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 once thought about.

How does a satellite know where it is?

On the ground this is easy to answer: a glance at your phone will do it.

Your phone has navigation, and navigation relies on twenty-odd dedicated satellites constantly beaming signals down at the ground.

Your phone picks up several of those signals at once, compares the difference in when they arrive, and works out where you are standing.

But that system was designed for the ground in the first place.

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

A satellite flying under that umbrella can still catch a signal.

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

So in deep space a satellite has only two roads: wait for people on the ground to tell it where it is, or work it out itself.

Last week the American space agency published the result of an experiment that took the second road.

It ran on a group of small satellites, with a partner company spun out of a university.

The method, stripped down, is plain to the point of being charming.

The satellite already had cameras on board.

In the past those cameras were used to photograph the ground, or the companions in its formation.

This time, they had it look up and photograph something else.

Photograph what?

Everything up there that can be seen.

Other satellites — and space junk.

Humans have left tens of thousands of pieces of rubbish in low Earth orbit: dead satellites, rocket debris, fragments from collisions.

They are not drifting at random.

Every piece sits obediently on its own orbit, and its position can be calculated.

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

And so the logic falls out.

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

It photographs a second and a third, and the combinations that fit all of them at once get fewer and fewer.

Once enough reference points match, the satellite can work backwards to which orbit it is on, and where on that orbit.

The principle is exactly the same 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 humans threw into the sky themselves.

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.

For the whole run, nobody on the ground was directing it.

It found itself — and along the way it made that catalogue slightly more accurate too.

Finally, the layer of this I find most elegant.

Space junk has always been purely bad news.

Every so often a report says the debris in low Earth orbit is increasing, getting more dangerous, and could smash a working satellite at any moment.

All of that holds. Not one word of it is exaggerated.

But in this experiment, that same pile of fragments turned into markers by the roadside.

The things did not change. What changed is the angle you look at them from.

A heap of rubbish nobody wanted became, when it was needed, the only free coordinate system in the sky.

Think about it:

Something everyone treats as a nuisance — could it, at the same time, be an answer?