Exochemistry is defined here as "the science of how to perform chemical reactions and processes in space". It is a necessary prerequisite for the full exploration and development of other planets and their satellites (referred to henceforth as "moons"). It must be emphasized that this subject is distinct from the traditional study of the "chemistry of space" (including astrochemistry and cosmochemistry), which involves simply observing natural chemical phenomena that occur in space and providing scientific explanations for them.

At present humankind has very little knowledge of exochemistry, but the following guiding principles are offered here:

1. Resources should be obtained from local sources whenever it is efficient to do so.

This principle, often referred to as "In Situ Resource Utilization" or ISRU, is already widely accepted by scientists studying space exploration. One example is the manufacture of rocket propellant on Mars, to supply fuel for the return leg of a trip to Mars. In many cases, the raw materials we need are available at or close to the planet or moon we are visiting. As of 1995, it cost around $13 million to launch one tonne of material into low Earth orbit (LEO), and still more to transport it to some distant planet. If a small automated chemical plant were put in place on the planet, it could provide many tonnes of material for many years to come, for very little cost by comparison.

2. Chemical processes should be designed for the local environment.

Clearly, the environment on the surface of the Moon or of Titan is very different from that of Earth. We must design processes (from scratch, if necessary) that are totally adapted for the environment in which it is to be run. In many ways, this can be regarded as an extension of the terrestrial concept of "green chemistry", whereby chemical processes on Earth are completely redesigned to fit in with the Earth environment.

Why should we not simply try to adapt our existing terrestrial processes? In some cases this may be appropriate, but these processes were designed using certain assumptions. These include the following: limitless supplies of water and elemental oxygen (O₂), plentiful supplies of organic alkanes (oil), elemental carbon (coal) and energy (chiefly as heat), as well as Earth's gravity, atmospheric pressure and "room temperature." Even on our nearest neighbor the Moon, few of these assumptions are valid. If we had always lived on the Moon, we would naturally design our processes for the Moon, because in that way we would make the most effective use of the Moon's resources. Therefore, if we plan to use our limited access to the resources of other worlds wisely, we should make sure our methods are designed for those worlds, not for our own.

3. We should expect the unexpected.

Although we regard our knowledge of chemistry as fairly comprehensive, this really only applies to conditions close to those typically found on Earth. The further we venture away from these conditions, the more likely we are to encounter strange new phenomena. We have encountered superconductivity and Bose-Einstein condensates, but who knows what other bizarre phenomena may be found when we fully explore chemistry in unusual environments? What is liquid metallic hydrogen like as a medium? How does a torrent of plasma at 10⁶ K affect the matter it encounters? We know a small amount about these scenarios at present, but we will no doubt discover completely new concepts once we develop ways to perform chemical reactions in such environments.