Effects on satellites
- Related Topics:
- solar cycle
- geomagnetic storm
There are two main space weather concerns for Earth-orbiting satellites: radiation exposure and atmospheric satellite drag. Radiation exposure is the interaction of charged particles and electromagnetic radiation with a spacecraft’s surfaces, instruments, and electronic components. Satellite drag can have a serious impact on the orbital lifetime of low-Earth-orbiting satellites.
Radiation exposure
Satellites in Earth orbit are exposed to significant amounts of high-energy electromagnetic radiation and charged particles that do not reach Earth’s surface on account of its protective atmosphere. The space environment around Earth is filled with energetic charged particles that are trapped in the Van Allen radiation belts. The spatial extent, the energy, and the amount of radiation in the Van Allen belts are controlled by space weather, with large increases in their size and amount of radiation occurring during large geomagnetic storms. Although satellites usually do not orbit directly in the Van Allen belts, these charged particles have a significant impact on the design of spacecraft and space instrumentation.
For example, high-energy electrons can penetrate spacecraft and deposit their charge in the dielectric (insulating) material of electronic circuit boards. If enough charge is built up, a discharge can break down the material, causing the electronic component to fail. This can have catastrophic consequences if the damaged electronic circuit controls a critical component of the spacecraft.
Atmospheric satellite drag
Though the uppermost layer of Earth’s atmosphere, the thermosphere, is extremely tenuous compared with the dense lower layer at the surface, it is not a perfect vacuum. Indeed, the density of the gas a few hundred kilometres above Earth’s surface is appreciable enough that over time it can lower the altitude of an orbiting satellite. Since the satellite’s velocity and the neutral gas density increase with decreasing altitude, the amount of drag quickly increases, causing a satellite to reenter Earth’s atmosphere and either burn up or crash to the surface. The density of the upper atmosphere at any given altitude varies with the amount of solar radiation it receives, and the amount of solar radiation in turn varies either day-to-day depending on solar activity or over the 11-year solar cycle. Between solar minimum and solar maximum, the temperature of the thermosphere roughly doubles. The upper atmosphere extends farther during solar maximum, and its density at any given altitude increases. In general, a satellite must have an altitude of at least 200 km (120 miles); otherwise, the high thermospheric density will prevent the satellite from completing more than a few orbits. Even the Hubble Space Telescope and the International Space Station (ISS), which orbit at altitudes of about 600 and 340 km (370 and 210 miles), respectively, would eventually reenter Earth’s atmosphere if they were not continuously reboosted to their original orbits.
Effects on crewed spaceflight
One major hazard of crewed planetary exploration is high-energy radiation, for the radiation that affects the electronic components of satellites can also damage living tissue. Radiation sickness, damage to DNA and cells, and even death are space weather concerns for astronauts who would make flights to the Moon or the multiyear journey to Mars. Solar energetic particles and cosmic rays are difficult to predict or protect against. Large solar storms, such as from flares and CMEs, can produce lethal radiation environments on the Moon or in interplanetary space. Shielding of spacecraft and surface laboratories on the Moon and Mars would be a critical component for any such human spaceflight effort. Even in low Earth orbit within the magnetosphere, astronauts on the ISS receive a dose of radiation equivalent to about 5–10 chest X-rays per day, which causes an increased risk of cancer.
Effects on satellite communications and navigation
Communication from the ground to satellites is affected by space weather as a result of perturbations of the ionosphere, which can reflect, refract, or absorb radio waves. This includes radio signals from Global Positioning System (GPS) satellites. Space weather can change the density structure of the ionosphere by creating areas of enhanced density. This modification of the ionosphere makes GPS less accurate and can even lead to a complete loss of the signal because the ionosphere can act as a lens or a mirror to radio waves traveling through it. Because the ionosphere has a different refractive index from the layers above and below it, radio waves are “bent” (refracted) as they pass from one layer to another. Under certain conditions and broadcast frequencies, the radio waves can be absorbed or even completely reflected. Sharp and localized differences (or gradients) in the density of the ionosphere also contribute significantly to the effects of space weather on satellite communication and navigation. These gradients become most pronounced during geomagnetic storms.
Effects on Earth’s surface
The greatest potential damage caused by space weather in economic terms would be the destruction of infrastructure required for continent-sized power distribution systems. The electric currents driven by the coupling of the solar wind and the interplanetary magnetic field with the geomagnetic field—which during the 19th century flowed through telegraph wires—can now find their way into electric power transmission lines with potentially devastating consequences. The enhanced currents can damage or destroy electrical transformers, causing a cascade of power failures across a large portion of the electric grid. For example, a storm during the 1989 solar maximum caused a massive power outage in Canada when transformers failed in Quebec. It has been estimated that if a geomagnetic storm like that of 1859 hit today, a large fraction of the North American power grid could be disabled, with estimated recovery times of months to years and financial losses of hundreds of billions of dollars.
Forecasting
The U.S. government has developed a Space Weather Prediction Center (SWPC) as part of the National Oceanic and Atmospheric Administration. The SWPC is based in Boulder, Colo., and observes the Sun in real time from both ground-based observatories and satellites in order to predict geomagnetic storms. Satellites stationed at geosynchronous orbit and at the first Lagrangian point measure charged particles and the solar and interplanetary magnetic fields. Scientists can combine these observations with empirical models of Earth’s space environment and thus forecast space weather for the government, power companies, airlines, and satellite communication and navigation providers and users from around the world.
Mark Moldwin