Solar Panel Degradation:
It has been known since the early years of the space program that energetic particles from the sun and cosmic rays from elsewhere in space, produce physical damage to silicon-based solar cells - the most common and lowest-cost technology used for satellite power generation. Because the power output of solar cells is monitored almost literally from minute to minute in a satellites lifespan, detailed studies have been conducted that show how this degradation progresses over time. The figure below shows the power delivered by the solar panels on the Solar Heliospheric Observatory, based on a similar figure published in an article by Paul Brekke ('Effects of Space Weather on Technology Infrastructure', 2003, NATO Science Series vol. 176 p. 116)
The graph shows a decline by 14% in the first seven years of the satellite's operation. Individual degradation events caused by specific solar proton events can also be seen. The largest drop in power was caused by the July 14, 2000 and the November 4 and 23, 2001 Solar Proton Events which reduced performance by nearly 2% in each instance. Without the solar proton events, the degradation would have been approximately 10% due to the generally constant cosmic-ray background rates found at the satellite's location at the 'L1' point. These two events cost the satellite two years of ordinary lifespan. This is the typical rate that satellite designers expect, and upon which estimates of satellite lifetimes are based. It is expected that solar panels will degrade by about 20-25% during the 10 to 15-year lifetimes of modern GEO satellites. Solar panel are oversized at launch by 25% to allow for this loss of power at the end of the mission lifetime. The problem is that solar flares and proton storms can upset these calculations and cause a satellite to end its service several years earlier, at a large cost to profit margins.
Premature Atmospheric Reentry:
If you visit the Space Track resource at the NASA Goddard Space Flight Center and examine the annual catalog of space debris and satellites that reenter the atmosphere each year, you will see that at least for satellites in LEO orbits, their lifetimes are eventually terminated by atmospheric drag. This is a severe problem for satellites with orbits below 500 km. The International Space Station, for example, is in a Space Shuttle-accessible orbit with an altitude of 219 miles (perigee 354 km). It requires re-boosting several times every year because atmospheric drag is constantly decreasing its altitude by a kilometer every 12 days. For example, in May 2000, the Space Shuttle used its thrusters to 're-boost' the ISS by adding a whopping 43 km (27 miles) to its current orbit. This figure shows how quickly the changes can add up:
decay process has been extensively modeled by NASA and military scientists
in an effort to predict when their LEO satellite systems will be entering
the atmosphere. The reason this is crucial to know is that uncontrolled
reentries are potentially very dangerous for larger satellites that may
not fully burn up. Large fragments (like the pieces from Skylab in July,
1979) can reach the ground, and were this to happen over densely populated
areas, property damage or even injury could result. The above figure,
appearing in the May 30, 2000 edition of Science@NASA,
comes from NASA/Marshall's Larry Kos who used a computer model of the
ISS orbit to project its orbital decay resulting from atmospheric drag.
During solar maximum conditions, the ISS loses 400 meters of altitude
per day (147 km/yr) , while during sunspot minimum conditions the rate
is only 80 meters per day (28 km/year). The software that produced this
approximate prediction is called LTIME (i.e., Lifetime). It has been developed
and improved, most recently by Mr. Jim McCarter, over the last 30+ years
at the Marshall Space Flight Center. Input parameters include the assembly
stage of the space station (solar panels, e.g., would increase drag) and
the response of the atmosphere to the solar cycle.
SNOE - As an example of a carefully-studied satellite that shows this atmospheric friction in operation, the $5 million, student-built, SNOE satellite was launched by the University of Colorado on February 27, 1998. It is a small satellite with a mass of 115 kg and a hexagonal size of about one-meter. The original circular orbit had an initial altitude of 556 km. The decay of its orbit was carefully studied for the first 1050 days, during which its altitude was reduced to 521 km. The decay rate was initially rather uniform for the first 650 days, with a decrease averaging 0.6 km per month, but between 650-1050 days it increased to 1.7-km per month by late 2000. After a successful 6-year mission and 32,248 orbits, the satellite finally burned-up at an altitude near 200 km on December 13, 2004. The plot below is from a presentation by the SNOE Team at the University of Colorado:
Solar storms heat the upper atmosphere causing increased drag which accelerates orbit decay. This solar modulation of LEO satellite decays can be easily seen by simply plotting the number of reentering satellites tracked, for example, by NORAD (and listed in Sat Tracks), and plotting them against the sunspot cycle. Such a plot will look like this:
The red curve is the sunspot cycle ( yearly average divided by 4). The dark curve is the number of reentered satellites each year. It is pretty clear that as the sun enters a phase with more intense activity near sunspot maximum, the number of satellite decays increases significantly. This is because solar activity (sunspots) is a strong source of X-rays which enter the upper atmosphere and cause excess heating. This heating expands Earth's atmosphere so that LEO satellites experience a denser atmosphere and more air resistance. We have seen how a single solar flare (solar proton) event can cause measurable damage and reduction of lifetime to a satellite's solar panels.
Can the decay rate of a LEO satellite be affected by an individual solar storm in which the level of X-rays suddenly increases by 10-100 times above the usual solar levels for several hours? Yes, indeed, this can happen, and that's a big reason why operating LEO satellites usually have active altitude control systems.
CBERS-2 (or ZY-1B) - The second, China-Brazil Earth Resources Satellite was launched on October 20, 2003 into a LEO orbit with an initial, nominal, altitude of 770 km. Their press release for December 9, 2003 announced that the satellite was not affected by the particles and radiation events associated with the late-October 2003 storms. The figure below which appeared in the December 9 press release shows that the altitude of the CBERS-2 satellite was significantly affected by the solar storms between October 27-28 causing a 70-meter loss in altitude which had to be corrected.
ASCA (Astro-D) - This Japanese X-ray satellite was launched into a 538 x 645 km elliptical orbit on February 20, 1993. Following a large solar flare on July 14, 2000 and a CME release that triggered the 'Bastille Day' storm on July 15, the atmosphere heating from this geomagnetic event caused the satellite orientation to change so violently that the attitude control system could not keep up with the changes. The satellite put itself in a 'Safe' mode, however the satellite started spinning slowly. Japanese ground controllers tried, but failed, to stop the spin because the batteries had not been fully charged since the solar panels were no longer pointing fully at the sun. The tumbling increased until the atmospheric friction finally accelerated its orbital decay. The plot below from Dr. Koji Mukai (USRA) at the U.S. ASCA Guest Observer Facility, shows the history of the decay:
Starshine-3 - This student-built satellite designed to last 8 months in orbit was launched on September 29, 2001 and placed in a LEO orbit with an initial altitude of 470 km. After making 7434 orbits of Earth, it burned up in the atmosphere on January 21, 2003 - exceeding its design life by eight months. Its purpose was to specifically study the affects of space weather on the atmosphere and its impact on satellite decay. This plot, created by Chris Peat at Heavens Above, shows the familiar acceleration of the decay during the latter few months. No specific solar storm event was responsible.
Sudden Event Upsets
The 23rd Cycle - Chapter 6 : "High-energy particles such as cosmic rays or protons from solar flares, do considerable internal damage to spacecraft. At the atomic scale, to an incoming proton, the walls of a satellite look more like a porous spaghetti colander than some solid wall of matter. When high-energy protons do manage to collide with atoms in the walls of the satellite, they produce sprays of secondary, energetic electrons that penetrate even deeper into the interior of the satellite, producing what engineers call 'Internal Dielectric Charging'. As the charging continues, eventually the electrical properties of some portion of the satellite breaks down and a discharge is produced. In a word, you end up with a miniature lighting bolt that causes a current to flow in some part of an electrical circuit it's not supposed to. As anyone who has inserted new boards into their PC can tell you, just one static discharge can destroy the circuitry on a board. Beyond actual physical damage, these particles can also change information stored in a computer's memory.
Microscopic current flows can flip a computer memory position from '1' to '0' or cause some components, or an entire spacecraft system, to switch-on when it is not supposed to. When this happens, it is called a 'Single Event Upset' or SEU, and like water they come in two flavors: hard and soft. A hard SEU actually does unreparable physical damage to a junction or part of a microcircuit. A soft SEU merely changes a binary value stored in a device's memory, and this can be corrected by simply 'rebooting' the device. Engineers on the ground cannot watch the circuitry of a satellite as it undergoes a discharge or SEU event, but they can monitor the functions of the satellite. When these change suddenly, and without any logical or human cause, they are called 'Satellite Anomalies'. They happen a lot more often than you will ever read about in the news media.
Gordon Wrenn is the Section Leader of the Space and Communications Department of DRA Farnborough in England. Some years ago, he looked into a rash of unexpected changes in an unnamed, commercial, geosynchronous satellite's pointing direction. The owners of the satellite let him look at their data under condition that he not divulge its name or who owned it. This particular satellite experienced many SEUs in its attitude sensor system. When the SEUs were compared to the radiation sensor data from the GOES-7 and METEOSAT-3 satellites, it was pretty clear that the anomalies followed along with increases in the number of energetic electrons detected by GOES-7. These insights, however, cannot be uncovered without cooperation from the satellite owners. The specific way that energetic particles cause internal dielectric charging can only be ferrited-out when satellite owners provide investigators with satellite data as Wrenn explains,
"Prompt and open reporting offers the opportunity to learn from others' mistakes. Sometimes the lesson can be fairly inexpensive; Telsat Canada were not so fortunate [with the loss of the Anik satellites]"
More readily available data on this problem can be had from government research and communication satellites because the information is, at least in principle, open to public scrutiny if you happen to know who to talk to, or can extract the information from thousands of technical reports. The first satellite in the NASA, Tracking and Data Relay Satellite System (TDRSS-1) was launched in April 1983, and from that time onwards, the satellite has been continuously affected by soft SEUs. The satellite anomalies affected the spacecraft's Attitude Control System, and like mosquitoes on a warm day, they remain a constant problem today. The SEUs have been traced to changes in the computer's RAM, and the most serious of these SEUs were considered mission-threatening. If left uncorrected, they could lead to the satellite tumbling out of control. Ground controllers have to constantly keep watch on the satellite's systems to make certain it keeps its antennas pointed in the right direction. This has become such an onerous task that one of the ground controllers, the late Don Vinson, once quipped, "If this [the repeated SEU's] keeps up, TDRS will have to be equipped with a joystick"
From USGS Landsat - When SEUs happen in memory areas reserved for commands, operational errors can result that lead to unplanned and sometimes catastrophic problems. SEUs can also happen in areas reserved for data which allow them to be 'visualized'. SEUs on the Landsat 7 ETM+ take the form of a sudden bright pixel (which may or may not saturate high). Often two or more pixels are affected at the same time when the high energy particle hits the detector array between adjacent detectors. After the initial bright spike there is a single dark pixel as the affected detectors recoil in bright target recovery. These dark pixels often saturate low (recording a value of zero); detection of low saturated pixels is the primary means of discovering these artifacts within the data. Below is an example of a single event upset in Band 3 of a path 217, row 75 scene collected on 3/31/2000. At least three detectors were affected by this event; all had a sudden spike, and two of them then saturated low due to bright target recovery behavior.
tThe vast majority of single event upsets found have occurred near the South Atlantic Anomaly. The geographic plot below shows scenes with less than 10 saturated pixels in each of Landsat imaging bands 1, 2, 3 and 4.
AA variety of satellite anomaly archives are available, including the extensive archive at the National Geophysical Data Center which was produced by Daniel Wilkinson for investigating the correlation between these anomalies and space weather events.