The environment on Earth’s surface has commonalities and differences from the space environment, past Earth’s atmosphere; this boundary is defined by the Karman line. The Earth’s atmosphere protects us, ground dwellers, from an immense amount of cosmic radiation, plasma, and micrometeoroids. Earth’s magnetic field protects us from solar wind particles, part of the phenomena of space weather [NASA]. In the space environment and common to our surface environment, spacecraft have to interact with gravity, electromagnetic radiation (in differing doses), atmospheric particles (in differing density). Although the spacecraft does technically interact with atmosphere particles past the Karman line, the spacecraft functions in a near-vacuum, approaching a truer vacuum farther into space. In this section, we will define each physical phenomenon (with equations!), discuss how each physical phenomenon affects the spacecraft subsystems, and explore the dominance of each phenomenon in orbital regimes.

Studying the Space Environment

There are numerous satellites that collect data, assess, and report on “Space Weather.” Here are a few:

The Sun

The single largest influence on the space environment and space weather is the Sun. Solar activity that affects the Earth and spacecraft results from natural phenomena occurring within the magnetically heated outer atmospheres in the Sun. These phenomena take many forms, including solar wind, radio wave flux, energy bursts such as solar flares, coronal mass ejection (CME) or solar eruptions, coronal heating, and sunspots. The Sun’s activity goes through a regular 11-year cycle of solar activity.

Some statistics about the Sun:

• Diameter: 1.4×10⁶km
• Mass: 2×10³⁰ km
• Surface (Photosphere) Temperature: ~5.5×10³ deg. C
• Mean Distance from Earth: 1.5×10⁸ km
• Rotation Rate: ~30 days (longer at the poles, shorter at the equator)
• Current Chemical Constituency:
  • Hydrogen: 75%
  • Helium: 25%
  • Everything else: <1%
• Energy Conversion:
  • Hydrogen fusion produces Helium (~5×10⁹ kg/sec)

The Sun produces electromagnetic radiation and particles. This solar output is the primary determinant of the space environment that spacecraft encounter. The sun releases electromagnetic (EM) energy across the electromagnetic spectrum from long-wavelength radio waves to short-wavelength x-rays and gamma rays. Increases in solar radiation can provide early warning of solar events that have sent large quantities of particles into space.

Electromagnetic radiation from the Sun travels at the speed of light, reaching Earth’s orbit in ~8 minutes. Charged particles emitted by the Sun travel much more slowly, arriving anywhere from 30 minutes to 4 days after the electromagnetic radiation.

EM energy contained in either electromagnetic radiation or charged particles can pose various hazards to spacecraft including:

• Degradation of solar arrays, polymer materials, and microelectronics
• Attitude perturbations (especially at GEO and greater altitudes)
• Orbit decay through atmospheric heating (for LEO satellites)
• Transmission signal interference

Electromagnetic Radiation

Ultraviolet

Photons of UV light are more energetic than photons of visible light. When this high-energy UV radiation reaches the Earth it causes heating of the atmosphere. This heating drives an expansion of the atmosphere and an increase in atmospheric density for a given altitude. Large increases in solar UV radiation are associated with solar flare activity.

Damage Due to Ultraviolet Radiation

Long-term exposure to UV radiation has been shown to cause significant changes in the optical and mechanical properties of various materials (such as changes in color, thermal properties, brittleness, and opaqueness). Most spacecraft use beta cloth as the outer layer of multi-layer insulation (MLI), protecting spacecraft components from the space environment.

X-Rays

X-rays are produced when electrons that have been accelerated by solar activity make a close pass near a solar wind proton.

Gamma Rays

Gamma Rays are produced when protons accelerated by solar activity strike an atom in the solar wind.

Gamma rays, found at very low background levels when the sun is ‘quiet,’ also provide early warning of solar flare activity.

Gamma-ray imaging of a June 1991 solar flare by the CGRO spacecraft showed that gamma-ray production can last up to an hour after the initial flare is sighted. (Here CGRO was on the night side of its orbit during the first 40 minutes after the flare was detected.)

Radio Waves

Measurement of solar electromagnetic radiation in the radio region is useful in determining sunspot levels. Radio emission from the sun at a wavelength of 10.7 cm (often called the “10 cm flux”) has been found to correlate very closely with the level of sunspot activity. Since this flux is easy to measure, it has replaced the sunspot “count” in many cases as the indicator of solar activity.

Solar Electromagnetic Radiation Summary

• The sun produces energy across the electromagnetic spectrum.  This output varies, depending upon whether the sun is ‘quiet’ or ‘active’ at a given time.
• Since electromagnetic radiation travels at the speed of light, identification of solar events that are depositing large quantities of charged particles into the solar wind can be made hours to days before such particles reach Earth orbit.
• A number of wavelengths provide early warning of increased solar activity:
  • long-wavelength radio waves can indicate an increase in sunspot activity
  • short wavelength UV, x-rays, and gamma rays can indicate an increase in solar activity
• The other factor of solar electromagnetic energy that is important to flight operations is the pressure that radiation exerts on satellites.  If not properly counterbalanced, solar pressure can produce attitude control anomalies.

Solar Radiation Particles (Solar Wind)

In addition to producing electromagnetic radiation, the Sun also produces a variable stream of particles that can affect satellite operations.

Composition:

• The solar wind is a plasma (an ~equal mixture of positive and negative charged particles).
• Positively charged ions – hydrogen and helium nuclei (~50%)
• Electrons (~50%)

Speed:

• Nominal wind velocity fluctuates between 200 km/s and 600 km/s (so the particles reach the Earth in 3-9 days).

Temperature:

• >100,000 C

Particle Density:

• Nominally ~10 ions & electrons per cubic centimeter.  (Typical atmospheric density at the surface of the Earth is 1022 times larger than this solar wind density.)

Variation:

• Increased solar activity can dramatically change the density, speed, and temperature of the solar wind.

Spacecraft Hazards Due to Solar Wind Plasma:

• Electronic system ‘bitflips’ and more serious component damage
• Spacecraft charging
• False sensor readings

Solar Radiation Pressure

For GEO satellites and interplanetary spacecraft, solar radiation pressure dominates the ‘drag’ that a satellite experiences similar to the way that atmospheric drag affects LEO satellites. Below 800 km altitude, atmospheric drag accelerations are greater than solar radiation pressure. Above 800 km, solar radiation pressure is dominant. Satellite geometry and the surface area exposed toward the Sun determine what effect solar radiation pressure has on the satellite. Solar radiation pressure may result in torques, rotation, and reorientation of GEO satellites.

Solar Radiation Torque

The solar radiation torque effect on the spacecraft is similar to that of aerodynamic torque, but the collision occurs with photons instead of air molecules. The mean momentum flux from the Sun, P, is:

where Fe is the solar constant (energy flux from Sun), c is the speed of light, and S is the unit vector from the spacecraft to the Sun. Radiation from the Sun can be completely absorbed, specularly reflected, or diffusely reflected, the probabilities of which are called the coefficients of absorption, where Ca + Cs + Cd = 1.

Cosmic Rays

In addition to the solar wind, very high energy (MeV and GeV range) ions are produced by our Sun (and by sources external to our solar system).  These high-energy ions are called Cosmic Rays. Although the flux of cosmic rays is very low, these particles are very dangerous because they include heavy, energetic ions of elements such as iron moving at close to the speed of light. Cosmic rays can cause intense ionization as they pass through matter, are difficult to shield against, and therefore constitute a significant hazard. Solar cosmic ray flux increases correlate well with solar flare events.

As the sun becomes more quiescent nearing solar minimum, there is less turbulence in the solar wind and in the magnetic field which is embedded in it.  Cosmic rays then find easier access to the inner solar system, resulting in an increase in the number of cosmic rays seen both in Earth orbit and at the Earth’s surface.

Solar Particles Summary

• The solar wind is primarily composed of plasma (an ~equal mixture of positively and negatively charged electrons).
• The density and composition of this plasma when it reaches Earth orbit can be dramatically affected by increases in solar activity.
• These charged particles are dangerous to satellites, especially in GEO and higher orbits.
• Cosmic rays are the very high energy ions (from the Sun and other sources outside the solar system)  that are repelled away from the Earth during periods of high solar activity but can penetrate down to the upper fringes of the atmosphere during ‘quiet’ solar periods.

Variations in Solar Output

It is important for spacecraft operations personnel to monitor variations in solar output since these variations have a significant impact on the space environment and on the hazards that spacecraft may encounter.

Solar output varies in predictable cycles known as solar cycles. The length of solar cycles can vary; however, a TYPICAL solar cycle is usually 11 years in duration. A typical solar cycle includes a 4-year build-up in solar magnetic activity (at the peak of which the Sun’s magnetic poles reverse) followed by a 7-year decline in solar magnetic activity.

Sunspots

Sunspots are sites of very intense magnetic fields in the Sun’s photosphere (the visible surface of the Sun). Sunspots are ~ 2,000 degrees cooler than the surrounding photosphere and can be up to 50,000 km in diameter. Individual sunspots last from a few hours to a few days. Solar flares and coronal mass ejections, the two primary sources of solar output, are linked to sunspot magnetic disturbances.  The third source of solar output, coronal holes, is discussed later in this chapter but is not believed to be directly related to sunspots.

The solar cycle is evident in measurements of solar sunspot numbers.  Sunspot activity follows the solar cycle.  In fact, the terms solar cycle and sunspot cycle are often used interchangeably.

Solar Flares

Solar flares are eruptions – sometimes spectacular eruptions – in the Sun’s chromosphere that release built-up magnetic energy and emit radiation across the electromagnetic spectrum. Solar flare radiation heats and accelerates particles in the solar wind (at temperatures from 10-30 x 106 °C). A visible brightening of the sun near a sunspot is usually an indication that a solar flare has occurred. Flares typically last from a few minutes to a few hours. High energy protons from a flare can reach Earth within 30 minutes (~1/3 the speed of light) of the flare initiation.  This is called a Solar Proton Event.

The primary difference in the energy spectrum between the ‘quiet’ sun and solar flares is the dramatic increase in energy in the short wavelengths (UV, X-rays, and gamma rays):

• X-rays result from the energizing of electrons during the flare
• Gamma rays result from the energizing of high-energy protons and heavier ions.

Solar Flare Profile

• Pre-flare Stage: X-ray and gamma-ray energies barely detectable
• Impulsive Phase: High energy x-ray and gamma-ray spectra show large fluctuating spikes while the low energy x-ray spectrum gradually rises
• Gradual Phase: High energy x-ray and gamma-ray levels begin to taper off as do low energy x-rays (although at a significantly slower rate)

Solar flares are classified according to x-ray output, specifically according to the order of magnitude of the peak burst intensity (I) measured at the earth in the 0.1 to 0.8 nanometer (x-ray) wavelength band.

Solar flare activity (for 24 hour period) is rated in the following categories:

• Very Low: X-ray events less than C-class
• Low: C-class x-ray events
• Moderate: Isolated (one to four) M-class x-ray events
• High: Several (5 or more) M-class x-ray events, or isolated (one to four) M5 or greater x-ray events
• Very High: Several (5 or more) M5 or greater x-ray events

Coronal Mass Ejection

A Coronal Mass Ejection (CME) occurs when a solar prominence – a bubble of trapped coronal gas – ruptures and the trapped coronal gas is released into the solar wind.

CMEs are a daily occurrence but a large one typically only occurs once or twice per year.  Large CMEs can dump large quantities (typically 10¹² to 10¹³ kg) of high-energy ions and electrons into the solar wind. The number of CMEs varies with the solar cycle from 0.5/day during solar minimum to 2.5/day during solar maximum. CMEs are usually associated with large groupings of sunspots, however, they are NOT usually associated with solar flares. CMEs can accelerate the solar wind to between 400 and 2,000 km/s. It takes 2-4 days for the energetic particles ejected by a CME into the solar wind to reach Earth’s orbit.

Coronal Holes

Coronal Holes form in the Sun’s corona – most often near the Sun’s magnetic poles – as a result of broken magnetic field lines and allow coronal material, particularly low-energy electrons, to escape into the solar wind.

Coronal holes most often form near the Sun’s poles, however, sometimes coronal holes also form at lower solar latitudes.  This seems to be more prevalent in the years following the solar maximum. 

Where to Get Information

There are a few primary sources of information on variations in the solar output:

• NOAA’s Space Environment Center in Boulder (Colorado) is the “Nation’s official source of space weather alerts and warnings for disturbances that can affect people and equipment working in the space environment.”   This website provides information (usually updating every ~15 minutes) on the solar wind and the solar EM output.

http://www.sec.noaa.gov/

• NASA Goddard Space Flight Center (GSFC) provides predictions for the next 20 years on the 10 cm radio flux.  These predictions are used by most LEO satellite projects to estimate atmospheric drag and orbit re-boost/tracking requirements.

http://envnet.gsfc.nasa.gov/

Upon first entering “Today’s Space Weather” at the NOAA Space Environment Center website, you see a photo of the sun in H-alpha (a good place to see solar flares, prominences, or CMEs).

National Solar Observatory (NSO) Kitt Peak coronal hole images can be found at:

http://www.nso.noao.edu/synoptic/synoptic.html

Upon visiting this website you are asked to select the ACE instrument from which you would like a plot.  To see cosmic ray data, select the Solar Isotope Spectrometer (SIS) and then specify the period to be plotted.  (A 7-day plot was specified in the figure.)

Daily updates of the 10cm solar flux can be also be found at the Canadian Solar Radio Monitoring website: http://www.drao.nrc.ca/icarus/www/sol_home.shtml

10 cm Radio Flux Predictions

Predictions of future radio flux can provide insight into future solar activity levels. GSFC provides predictions on 5-day centers for the next ~20 years. This information is used to predict atmospheric drag and determine when upcoming LEO spacecraft to orbit adjustment maneuvers will be required. Any variation of the measured flux from these predictions will affect the frequency of maneuvers. GSFC EnviroNET at Environmental Planning | National Aeronautics and Space Administration (nasa.gov)

Variations in Solar Output Summary

• Alerts of potentially dangerous changes in solar output can be found at the NOAA Space Environment Center website:  http://www.sec.noaa.gov
• It is also possible to be put on SEC email distribution lists to receive daily updates of space weather automatically.
• The solar output information can be used to identify potentially hazardous solar events as they are occurring or, in some cases, before they occur.
• This information can also be used after a satellite anomaly has been identified to assist in the root cause analysis of the anomaly.
• Make sure when using space weather data to predict the hazard to your spacecraft that you take into account the location of the satellite collecting the data vs. the location of your satellite.

The Earth’s Magnetic Field

The heating and cooling of liquid metals in the Earth’s core are thought to be the driver behind the Earth’s magnetic field. Close in (within ~56,000 km of the Earth’s surface) on the dayside this field resembles a typical dipole magnetic field. Beyond ~56,000 km the magnetic field is compressed and elongated by interaction with the solar wind.  (This will be covered in the next section.) 90% of the measurable field is located beneath the Earth’s surface.

The Sun-Earth Connection

The solar wind, composed mainly of charged particles, has difficulty penetrating the Earth’s magnetic field and so attempts to flow around it.  In the upwind direction (sunward) the magnetic field is compressed and a bow shock is formed.  In the downwind direction (anti-sunward) the magnetic field elongates, forming a magnetotail. The cavity formed around the Earth by the interaction between the solar wind and the Earth’s magnetic field is known as the magnetosphere.

Solar wind particles can enter the Earth’s magnetosphere at two locations:

• Through the polar cusps (producing the Aurora effects)
• Through the ‘back door’ (resulting in particle addition to the outer Van Allen radiation belt)

Van Allen Radiation Belts

The Van Allen radiation belts are doughnut-shaped regions of high-energy particles encircling the Earth, held in place by the Earth’s magnetic field.  (These radiation belts are named after Dr. James Van Allen, the American physicist whose Geiger counter on the first successful U.S. satellite, Explorer I, first detected these belts in 1958. Dr. Van Allen was a professor at the University of Iowa).

Inner Belt:

• Formed by cosmic radiation and solar wind
• Composed primarily of high energy protons (10-100 MeV)
• Very intense, compact, and fairly stable
• Increases in intensity during solar minimum.

Outer Belt:

• Composed of ‘trapped’ solar wind plasma (typically <50 KeV)
• Greatly influenced by fluctuations in solar activity (e.g., solar storms); particle density can increase by a factor of 10 to 1,000 over a short period (minutes)

South Atlantic Anomaly (SAA)

Although most of the inner Van Allen radiation belt stays above 500 km altitude, the belt does “dip” down to nearly 250 km in a region known as the South Atlantic Anomaly (SAA).  This “dip” in the inner Van Allen belt occurs because the Earth’s magnetic field is not centered on the Earth’s core but a few hundred miles off the Earth’s core, away from the SAA.  The boundaries of the SAA vary with altitude (down to 250 km).  At 500 km the SAA ranges from -90 to +40 longitude and -50 to 0 latitude. When low-earth orbiting satellites fly through this region of dense, high-energy proton radiation in the SAA, electronic components and instruments can be disrupted.

Geomagnetic Storms

When solar energy input into the magnetosphere increases – entering the magnetosphere at the polar cusps or the rear of the magnetosphere – geomagnetic storms can result.  A geomagnetic storm will usually begin from one to four days after a coronal mass ejection (CME) is first seen and may last from a few hours to days. Also, solar flare storms will usually begin from a few hours to a day after the flare is first seen and may last from a few minutes to a few hours.

At Earth, geomagnetic storms can cause:

• Increases in auroral activity
• Power grid failures
• Communication blackouts
• Heating and expansion of the atmosphere

In 1997 particles from a large CME on January 6th reached Earth on January 10th, striking and compressing the magnetosphere so that the bow shock “pushed in” to ~36,000 km – geosynchronous orbit – altitude.  This resulted in:

•   Great auroral displays
•   Disruption in radio communications
•   The loss of the geosynchronous Telstar 401 satellite

Within a ~1 day, the storm had passed.

The Kp-Index is a planet-wide average measurement of how disturbed the Earth’s magnetic field is.  (Measurements are taken at numerous ground-based sites.) The Kp-Index is averaged over 3 hours. A Kp-Index of 4 or greater indicates that a geomagnetic storm is occurring.

Magnetic Substorms

When kinetic energy from the solar wind is converted into magnetic energy and stored in the Earth’s magnetotail, this build-up of energy can be released as a Magnetic Substorm. A magnetic substorm releases an energized plasma (5-50 keV) that is injected toward the Earth (from the anti-sunward direction). This hot plasma can extend into geosynchronous orbit where satellites may see a hundred-fold increase in the particle environment. Substorms occur predominantly during geomagnetic storms.

Magnetic substorm phases include:

• Growth Phase:  This phase usually lasts about 1 hour during which an increase in solar wind kinetic energy causes the storing of this energy as magnetic energy in the Earth’s magnetic field.
• Expansion Phase:  Usually lasts 30 minutes (10 minutes to 2 hours).  The beginning of this phase is the onset of the substorm.   Plasma release toward Earth reaches geosynchronous orbits (10s of keVs).  In addition, auroral activity increases on the night side.
• Recovery Phase:  Usually lasts about 1 hour during which time the magnetic field returns to normal and the plasma flux abates.

Sun-Earth Connection – Where to Get Information

K-Index data is updated every 15 minutes at the NOAA SEC website:  http://solar.sec.noaa.gov/rt_plots/kp_3d.cgi

In addition to the Kp-index, there are a number of measurements of variations in other components of the Earth’s magnetic field (known as the Ap-index, and the Hp-index).  Information on the Hp-index (as measured at GEO) can be found at the NOAA SEC website:  http://www.sec.noaa.gov/today.html

Physical Phenomena Definition

Gravity

^{Apollo 15 feather and hammer drop. Video courtesy of NASA.}

All things with mass or energy experience gravity, the phenomenon that brings objects or light toward (or gravitates toward) one another. For objects in space with a large enough mass, the proximal smaller mass will be brought toward the larger mass, like the sun attracting planets and like an apple falling toward Earth. The larger mass moves toward the smaller mass too but if the mass difference is large, the large mass’ movement is imperceivable. Discovered in 1687, Newton’s law of gravitation related the “forces which keep the planets in their orbs must [be] reciprocally as the squares of their distances from the centers about which they revolve: and thereby compared the force required to keep the Moon in her Orb with the force of gravity at the surface of the Earth; and found them answer pretty nearly.”

Where F is the force, m1 and m2 are the masses of the objects interacting, r is the distance between the centers of the masses and G is the gravitational constant. For our interest in satellites, the force of gravity on a spacecraft from orbiting a planet is related to the mass of the spacecraft, the mass of the planet, the gravitational constant, and the distance between the center of the spacecraft and the center of the planet. The mass to force relationship is straightforward: the more massive the satellite, the more gravitational force, and vice versa. The interesting relationship in this equation is the inverse squared relationship between force and distance. As the satellite moves farther away, the force of gravity steeply drops off, which explains why we need a rocket to get off the Earth’s surface but we only need spacecraft thrusters once we’re in orbit. We care about gravity in spacecraft design because we want to know how fast the spacecraft is moving in its orbit or how fast the spacecraft needs to move to be captured or escape a planet’s gravity. Let’s assume for all cases, the mass of our satellite is significantly less than the planet we’re orbiting, which is a very reasonable assumption. For the simplest circular orbit of very little eccentricity, the velocity may be approximated as:

Where is the orbit velocity, M is the planet’s mass, and the other variables carry over from the gravity equation.

For closed orbits, the spacecraft’s orbital period is given by the formula: T = 2r3GM.

A direct analogy may be made to the satellite revisit period, which is the time elapsed between observations of the same point on earth by a satellite.

Fun history: astronomers couldn’t directly measure how far away the other planets were but they could observe the time it took for the same planet to complete an orbit around the sun by observing the sky. Astronomers would use a reorganized version of the orbital period formula r = (GMT242 )1/3 to calculate the distance of the planet from the sun.

If we were to calculate the escape velocity of a spacecraft leaving a planet’s surface, the escape velocity for that body, at a given distance, is calculated by the formula:

More generally for orbits of any eccentricity, the instantaneous orbital speed of a body at any given point in its trajectory takes both the mean distance and the instantaneous distance into account:

Where μ is the standard gravitational parameter of the orbited body, r is the distance at which the speed is to be calculated, and a is the length of the semi-major axis of the elliptical orbit.

Now that we know how to calculate the instantaneous velocity of a spacecraft, we can calculate satellite revisit time, apply basic knowledge of orbits to propulsive systems, and calculate the effects of other environmental phenomena that depend on spacecraft velocity. Relevant parameters: altitude, eccentricity, planet mass

Outputs: orbit velocity, orbital period

Atmosphere

Our atmosphere consists of a rich abundance of nitrogen, oxygen, argon, carbon dioxide, etc. [Wikipedia]. While we humans may appreciate this fluid as a medium for life, spacecraft view our atmosphere as more of a burden of particles to push through (producing aerodynamic forces); think of capsules, space shuttles, or rockets that must re-enter our atmosphere.

^{LDSD: Supersonic Test Flight. Video courtesy of NASA/JPL}

^{Video of Space Shuttle Heat Protection – Last Flight of Spaceshuttle Columbia. Video courtesy of BBC.}

^{Video of Falcon 9 First Stage Reentry Footage from Plane. Video by SpaceX.}

Significant heat builds up from the spacecraft hitting many, many, many atmospheric particles at high speed.

Atmospheric Layers

The atmosphere varies in air pressure and density, decreasing with altitude. The atmosphere may be broken into different layers corresponding to temperature behavior:

• Troposphere
  • 0 to 12 km (0 to 7 miles)
  • 80% of the atmosphere’s mass
  • this is where weather occurs
• Stratosphere
  • 12 to 50 km (7 to 31 miles)
  • 19% of the atmosphere’s mass, including the ozone layer
  • this is where commercial airliners like to fly
• Mesosphere
  • 50 to 80 km (31 to 50 miles)
  • This is where meteors burn up
• Thermosphere
  • 80 to 700 km (50 to 440 miles)
  • This is where the ISS flies
  • solar activity plays a major role in the temperature and density of this region
• Exosphere
  • 700 to 10,000 km (440 to 6,200 miles)
  • The upper limit of the atmosphere where it merges into space

We care about the region past the Karman line, starting at 100 km, which includes the Thermosphere and Exosphere. Past the Exosphere’s altitude of influence at about 10,000 km, “the influence of solar radiation pressure on atomic hydrogen exceeds that of Earth’s gravitational pull” [Wikipedia].

The Thermosphere and Exosphere are the layers that primarily impact satellite operations.

Thermosphere

• Temperature increases with altitude up to 700-1200º C. This increase in temperature is due to the absorption of solar radiation by the limited amount of remaining molecular oxygen. Small changes in solar radiation can greatly affect the temperature in this region.
• The major atmospheric components in this region are still nitrogen and oxygen.  At this extreme altitude, gas molecules are widely separated (density at 300 km of 109 /cm3 compared to 1019 /cm3 at sea level).
• In the majority of the thermosphere, spacecraft experience increased drag that causes orbital velocities to increase, altitude to decrease, and eventual re-entry into Earth’s atmosphere.
• https://ccmc.gsfc.nasa.gov/RoR_WWW/SWREDI/2015/SatDrag_YZheng_060415.pdf

Exosphere

• The exosphere is the transitional zone between Earth’s atmosphere and interplanetary space.
• At exospheric heights, molecular escape from the earth’s atmosphere is significant. Lighter atoms and molecules can escape at lower altitudes than heavier ones.
• Solar winds start stripping away the exosphere. As this layer’s atoms and molecules are so far apart, the spacecraft experiences near-vacuum and is not significantly affected by atmospheric drag. This layer is very cold, affecting the spacecraft bus subsystem survivability.

Relevant parameters: pressure, density, temperature, orbital velocity, spacecraft area

Outputs: orbital velocity

The Ionosphere

You’ll notice that the Ionosphere is not one of the atmospheric layers shown or mentioned so far, even though it is probably the best known. That is because, unlike the other layers, it is not based on molecular density or temperature. Beginning at about 90 km, there is a significant quantity of ions and electrons in the atmosphere (mainly created by interaction with solar x-ray and UV radiation). This atmospheric region of charged particles, overlapping with the Mesosphere, Thermosphere, and Exosphere, is called the Ionosphere.

The motion of ions and electrons in the ionosphere produces an electrical current that heats and increases the density of the atmosphere. As shown below, the peak density of the ionosphere is between 200 and 600 km (the F2 region).

Charged particles in the ionosphere are also able to reflect radio waves, allowing RF transmission ‘over the horizon.’

• The highest frequency that can be reflected varies as the amount of charged particles in the Ionosphere varies – with the typical maximum reflective frequency as low as 2Mhz during the night and as high as 10 to 15 Mhz during the day when ionization is greatest.

Charged particles from the sun in the ionosphere cause auroral activity and LEO spacecraft hazards.

Vacuum

There are three main problems for spacecraft caused by the near-vacuum of space:

• Out-gassing
  • Tiny bubbles of gas trapped in materials under atmospheric pressure are released in a vacuum. Not a problem for materials, but can create a film of dirt over the lens and sensors.
• Cold welding
  • In space, there is no longer a tiny cushion provided by air between surfaces. As a consequence, the raw materials may weld together. The process is usually reversible, but it is better to avoid it with surface treatments (anodization).
• Heat transfer
  • No conduction or convection, only radiation.

There are advantages of operating in the near-vacuum of space:

• RF Signal Attenuation
  • Radiofrequency (RF) signals transmitted through the atmosphere suffer degradation of signal (called attenuation). The amount of attenuation depends on the frequency band and some bands are also adversely affected by atmospheric conditions such as rain or snow. RF signals traveling in the near-vacuum of space do not suffer from this attenuation.
• Drag
  • In the near-vacuum of space, the distribution of molecules is very sparse resulting in little or no effects of drag on the spacecraft caused by collisions with the gas molecules. The lower the orbit, the less perfect is the vacuum, and the greater the drag on the spacecraft.

Even the remote areas of interplanetary or interstellar space, far from any planets, are not true vacuums and have widely dispersed particles, molecules, and photons, known as the interplanetary or interstellar medium.

Space Radiation and Particles

Electromagnetic radiation and particles both have an effect on spacecraft in both Earth orbit and deep space. The figure shows the typical radiation and particles with their dimensions that affect spacecraft.

Single Event Effects

When a single, high-energy proton/ion penetrates a satellite surface and encounters an electronic device, it may decelerate and lose energy through an ionization process.  This results in a short pulse of current in the impacted device, known as a Single Event Effect (SEE). The SEE experienced by the satellite will depend on:

• The energy of the impacting proton/ion
• Path length over which the charge was deposited
• The sensitivity of the impacted circuit to the pulse of current

Older spacecraft were not as prone to Single Event Upsets since only large ions had the energy/mass to damage the robust electronics. In the past 30 years, the miniaturization of spacecraft electronics has resulted in vulnerability to disabling by single protons (which are much more prevalent in the space environment than heavier ions).

The SEE can cause either a hard error or a soft error:

Soft Errors

The damage is temporary and non-destructive. This includes:

• Single Event Upset (SEU or “bit flip”) – a change of state or transient induced by an ionizing particle impacting a device. This may occur in digital, analog, and optical components or may have effects in surrounding circuitry. These are “soft” bit errors in that a reset/rewriting/re-powering of the device normally restores the device to normal operations. Multiple SEUs occurring ‘simultaneously’ in a byte/word are referred to as Multi-Bit Errors.
• Single Event Latchup (SEL) – a potentially destructive condition where the component draws excessive current (basic shorts to ground). In traditional SEL, the device current may exceed the device’s maximum specification (typically hundreds of milliamps) and destroy the device if not current-limited. A “micro latch” is a subset of SEL where the device current remains below the maximum specified for the device. Removal of power to the device is required in all non-catastrophic SEL conditions in order to recover device operations.

Hard Errors

The damage is permanent and functional in nature. This includes:

• Single Hard Error (SEE) – an SEU that causes a permanent state change such as a ‘stuck bit’ in a memory device.
• Single Event Burnout – non-reversible device failure due to a high current state in a power transistor.
• Single Event Gate Rupture- a destructive rupture of a gate insulator.

What Causes SEEs?

Van Allen Belt Trapped Protons (including the SAA)

Proton density in the inner Van Allen belt is cyclic with the maximum density coming at solar minimum and minimum density at solar maximum. At the outer edge of the inner belt (7,000 km to 14,000 km), there is an area where high-energy protons from solar flare/geomagnetic storms can become trapped for periods of 6-8 months.  This is most likely to occur when the inner belt is at its weakest levels.

Cosmic Ray Ions

The very high energies of these particles make them a risk even though their flux is very low.   Flux density for these particles within the magnetosphere varies with the solar cycle, reaching a peak during solar minimum. Polar orbits are especially at risk from these high-energy particles.

Solar Flare Ions

The probability of occurrence of these events follows the 11-year solar cycle. Solar flare events can last from hours to days and the high-energy protons they emit can reach Earth within ~30 minutes of the flare occurrence. Solar flare ions typically do not have enough energy to penetrate farther into the magnetosphere than the outer Van Allen belt.

Danger to LEO Satellites

The inner Van Allen belt poses the greatest high-energy proton risk to LEO satellites (with peak proton density during solar cycle minimums). The inner Van Allen belt reaches from ~500 km to 5,500 km with a maximum proton density occurring at ~5,000 km. Many low-Earth orbiting missions are exposed to high-energy protons in each orbit when passing through the South Atlantic Anomaly (SAA).  At polar latitudes the cusps in the magnetosphere allow solar flare and cosmic ray protons to reach down to LEO.

Danger to Geosynchronous Satellites

Geosynchronous satellites are at risk from high energy protons/ions from the solar wind (with peak density during solar flare periods) and from cosmic rays during solar minimum. During periods of very high solar activity, the magnetosphere may become compressed on the sunward side, placing GEO satellites in this area outside of the magnetosphere and directly in the path of high-energy solar protons.

Outside of the magnetosphere, the risk from cosmic rays remains constant (and  > than the risk to GEO satellites during solar minimum) while the risk from solar protons varies with the solar cycle and with distance from the Sun. In addition, other planets such as Jupiter, have radiation belts that can present a SEE threat for satellites in orbit around them.

SEE Mitigations

By Design

Single event effects vary based on the type of devices that are impacted (e.g., power converter, memory storage).  During the spacecraft design process, a determination is made of the number of SEEs that can be tolerated by each spacecraft component that is at risk for single event effects. Designers can partially protect the satellite against SEEs by shielding and other satellite design features (error detection/correction, watchdog timers, redundancy).

Shielding

Although not providing total protection, increased shielding (at the expense of increased overall spacecraft mass) will reduce the number of SEEs encountered by a spacecraft. SEU vulnerability on TDRS-1 resulted in shielding design improvements on subsequent Tracking and Data Relay Satellites.

Error Detection and Correction (EDAC)

Spacecraft solid-state memory devices typically incorporate some type of scheme for detecting and correcting SEUs.  Sometimes the rate of this memory ‘scrubbing’ can be adjusted to minimize the chance that a multiple-bit error will occur prior to the initial SEU being corrected by the EDAC software.  There are a number of EDAC methods employed including:

Watchdog Timers

Many spacecraft have “I’m OK” timers that are reset within a given timeframe as long as the ‘sender’ is healthy.  If disabled by a SEE, the “I’m OK” signal is not sent and the spacecraft is reconfigured automatically as necessary to remain safe within the context of the identified anomaly.

Redundancy

Redundant components can provide backup capability in case a destructive SEE occurs.  In addition, redundant components run in parallel may provide a ‘voting’ capability where polling and comparison are performed to determine if components are working properly.

Example of SEE Mitigation

The Oriented Scintillation Spectrometer Experiment (sensitive to energies from 50 keV to 10MeV) on CGRO carries a separate charged particle monitor  (CPM) detector. The event rates in the CPM detector provide a monitor of the high-energy charged particle environment for OSSE.

Additionally, the CPM provides detection of the spacecraft entry into the South Atlantic Anomaly (SAA).  The spacecraft is designed to turn off the OSSE detectors during traversals of the SAA; this charged particle monitor, however, remains on to provide integral charged particle dose monitoring for background modeling.

High-Velocity Impacts

One danger that faces all spacecraft, especially in Earth orbit, is the possibility of a high-velocity impact with a piece of matter, which can be either of artificial origin (e.g., space debris) or natural (meteoroids). If a satellite is unlucky enough to be in the wrong place at the wrong time, the damage that will be done will depend on the impact velocity, size/mass of the particle, the angle of strike, and the material composition. We’ll examine the nature and likelihood of these threats, then look at the damage that the impact can cause.

Space Debris

Space debris, also called space junk, is an artificial material that is orbiting Earth but is no longer functional. The source is usually from spent rocket boosters, defunct satellites, or pieces of these objects after an explosion or break-up, as well as pieces discarded (intentionally or unintentionally) by spacecraft during missions. Satellite collisions (either unintentional or as a result of an anti-satellite test) can produce hundreds or thousands of pieces of space debris.

Due to high impact speed in space (~10 km/sec in LEO), even sub-millimeter

debris pose a realistic threat to human spaceflight and robotic missions

10 km/sec = 22,000 miles per hour (the speed of a bullet ~1,500 miles per hour)

• The mission-ending threat is dominated by small (mm-to-cm sized) debris impacts
• Total mass: >8000 tons LEO-to-GEO (~3000 tons in LEO)

Damage Caused by Impacts

Debris can cause a range of damage. Man-made particles less than 1mm in diameter will not penetrate a spacecraft’s skin but may damage the surface they impact. Natural particles even below 1 mm in diameter may penetrate into a spacecraft, creating a plasma that can result in charge/discharge phenomena. The different types of impacting debris damage are:

• Spallation
• Cratering
• Penetration
• Cracking
• Arcing
• Fragmentation
• Erosion
• Catastrophic

Spallation

In addition to the damage that can be done by impacting debris itself, spacecraft are also susceptible to particles created by spallation. Spallation is the process of material from the satellite itself being flaked/chipped off and energized by the impacting object. Spallation is more prevalent in impacts from micrometeoroids where the high impact velocities bring the particle in with a great deal of energy. These ‘secondary’ particles can spread out and cause additional damage.

Spallation particles can be created by:

• Cratering:  In this case, the impacting particle does not break through the surface but does knock loose and energize spacecraft material on the inside surface of the impact zone.
• Penetration:  In this case, the impacting particles pass through the material but some of the energy is transferred to the surrounding spacecraft material (which flakes/chips off).

Cratering

Typically seen when LEO satellites are impacted at low impact velocities by small  (<1mm) man-made debris particles Although the impact may not damage the structural integrity of the impacted surface, it may spall off secondary particles and shock waves which can then damage internal spacecraft components.  The impact may also change the properties of the impacted surface:

• Thermal Changes: Debris may cause de-lamination and removal of surface coatings well beyond the initial impact sight.  (Approximately 5% of the LDEF thermal control coatings were removed over the 6-year mission.)
• Optical Changes: Debris in the 10 to 100-micron range can significantly alter optical properties.

Penetration

If a particle is able to penetrate the exterior skin of a satellite, it will probably enter the satellite in a fragmented or liquified state over an area much larger than the original penetration sight.

For example, if a 1 cm diameter aluminum sphere strikes a typical  0.5 cm thick aluminum spacecraft wall at 10 km/s, the impact would fully melt and partially vaporize the impactor and create a 2.7 cm diameter hole.  Once inside the spacecraft, this material would apply enough pressure on interior components to destroy almost everything within 15 cm of the surface. Even particles down to 0.75mm in diameter impacting at 10 km/s on a solar array or high gain antenna drive housing could cause spallation that could potentially jam the motor.

Cracking

Impacts on optical surfaces can result in cracking. Solar cell cover glass is particularly susceptible to this type of damage since it provides a large potential impact area. Cover glass cracking can significantly reduce the transmission capability of the glass, resulting in reduced power generation capability. Man-made objects (nuts, bolts, paint chips, etc.) are usually the cause of this kind of damage.

Arcing

Small particles that either penetrate or crater (with spallation created) the surface of a spacecraft may generate plasmas that can cause internal spacecraft component charging and arc discharge phenomena.  This phenomenon may be seen with particles penetrating the spacecraft body or with particles impacting the solar array surfaces. The high impact velocities associated with natural debris are the primary cause of this anomaly.  In fact, this is the most significant risk posed by meteor shower particles.

Fragmentation (and Catastrophic Impact)

Through modeling and impact tests it has been estimated that if the ratio of the kinetic energy of the impacting debris to the mass of the target satellite is 40 J/g, the impacted satellite will totally break apart.

For example, a 0.1 kg piece of debris impacting at 10 km/s would probably not completely break apart a 400 kg spacecraft, BUT, a 0.5 kg piece of debris at the same impact velocity or the 0.1 kg debris impacting at 13 km/s probably would (break apart the spacecraft). Complete fragmentation is also dependent on where the debris strikes the spacecraft.  If, for example, the debris strikes a solar array, probably only the array would be destroyed.

Tracking the Debris

NASA’s primary method for identifying orbital debris between 1 and 30 cm in diameter is by using ground-based radar and optical telescopes. Inspection of spacecraft components returned to Earth (or photographs taken in space) can provide some idea of the population of even smaller debris particles.

Other Sources for High-Velocity Impacts

The other main source of spacecraft impacts is meteoroids.

Interesting Fact:

What is the difference between a “meteoroid”, “meteor”, and “meteorite”? Many people think they are just interchangeable words for the same thing, but they are wrong. Here are the definitions:

A meteoroid is a small rocky or metallic body in outer space that is significantly smaller than an asteroid and varies in size from a few millimeters to about a meteor. Smaller than this they are known as “micrometeoroids.”

A meteor is a meteoroid that has entered and is passing through the Earth’s atmosphere that leaves a trail as it burns up and is commonly known as a “shooting star.”

A meteorite is the remains of a meteor that has landed on the Earth and is now a special type of rock. Most of the meteors burn up before they reach the Earth’s surface, but the larger ones can survive to become the rock known as a meteorite. By analyzing these rocks, scientists can tell if they are of Earth origin or from outer space, even from another planetary body such as the Moon or Mars. With the latter, sometimes they will find small pockets of Martian gases trapped inside the meteorite.

Meteroids and Micrometeoroids

• Average Velocity:  20-40 km/s
• Average Impact Velocity:  70+ km/s
• Average Size:  <1 cm diameter (therefore the term “micrometeoroid”)
• Location:  scattered through space – they begin to burn up as they enter the Earth’s atmosphere at ~80-110 km altitude

Meteor Showers

The meteoroids that pose the greatest risk to satellite operations are ‘shower’ meteoroids since they have a much higher flux density (compared to sporadic meteoroids) although this increased density only persists for the duration of the ‘shower’ (hours/days). These small, high-velocity meteoroids are the debris left behind by a comet as it passed near the Sun. The meteor ‘shower’ (or ‘storm’ if the density is very high) results from the Earth passing through one of these cometary ‘trails’. Meteor showers occur each year with yearly variations in intensity based on the location of the Earth in comparison to previously deposited trails.

Yearly meteor showers that usually provide a significant meteoroid flux at Earth orbit (with 1999 peak flux data) include:

Leonids

During a meteor shower, the perceived focal point where the meteors seem to be emanating from is in the constellation that gives the shower its name. The meteor shower that has received a great deal of publicity over the past few years is the Leonids. Leonid showers are caused each year by the passage of the Earth through the area in space where the comet Temple-Tuttle deposited debris during one of its passes through the solar system.

These debris trails start out fairly dense and compact, but over a period of years, they broaden out and diffuse as they are affected by the gravitational pull of the sun and planets (particularly Jupiter). A Leonid meteor ‘storm’ can occur every ~33 years as the Earth’s trajectory takes it through a close approach with recently deposited debris trails.

Zenith Hourly Rate

The term Zenith Hourly Rate (ZHR) refers to how many light trails a viewer would see if they were looking directly at the zenith for one hour. (A meteor ‘storm’ is defined as a shower with a ZHR of >1,000.) During the Leonid storm of 1966, the flux of meteors entering Earth’s atmosphere was estimated to have peaked at ~200 meteors/second.

Where to Get Information About Meteor Showers and Orbital Debris

To calculate the expected Leonid flux in your satellite’s orbit, you can use the Leonid fluence calculator located at NASA’s Marshall Space Flight Center Space Environments and Effects office at http://see.msfc.nasa.gov/

You will need to fill out a form to be given access to the downloadable version of the fluence calculator.  (The form ensures that the user is working within the United States for a U.S. company/federal agency.)

Given a spacecraft’s orbital parameters and skin thickness, the Leonid fluence calculator provides information on exposure time (per orbit) and the flux rate of Leonids large enough to penetrate the spacecraft (during the peak flux period).

Information on changes in the orbital debris environment can be found at the website of NASA’s Orbital Debris Program Office at https://orbitaldebris.jsc.nasa.gov/

Designing Your Spacecraft for High-Velocity Impacts

Modeling is done prelaunch to determine the primary spacecraft risk areas to determine where shielding or other measures might be employed to reduce overall risk.

To reduce the risk of orbital debris damage to the Space Shuttle front windows and thermal radiators, the orbiter flies as often as possible with the tail forward and the payload bay doors partially closed.  This configuration does not protect against micrometeoroids (perceived to be a much smaller risk) that may impact the orbiter from any direction.

Mission Design Considerations for Orbital Debris and Impacts:

Spacecraft Charging

‘In-space charging effects are caused by interactions between the in-flight plasma environment and spacecraft materials and electronic subsystems. Possible detrimental effects of spacecraft charging include disruption of or damage to subsystems (such as power, navigation, communications, or instrumentation) because of field buildup and electrostatic discharge (ESD) as a result of the spacecraft’s passage through the space plasma and high-energy particle environments. Charges can also attract contaminants, affecting thermal properties, optical instruments, and solar arrays; and they can change particle trajectories, thus affecting plasma-measuring instruments.’ [Yuen]

Absolute Charging

The spacecraft has an electric potential that is at a different level than the potential of the plasma through which the spacecraft is traveling. Effects can include:

• Enhance surface contamination which can degrade thermal properties
• Compromise scientific instruments trying to ‘measure’ properties of the space environment

Differential Charging

Different surfaces of the spacecraft are at different potential levels. This may result in an electrostatic discharge (ESD) ‘arcing’ between areas of different potential.  Arcing may result in:

• Physical material damage
• Electromagnetic interference (EMI)
• Long-term degradation of exterior surface coatings
• Vehicle torquing/wobble

Spacecraft charging is a predominant factor for:

• Satellites outside the Earth’s magnetosphere where charged particles from the sun arrive at the spacecraft unimpeded
• GEO satellites where magnetic substorm particles can easily reach these altitudes.  (Remember, in some cases the magnetosphere can be pushed toward Earth by magnetic storms, exposing GEO satellites to the full solar particle flux and greatly increasing the potential for spacecraft charging.)

Although less significant of an issue, LEO spacecraft can also be charged by ions trapped in the ionosphere or by collision with micrometeoroids.

Sources of Spacecraft Charging

The charged particles that cause spacecraft charging are typically lower in energy than the particles that cause SEEs (from a few 10s of keV to ~3 MeV). Primary sources of spacecraft charging include:

• Magnetic Substorms: Inject large quantities of 5-50 keV electrons into geosynchronous orbit
• Coronal Mass Ejections: Some of the electron flux that arrives a few days after a CME is low enough in energy to produce charging
• Coronal Holes: Produce large quantities of low-energy electrons
• Micrometeoroids: Small, high-energy particles create plasma when they strike a spacecraft.  (This will be covered in the ‘Debris Impact’ section.)

Categories of Spacecraft Charging

Spacecraft Charging Can Be Divided Into Three Categories:

• Surface Charging
• Internal Charging
• Low Altitude Charging

Surface Charging

Surface charging is caused by:

• Low Energy Plasma
• Geomagnetic substorm electrons 1 – 100 KeV
• Photo  Emission electrons resulting from impingement of Solar EUV and x-ray radiation

Surface Charging Risk Locations/Periods

• During/after geomagnetic substorms causing injection of 1-100 KeV electrons into the magnetosphere
• LEO (low risk):  Orbit midnight through orbit dawn
• Polar orbits <1,000 km
• Primarily a concern in GEO orbit
• Spacecraft that have periods with large amounts of self-shadowing
• Spacecraft that pass near one of the large outer planets (Jupiter, Saturn, Uranus, Neptune)

While in eclipse, the spacecraft may charge to tens of kilovolts as electrons ‘stick’ to the surface

In sunlight, illumination of the vehicle’s skin dislodges electrons from the surface (photoemission) and the skin develops a relative positive charge. The electrons may form a negative plasma cloud around the vehicle. If the spacecraft surface is well-grounded then absolute charging will result.

If the spacecraft surface is not well-grounded (it may have started out grounded – but – something like a protective blanket may have become detached from the grounding system while in orbit) and if some areas of the spacecraft are in shadow then differential charging will result. In either case, a discharge may be initiated by either a change in solar illumination, a change in the particle environment, or onboard electrical activity.

Internal Charging

High energy electrons are very dangerous to satellite operations since they can be extremely energetic (> 1 MeV) and can bury themselves deep inside a spacecraft in the dielectric materials that compose much of a satellite’s electronic components. If the electron-induced charge on these dielectric materials builds up faster than the charge can dissipate, differential charging of the material versus its surroundings can result in a discharge.

Electrical activity on the satellite (which may, in many cases, result from commanding) may provide the trigger to initiate a discharge. High energy electrons are very dangerous to satellite operations since they can be extremely energetic (> 1 MeV) and can bury themselves deep inside a spacecraft in the dielectric materials that compose much of a satellite’s electronic components. If the electron-induced charge on these dielectric materials builds up faster than the charge can dissipate, differential charging of the material versus its surroundings can result in a discharge.

Electrical activity on the satellite (which may, in many cases, result from commanding) may provide the trigger to initiate a discharge.

Internal Charging Examples

In January of 1994, two Canadian comms satellites, Anik E-1 and Anik E-2 experienced problems within a day of each other after having been exposed to high-velocity electron flow. Both satellites suffered momentum wheel control circuitry anomalies.

Anik E-1 was recovered in 8 hours but it took six months to recover Anik E-2. Investigations attributed the spacecraft anomalies to spacecraft internal charging/discharging.

In May of 1998, NASA’s SAMPEX and POLAR satellites identified a large increase in solar wind electrons produced by a number of CMEs. There were a number of anomalies (dates shown in the following figure).  The loss of attitude control on the GEO Galaxy 4 communication satellite (knocking out 90% of all US pagers) occurred during this electron flux event.

Low Altitude Charging

Although spacecraft orbiting at low altitudes must also be concerned with charging, the plasma they move-in has been historically more difficult to characterize analytically than the GEO environment. Analysis of limited spacecraft data points to spacecraft charging of between ~200 and 500 V (the minimum thought to be required for arcing to occur) being fairly common in LEO polar-orbiting spacecraft. This charging of polar-orbiting spacecraft is also tied to increases in solar activity (specifically, increased auroral activity and increased ionospheric density). The charging that occurs is the same type of surface charging that occurs in GEO satellites (predominantly differential charging between areas in/out of shadow). There has also been some analysis of differential internal charging in LEO satellites at all latitudes where high voltage power sources (>40kV) were used.

Spacecraft Charging Mitigation

Although there is seldom much-advanced warning of geomagnetic substorms, it is possible to get advanced information on CMEs and Coronal Holes that may be pumping low-energy particles into the solar wind.  (Remember, surface charging is predominant when the particle energies are <100 keV, while internal charging occurs at the 100 keV to ~3MeV particle energy levels.). It is also a good idea to assess the susceptibility of your spacecraft to surface charging resulting from the spacecraft design and operational profile (orbit/shadowing). If large geomagnetic storms are occurring/predicted, it may be a good idea to turn down/off high voltage devices (>100V) to reduce the risk of discharge arcing (especially in polar & GEO orbits).

For polar-orbiting satellites, operations engineers should monitor solar activity, ionospheric activity, and auroral activity. Constantly updating plots of auroral activity for both poles (in ergs/cm2/s) is available at the NOAA Space Weather website: https://www.swpc.noaa.gov/

Spacecraft charging, arcing

http://assets.press.princeton.edu/chapters/s9500.pdf

https://www.spenvis.oma.be/help/background/charging/charging.html#SPI

From Akin:

Planetary environments – deep space?

• Temperature
• Electromagnetic radiation
• Gravitation
• Atmospheric particles
• Newtonian flow
• Solar wind particles
• Ionizing radiation
• Micrometeoroids/orbital debris
• Spacecraft charging

From space environment implications for spacecraft design:

Effects on the Spacecraft

Besides the physical effects that the space environment has on the spacecraft as we have discussed, most of the factors we have mentioned impart a force on the spacecraft, which is usually free to rotate, so the forces are normally experienced as torques on the spacecraft and are called disturbance torques. This is a measure of how much the spacecraft is affected by the environmental factor. The effect of the factors depends on the altitude at which the spacecraft is flying. Some factors, such as atmospheric molecules, have the strongest effect at low altitudes, while at higher altitudes, solar pressure becomes predominant. The figure shows the relative magnitudes of the disturbance torques at different altitudes.

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