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Hazards in Space
RadiationRadiation describes the flow of energy through the universe. In orbit, a satellite and its electronics are exposed to high-energy particles from different sources. The most common sources of exposure to ionizing radiation are the Van Allen radiation belts and cosmic radiation.Cosmic rays come from all directions and consist of approximately 85% protons, 14% alpha particles, and 1% heavy ions, together with x-ray and gamma-ray radiation. Most ionization effects are caused by particles with energies between 108 and 2*1010 eV.Solar particle events come from the direction of the sun and consist of a large flux of high-energy (several GeV) protons and heavy ions, again accompanied by x-ray radiation.The Van Allen radiation belts contain electrons (up to about 10 MeV) and protons (up to 100s MeV) trapped in the geomagnetic field. The particle flux in the regions farther from the Earth can vary wildly depending on the actual conditions of the sun and the magnetosphere.Secondary particles result from interaction of other kinds of radiation with structures around the electronic devices.The figure below gives an idea of the many sources of radiation and possible effects on a wide variety of devices.Radiation effects in the interior of satellites are often grouped into three categories [2]:
Total Ionizing Dose
These effects in electronics are the result of damage that usually builds up over a long period of time in an insulating region of an electronic device. This changes the device properties, which results in performance degradation and eventually can cause the device to fail completely.
Displacement Damage
Displacement damage is a cumulative effect in the electronic device’s semiconductor material. These effects also cause the device to deteriorate at first, possibly failing if exposed to enough radiation.
Single event effects
Single event effects are caused by the passage of a single particle through a sensitive region in an electronic device. There are many types of single event effects, both benign and destructive. Sometimes, a single event effect is so small that it goes unnoticed. At the other extreme, it could cause a system in a satellite to shut down.
Satellite Charging and Discharging
Near the Van Allen radiation belts and the magnetotail, plasma bombardment occurs due to varying plasma density, resulting in a negative electrostatic charge on the surface of the satellite
The photoelectric effect due to solar radiation liberates electrons on a satellite's surface, resulting in a positive charge on the satellite's sunlit side.
A satellite will usually have a negative potential on shaded areas (due to plasma charging) and a positive potential on sunlit areas (due to the photoelectric effect). If the surface of the satellite is conductive, a current will develop to cancel these potentials. On a non-conducting surface, the charge separation is maintained until voltage exceeds the resistive threshold of the material, leading to a sudden electrostatic discharge.
At GEO, satellites are most vulnerable to charging/discharging because they typically move in and out of the upper regions of the Van Allen Radiation Belts and the Earth's magnetotail; the low plasma density around the satellite does not allow the charge to bleed off or neutralize before a discharge occurs.
Sudden electrostatic discharge (high current or arc) can cause hardware damage, such as blown fuses, damaged transistors and capacitors, vaporized metal parts, structural damage, and breakdown of thermal coatings.
These effects translate into failures such as:
False commands
On/Off circuit switching
Memory changes
Solar cell degradation
Degradation of optical sensors
Deep Charging
Deep charging of a satellite occurs when cosmic ray particles pass through a satellite and ionize atoms within through collisions. Some of these particles are solar in origin, but the majority are galactic and with no preference to time or light conditions. They do show some dependence on the solar cycle.Particle CollisionHigh energy solar flare particles and galactic cosmic rays can cause direct damage to the surface of a satellite, such as vaporization of surface materials and structural damage. These particles can also enter star or horizon sensors and mimic reference points, leading to false readings and loss of orientation. Consequences include misalignment of antennas and solar panels, overcorrection of orbit, etc.OutgassingOutgassing is a phenomenon where molecules of material evaporate into space. Although many materials experience outgassing, composites and other materials made with volatile solvents are particularly susceptible. These include electronic microchips, plastics, glues and adhesives. As a result of outgassing, the material’s physical properties can change. Furthermore, the evaporating molecules can form a thin film over other components of the satellite (such as solar panels), thereby affecting their performance.Tin Whiskers [3]Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish. One common example of this is lead-free solder.These whiskers have been observed to grow to lengths of several millimeters (mm), and in rare instances, to lengths in excess of 10 mm. Numerous electronic system failures have been attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements maintained at different electrical potentials.
The image below is an example of a small tin whisker shorting out two terminals of a surface mount component.
Tin whiskers pose a serious reliability risk to electronic assemblies, particularly in space. To date, there are reports of at least three tin whisker induced short circuits that resulted in complete failure of on-orbit commercial satellites [3]. Four partial failures have also occurred [3].
The general dangers of tin whiskers fall into three categories [3]:
Short Circuit
In low voltage, high impedance circuits, a stable short circuit may occur because there is insufficient current available to fuse the whisker open. Depending on a variety of factors including the diameter and length of the whisker, it can take more than 50 mA to fuse open a tin whisker.
Metal Vapor Arc
If a tin whisker initiates a short in an application environment possessing high levels of current and voltage, then a very destructive phenomenon known as a metal vapor arc can occur. When the arc occurs, the solid metal whisker is vaporized into a plasma of highly conductive metal ions, more conductive than the solid whisker itself. This plasma can form an arc capable of carrying hundreds of amps. Such arcs can be sustained for long duration (several seconds) until interrupted by circuit protection devices (e.g., fuses, circuit breakers) or until other arc extinguishing processes occur. In the low pressure environment of space, less power is required to initiate and sustain a whisker-induced metal vapor arc than in an air-filled environment.
Metal vapor arcs in a vacuum are reported to have occurred on at least three commercial satellites resulting in blown fuses that rendered the spacecraft non-operational.
Debris/Contamination
Whiskers or parts of whiskers may break loose and bridge isolated conductors or interfere with optical surfaces.
Space Debris
Space debris refers to non-operational manmade objects of any size (varying from entire satellites to small chips of paint) in space. Of the almost 10,000 manmade items in space currently tracked and catalogued, only about 5% are operational space systems. The rest is space debris.
Space debris smaller than approximately 2 cm cannot be detected and tracked reliably, so a more realistic estimate suggests the presence of 40,000 to 80,000 pieces of debris in orbit around the Earth. Most debris is small, but it can travel at relatively high speeds.
Although space debris is concentrated in LEO, atmospheric drag at GEO is low, so the debris is present for a much longer time. A dense object could do catastrophic damage if it were to hit the satellite even at a low relative velocity, such as those encountered at GEO.
Meteorites
Meteorites consist of natural material originating from asteroids and elsewhere in space. Although major impacts are not very common in satellite systems, frequent micro-meteorite collisions can cause extensive damage to metal surfaces.
Most meteorites are too small and travel too fast to be detected in time for satellite controllers to direct a satellite to change it's orbit to avoid collision.
Note: This brief overview of the potential hazards in space is adapted from [1].
REFERENCES
[1] http://www.fas.org/spp/military/docops/army/ref_text/chap5im.htm
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