Satellites get a new type of armour

Meaning:Debris is the scattered bits and pieces left behind when something breaks apart — snapped branches after a storm, or metal and glass after a crash. It is leftover junk, spread around, that nobody is keeping track of. In this article it means space debris: the dead satellites, spent rocket parts and flecks of paint still circling Earth after decades of launches.

Meaning:A projectile is an object that has been thrown, fired or launched and is now flying on its own — a pitched baseball, an arrow, a bullet. It shares a root with the word project, which once meant to throw forward. In this article the projectiles are the pea-sized aluminium pellets that engineers fire from a gas gun to copy what space junk does to a satellite.

Meaning:Conventional means the usual way — the standard version that people already use, as opposed to a new or unusual one. It is not praise and not an insult; it simply marks something as the familiar option. In this article a conventional Whipple shield is the ordinary two-layer aluminium design engineers have built for decades, and it is what every new material has to beat.

A fleck of paint is harmless. You could catch one on your fingertip. But a fleck of paint in orbit can blast a crater in a satellite, and the only thing that changed is how fast it is moving.

Physicists call the energy stored in a moving object its kinetic energy, and the rule behind it is strange the first time you meet it. Double an object's mass and you double that energy. But double its speed and you multiply the energy by four. Ten times faster is a hundred times the energy. Speed counts for far more than size.

Picture two tennis balls. One rolls off a table and taps your shoe, and you barely feel it. The other comes off a serving machine at 100mph and hits your shoe, and you are limping to the nurse. Same ball, same mass — only the speed is different. Now push that ball to seven kilometres a second, the speed of junk in low orbit, which is about eight times the speed of a rifle bullet. At that speed it would not bruise a satellite. It would tear a hole straight through.

So when you want to know how much damage a moving object can do, ask how fast it is going before you ask how big it is.

Every material is a trade-off. Steel is strong but heavy. Plastic is light but soft. Ceramic is hard but shatters. For centuries, building something meant picking the least-bad single material for the job — until people began locking materials together instead.

Materials scientists call the result a composite: one material built from two or more different materials joined so the mixture does something neither could do alone. The word just means put together from parts. Usually one part is a stiff fibre and the other is a softer material that holds those fibres in place. Kevlar, the stuff in bullet-proof vests, is a mesh of extremely tough fibres. Nextel, its partner in this article, is ceramic drawn into threads and woven like cloth, which makes it hard in a way Kevlar is not. Bond the two and you get a sheet that is hard on the outside, tough on the inside, and still light enough to launch.

You already own composites. A bike helmet is stiff foam under a hard plastic shell. When your head hits the pavement, the shell spreads the force of the blow across a wide area instead of one point, and the foam crushes slowly underneath, which stretches the stop out over a longer time. Neither layer works by itself: bare foam tears open on a rock, and bare plastic passes the whole jolt through to your skull. Together they do the job, at a weight you can wear all afternoon without noticing.

That is the whole trick. When no single material has everything you need, build one that does.