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.


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.

Space weather for space situational awareness

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.


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 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.


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.


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 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.


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.


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.

Space radiation classification

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.