because the distances you're going and the speeds you're going, a tiny change in direction can result in a huge change in how far it's going to miss the earth. and that could be vital. a 160—metre wide rock like diomorphos could be catastrophic for populated areas. bigger asteroids that are kilometres across have the potential to devastate the planet. the hope is this mission will show us if it's possible to stop future collisions. the spacecraft will arrive at its destination next autumn. rebecca morelle, bbc news. caroline smith is head of earth science at the natural history museum and joins me now. caroline, hello. is itjust a matter of time until a catastrophic asteroid crashes into us? to be honest, it _ asteroid crashes into us? to be honest, it is. _ asteroid crashes into us? to be honest, it is. it— asteroid crashes into us? to be honest, it is. it has— asteroid crashes into us? to be honest, it is. it has happened l asteroid crashes into us? to be| honest, it is. it has happened in the geological past and we know it will happen in the future, so this is a very important mission that nasais is a very important mission that nasa is doing and it will be a