Denise Beisecker, DLR Institute of Technical Physics; Frederic Seiz, German Aerospace Center, DLR; Nils Bartels, German Aerospace Center, DLR; Wolfgang Riede, German Aerospace Center, DLR; Maciej Sznajder, German Aerospace Center, DLR; Tom Sprowitz, German Aerospace Center, DLR; Thomas Renger, German Aerospace Center, DLR
Keywords: Space Debris, Space Aging, Charged Particle Radiation, Space Debris Characterization, Complex Irradiation Facility, Surface Degradation, Optical Property Degradation
Abstract:
The space environment is hazardous due to its extreme temperatures, electromagnetic radiation and its freely propagating charged particles. In the Earth orbits the dominant source of this radiation is the Sun. During the lifetime of a satellite, its systems are protected by various shielding materials. However, there is no material, which is absolutely unaffected by the space environment. Here, aging studies are a key point to improve satellite designs and material choices.
Besides the natural environment, space debris poses a permanent and increasingly threat to operating spacecrafts. Today, space debris is observed mainly with radar systems and optical telescopes. In terms of space debris identification the optical fingerprint contains valuable information. Nevertheless, decaying optical properties influences the optical response of the objects. Here, aging studies deliver a great opportunity to deepen the understanding of the fingerprints and in adding possibilities to obtain more information like the age of catalogued objects.
In terms of space debris disposal, ground-based orbit modification, such as photon momentum transfer and laser ablation, is a field of research with constantly growing importance. To successfully apply these technologies, a good understanding of the surface properties, which includes aging behavior, is required to predict the energy and impulse transfer from the laser into the material. It is expectable that irregular surfaces due to aging show different behavior than the theory predicts for unaged surfaces.
We experimentally investigate the surface degradation of typical space debris materials due to low energetic charged particles, typically encountered in the Van-Allen-Belts. This becomes increasingly important since the mostly congested Low Earth Orbits (LEO) are intersecting with the Inner Van-Allen-Belt. The most populated orbit is the Sun Synchronous Orbit (SSO) with an orbital height around 800 km.
The space environment in the solar system is mainly dominated by the Sun, which emits electromagnetic radiation and charged particles. The harming LEO environment consists especially of vacuum UV light (VUV), atomic oxygen (ATOX), electrons, protons and other element atoms and ions. ATOX is the dominating species up to 650 km. VUV and protons in different energetic states are present in higher orbits. Charged particles, mainly protons, are caught in the Inner van-Allen-Belt.
In order to improve the knowledge about space debris, the degradation of the optical properties becomes interesting. The interaction of charged particles, ATOX, and VUV causes visible corrosion, surface defects and material degradation. Any change of a surface results in a change of its optical properties. The ISS mission MISSE has already investigated the optical degradation of materials under ATOX exposure. The results clearly show significant changes in the samples appearance, which is also related with changes in their reflectivity. This can impressively be seen at the Black Kapton sample, which has its black layer totally eroded. Comparable results were found by Engelhart 2019, whom has ground-based investigated the impact of GEO electrons on absolute reflectivity of thin foils depending on the dose. Skurat et. al. showed, that polymer films, which were exposed to UV radiation on a Salyut station, have significantly decayed in their mechanical properties.
The present study focusses on researching these changes. In particular, we investigate surface changes due to low energetic protons and electrons. Low energetic here refers to energy levels of 100 keV and below. These particles have a low penetration depth into the materials and, therefore, can lead to strong surface modification.
Prior to the experiment, we simulate a characteristic radiation flux using the SPENVIS tool (ESA SPace ENVironmental Information System). For the simulation we chose the most populated orbit (SSO at 800 km) since it represents the conditions of the major LEO population. As a result, an integrated proton flux of 6.7*10³ p+ cm-2 s-1 is expected at energies of 100 keV, as it is shown in figure 1. Additionally, the simulation shows a high flux of low energetic electrons which varies their maximum depending on the solar cycle. Based on this result, the experimental simulation will be conducted under proton irradiation for an orbit resident time of 100 years. A measurement with electrons is conducted as well.
Figure 1: p+ and e- flux at 800 km SSO
The choice of samples is for typical materials of satellite structures. These are two aluminum alloys (6061, 6082), multilayer insulation foil, epoxy based CFRP, Acktar Black coated aluminum, PTFE and titanium. The size of a sample is 21×21 mm². They are glued with double sided UHV (ultra-high vacuum) approved tape to the sample holder (see figure 2). All samples are simultaneously and uniformly irradiated.
Figure 2: Samples in the UHV chamber of the CIF
Realistic space conditions can be created in the Complex Irradiation Facility (CIF) in Bremen, Germany. The facility consists of an ultra-high vacuum sample chamber with a sample holder that can be cooled or heated. The radiation sources which are linked to the sample chamber are a particle accelerator for protons and electrons and solar generator consisting of Xenon, Deuterium, and Argon light sources. Samples can be irradiated with multiples of the solar constant.
An optical set-up for detecting the surface modification due to changes in a samples albedo was developed. This system, called LAMBDA (laser camera albedo analyzer), records surface changes via variations in brightness of reflected laser irradiation on sample surfaces. The great advantage of an optical system is that induced surface modifications can be measured directly in the sample chamber during proton irradiation. The LAMBDA system uses a cw HeNe laser beam to illuminate the sample surfaces. As the surfaces are polished before the charged particle irradiation starts, this refers to the change of a specular reflecting surface towards a Lambertian surface. A Canon EOS 5D Mark IV camera continuously photographs the samples. The detected brightness is increasing while the sample surfaces are changing towards Lambertian surfaces.
The resulting surfaces are further investigated with reflectometers, Fourier-Transform Spectrometers and REM/EDX in order to quantify the changes of the optical properties due to low energetic proton and electron irradiation.
Date of Conference: September 15-18, 2020
Track: Orbital Debris