OVERVIEW
The space around the Earth is populated by a variety of electromagnetic radiation and particulate matter from atomic nuclei through man-made objects to large natural material such as meteoroids, asteroids and comets. Some of these objects, particularly artificial satellites and debris, may be in orbit around the Earth while other objects may be passing through the near-Earth region.
Space situational awareness (SSA) refers to a knowledge of this material and radiation with particular reference to the effect it might have on the Earth, the near Earth environment and artificial satellites we have launched into space. It also refers to operations conducted in space that could affect operational satellites.
We might note that the above definition is that normally accepted by the European community whereas the USA more commonly restricts the definition to man-made objects orbiting in and around the Earth. As space weather in particular has the capacity to damage satellites just as much and on a larger scale than collisions between satellites or debris we feel the broader definition is more appropriate. We thus refer to the SSA triad.
We might also refer to the statement by a senior US Air Force General who stated that useful SSA should not only be concerned with what is in orbit at the present time but what will be in orbit in the future. If we accept this we must also accept that human intelligence (HUMINT) is just as important as imagery intelligence (IMINT).
ORBITAL SPACE OBJECTS
Also referred to as Resident Space Objects, OSO are currently the most studied group of the triad because of the generation of space debris and its potential effects on our activities in Earth orbital space. In particular, the Kessler syndrome whereby increasing collisions could lead to a chain reaction in which orbital space, particularly in the low Earth orbital regime (LEO) , becomes unusable.
However, collisions are not the only factor that leads to the production of space debris (see diagram below).
The magnitude of the problem can be grasped when it is realised that the launch of a large satellite can produce over 100 items of debris larger than 10 cm in size and the collision between two satellites can result in several thousand debris fragments larger than 10cm. This, together with the knowledge that a 10 cm debris fragment is sufficient to totally destroy an active satellite in a collision, and a one cm object will result in serious damage.
Currently, a 10cm object can be detected by continuous space surveillance systems, but a 1cm object is usually below the detection limit. Radar and optical detection of OSO is essential to enable and update a catalog of orbital objects for collisional assessment (CONASS) and where possible collision avoidance (COLA).
The current status of the near Earth orbital environment for objects >10cm in size is shown by the graph below produced by the NASA Orbital Debris Program Office (ODPO).
A large number of space sensors, including many in Australia, are used to track OSO for SSA purposes. Various types of radar and optical sensors are shown below.
Collisions can cause damage and increase the population of space debris but they are not the only problem associated with space debris. In Space Science OSO can be mistaken for or mask other phenomena. In astronomy space debris can ruin long exposure optical images, cause transients in radio telescopes and over long periods of time increase sky glow and hide distant and faint objects. There is also a minor problem from the reentry of large space debris (as well as large natural objects) into the atmosphere and Earth’s surface.
SSA AND SPACE WEATHER
The diagram below shows the range of effects from the Earth’s environment, the Sun, the Solar System and the Galaxy which affect the Earth’s orbital environment and the objects that orbit in it.
This graphical summary of space weather also includes the macroscopic bodies of the solar system which include NEOs (Near Earth Objects) and the study of these is undertaken under the heading of Planetary Defence which is now under the umbrella of the United Nations IAWN (International Asteroid Warning Network).
SUMMARY
A comprehensive awareness of the space around the Earth is quite formidable and covers a range of disciplines. The monitoring and analysis of this environment is thus normally the responsibility of various organisations. Comprehensive surveillance of the Earth orbital space and of space weather has been the province of the military in the USA until recently but this is beginning to change. Unfortunately comprehensive CONASS is only possible with open access to all sensed orbital objects down to 1 cm in size and this is currently impossible due to sensor and intelligence limitations.
Australian Space Academy