By bouncing laser light off of mirrors on the lunar surface and recording the amount of time it takes to make the trip, scientists can determine the distance to the moon. The technique is known as lunar laser ranging. By keeping close track of your distance and thereby your position in relation to the moon over time, you can track the movements of the continents on earth.
Long-term measurements like these have been made by the McDonald Observatory for over 30 years. The first system it used was rushed into service in 1969 and had to be aimed with a telescope. For sixteen years, the observatory could use this laser ranging system to determine the distance to the moon within 10 centimeters. In 1983, the system was replaced with a new state-of-the-art laser ranging system and by 1989, when it was completed, the observatory had increased this precision to within one centimeter.
All of this requires lasers with the ability to emit pulses of light only a few billionths of a second long, computers with equally accurate timing and, of course, mirrors – on the moon.
This last is a tall order. Several were deposited by Soviet lunar landers, but most were placed there by the Apollo astronauts starting in 1969. Getting to the moon requires that you attain a great enough speed to escape the pull of the Earth’s gravity – around seven miles per second – and reach it by accelerating at a rate that won’t damage your cargo of instruments and/or humans. So far the only way to do this is by using a rocket.
The rocket responsible for the moon landings was the Saturn V, the largest machine ever built by man. It stood 363 feet high, weighed 6.2 million pounds and produced 7.5 million pounds of thrust on liftoff. It just barely reached escape velocity.
Two million pounds of the rocket’s weight consisted of liquid hydrogen and liquid oxygen, which were ignited in stages in what amounted to a series of controlled explosions. To keep the rocket from tearing itself apart, all of this combusting liquid and gas must be released as it expands. To give the rocket any kick, it must be released in a controlled fashion in a single direction.
The solution to this engineering problem came long before the Apollo program from a Swedish entrepreneur by the name of Gustav de Laval, who developed 92 Swedish patents and founded 37 companies before his death in 1913. One of his inventions was the Laval nozzle, which focuses the expanding gas leaving a rocket engine and accelerates it beyond the speed of sound.
In the event that something did go wrong on the launch pad, igniting all the fuel at once and causing the rocket to explode, astronauts were supposed to climb out of their space capsule and dive down a sort of laundry chute coated in Teflon. The slide let out hundreds of feet below the ground in a rubber room designed to withstand the impact from the explosion of 2,000 tons of rocket fuel. After the burning and exploding parts were over, the astronauts would begin a long hike to civilization miles away by way of a narrow concrete tunnel underground, illuminated every few yards by bare light bulbs hanging from the ceiling. Of course, all of this was mainly for the astronauts’ peace of mind. In all likelihood, no one would have made it out of the capsule in a real disaster.
The Saturn V rocket is actually a series of three rockets stacked on top of one another. Each of these is called a rocket stage. As the fuel in each stage of the rocket is used up, the stages become dead weight and must be dumped for the next rocket to function properly. Removing something this large and bolted on this well is a difficult proposition. To do this, engineers use a set of controlled explosions delivered by specialized explosives called shape charges.
As it turns out, the shape of an explosive has a lot to do with where the bulk of the explosion is directed. And once you can direct the force of an explosion, you’re free to use smaller explosives and use them for more sensitive applications. In the case of the Saturn V, shape charges were used in such a controlled manner that they could blow off an entire stage of the massive rocket and get it clear of the vehicle without damaging the next stage.
Shape charges are used in more mundane applications as well. They are used to blast away pieces of mountains to make room for highways, in conventional weapons and in demolition. If you’ve ever seen footage of an enormous skyscraper being demolished and tumbling into its own basement, you can respect the amount of precision these devices are capable of.
Perhaps the most demanding and horrifying use of shape charges is the role they play as the fuse for plutonium-based nuclear warheads and thermonuclear devices. But that’s a topic for another column…
Josh Braun is the Daily Nexus science and technology editor.