The problem is speed doesn’t matter. We wouldn’t know until after it hit us. Gamma-ray bursts move at the speed of light. No warning system could alert us in time. By the time we detected it, Earth would already feel its effects.

The burst would arrive in two stages. First comes the initial blast of gamma rays. This would hit our atmosphere and strip away the ozone layer on one side of Earth. The rays would convert nitrogen and oxygen in our air into nitrogen dioxide. The sky would turn brown. But we wouldn’t see this coming.

Second stage would be worse. The gamma rays would set off an atmospheric series of chemical reactions. This would produce nitric oxide. More ozone would be deleted by the nitric oxide. One would see this over several months. The ozone layer would progressively disappear.

Most surface life on Earth would die. Not from the gamma rays itself; they wouldn’t make ground contact.

Death would come from ultraviolet radiation from our own sun.

Without the ozone layer, nothing would stop it. Plants would die first. Animals would follow.

We know this from studying previous mass extinctions. The Ordovician extinction might have come from a gamma-ray burst. The evidence sits in rock layers. They show a pattern of extinction consistent with ozone depletion.

The good news is that these bursts are rare. The nearest star capable of producing one that could harm Earth is thousands of light-years away.