Now, as I speak, a spacecraft is making a two-year journey to an asteroid near Earth that NASA blasted a crater into back in 2022. It's all happening in the name of planetary defence. So, how do you kill an asteroid? And why? Volcanologist turned science journalist Robin George Andrews has this handy guide. In

the early hours of June 30th, 1908, in a remote corner of Siberia, the sky suddenly erupted with a preternatural fury. A few nomads sojourning in the area were flung into the air by a thundering howl. Someone 25 miles away was propelled backwards through his hut, and in a heartbeat, 80 million trees were knocked flat as an area the size of Greater London was decimated. This pandemonium was caused by a relatively diminutive asteroid, one just 50 metres long, and its mid-air blast was akin to a 12 megaton nuclear explosion. Although asteroid strikes aren't a day-to concern, they do happen if you wait long enough, and space rocks 140 meters long are of chief concern to planetary defense researchers. They are hard to detect and find their way to Earth far more frequently than the giant, globally destructive ones. They won't end civilization, but they could vaporize a city, which is why these relatively bijou asteroids are known as city killers. So if one were heading our way and telescopes caught it in time, how would we stop it? The most popular method is something known as a kinetic impactor, an uncrewed spacecraft that would crash into the asteroid at breakneck speeds, causing it to recoil and change its orbit around the sun. Fortunately, we already know this technique works. In September 2022, NASA piloted a car-sized spacecraft into a harmless, city-killer asteroid named Dimorphos. The mission, known as the Double Asteroid Redirection Test, or DART, aimed to change Dimorphos' orbit around a larger asteroid named Didymos and it did so with aplomb. I was there at mission control when it happened. Hundreds of scientists cheered, roared and cried with joy as they watched in real time their spacecraft's last moments as it hit a perfect bullseye on Dimorphos. You must be careful though, if you punch an earthbound asteroid too hard it will fragment into several smaller but still dangerously sized pieces, effectively turning a cannonball into a shotgun spray. For bigger asteroids, or for smaller earthbound asteroids discovered with little warning time, something with more oomph may be required. A nuclear weapon. Park a nuke-armed spacecraft next to the asteroid, detonate, and one side of it will become severely irradiated. That side will shatter and jettison debris into space, pushing the asteroid away from Earth as if it were a rocket. If the asteroid were discovered too late to deflect it away from the Earth, we may try to completely vaporize it with ever more powerful nuclear detonation. A Hail Mary approach that risks turning the cannonball into a now radioactive shotgun spray. Alternative ways to save the planet are gentler but require far more advanced warning to work. One such idea is a gravity tractor, a supermassive spacecraft that would fly out to the earthbound projectile, orbit it, then slowly fly in a different direction using its immense gravity to yank the asteroid out of its earthbound racetrack. For now though, most of these planetary defense techniques are hypothetical. Only one, the kinetic impactor, has been tested in deep space and we can thank our lucky stars that it worked wonders. Not content on leaving the task of saving the world to NASA, the European Space Agency is also stepping up to the plate. This month, it launched the Hera spacecraft. And liftoff. Go Hera, go Falcon, go SpaceX. A robotic detective which, in 2026, will arrive at Dimorphos, the asteroid DART impacted, to check America's homework while getting to know the very sort of asteroid that imperils Earth inside out. We live in a timeline where completely cancelling out an entire category of natural disaster is now possible. Planetary defense is a global endeavor. When it wins, everybody wins.

And that is all we have time for this week. You've been listening to BBC Inside Science with me, Victoria Gill. The producers were Sophie Ormiston, Ella Hubber and Gerry Holt. Technical production was by Diffan Rose. The show was made in Cardiff by BBC Wales and West in partnership with The Open University. And I will see you next week for some spooky science. So until then, thanks for listening and bye-bye.