The Secret History of the Supernova at the Bottom of the Sea
In February 1987, Neil Gehrels, a young researcher at NASA’s Goddard Space Flight Center, boarded a military plane bound for the Australian Outback. Gehrels carried some peculiar cargo: a polyethylene space balloon and a set of radiation detectors he had just finished building back in the lab. He was in a hurry to get to Alice Springs, a remote outpost in the Northern Territory, where he would launch these instruments high above Earth’s atmosphere to get a peek at the most exciting event in our neck of the cosmos: a supernova exploding in one of the Milky Way’s nearby satellite galaxies.
Like many supernovas, SN 1987A announced the violent collapse of a massive star. What set it apart was its proximity to Earth; it was the closest stellar cataclysm since Johannes Kepler spotted one in our own Milky Way galaxy in 1604. Since then, scientists have thought up many questions that to answer would require a front row seat to another supernova. They were questions like this: How close does a supernova need to be to devastate life on Earth?
Back in the 1970s, researchers hypothesized that radiation from a nearby supernova could annihilate the ozone layer, exposing plants and animals to harmful ultraviolet light, and possibly cause a mass extinction. Armed with new data from SN 1987A, Gehrels could now calculate a theoretical radius of doom, inside which a supernova would have grievous effects, and how often dying stars might stray inside it.
“The bottom line was that there would be a supernova close enough to the Earth to drastically affect the ozone layer about once every billion years,” says Gehrels, who still works at Goddard. That’s not very often, he admits, and no threatening stars prowl the solar system today. But Earth has existed for 4.6 billion years, and life for about half that time, meaning the odds are good that a supernova blasted the planet sometime in the past. The problem is figuring out when. Because supernovas mainly affect the atmosphere, it’s hard to find the smoking gun,”Gehrels says.
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