A substantial fraction of our data on many Solar System objects has been obtained by close-up studies conducted by spacecraft. This Appendix starts with a short section on rocketry (how a rocket works). Section F.2 contains tables listing many of the most significant lunar and interplanetary spacecraft and astronomical observations in space. This appendix further includes diagrams of two historically significant spacecraft (Figs. F.1 and F.5), and two historic images (Figs. F.2 and F.6).

Rocketry

The principles of ‘rocket science’ are actually quite simple, although many practical aspects of ‘rocket engineering’ are far more complicated. A rocket accelerates by expelling gas (or plasma) at high velocity. Conservation of momentum implies that the velocity, ν, of the rocket of mass M (which includes propellent), expelling gas at velocity νexp and rate dM/dt satisfies:

where Fext accounts for all external forces on the rocket. Equation (F.1) is known as the fundamental rocket equation.

In a uniform gravitational field that induces an acceleration gp with no other external forces, the rocket equation reduces to

Integrating equation (F.2) and setting v = 0 at t = 0 gives

where M0 is the mass at t = 0, and there is a minus sign in front of the last term in equation (F.3) because the gravitational force is directed downwards. Note that there is a premium to burning fuel rapidly – the shorter the burn time, the greater the velocity for given ejection speed and mass.