Another piece of space junk plunged into the ocean this week, this time off the coast of Sri Lanka. No one was hurt, but it could have been an unlucky Friday the 13th if it had hit the island.

It underlines how the time has come to end our 50-year-old habit of building throwaway rockets that clutter space with junk.

The object that fell to Earth was on an orbit that carried it between Earth and the far side of the moon, so it was most likely a booster stage from a moon mission. That means it was either American, Russian, Chinese or Japanese, which are the only countries to shoot things at the moon.

So far, the fallen object hasn't been identified, which is odd because the space command watching for things that could threaten Earth is pretty good at identifying wayward craft. (It's also possible this was from a military mission and no one is talking.)

In any case, the fundamental problem is that everything we send into space leaves something behind, which can come back down on our heads or run into something else already up there.

It's a problem that is only going to get worse, unless we come up with new technology for leaving the planet.

Throwaway rockets

WT1190F space junk

WT1190F, a piece of space junk that dropped to Earth on Nov. 13, was first detected on Oct. 3 by the Catalina Sky Survey, a project based at the University of Arizona that searches the sky for potentially hazardous asteroids. (Catalina Sky Survey)

This philosophy of throwaway rockets goes right back to the earliest days of space flight and early pioneers, such as German rocket scientist Wernher von Braun. When he came to the United States to develop rockets to put people into space and on the moon (and eventually, he hoped, on to Mars), von Braun knew that getting up there requires a vehicle that must reach enormous speeds to escape the gravity of Earth.

Just climbing out of the atmosphere and reaching orbit requires a speed of 29,000 km/h. You have to go faster than that to reach the moon, and even faster to reach Mars.

Unfortunately, going fast takes a lot of fuel. In fact, 90 per cent of a rocket's takeoff weight is fuel. But fuel is heavy, which means you need bigger engines, which require more fuel, which adds more weight — it's an escalating problem known as the rocket equation.

That is why rockets are so huge.

Von Braun realized that most of the fuel would be used up at the beginning of the flight, just getting off the ground, which means that after the first few minutes, the rocket would be carrying very large but empty fuel tanks.

So, why drag them all the way to the moon or Mars? Why not throw them away, to save weight for the rest of the journey?

This technique reached extremes in the Saturn 5 moon rocket, which, standing on the launch pad, was twice as tall as Niagara Falls. After dropping pieces of itself into the Atlantic, in space on the way to the moon, on the surface of the moon and on the way home, the tiny capsule — barely big enough to seat three astronauts — splashed down in the ocean. It was a tiny fraction of the original machine.

That's the equivalent of buying a brand new car, driving it around until it runs out of gas, throwing the whole thing away and only keeping the steering wheel. (And the next time you want to go for a drive, you need a brand new car.)

Solving the space junk problem

Saturn 5 rocket

The Saturn V launched the Skylab space station in 1973, and created a large amount of space junk in the process. (NASA)

As ridiculous as that sounds, that is exactly how the rocket industry has operated since the 1950s. And that is why space is littered with so much junk.

Ironically, the one system designed to be reusable, the American space shuttles, turned out to be far more expensive to fly because they were much more complicated. So, the next generation of U.S. rockets, called SLS, goes back to the old disposable philosophy. It will be the largest rocket ever built ... then thrown away after one use.

If we want to reduce space junk, the first step is to stop building machines that discard parts of themselves all over the place. So far, the only company doing that is Space X, which intends to return its first-stage boosters to a floating platform at sea.

Twice already, those attempts have failed. But they do expect success, which will not only reduce pollution in space but also save the company a lot of money, by reusing those boosters instead of building new ones from scratch.

The three Rs — reduce, reuse, recycle — have been employed for decades to handle our garbage here on the ground. It's time now to apply those same rules in space, because what goes up can come down — in ways we might not like.