On a hot evening in a tent on the Welsh borders, physicist Laura Grego said something that stayed in the air long after the applause: "People have to be aware of what is happening in space. Public knowledge has to catch up." Three weeks later, SpaceX floated on the stock exchange and briefly made Elon Musk the world's first trillionaire. The timing felt like a punchline.

So what exactly should we know? Quite a lot, it turns out. Grego, speaking from Cambridge, Massachusetts, where she leads security research at the Union of Concerned Scientists, argues that the average person's relationship with the night sky is on the edge of a radical change. For most of human history, looking up has meant wonder — stars, silence, a sense of belonging to something vast.

But SpaceX has now applied to a federal regulator for permission to launch up to one million satellites. One million. Add that to tens of thousands already planned by other companies, and the sky you see tonight may soon look very different from the one your grandparents knew. Grego arrived at physics by hunting for the hardest subject she could find at university — "a very midwestern perspective", she says with warmth.

She trained as an observational cosmologist before her parallel life as an activist drew her toward questions of global security.