NOAA / Space Weather Prediction Center
Like ships at sea, satellites sail the ocean of space. And, like their terrestrial counterparts, satellites must endure severe storms in the environment in order to perform their mission. We have learned much about the space environment since the beginning of the space age in 1957. This knowledge has allowed satellite designers and manufacturers to construct spacecraft that are largely impervious but not entirely immune to variations in the space environment.
The major obstacles to mission success remain the hazards during launch and early spacecraft deployment. Once the spacecraft is deployed and on-station, the spacecraft operator must then be vigilant against other hazards that might endanger the mission. Space weather is one of these hazards and it should be monitored to help ensure spacecraft health and to minimize outages.
Space weather effects on satellites vary according to orbit, spacecraft local time, spacecraft position relative to certain regions in space, stage of the 11-year sunspot cycle, and many other factors. Effects can range from simple upsets, that are easily recovered from, to total mission failure. A major spacecraft insurance company recently estimated that over $500,000,000 in insurance claims were disbursed during the period 1994-1999 due to on-orbit failures related to space weather. Obviously, space weather is of concern to those pursuing commerce and discovery in space.
Since its inception in 1965, the Space Weather Prediction Center (SWPC) has provided services to the satellite community in the form of data, alerts, warnings, watches, and satellite anomaly assessments. Satellite operators and manufacturers routinely utilized those services. With the exception of specialized assessments of the environment at the time of specific anomalies, all those services continue and are continually reviewed for their efficacy. The SWPC has performed over 300 anomaly assessments. Of those, approximately one third appeared to have been caused by variations in the space environment. Many other anomalies occurred that may have been related to space weather but causality often times is difficult to determine. In the past, some operators and manufacturers were reticent in openly discussing problems with their spacecraft. However, this is changing and the industry has become more open.
Finally, there has been discussion regarding what actions a spacecraft operator would take when faced with the prospect of space weather storms. It has been the experience of the SWPC that rarely will an operator take proactive steps to protect his space asset based upon a warning of an impending disturbance or an alert of an actual storm arrival. This is not to say that this information is not important or of extreme interest. Most spacecraft are designed to survive severe environmental conditions and survive most of them do. However, almost all of the major satellite control centers have subscribed to SWPC services for the important reasons of situational awareness and satellite anomaly resolution. Just as sailors must know the conditions they are facing, so must space farers.
Spacecraft anomalies are grouped into broad categories based upon the effect upon the spacecraft. A list of potential effects follows:
* SWPC has applicable data/alerts/warnings/watches/products
In the following sections, each of the topics for which SWPC has applicable services, will be discussed.
Surface charging to a high voltage does not usually cause immediate problems for a spacecraft. However, electrical discharges resulting from differential charging can damage surface material and create electromagnetic interference that can result in damage to electronic devices. Variations in low energy plasma parameters around the spacecraft, along with the photoelectric effect from sunlight, cause most surface charging. Due to the low energy of the plasma, this type of charging does not penetrate directly into interior components. Surface charging can be largely mitigated through proper materials selection and grounding techniques.
Surface charging occurs predominantly during geomagnetic storms. It is usually more severe in the spacecraft local times of midnight to dawn but can occur at any time. Night to day, and day to night transitions are especially problematic during storms since the photoelectric effect is abruptly present or absent, which can trip discharges. Additionally, thruster firings can change the local plasma environment and trigger discharges.
The common measure for geomagnetic storms, and hence the occurrence of surface charging, is the K index. This index is a 3 hourly measure ranging from 0-9 (0=quiet, 9=severely disturbed.). It is derived from ground-based magnetometer data and is used as a surrogate for actual plasma measurements at satellite altitudes. In general, surface charging effects begin at the K=4 to K=5 level. Charging is probable at K>=6 (see Today's Space Weather). Geomagnetic substorms can be somewhat localized in space so the use of the planetary K index (Kp) may mask the severity of effect upon a specific spacecraft. Some claim better correlation is achieved using data from a ground-based magnetometer at the "footpoint" of the magnetic field line that passes through the affected spacecraft. The Los Alamos National Laboratory (LANL) also has sensors on several geosynchronous (GEO) spacecraft that directly measure the appropriate particle energy ranges to determine if surface charging is probable. (see LANL Web site)
This phenomenon is a problem primarily for high altitude spacecraft. At times, when Earth is immersed in a high-speed solar wind stream, the Van Allen belts become populated with high fluxes of relativistic (>~1 MeV) electrons. These electrons easily penetrate spacecraft shielding and can build up charge where they come to rest in dielectrics such as coax cable, circuit boards, electrically floating radiation shields, etc. If the electron flux is high for extended periods, abrupt discharges (tiny "lightening strokes") deep in the spacecraft can occur.
High fluxes of these electrons vary with the 11 year solar cycle and are most prevalent late in the cycle and at solar minimum. Occasionally, high-energy electron events recur with a 27-day periodicity - the rotation period of the Sun. Discharges appear to correlate well with long periods of high fluxes. At these times, charge buildup exceeds the natural charge leakage rate of the dielectric. The charge builds and discharge occurs after the breakdown voltage is reached. In the past, some energetic electron enhancements at GEO have approached two weeks in duration. It was at the end of one of these long duration enhancements in 1994 that two Canadian satellites experienced debilitating upsets.
SWPC operates electron flux sensors on the GOES GEO spacecraft. These instruments measure electron fluxes of >0.6 and >2 MeV. (See GOES electron data ) Historically, deep dielectric discharges begin to occur when the
>2 MeV fluxes exceed 1000 particles/cm/sec/ster. In general, fluxes become elevated for all GEO spacecraft at the same time. However, there is a diurnal variation where fluxes peak by approximately an order of magnitude for spacecraft at local noon.
Single event upsets occur when a high-energy particle (>~50 MeV) penetrates spacecraft shielding and has the misfortune to hit a device in just the wrong way to cause disruption. This is generally a hit or miss situation. Effects can range from simple device tripping to component latch-up or failure. Particle bombardment of memory devices can also change on-board software through physical damage or through deposition of charge resulting in a "bit flip." There are two natural phenomena that cause this type of problem - Galactic Cosmic Rays (GCRs) and Solar Proton Events (SPEs).
Galactic cosmic "rays" are actually particles, sometimes with high Z number (nuclear mass) and energies exceeding GeV levels. Fortunately, the flux of GCRs is relatively low so the resulting SEU rate is also low. GCR fluxes are highest by approximately 25% during solar minimum. It is at this time that the Sun expels little solar material and magnetic fields to detect the incoming GCRs prior to arrival at Earth.
Solar Proton Events at Earth can occur throughout the solar cycle but are most frequent in solar maximum years. SPEs result from powerful solar flares with fast coronal mass ejections. During an SPE satellites experience dramatically increased bombardment by high-energy particles, primarily protons. Fluxes of particles with energies > 10 MeV, can reach 70,000 protons/cm2/sec/ster. SEU rates increase with high fluxes since there is a higher likelihood of impact with a sensitive location. High-energy particles reach Earth from 30 minutes to several hours following the initiating solar event. The particle energy spectrum and arrival time seen by satellites varies with the location and nature of the event on the solar disk.
The SWPC has high-energy proton detectors on the GOES GEO and NOAA polar-orbiting satellites. (See GOES proton plots and NOAA/POES plots) SEUs occur whenever these fluxes become elevated. Higher energies penetrate further into satellite interiors. Spacecraft at GEO experience roughly the same SPE fluxes as in nearby interplanetary space. High inclination, Low Earth Orbit (LEO) spacecraft experience highest fluxes when in the auroral zone (~>65 degrees latitude) above Earth. On rare occasions, when an SPE is in progress and a severe geomagnetic storm drives the auroral zones equatorward, lower inclination LEO satellites can experience difficulties.
Spacecraft in LEO experience periods of increased drag that causes them to slow, lose altitude and finally reenter the atmosphere. Short-term drag effects are generally felt by spacecraft <1,000 km altitude. Drag increase is well correlated with solar Ultraviolet (UV) output and additional atmospheric heating that occurs during geomagnetic storms. Solar UV flux varies in concert with the 11-year solar cycle and to a lesser degree with the 27-day solar rotation period. Geomagnetic storms are sporadic, but most major storms occur during solar maximum years.
Most drag models use radio flux at 10.7 cm wavelength as a proxy for solar UV flux. (Before long, the GOES spacecraft will have continuous UV monitoring) Kp is the index commonly used as a surrogate for short-term atmospheric heating due to geomagnetic storms. In general, 10.7 cm flux >250 solar flux units and Kp>=6 result in detectably increased drag on LEO spacecraft. Very high UV/10.7 cm flux and Kp values can result in extreme short-term increases in drag. During the great geomagnetic storm of 13-14 March 1989, tracking of thousands of space objects was lost and it took North American Defense Command (NORAD) many days to reacquire them in their new, lower, faster orbits. One LEO satellite lost over 30 kilometers of altitude, and hence significant lifetime, during this storm.
Spacecraft "age" through continual bombardment by GCRs, trapped radiation, and SPEs. There are several models used to estimate the total dose expected in various orbits and at different stages of the solar cycle. These models provide total dose estimates that are helpful in estimating the lifetime of an operational satellite. The total dose a satellite receives from GCRs is relatively constant. Solar cycle variations in trapped radiation are also reasonably well modeled. SPEs are most prevalent during the solar maximum years but their time of occurrence and severity are very difficult to model.
Spacecraft components are manufactured to withstand high total doses of radiation. However, it is important for the satellite operator to know how much dose each spacecraft in his fleet has endured. This knowledge allows for reasoned replacement strategies in an industry with very long manufacturing lead times.
Spacecraft power panels are physically and permanently damaged by particles of energy high enough to penetrate their surfaces. During one large high-energy SPE, several percent of power panel output can be lost. This shortens the overall lifetime of the spacecraft or at least entails power management problems as the spacecraft nears its end of life. Recent developments in the manufacturing process have made SPEs less of a problem, but power loss still occurs in these new panels.
The Sun is a strong, highly variable, broad-band radio source. At times, the Sun is within a side-lobe or even the main beam of a ground antenna looking at a satellite, usually pointed within about 1 degree of the Sun. If the Sun happens to produce a large radio burst during that time, the signal from the spacecraft can be overwhelmed. Large solar radio bursts occur most frequently during solar maximum years. An operator should be aware of when the Sun is in close proximity to the satellite being tracked. The SWPC, through reports from U. S. Air Force radio observatories, catalogs solar radio burst occurrences. (See Activity Summaries and Solar Event Lists )
At times, the ionosphere becomes highly irregular causing satellite signals to band inhomogeneously when they transit this disturbed medium, and scintillate at the receiver. Strong geomagnetic storms can cause scintillation in the auroral zones. Additionally, scintillation is problematic for signals traversing the equatorial ionosphere. In this area, large rising turbulent plumes form in the afternoon and evening ionosphere, resulting in rapidly varying, significant signal loss. Not only does this affect telemetry up/downlink but, GPS users can lose tracking of enough spacecraft so as to make location finding difficult.
Some spacecraft use Earth's magnetic field as an aid in orientation or as a force to work against to dump momentum and slow down reaction wheels. During geomagnetic storms, dramatic unexpected changes in the magnetic field observed by the satellite can lead to mis-orientation of the spacecraft. Some effects have been reported at Kp values as low as Kp=4. Usually, problems are not experienced until Kp>=6 occurs.
GEO spacecraft also experience a unique occurrence termed a Magnetopause Crossing. The sunward boundary of Earth's magnetic field (magnetopause) is usually located approximately 10 Earth radii from Earth center. Variations in the pressure (due to changes in the velocity, density, and magnetic field) of the incoming solar wind change the location of that boundary. Under solar wind conditions of high velocity and density and strongly southward magnetic field, this boundary can be rammed to inside the altitude of GEO orbit at 6.6 Earth radii. A GEO spacecraft on the sunward side of Earth can be outside the (compressed) magnetopause and in the (modified) solar wind magnetic field for minutes to hours. When the magnetopause is inside 6.6 radii, GEO spacecraft are within the magnetosheath between the bow shock and the magnetopause. Magnetic sensors on board become confused as the detected magnetic field drops from ~200 nanoTesla to near zero and its sign changes erratically. The GOES spacecraft have magnetometers on board that unambiguously identify crossings at their positions. (See GOES Magnetometer plots). However, since magnetopause compression is time varying, and different spacecraft are at different longitudes, a GOES satellite may not observe a crossing experienced by others, and conversely.
During Solar Proton Events, photonic devices such as CCDs and some star trackers experience a noise floor increase. For star trackers, this noise can result in orientation problems. Streaks and extra "photo electrons" in imaging CCDs can compromise data quality. (See GOES Proton Flux plots)
It is currently impossible and uneconomic to design a spacecraft that is entirely immune to variations in the space environment. Until authenticity is achieved, it is vital for the satellite operator to be aware of the seas he is navigating. SWPC data, alerts, watches, warnings, and products provide situational awareness and are essential in anomaly resolution.
There will be future satellite problems, and even failures. But the gains and profits to be had are too great to forego the challenges. Space-faring nations and commercial enterprises will continue to brave the stormy, and at times dangerous, ocean of space. The SWPC will be with these explorers to make their voyages as successful and profitable as possible.
Space Weather Prediction Center -- Satellite Operators User Group page
International Clearing House for Space Weather Information
National Geophysical Data Center - Solar-Terrestrial Physics
Space Science Institute - Space Weather Center
Space Weather: A Research Perspective
Marshall Space Flight Center: The Space Environment and Effects (SEE) Program
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