The space shuttle main engines, among many other rockets, use the very simple chemical reaction between hydrogen and oxygen to produce thrust. When launched, the shuttle's main engines burn 530,000 liters (-140,000 gallons) of liquid oxygen and 1,400,000 liters (-380,000 gallons) ofliquid hydrogen. This, combined with the orbiter's solid rocket motors, produces enough thrust force to lift the shuttle's 2-million-kilogram (4.4-million-pound) weight from the surface of the Earth to low Earth orbit. These two simple chemical ingredients, hydrogen and oxygen, may be available on the Moon for our use.

Lunar rock contains oxygen in abundance. Using concentrated sunlight, provided by solar concentrators like those described in Chapter 12 for solar-thermal rockets, the rocks could be superheated to the point where the oxygen is released and collected. This would be the easy part. Oxygen-bearing rock appears to be available anywhere on the Moon. Finding hydrogen on the moon will be much more difficult.

Hydrogen, with an atomic number of 1, is the lightest of all elements. Not much energy is required to get hydrogen atoms moving at speeds that exceed the lunar escape velocity (the speed at which an object will escape from the gravity of a planet or moon, never to return). Over the billions of years since its creation, any atmospheric hydrogen the Moon might once have had has long gone. On the Earth, in addition to atmospheric hydrogen, there is water. One water molecule is composed of two hydrogen atoms and one oxygen atom. Our vast oceans and lakes provide a huge supply of hydrogen to meet our needs. Until fairly recently, the Moon was considered the ultimate desert—totally bereft of water. Then the Lunar Prospector Mission provided tantalizing hints that water ice might be preserved deep in craters near the lunar poles. Lots of ice, perhaps as much as 300 million tons ...

If there is water on the Moon, then its hydrogen component can be easily obtained by passing an electrical current through the water in a process known as electrolysis, liberating both the hydrogen and oxygen. The process is not complicated and is used regularly in earthly chemical plants and science classrooms.

Once the two gases are collected, the ingredients are in place for fueling chemical rocket engines to continue our journey.

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