Plans for ensuring survival of SBL optics in the low earth orbit environment

Abstract

This paper discusses plans and initial results of efforts to characterize optical coatings, coating materials and optical substrates planned for use in the Space Based Laser (SBL) Integrated Flight Experiment (IPX) for their endurance in the low earth orbit (LEO) space environment. High energy laser (HEL) systems such as IPX place very challenging requirements on their constituent optics, and the degradation due to the natural LEO environment creates additional risk to system performance. The SBL program has chosen to mitigate this risk by use of ground and flight experiments. The flight experiment is participation in the collaborative experiment, Materials International Space Station Experiment (MISSE), a program funded by NASA and the U.S. Air Force Research Laboratory. The ground experiments are to use laboratory facilities to simulate the atomic oxygen exposure of the LEO environment. This paper begins with a discussion of the LEO environment and the potential effects on IPX optical components and materials. The next section gives an overview of the IPX and the optical components that make up each of the program elements. Next, an overview of the planned ground experimentation and participation in the MISSE collaboration is given. The final section gives an example of laboratory measurements madeon the samples with SBL legacy as a preview to the results expected from the flight experiment. The first flight of MISSE samples to Space Station Alpha is Summer 2001, with a return to Earth in June 2002. The Low Earth Orbit Environment and the Optics Challenge to IPX High energy laser (HEL) systems require extreme performance from their optics in order to meet system goals. Typically, HEL optics are stressing for high damage resistance, high reflectance, low absorptance, scatter and aberration along with predictable spatial uniformity. Producing such optics has been a main concern of the HEL community since the discovery of the laser. Usually, just building the optics is challenge enough. The space based laser (SBL) integrated flight experiment (IPX) program must make these optics and have them meet mission requirements in the challenging environs of low earth orbit (LEO). The LEO environment can be characterized by exposure to the neutral thermosphere (ambient atmosphere), charged plasma, the thermal environment (including diurnal cycles), exposure to solar radiation, debris/micrometeorites and ionizing radiation. The neutral thermosphere, is the atmospheric layer above an altitude of approximately 100 km. At an altitude of 80 kilometers or greater, solar ultra-violet (UV) bombardment of the tenuous atmosphere initiates chemical reactions and breaks down molecular species to their atomic components. In this region of the atmosphere, the concentration of individual species is essentially determined by weight segregation. At the expected altitude of IPX, ~480 km, the predominant species is atomic oxygen (AO). The abundance of neutral species at altitude is shown in Figure 1.