Rupert GruberHiding somewhere in the gloom of space, there is a gigantic asteroid on a collision course with our planet. If we dont spot it and somehow thwart its arrival, it will pierce Earths atmosphere at 60,000 kilometres per hour and hammer into the ground, vaporising anything it touches.With millions of asteroids hurtling through the inner solar system, the threat is inevitable: sooner or later, an impact will happen. But that doesnt mean Earth has to be a sitting duck. A global community is engrossed in planetary defence, carefully planning how to fend off these extraterrestrial interlopers when they show up or at least minimise the carnage.Among other things, this work has involved scanning the sky for threats and testing missions to knock asteroids off course. But it also includes a surprising amount of role-playing, where teams war-game asteroid impact scenarios. In the real world, we havent really gotten to this stage where we actually have to design and build missions, so thats why we need these exercises, says NASAs Paul Chodas, who runs many of the role-plays. They make you think about details that you would not otherwise think about.Over the coming paragraphs, you will be in the hot seat in a choose-your-own adventure version of one of these role-playing games. You will decide how to react as the asteroid bears down upon us. Whether you opt to slam a spacecraft into it, use sunlight-absorbing paint to shift its path, or simply blow it to smithereens, you will find we have more options