for Upilex S after more than 1 GGy exposure Even though care was taken, the PET films were found to be very delicate to handle. Several 500 samples were broken during demounting from the sample holder. The PEN and PI films were better in 400 that respect and this allowed re-exposing identical samples in subsequent test runs. 300 The thermo-optical properties (α/ε) of the Al side 200 stayed unchanged within the uncertainty for all

Failure [MPa] Stress 100 samples. For the PEN films an intrinsic small increase/damage of the solar absorptance in the

0 UV/VIS range was noted. This damage was BOL 1.02 GGy however not increasing in subsequent exposures. This implies that there is no strict correlation Fig. 9 Failure Stress for Upilex S after more than 1 between thermo-optical degradation and GGy exposure mechanical degradation. The reason for that is

12000 found in different radical interactions and the creation of chromophoric clusters in the UV/VIS 10000 range. These clusters appear from 12 MGy absolute

8000 dose exposures onwards and appear to be saturated in the UV/VIS range. No bleaching 6000 recovery was detected by UV/VIS/NIR analysis which shows that those chromophoric clusters are 4000

E Modulus [MPa] stable regarding air exposure. 2000 The solar absorptance of Upilex S film was found to 0 BOL 1.02 GGy be more affected than PEN during all test campaigns. It was found that higher absolute doses Fig. 10 E-Modulus for Upilex S after more than 1 lead to more degradation. The increase followed an GGy exposure asymptotic curve. The data also suggest a weak dose rate dependence, higher dose rates lead to 7. Thermo-optical assessment somewhat more damage and a slight increase in α. Most of that degradation was however found to be As mentioned earlier separate 15 mm wide and 40 recoverable as Upilex bleaches upon air exposure mm long samples were used for these gaining nearly the original value after storing in the measurements. The thermo-optical properties were laboratory condition. This fits well to previously measured with instruments from AZ Technology. reported results in [28] where that recovery is For the solar absorptance (α) a Laboratory Portable deemed to be caused by the oxygen diffusion and Spectroreflectometer (LPSR) that measures the subsequent reaction with radicals in the polymer total reflectance in the wavelength range between film. It was also found that this effect is light 250 nm and 2800 nm was used. The infrared sensitive. An example of that is shown in Fig. 11 reflectance was measured to determine the thermal where ∆ α is plotted versus time. Here samples emittance (ε). For that a portable Emissometer were exposed up to 1.02 GGy and were physically model TEMP 2000 was used. This instrument gives separated for recovery. The samples depicted in blue were stored in a closed dark cabinet whereas In terms of thermo-optical stability some effects the samples in red where stored under normal were noted on PEN and more effects were noted on laboratory lighting. Already after a few hours of the tested PI film. The first seems to reach a recovery a distinction is noticeable and after more saturation at 12 MGy whereas the UV/VIS than 100 hrs a clear distinction between “dark” and degradation increases with increasing absolute “light” samples can be made. At the extreme dose dose. This degradation is mostly recoverable and is test of 1.02 GGy, the thermal emittance is light sensitive. noticeably changing indicating a change in the chemical structure of the material. All those results Therefore it is important to understand the radical will be summarised in detail in a forthcoming paper. interactions, the generation of chromophoric clusters and the related time constants of their interaction and decay. This will depend on the physico-chemical structure of materials and will be material dependent.

Acknowledgement

The ESA Douglas Marsh Fellowship is gratefully acknowledged for enabling this programme. NASA’s Space Environmental Effects Branch part of the Materials & Processes Laboratory in the Engineering Directorate of MSFC is also gratefully acknowledged as the host organization. Beside the

Fig. 11: Influence of Recovery of Upilex S between co-authors, the support by Ch. Semmel and J. Scott light/dark storage after exposure to more than 1 Miller for dose rate calculations and fruitful GGy. discussions with T. Schneider and J. Vaughn are acknowledged. 8. Summary and Conclusion 9. References Ultra-thin materials for Gossamer Structures like solar sails will enable those missions but they are 1. Tsander, F.A., 1969, "From a Scientific Heritage", Translation, pp. 1-92, NASA TT F-541, very challenging to test. By testing as many National Aeronautics and Space Administration, samples as possible and by following good Washington, DC. procedures many of the technical challenges that those materials pose can be overcome. 2. Blagonravov, A.A., Editor, "K.E. Tsiolkovsky Selected Works", Translation by G. In this work absolute dose and dose rate effect were Yankovsky, pp. 140-163, Mir Publishers, , USSR. assessed on ultra-thin polymer films. We assessed 1968 Al-coated and uncoated 7.5 µm thick PI films, Al coated 1.87 µm thick PET and 2.1 µm thick PEN 3. Tsander, F.A., "The Use of Light Pressure for Flight in Interplanetary Space, Problems ofFlight by Jet films. The majority of the work was performed on Propulsion", L.K. Korneev, Editor, pp. 303-321, Israel the 2.1 µm thick PEN film though. For that material Program for ScientificTranslations, Jerusalem, Israel. it was found that up to medium rate of damage, 12 1964 MGy and 23.3 MGy, no dose rate dependence was found. For higher absorbed damage a small trend 4. McInnes, C.R., "Solar Sailing: Technology, Dynamics regarding an inverse dependence of dose rate was and Mission Applications", Praxis Publishing, Chichester, noted, i.e. lower dose rates lead to more damage. United Kingdom, 1999 This implies that higher dose rates could yield favourable results as lower dose rates result in 5. P.A. Gray, M.R. Carruth, D.L. Edwards, “Photon Flux Amplification For Enhancing Photonic Propulsive Forces, more damage. This could be explained by the 33rd Plasmadynamics & Laser Conf, Maui, Hawai, USA, various time constants of the radical interactions of 20-23 May 2002 (AIAA-2002-2177) that material. It was also found that the dominant degradation mechanism for that material is chain 6. P.A. Gray, D.L. Edwards, M.R. Carruth, “Laser Photon scission and that the damage is increasing with Force Measurement Using A CW Laser, 33rd increasing absolute dose. The 50% damage value Plasmadynamics & Laser Conf, Maui, Hawai, USA, 20-23 of the elongation for that material is below 12 MGy. May 2002 (AIAA-2002-2178)

7. Souza, D.M., 1994, "Space Sailing", pp. 1-63, Lerner Publications Company, Minneapolis, MN. 23 Iwata M. et al, “Evaluation of New Thermal Control Material for Interior Planet Missions, SAE paper 981687, 8 Mc Innes C.R., Delivering fast and capable missions to 1998 the outer solar system, Advacnces in Space Research 34 (2004) 184-191 24 C.O.A. Semprimoschnig, S. Heltzel, A. Polsak, M. v. Eesbeek, “Space Environmental Testing of Thermal 9 D. Edwards, M. Hovater, W. Hubbs, P. Gray, G. Wertz, Control Foils at Extreme Temperatures W. Hollerman, “ CHARACTERIZATION OF CANDIDATE High Performance Polymers, Vol. 16, No. 2, 207-220 SOLAR SAIL MATERIAL EXPOSED TO SPACE (2004) ENVIRONMENTAL EFFECTS”, 42nd AIAA Aerospace Sciences Meeting and Exhibit, 5 - 8 January 2004, Reno, 25 S. Heltzel, C.O.A. Semprimoschnig, “A detailed study Nevada, USA (AIAA 2004-1085) on the thermal endurance of Kapton HN and Upilex S”,High Performance Polymers, pp. 235-248, volume 16, 10 W. Hollerman, T. Albarado, M. Lentz, D. Edwards, W. number 2, June 2004 Hubbs, Ch. Semmel, “IONIZING RADIATION EXPOSURE MEASUREMENTS FOR CANDIDATE 26 S. Heltzel, C.O.A. Semprimoschnig, M.R.J.v. Eesbeek, SOLAR SAILS”, 39th AIAA/ASME/SAE/ASEE Joint “INVESTIGATION OF THE DEGRADATION OF Propulsion Conference and Exhibit, 20-23 July 2003, THERMAL CONTROL MATERIALS BY THERMAL Huntsville, Alabama, USA (AIAA 2003-4660) ANALYSIS”, 10th ISMSE, The Netherlands, 2006

11 D. Edwards, W. Hubbs, T. Stanaland, A. Hollerman, 27 C.O.A. Semprimoschnig, S. Heltzel et al., “The ESA, Ch. Semmel, “Characterization of Candidate Solar Sail Mission – from a materials engineering Materials Subjected to Electron Radiation”, 9th ISMSE, perspective”, 10th ISMSE, The Netherlands, 2006 Noordwijk, The Netherlands, 16-20 June, 2003 28 M. Iwata, F. Imai et al, “Fundamental Research to 12 H. Wilski, “The Radiation Induced Degradation of Establish Ground-Test Methodology of Thermal Control Polymers”, Rad. Phys. Chem. Vol 29, No 1, pp1-14, 1987 Film, 36th AIAA Thermophysics Conference, 23-26 June 2003 (AIAA 2003-3908) 13 H. Wilski, “Radiation Stability of Polymers, Rad. Phys. Chem. Vol 35, No 1-3, pp 186-189, 1990

14 Gillen K.T., R.L. Clough, J. Polym. Sci, Polym. Chem. Ed. 23, 2693, (1985)

15 R.L. Clough, K.T. Gillen, C.A. Quintana, J. Polym. Sci, Polym. Chem. Ed. 23, 359, (1985

16 R.L. Clough, “High-energy radiation and polymers: A review of commercial processes and emerging applications, Nucl. Inst. and Meth. in Phys. Res. B 185 (2001) 8-33

17 R. Clough, Encyclopedia Polym. Sci. Eng 13 (1988) 667

18 Russell et al, ‘Simulated space environmental testing on thin films”, NASA/CR-2000-210101, April 2000

19 Wooldrige et al, “Effects of manufacturing and deployment on thin films for the NGST sunshade, AIAA, 2001-1349

20 Yokota R., “Recent trends and space applications of polyimides”, J. of Photopolymer Sc & Techn., Vol 12, 2, 209, 1999

21 Matsumoto T, “Nonaromatic polyimides derived from cycloaliphatic monomers, Macromolecules, 32, 4933, 1999

22 Hasegawa et al., “Structure and properties of novel asymmetric biphenyl type polyimides. Homo and copolymers and blends, Macromolecules, 32, 387, 1999