Solar Weather Effects on Satellites
Every once in a great while, a report surfaces about a communications satellite which has been partially or completed disabled as the result of a sudden knockout blow delivered by the sun. The first thing to keep in mind is that these things do happen. The second thing to keep in mind is that they happen very rarely.
Out of the hundreds of satellites successfully launched over the past 5 decades, only a mere handful have succumbed to any form of so-called “solar weather” which satellites are designed and built to withstand.
The study of solar weather is ongoing, and, operators are constantly monitoring the sun’s activities and improving their ability to respond to the impact of solar events.
Satellite operators focus on four elements of solar weather that can affect satellite communications: solar wind, coronal holes, coronal mass ejections (CME’s) and solar flares.
The solar wind is constant but varies in intensity, while the other three solar phenomena come and go. The goal in terms of space infrastructure has been to identify and effectively counter the sun’s link to so-called single-event upsets (SEUs) which happen whenever the performance of one or more spacecraft components abruptly changes without warning.
SEU’s are not apt to be caused by the solar wind itself which is relatively low in energy and seldom penetrates the outer layers or protective skin of a spacecraft. Instead, solar flares, CME’s and coronal holes—their powerful reach often extends beyond the orbit of Mars—can be disruptive.
When solar storms erupt, they can bombard a satellite with highly – charged particles and increase the amount of charging on a spacecraft’s surfaces. When CMEs occur in the Sun’s corona or outer atmosphere, for example, a huge amount of plasma and magnetic energy is emitted.
The huge and quite visible explosions on the sun are known as solar flares – the most extreme form of solar storms. They discharge large amounts of radiation and a highly charged cloud of protons in particular. X-ray observations provide an important early warning for astronauts in orbit, while slower – moving CMEs often trail behind subject to the sun’s magnetic field.
CME’s follow a curving path as they leave the sun. Because of this, the CME may not actually impact satellites at all. When a CME impacts the earth, the earth’s magnetic field compresses on one side and stretches out on the other. This can result in dazzling auroral displays over the poles, for example. Fortunately, most CMEs last only 3 days or less.
Thankfully, the sun is fairly predictable in this regard, and sunspot activity takes place in 11-year cycles with the maximum or most intense stage lasting about 2 years, and the least intense stage lasting about 5 years.
Since 2006, we have experienced the least active period of major solar weather events in recent history. In other words, the sun has been very quiet lately.
Coping with electrostatic discharges from the sun that can potentially disrupt satellite services are part of the everyday reality of the satellite world. Losing solar power is not a serious concern whereas losing total control and command of a satellite as the result of solar weather is the most severe effect.
Solar panels on satellites are the most affected components, and normal erosion rates for solar panels are usually 0.3% to 1% per year.
A solar storm can reduce solar panel performance by 3% to 5% in a day, but since this phenomenon is well understood, spacecraft manufacturers increase the tolerances by design, and attach larger than needed solar panels to satellite in order to allow for losses during the anticipated solar storms.
The body of a communications satellite, which contains vital control and communication components, is specially adapted using special materials as well as active and passive measures so as to be highly resilient. A so-called “Faraday Cage” protects the satellite’s internal equipment from external electrical charges. High – energy particles discharged by the sun rapidly lose strength as they pass through the multiple layers of a spacecraft’s body or bus as well. There, they encounter a series of specially designed circuit dividers, individual compartments, and other unique structural elements that act as barriers.
The disruptive nature of solar weather impacts far more than satellite operations, and adversely affects terrestrial power and communications grids.
For these and other reasons, a considerable amount of manpower and money has been devoted to monitoring the sun’s activity, and more research into solar phenomena in general is planned in the future. Among other things, one benefit has been a steady improvement in our ability to rapidly detect and track these solar events using powerful observation and detection systems both on the ground and in space.
NASA, the U.S. National Oceanic and Atmospheric Administration and the U.S. Department of Defense oversee much of this activity. For example, besides NASA’s twin Solar Terrestrial Relations Observatory (STEREO) spacecraft, the Air Force Research Laboratory launched the Communication/Navigation Outage Forecasting System (C/NOFS) satellite several years ago to forecast the presence of ionospheric irregularities caused by the sun that adversely impact communication and navigation systems. Space and ground-based measurements have been taken to help determine how the plasma irregularities affect the propagation of electro-magnetic waves, among other things.
Satellites depend upon the sun, and satellite operators have steadily developed tools and techniques which allow them to ensure the operational integrity of all satellites in the face of all forms of solar weather. That weather changes over time, while satellite performance and design gets better and better. Thanks to proper planning, design and execution, the survival rate of satellites is quite remarkable.