Overview

To say that the space environment is not well understood is an understatement. Serious scientific investigation of space has been underway for less than 50 years, and only a few hundred people have visited the shallow sphere of space we call low Earth orbit. Just 27 men have ventured beyond Earth orbit to the Moon, a mere 300,000 km distant, and only four unmanned spacecraft have left the confines of the solar system.

So, how should we define the space environment? As with all definitions and certainly with definitions of the Earth's environment it depends on one's viewpoint. For example, from a geomorphological perspective the Earth can be said to have many different environments: polar, tropical, and temperate; highland, lowland, and subterranean; land, sea, and air. Likewise, from a social perspective, different areas of the Earth might be described as rural or urban; natural or manmade; uninhabited or overpopulated.

However, if we confine ourselves to simple definitions of the word environment, which include "surroundings" and "external conditions," we find that these are readily transferable to space.

Surroundings

In simple, admittedly geocentric terms, the space environment can be defined as that part of the universe beyond the Earth's atmosphere (because the atmosphere is considered part of the Earth's environment). Of course, in universal terms, the Earth is just another planetary body and is therefore part of the space environment. That aside, the geocentric definition of the space environment depends on where the atmosphere ends and space begins.

To assist that decision, it...