Most people don’t have much of an understanding of orbital mechanics. That’s why science fiction movies can get away with space chase scenes where spaceships fly like airplanes, in wide arching curves, chasing one another and shooting at each other with laser beams.

Examples of this are all the Star Wars movies. Han Solo flies his ship like it is a bumper car, and the boxy enemy fighters fly after him like airplanes in an air show. The movie Armageddon with Bruce Willis shows a space shuttle on steroids flying through the tail of a comet, with icebergs of all sizes tumbling about, and the ship curving around them and dodging them like a bat navigating a cave.

That’s not how it works. Objects in space can only move in straight lines. They are not flying, as flight implies air, but rather they are falling. And objects fall in straight lines. Well, almost straight lines, or better, ballistic trajectories. The only exception: orbits are actually round or elliptical, but the curvature of an orbit is so slight that the eye would never be able to notice it.

Two movies come to mind that represent space travel more realistically. One is the old 2001: A Space Odyssey. The other, more recent one is Mission to Mars. It has some great rescue scenes where the rules of orbital mechanics are followed and actually contribute to the plot.

The fact is: objects in space travel in straight lines. And if that space is near Earth, those objects are by definition in some kind of orbit, or else they’d be falling to Earth and burn up in the atmosphere. Objects in low Earth orbit travel very fast.

Let’s move from science fiction to real science: The International Space Station orbits at an altitude of 380 kilometers. Since orbital speeds, assuming circular orbits, depend only on the mass of the orbited object, in this case the Earth, and the distance of the orbiting object from the center of the orbited object, the speed at 380 kilometers is a constant 7.68 kilometers per second or 17,172 miles per hour.

In comparison, the very fastest rifle bullets travel at a speed of 1.5 kilometers per second or 3,354 miles per hour. Most rifle bullets are much slower. So the international space station is traveling more than 5 times as fast as the fastest rifle bullet. When the space shuttle docks with the space station, the two objects both travel at that speed right next to each other. Of course, to do that, the space shuttle certainly does not have to face with its nose forward, bottom down. Most of the time it maintains some drastically different orientation.

The Hubble Space Telescope orbits at an altitude of 600 kilometers and its orbital speed is therefore somewhat slower: 7.56 kilometers per second.

What does all this have to do with space debris?

NASA is worried about sending the shuttle on one of its last missions to the Hubble for a last set of repairs. As it turns out, in 2007, the Chinese “shot down” a satellite, causing a huge debris field in space. Also in 2007, a Russian satellite failed and broke up in space. These two events caused a huge increase in the amount of space debris at that altitude.

When an object breaks up in orbit, whether that is due to being exploded on purpose (Chinese) or an accident (Russia), the pieces, large and small, keep “floating” or rather, falling at those tremendous orbital speeds. Since they are in orbit, they will stay there for many years to come. The higher the orbit, the longer the stability. An object orbiting at 600 kilometers altitude will remain there for decades.

Now imagine a small piece of such debris, say a nut, like a lug nut on your car wheel, traveling at 17,000 miles per hour. Let’s say the space shuttle is traveling in the opposite direction, also at 17,000 miles per hour. Now the combined rendezvous speed of the two objects is 34,000 miles per hour. A lug nut, at that speed, hitting the space shuttle would cause the shuttle to disintegrate instantly. The energy would be enormous. Even an object the size of a pinhead, a piece of dust really, would be fatal.

With the amount of space junk out there, NASA has calculated that the chances of a catastrophic accident at the altitude of the space station is 1 in 300 and at the altitude of the Hubble Space Telescope is 1 in 160. That’s pretty scary odds, if you are an astronaut and your life is on the line.

Given all this, I wonder what the Chinese were thinking when they blew up that satellite?