2.11 SPACE WEATHER EFFECTS ON SPACECRAFT SYSTEMS Janet L. Barth* NASA/Goddard Space Flight Center, Greenbelt, Maryland ABSTRACT use of rapidly evolving, miniaturized technologies. Finally, space agencies and industry are Space-based systems are developing into developing missions that must operate in critical infrastructure required to support the quality challenging space environments. For example, of life on Earth. Hence, spacecraft reliability is a earth science missions that seek to understand serious issue that is complicated by exposure to complex global change processes require global the space environment. Complex mission designs coverage that cannot be achieved in low earth along with rapidly evolving technologies have orbits (LEOs). However, placing spacecraft in the outpaced efforts to accommodate detrimental higher altitude regions of medium earth orbits space environment impacts on systems. (MEOs) and geostationary/geosynchronous orbits Hazardous space environments, the effects on (GEOs) results in a significant increase in radiation systems, and the accommodation of the effects exposure. Europe’s global positioning satellite are described with a focus on the need to predict system and NASA’s Living With A Star (LWS) space environments. Program also plan multiple spacecraft in high 1. INTRODUCTION radiation regions of the magnetosphere. Our increasing dependence on space-based The Sun emits time-varying magnetic fields, systems demands that we increase their reliability, plasmas, and energetic particles. This solar ideally achieving “all weather” space systems. variability drives changes in the interplanetary Reliability cannot be increased without careful environment which then interacts with the Earth’s management of the effects of the space magnetic field and outermost atmosphere to environment on systems. Mitigation must include produce changes in the near-Earth space both design accommodations and operational environment. The space environment and its counter-measures. Mitigating space environment solar-induced changes interact with spacecraft effects requires simulations of the effects with and instrument components and can cause accurate models of the environment and anomalies resulting in loss of data, degradation of environment interactions. The goal of simulations capability, service outages, and, in extreme cases, is to accurately predict the level of hazard. As the loss of spacecraft. shown in Fig. 1, uncertainties in the simulation Revolutionary changes have occurred in process translate directly into design margins that space-based systems since the first exploratory add overhead and can preclude the use of some missions of the “space age”. First, humanity is technologies. increasingly reliant on space-based assets. In addition to the research functions that are Space performed in space in the areas of space science, earth science, human exploration of space, and Damage Space or aeronautics and transportation; critical services Environments Interference are also space-based, including navigation, telecommunications, defense, space environment monitoring, and terrestrial weather monitoring. Second, the performance demands of reconfigurable systems, constellations of small Simulation Spacecraft Structural Model Space Component Testing in Laboratories Prediction spacecraft, large deployable structures, imagers, Environment of Hazard Component Testing in Space Models Level and on-board computing increase the complexity Interaction Models of spacecraft and payloads which often require the Uncertainties Design Margins * Corresponding author address: Janet L. Barth, NASA/Goddard Space Flight Center, Code 561.4, Fig. 1: Accommodation of space environment effects Greenbelt, MD 20771, on spacecraft requires ground simulations that include email: [email protected]. models of environments and effects. Before discussing environment effects and temperature of the corona inputs sufficient energy effects accommodation, a review of space to allow electrons to escape the gravitational pull environments that are hazardous to spacecraft will of the sun. The result of the electron ejections is a be given. More detailed discussions of the charge imbalance resulting in the ejection of hazardous environments can be found in Barth protons and heavier ions from the corona. The (1997), Dyer (1998), and Mazur (2002). ejected gas is so hot that the particles are homogenized into a dilute plasma*. The energy 2. THE SPACE ENVIRONMENT density of the plasma exceeds that of its magnetic Spacecraft are exposed to a multitude of field so the solar magnetic field is “frozen” into the environments that are not present at the surface of plasma. The electrically neutral plasma streams the Earth, including micrometeoroids and orbital radially outward from the sun at a velocity of debris, ultraviolet irradiation, neutral particles, cold approximately 300 to 900 kilometers per second and hot plasma, and particle radiation. The with a temperature on the order of 104 to 106 K. interaction of these environments with spacecraft The energies of the particles range from systems cause degradation of materials, thermal approximately 0.5 to 2.0 keV/n. The average changes, contamination, excitation, spacecraft density of the solar wind is 1 to 30 particles/cm3. glow, charging, radiation damage, and induced 2.4 High-energy Interplanetary Particles background interference. There are large spatial and temporal variations in the constituency and In addition to the solar wind plasma, density of the environments; therefore, exposure interplanetary space contains high-energy charged to the environment is highly dependent on the particles called galactic cosmic rays (GCRs) that location of the spacecraft. For the purpose of are present at all times. Solar events that occur discussion of environmental definitions, missions sporadically (coronal mass ejections and flares) can be roughly categorized into LEOs, MEOs, can also be a source of high-energy particles. GEO, geosynchronous transfer orbits (GTOs), and GCR intensities rise and fall slowly with the interplanetary. solar cycle variations such that GCRs are at their peak level during solar inactive times and at their 2.1 Meteoroids and Orbital Debris lowest level during solar active times. The Meteoroids are primarily remnants of comet difference between the extremes of the GCR orbits. Several times a year Earth system fluence levels is approximately a factor of 2 to 10 encounters increased meteoroid exposure as it depending on the ion energy. Fig. 2 shows the intersects a comet orbit. Also, sporadic particles slow, long-term cyclic variation of the cosmic ray are released on a daily basis from the asteroid (C, N, O) fluences for a 20-year period as belt. Orbital debris consists of operational measured by the IMP-8 spacecraft. payloads, spent rockets stages, fragments of During active phases of the solar cycle, the rockets and satellites, and other hardware and numbers and intensity of coronal mass ejections ejecta. The United States Air Force Space and solar flares increases. These events can Command’s North American Aerospace Defence cause periodic increases in the levels of Command (NORAD) tracks over 7,000 objects in interplanetary particles that are orders of LEO that are greater than 10 cm in size, and there magnitude higher than the GCR environment. The are tens of thousands smaller objects. sharp spikes superimposed on the cosmic ray background seen in Fig. 2 are caused by solar 2.2 Ultraviolet Irradiation events. The particles consist of ions of all The sun is the natural source of ultraviolet (UV) elements but protons usually dominate the irradiation, which has wavelengths of about 100 to abundances. Solar particles diffuse as they move 400 nanometers. UV irradiance can penetrate the through the interplanetary medium. Therefore, the atmosphere to reach the surface of the Earth. UV solar particles abundances are a function of radial radiation diffuses with distance from the sun at a distance from the Sun. rate of 1/R2 where R is the radial distance from the The Earth’s magnetic field provides some Sun. protection to Earth-orbiting spacecraft from interplanetary particles by deflecting the particles 2.3 The Solar Wind The sun’s outer atmosphere, the corona, * extends several solar diameters into interplanetary Plasma is ionized gas in which electron and ion space. The corona continuously emits a stream of densities are approximately equal. Plasma is protons, electrons, doubly charged helium ions, distinguished from the energetic particle population in and small amounts of other heavy ions, that it does not cause radiation effects and has energies collectively called the solar wind. The high < 100 keV. as they impinge upon the magnetosphere. The fact, 99.9% of the solar wind particles pass around exposure of a spacecraft primarily depends on the the Earth’s magnetosphere. The flow of the solar inclination and, secondarily, the altitude of the wind around the flanks of the magnetopause trajectory. For example, interplanetary particles stretches the geomagnetic field in the anti-solar have free access over the polar regions where direction into a long tail of up to ~300 Earth radii field lines are open to interplanetary space. The altitude. Some tail field lines are not closed and penetration power of these particles is also a are connected to the solar magnetic field function of the particle’s energy and ionization embedded in the solar wind. state and of solar wind and magnetospheric conditions.