NASA 让卫星靠拍太空垃圾找到了自己。大家都嫌碎片危险，它成了天上唯一免费的路标。

Start with a question most people have never thought about.

How does a satellite know where it is?

On the ground this is easy to answer: look at your phone. Your phone has GPS.

GPS is twenty-odd satellites constantly beaming signals down at the ground.

Your phone picks up several of those signals at once, and works out where you are.

But GPS itself was designed for the ground.

Its signals point downward, like an umbrella, covering the Earth.

A satellite inside that umbrella can still use it.

Once you fly higher — out toward the Moon — the umbrella no longer covers you.

So in deep space, a satellite either relies on people on the ground to tell it, or it works it out itself.

Last week, NASA published a result of the second kind.

The experiment is called FALCON, running on a group of small satellites called Starling.

The partner is a company called EraDrive, spun out of Stanford.

The method, stripped down, is very plain.

The satellite already had cameras on board.

Those cameras used to photograph the ground, or the companions in its formation.

This time, they let it photograph something else.

Photograph what?

Everything visible up there.

Other satellites — and space junk.

There are tens of thousands of pieces of human rubbish up there: dead satellites, rocket debris, fragments.

They are not drifting at random. Every piece is on its own orbit, and its position can be calculated.

The ground has long had a catalogue recording roughly where these things are.

So the logic becomes clear.

The camera photographs a bright point, the satellite checks the catalogue, and guesses which piece it is.

It photographs a second, a third, and the combinations that fit get fewer and fewer.

Once enough of them match, the satellite can work backwards to where its own orbit is.

This is the same thing as finding your way in an unfamiliar city.

You do not know where you are, but you recognise three buildings, and then you do.

Except here the buildings are rubbish other people threw into the sky.

The result was better than expected.

In three days, that satellite worked out the orbits of more than two hundred space objects more accurately than before, on its own.

Nobody on the ground was directing it.

Finally, the layer of this I find most elegant.

Space junk has always been purely bad news.

Every year there are reports saying the debris up there is increasing, it is dangerous, it could smash a working satellite.

All of that is true.

But in this experiment, those same fragments became markers by the roadside.

Same pile of things. It depends what you take them for.

Think about it:

Something everyone treats as a nuisance — could it also be an answer?