INTRODUCTION
Space Situational Awareness (SSA) in its truest sense refers to a knowledge of our near-space environment, and thus includes a knowledge of space weather (the natural component of the space environment). Space weather affects systems both in space and on the ground. In this note we will concentrate on the aspects of space weather that affect space based assets and their products. We will consider both the positive and negative sides of space weather - that is, the benefits and opportunities as well as the hazards and the threats.
OVERVIEW
A narrow view of space weather considers the Sun as the sole source of disturbances in our space environment. However, particularly with respect to SSA, there are most definitely extrasolar sources that influence the environment around the Earth. And as we move assets beyond Earth orbit we need to consider these even more.
The diagram below attempts to provide an overview of those aspects of space weather that are significant for SSA. It considers four sources that both form and modify the space environment. These are (1) the Earth, (2) the Sun, (3) the Solar System and (4) the Galaxy. Each of these, and their mutual interactions will be discussed in the following paragraphs.
Joe Allen, working at the US National Geophysical Data Centre (NGDC), has compiled a list of satellite anomalies. In the two decades from 1971 to 1989, just under 3000 satellite anomalies were recorded. It is not always easy to ascertain the cause of each anomaly, particularly as manufacturers are usually reluctant to admit any vulnerability in their products, but it is estimated that up to 25% of all spacecraft failures are due to the space environment. This is an indication of the significance of space weather on our space assets.
THE EARTH
Although the Sun is often quoted as the prime (or even only) source of space weather, its role is essentially to modify what already exists in the near-Earth space environment. Even without a solar influence the nature of the Earth's gravity, atmosphere, and magnetosphere influence what we can do in space.
The mass distribution that we call the Earth determines the gravitational field that exists around it. On the one hand the gravitational field makes it difficult for us to leave the surface of the Earth, but on the other it makes it possible for satellites to orbit the planet without any expenditure of energy.
For many purposes we can regard the Earth as a sphere, but the difference between a true sphere and the actual mass distribution of the Earth changes the elements of a satellite orbit so that orbital predictions would be useless unless this difference is taken into account. Satellites have helped refine our knowledge of the terrestrial gravitational field, which in turn have allowed more accurate orbital predictions.
Rotation
The Earth's rotation makes possible one particular very useful orbit - the geosynchronous orbit. This orbit, where a satellite appears to be stationary (or nearly so) with respect to a point on the Earth, means that the ground station does not have to track the motion of the satellite, a considerable savings in hardware and cost.
The geosynchronous orbit also lies at an altitude where approximately one third of the Earth's surface is visible. If the geosynchronous orbit was at a much lower altitude it would not be nearly so useful. The tradeoff, of course, is the higher power required for communication from this orbit.
 This low cost non-tracking roof mounted dish receives TV signals from a satellite in geosynchronous orbit. |
 Geosynchronous satellites can see about one third of the Earth's surface. This shows the overlapping coverage of several geosats. |
Atmosphere
The Earth maintains an atmosphere by virtue of its gravity, with a seconday role being played by its magnetic field, as will be discussed later.
The atmosphere has both positive and negative aspects with respect to space assets. On the beneficial side it allows the return of spacecraft to the Earth's surface with minimal energy expenditure. After an initial deorbit burn, the atmosphere can be used to absorb the large kinetic energy of the returning craft, and allows aerodynamic lift for a soft landing (or the use of a parachute if the reentry vehicle does not have aerodynamic surfaces) without the expenditure of any fuel.
Gravity and thermal pressure work to give the atmospheric density an exponential fall-off with altitude. By the time we reach a height of 20 km, 99% of the atmosphere lies below us, and at orbital altitudes we have a better vacuum than most research laboratories can achieve.
This lack of atmosphere gives rise to three main problems. A pressure differential between the interior and exterior of a spacecraft (particularly if pressurised for humans) gives rise to a mechanical strain. Thermal control is more difficult in space. Heat can only be transmitted by radiation. The vacuum is not conductive and the lack of gravity in an orbiting satellite does not allow convection. A totally different mechanism of dissipating heat from electronic equipment must be employed. Heat pipes to radiative surfaces are one solution.
The vacuum also allows outgassing of materials that can return to the spacecraft and contaminate surfaces. This is why many satellites are assembled in "clean rooms" on Earth.
Although most of the atmosphere is gone at orbital altitude, enough still remains to create drag at low Earth orbit, slowly reducing the orbital energy and altitude of a satellite. Either fuel must be carried and used to occasionally reboost the satellite to higher orbit, or a reduced lifetime must be accepted. The graph below plots the altitude of the International Space Station over one year, showing frequent reboosts.
The residual atmospheric atoms also collide with spacecraft surfaces at hypervelocities, which produces physical sputtering of the surface, chemical attack from atomic oxygen and a spacecraft glow from large surfaces that can interfere with low-light experiments.
Ionosphere
The ionosphere is created by the ionising effect of solar extreme ultraviolet (EUV) and X-rays on the Earth's upper atmosphere. This occurs between the altitudes of about 100 to 500 km. The ionosphere is a weak plasma - only about one percent of the neutral atmosphere is ionised, but this is sufficient to affect radio signals passing through it.
A stable ionosphere enables high frequency communications between stations on the Earth's surface, but for space assets the ionosphere, which shows a strong diurnal and other variations, displays only undesirable effects.
Transionospheric communication and navigation signals suffer delays and distortion when passing through the ionosphere. Refractive effects displace objects from their true positions for space looking radars. Satellites that use polarisation as a means of multiplying the number of channels in a given frequency band can experience polarisation rotation that couples undesirable power into the orthognal channel.
The ionosphere is a dispersive medium and the magnitude of the distortions varies with frequency. Higher frequencies are affected less than lower frequencies. C-band frequencies are only infrequently affected, and higher bands are usually immune to ionospheric effects.
Geomagnetic Field
The Earth's magnetic field has two components. The internal component is generated within the Earth's core by movements of molten conductive material (mostly liquid iron). The external component is generated by particles circulating in orbit. The internal component shows a slow variation with time, on the order of years. The external component is subject to large variation as the solar wind and coronal mass ejections make rapid changes in the particles injected into the Earth's magnetosphere.
The internal component of the geomagnetic field is several orders of magnitude larger than the external component. Over mid-latitude Australia, the internal component is around 50,000 nanoTesla, whereas the largest external component rarely exceeds 500 nT.
Some space assets use the Earth's magnetic field for attitude stabilisation, and/or for gross locational determination. Tethers can be used to generate power from the field and may be useful to deorbit space assets at the end of their life, reducing the proliferation of orbital debris.
Radiation Belts
The Earth's magnetic field traps charged particles from the Sun and those of galactic cosmic rays, forming radiation belts around the Earth. These belts were discovered in 1958 by a radiation detector on the first US satellite. This was due to the space physicist James van Allen, and the belts are often known as the Van Allen radiation belts.
There are two main belts. The outer belt is mainly populated by solar charged particles. The inner belt is mainly populated by protons arising from the decay of albedo neutrons. These neutrons are produced by interaction of galactic cosmic rays with the Earth's atmosphere and then reflected back to space. They decay to positively charged protons with a half life of about 13 minutes.
The trapped radiation belts define the upper limit of low Earth orbit at around 2000 km. The outer belt ends (fortunately) just below geosynchronous orbit at 36,000 km altitude. The region between these two limits is one of high radiation levels that causes rapid deterioration of unhardened electronics and must be traversed rapidly by manned missions to the Moon or beyond to limit the radiation dose received. Global navigation satellites (such as GPS) that utilise this Medium Earth Orbit (MEO) regime must use radiation hardened electronics to survive in this environment. Gallium arsenide semiconductors replace silicon devices as they can withstand much higher radiation doses.
Because the Earth's magnetic field is centred about an axis that is offset and rotated with respect to the rotational axis, one part of the inner belt comes closer to the Earth than the rest. This occurs in a region to the south-east of South America and is termed the South Atlantic Anomaly (SAA). Low Earth orbit (LEO) satellites passing through this region experience higher radiation levels, and astronauts report seeing flashes when their eyes are closed. This occurs when ionising particles activate cells in the retina.
THE SUN
The Sun continually gives off both elecromagnetic and particulate emissions. It is convenient to divide these into quiet and active components. However, it is important to realise that the quiet component is not constant. It simply varies at a slower rate than the active component.
THE QUIET SUN
Emission from the quiet Sun spans the full electromagnetic spectrum. The particulate emission is usually referred to as the solar wind. Variations in both types of emission occur with periods of 11 years (the sunspot cycle period) and shorter term periods of around one month (the solar rotation period of about 28 days) and of several months (due to the growth and decay of solar magnetic features such as large sunspot groups and coronal holes). The period due to solar rotation is of course an observational artefact at the Earth's position as it orbits around the Sun.
Solar radio emission as detected at the Earth's orbit is measured in Solar Flux Units (SFU) where in SI units 1 SFU = 10-22 W m2 Hz-1. Solar radio flux increases with frequency, being about 10 SFU at 200 MHz and 500 SFU at 15 GHz. This variation is shown below.
The above graph indicates solar radio emission at the time of sunspot minima with no spots visible on the solar disc. Added to this is a slowly varying component which peaks around 3 GHz and can increase the flux at this frequency by 400% during times of large sunspot maxima. This emission does not come from the sunspots themselves, but from chromospheric plage associated with high magnetic fields.
The quiet radio emission from the Sun provides both an opportunity and a hazard. It is used as a calibration source for radars and microwave communication systems. This requires ground based calibrated solar radio telescopes that constantly monitor and report the solar radio fluxes.
It is also a source of interference to any electromagnetic system where the Sun appears in the beam. An example is the Sun outage phenomenon where geosynchronous TV downlinks are subject to this interference around the equinoxes, when the Sun appears behind a geosat for several tens of minutes over a period of a few days. The solar radio signal is typically 20 dB or so greater than the satellite signal, and no TV reception is possible during this time.
IR/Visible Radiation
Most of the solar electromagnetic output lies in the visible and infrared spectral bands. This is constant to within about 0.1% and at the Earth's orbit has a value of 1370 W m-2. It is thus a good source of power for space systems, and most satellites and spacecraft use solar photovoltaic cells to provide all their system power.
Infrared radiation from the Sun is also a factor in the temperature stabilisation of a satellite.
Since Peter Glaser's seminal paper in 1968, solar power satellites (SPS) have been investigated, whereby energy from the Sun is collected by huge arrays in space and converted to microwave energy for downlink to Earth. Although there are many technical and political hurdles to overcome before SPS can become a reality, the Japanese government has made it an integral part of their space policy.
UV/X-ray Emission
The extreme ultraviolet and soft x-ray emission from the Sun ionises the Earth's upper atmosphere creating a weak plasma known as the ionosphere. Different wavelengths of this emission interact with different atomic and molecular species in the atmosphere to produce the different ionospheric layers (D, E and F).
Ultraviolet radiation photons also have sufficient energy to release electrons from surface metals on satellites leaving them charged. If a differential charge of sufficent magnitude builds up between two different surfaces, a sudden discharge amy occur which can damage components in and around the discharge.
The Solar Wind
The Sun's outer atmosphere, the corona, has a kinetic temperature of 2 million Kelvin. The individual particle energies at this temperature can easily escape the large graviational field of the Sun and boil off into the interplanetary medium in a stream we call the solar wind.
In fact it is not the Sun's gravity but its coronal magnetic fields that control this outflow.
At the Earth's orbit, the solar wind consists of around 7 particles per cubic centimetre (7 million per cubic metre) travelling at an average speed of 400 km/sec. However, these values can vary by over a factor of two either way.
 |  Left: The Atmospheric Composition Explorer monitors the solar wind at the Earth-Sun Lagrange point (NASA). Above: The US Space Weather Prediction Center displays the critical solar wind data as dial pointers (NOAA/SWPC). |
The pressure produced by the solar wind modifies the shape of the Earth's magnetic field. Most of the time the magnetopause (the boundary between the solar wind and the Earth's magnetosphere) lies well outside geosynchronous orbit. However, during extreme conditions in the solar wind (usually produced by a transient coronal mass ejection) the magnetopause may be pushed inside geosynchronous orbit. This exposes geosats to a different particle and radiation environment and can cause problems to any satellite that relies on magnetic stabilisation to any extent. It has yet to be seen how such conditions will affect the new electric thrusters that will be used for geosat station keeping.
Compressed magnetopause position
(Images from IPS Radio and Space Services model)
The solar wind extends out and defines the far limits of the solar system or heliosphere. The heliopause is the boundary where the pressure of the solar wind is just countered by the pressure of the interstellar medium. Very high energy but very low frequency (~1 kHz) emissions are produced around this interaction. We cannot receive this emission at Earth because it is shielded by the plasma in the interplanetary medium. Distant spacecraft, such as Pioneer and Voyager, detect this energy.
THE ACTIVE SUN
The total energy emitted by the Sun varies by less than 0.1% and most of this energy is emitted in the infrared and visible parts of the electromagnetic spectrum.
However, this stability is not evident in either end of the spectrum. Both radio and x-ray emissions may vary in a transient fashion by up to six-orders of magnitude. This emission comes in bursts or flares and may last from minutes to hours.
The highest amplitude radio bursts occur at the lower frequencies. At 100 MHz, a very large radio burst may reach one million SFU, whereas at 20 GHz the largest bursts do not exceed 20,000 SFU due to a process known as synchrotron self-absorption.
Any space communication or navigation system that has the Sun in its beam (or even near the beam) during one of these large bursts will invariably be disrupted to some extent. In most case the link will fail completely. Fortunately, such events are rare and do not last long, hours at the most.
Before 2006 it used to be thought that the GPS global positioning system was immune to interference from even the largest solar radio bursts. GPS operates at L-band (1 to 2 GHz), and the largest solar bursts ever recorded in this band were around 100,000 SFU. It just so happens that calculations showed that this was about the threshold at which we might expect GPS degradation. And so it was thought that GPS was immune to solar electromagnetic interference (although it was well known that ionospheric irregularities in equatorial regions could cause signal dropout). GPS is more immune than most space EM systems because it uses spread-spectrum techniques that compress to a very narrowband of about 100 Hz in the receiving processor.
In December of 2006 the Sun surprised everyone by producing a series of extremely large bursts on two separate days. These bursts were estimated to have flux densities of one million SFU at L-band. Many GPS receivers in the sunlit sector of the Earth were affected by these bursts.
Although X-ray flares from the Sun cause additional ionisation in the lowest layer of the ionosphere (the D-region), this mostly affects sub-ionospheric communication, as the amount of additional ionisation is considerably less than that in the normal upper F-layer, which is the layer that usually affects transionospheric communication.
Apart from a slightly higher absorption (less than 1 dB) on space-ground circuits and the blinding of X-ray sensors on astronomical satellites, even the largest X-ray flares do not have the same deleterious effect as a very large solar radio burst.
Coronal Mass Ejections
The solar wind carries away from the Sun a mass of about one million tons (109 kg) per second. However, on occasion it can also spew out a large cloud of plasma with a total mass on the order of 5 billion tons. This can be seen using a white-light coronagraph on board a spacecraft, and is called a coronal mass ejection or CME. Occasionally a CME may be seen with a ground-based coronagraph on top of a high mountain, above most of the Earth's atmosphere which scatters sunlight to an extent that CMEs are normally invisible from the ground.
CMEs carry away solar magnetic fields that are trapped through the plasma and help keep it together.
These CMEs are millions of kilometres in extent and travel out from the Sun with velocities from about 200 to 2000 km/sec. If they are travelling toward the Earth their travel time may range from 1 to 5 days, although 2 to 4 is more common.
What happens when they arrive at the Earth depends on the direction of the magnetic field they carry along with them. If this field has a southward component, the CME will couple into the Earth's magnetosphere and large quatities of plasma particles will be dumped into the Earth's upper atmosphere. This creates a geomagnetic storm. This is seen by ground-based magnetometers.
During the storm the Earth's magnetic field will fluctuate wildy as particles move around the Earth with some precipitating to lower layers. A large storm may last for several days creating intense aurora, ionospheric storms and irregularities, neutral atmospheric density changes, spacecraft charging and other effects on space assets.
In March 1989, when a large CME caused one of the largest geomagnetic storms seen in 50 years, the US Air Force Space Surveillance Center in Colorado lost track of 30% of the space object population in low Earth orbit. What happened was that although the sensors still observed these objects, the geomagnetic storm had so increased the atmospheric density at LEO that the objects were not in their expected orbital positions. And so they became decorrelated. That is, the known space objects (active satellites and debris) could not be matched, or correlated, with the objects observed. It took the SSC three weeks to recorrelate the objects they had previously been tracking.
CMEs have the beneficial effect of shielding the inner solar system from the lower energy galactic cosmic rays.
Solar Energetic Particle Events
Every so often a large number of solar protons (hydrogen nucleii), together with a smaller percentage of heavier nucleii (mainly helium) are accelerated to energies of a few million electron volts (MeV) and even up to around 1 GeV (although this is rare). This acceleration seems to occur by at least three different mechanisms. It can be a direct acceleration near the Sun, associated with CME shock waves near the Sun and sometimes with CMEs out in the interplanetary medium.
Although Solar Particle Events (SPEs) rarely make it to the Earth's surface, occasionally a very high energy event (with protons to 100's of MeV) will produce what is then termed a Ground Level Event (GLE).
A lot more SPEs do affect space assets is Earth orbit. They can cause permanent degradation in solar cells, sensors and electronic equipment, and they pose a severe hazard to astronauts. Spacecraft in high inclination orbits are most affected as they receive little protection from the Earth's magnetic field in polar regions where the field lines are open.
One large SPE event in 1990 reduced the efficiency of solar cells in several communication geosats by up to 30%. This can represent $100 million per satellite in lost revenue.
On interplanetary trips, the highest energy SPEs represent a significant threat to humans, although because of their lower energies they are easier to shield than galactic cosmic radiation.
THE SOLAR SYSTEM
The main bodies of the solar system are the planets which orbit around the Sun, whose massive bulk provides the primary graviational field source controlling motion in the system.
However, there are numerous smaller bodies in the solar system. These include the asteroids and the comets and innumerable small pieces of rock and dust. Some of these pose hazards to heliospheric space operations, but others may offer opportunities in the way of material resources for future space operations.
The smaller bodies in the solar system usually come from pieces evaporated from comets or from asteroidal collision fragmentation.
The smallest particles are micron size pieces that are termed micrometeorites (or probably more correctly micrometeoroids) and collectively form a population known as interplanetary dust. The population of interplanetary dust particles (IPD) can be seen from Earth due to the sunlight they scatter. This is known as zodiacal light. It is seen most often and more clearly from near equatorial (tropical) sites. It appears during the evening (in the west) or morning (in the east) twilight hours as a cone of light stretching from the horizon, where the Sun has just set, up to sometimes 45 degrees in elevation. This is scattered light from IPD which lies mainly in the plane of the ecliptic between the Earth and the Sun. A much rarer phenomenon is the geigenschein which appears around the zenith at midnight. This is from IPD in orbit further away from the Sun than the Earth.
Micrometeorites are encountered as they pass through low Earth orbit, but the particle size is so small that impact damage is confined to very limited surface abrasion. Although a single particle thus does negligible damage, continued erosion of sensitive surfaces over time can reduce the efficiency of a sensor such as a telescope.
These particles also account for, along with meteoroid atmospheric ablation products, about 10,000 tons of matter which floats down to the surface each year. Some of this undoubtedly accounts for the nickel found on the ocean floor.
Meteoroids
Meteoroids are bodies from less than a millimetre to say 10 metres in diameter that orbit the solar system and occasionally are captured by the Earth's gravity. When they meet the atmosphere they burn up, producing the visual meteor phenomenon. A one gram meteoroid produces a meteor of around zeroth magnitude (the apparent brightness of our nearest star, Alpha-Centauri). At the beginning of the space age it was thought that meteoroids might pose a serious threat to space operations. This has not proved the case, although a Space Shuttle windscreen has been cratered by such an object. At the size of one centimetre diameter, where serious damage will result if a spacecraft suffers an impact, artificial space debris is now more prominent and a greater danger than natural meteoroids.
Asteroids
The dividing size between a meteoroid and an asteroid is not well defined, but could be considered to be about 10 metres. Asteroids are pieces of rock that vary from this size up to hundreds of kilometres in diameter. The smaller asteroids are much more prolific, with only a few thousand asteroids known or thought to exceed one kilometre in size.
Because of their small numbers, asteroids are unlikely to pose any significant impact hazard to spacecraft, even when they pass by the Earth below geosynchronous altitude. (See our analysis of this probability.) Asteroids are more likely to be useful as a source of raw materials in future space operations, and private companies have recently been established to explore this potential.
Comets
Comets are low density objects that, in contrast to asteroids, have highly elliptical orbits. It is thought that the source of comets is the Oort cloud, located right on the fringes of the solar system. It is estimated that this cloud contains about one billion comets. Occasionally, gravitational pertubations send one of these comets into the inner solar system. Some of these comets are cuptured by planetary gravitational fields (usually by Jupiter) and remain within the planetary system.
The US astronomer Fred Whipple in the early 1950's described a comet as being a 'dirty snowball', indicating the presence of a large quantity of ices, including water ice. It has been suggested that comets may have delivered water to the surfaces of planets in the past and that they could be used in the future as sources of water for large scale space operations.
Comets are rarer than asteroids and their capture, due to their highly elliptical orbits, will be much more difficult than asteroidal capture.
THE GALAXY
To most people concerned with the practicalities of space operations and space assets in the near-Earth space environment, the Galaxy (Milky Way) seems too distant to have any relevance to space situational awareness. This, however, is not so. The galaxy provides a vast playground of immense size where even small magnetic fields and gas clouds can accelerate masses of sub-atomic particles (mostly protons) to immense energies such that they provide a radiation hazard to space operations around the Earth and in the solar system. In fact, Galactic Cosmic Radiation (GCR) probably is the single most important limiting factor to human space travel beyond Earth orbit.
On the Earth's surface we are exposed to a continuous low level of ionising radiation. Some of this comes from materials that compose the terrestrial regolith, but some of it comes from space. This was proven by the Austrian physicist Victor Hess in 1911, when he took a very simple radiation detector up in a balloon. He found that for a few hundred metres the radiation level decreased, but that as he continued ascending the dose rate increased substantially. In fact, at the levels flown by commercial airliners, the radiation levels are 20 to 50 times that experienced on the surface. Although not a problem for passengers, air crew who frequently fly certain routes need to have their radiation dose monitored. Some countries have put laws in place which limit flying hours based on radiation dose. In space, the radiation dose received by astronauts is often well above the maximum levels accepted in industry.
Space radiation comes from three sources, (1) radiation trapped in belts or zones by a planetary magnetic field (eg the van Allen belts), (2) infrequent transient bursts of solar energetic particles and (3) galactic cosmic radiation.
Galactic cosmic rays are a relatively constant source of radiation, no matter where you are in the solar system, and have the highest energies of any of the three sources of space radiation.
Galactic cosmic rays are mainly protons with energies of several hundred MeV to over 1021 electron volts. Not even the largest particle accelerators on Earth (eg the Large Hadron Collider) can reach these energies. Particles are accelerated to these energies by galactic magnetic fields over 100,000 light year distances, and by interations with interstellar shock-waves associated with large plasma clouds. A small percentage of GCR are alpha particles and nucleii from atoms of higher atomic mass.
Galactic cosmic rays are not present in large enough numbers to be a bulk hazard to spacecraft materials and electronics, but an individual particle has enough energy to cause a single event upset (SEU), usually in semiconductor material. And humans are more sensitive to radiation than inorganic materials and so GCR does pose a considerable threat and even a limitation to human presence in space.
An SEU happens when a GCR particle, usually a proton in space, passes into a piece of semiconductor material - a transistor or integrated circuit which contains many active circuit elements.
The particle may pass through the material, creating a limited amount of ionisation in its path, or it may deposit all its energy near the transistor elements. When this occurs a large number of charge carriers will be created and an undesirable transient current may flow. This will create an unexpected signal in the processor or memory or other circuitry in which the transistor resides. In memory elements, what is known as a bit-flip may occur, wherein a 'one' may be changed to a 'zero' or vice versa. These upsets are usually transitory, but if they occur at the wrong time, they may be interpreted as a rogue command to the spacecraft. In the worst cases they may cause a signal latch-up which requires that power be removed from the system to correct the problem. This may or may not be possible. Several cases of total loss of a spacecraft (including a Russian Phobos mission to Mars) have been attributed to this mechanism.
On the surface of the Earth, humans are mostly shielded from GCR by the atmosphere and the geomagnetic field. In low Earth orbit the GCR dose is much larger, but the magnetosphere stills provides a measure of protection. Outside this protection the GCR dose is higher still.
The interplanetary magnetic field does prevent the lower energy GRC particles from making it to Earth orbit. The amount of shielding provided depends on the intensity of the magnetic fields carried out into the solar system by the solar wind and coronal mass ejections. The lowest energy of GCR which makes it to Earth orbit, and thus the total GCR intensity shows a variation from about 300 MeV to 1 GeV with a period of around 11 years, an inverse variation to the sunspot cycle. That is, when solar magnetic activity is at a minimum, GCR intensity is greatest, and vice versa.
The NASA Mars Science Laboratory (MSL) carried a radiation detector on its six month voyage to Mars. The following graph shows the radiation levels encountered during this time.
The constant background of about 350 microgray per day is due to GCR. The five peaks are due to solar energtic particle emission. Most SPE radiation is transitory and does not add significantly to the total dose (although large SPE events do). NASA/JPL estimate that the human dose equivalent on this six-month journey was 300 milliSievert (mSv). Allowing for a return trip and maybe a year on the Martian surface, the total dose to an astronaut would be around 1.2 Sv. This is 25% of the acute lethal dose that will kill 50% of a human population in 30 days (the LD50-30). GCR is probably the single most limiting factor to human space travel outside the Earth near-space environment.
CONCLUSION
There are many aspects of the natural space environment that must be considered for safe and reliable operation of space assets. Some aspects of the environment may be utilised to advantage and some must be mitigated. The most dangerous dimension of space weather is the radiation from high energy particles. The diagram below illustrates the various sources of space radiation.
High energy solar particle events pose the greatest hazard because of the high fluxes possible. The danger is to hardware and to humans. Low energy particles from coronal mass ejections are probably the next most hazardous space weather condition, and these occur significantly more frequently than SPEs. Galactic cosmic rays can cause spacecraft upsets (SEUs) and are definitely a limiting factor in long duration human presence in space.
At the current time, space weather poses a greater economic hazard to space operations than does the artificial space debris population in Earth orbit. This may change in the future as that population increases.
REFERENCES
* Adolph A Jursa (Editor), Handbook of Geophysics and the Space Environment, Air Force Geophysics Laboratory, National Technical Information Service Accession number ADA 167000 (1985)
* Alan C Trimble, The Space Environment - Implications for Spacecraft Design, Princeton University Press (1995)
* Daniel Hastings & Henry Garrett, Spacecraft - Environment Interactions, Cambridge University Press (1996)
* Henry B Garrett & Albert Whittlesey, Guide to Mitigating Spacecraft Charging Effects, Jet Propulsion Laboratory (2011), [Available from JPL]
Web Sites
* IPS Radio and Space Services: space weather for satellite operations from the Australian Space Weather Agency.
* European Space Agency: space situational awareness space weather straight from the Space Pole in Belgium.
* Space Weather Prediction Center: US source of space weather data with many space sensor feeds.
Australian Space Academy