Some rather smart scientists from the Lawrence Livermore National Laboratory (which has an excellent plan for planetary defense) have worked out that, for an asteroid of 90 meters, you need 10 years to lift it with a kinetic impactor with confidence prevent an earthly impact. So, to declare 2024 JR4, if it is 90 meters long and we only have a few years, we probably need a larger impactor spacecraft (but don't break it!) – or we need different kinetic impactors bend it AF (but each has to work perfectly).
Eight years until the impact is a bit tight. It is not impossible that the choice would be made to use a nuclear weapon to bend it; This can be geopolitically very awkward, but a core leg would give a greater deflection than an equivalent arrow -like spacecraft. Or maybe they would choose to try to evaporate the asteroid with something like a 1 megaton, that LLNL says it would work with an asteroid of this size.
Ars: So it's a bit late in the game to plan an impact mission?
Andrews: This is not an ideal situation. And humanity has never tried to stop an asteroid impact for real. I imagine that if 2024 JR4 became an agreed emergency situation, the Dart team (especially JHUAPL + NASA) would join forces with SpaceX (and other space agencies, especially ESA but probably others) to quickly build the right massive kinetic impactor (or impactors) and prepare for a deflection attempt nearly 2028, when the asteroid flies its next earth. But yes, eight years is not too much time.
A deflection could work! But it will not be as simple as the asteroid just hit very hard in 2028.
Ars: How important is NASA for planetary defense?
Andrews: Planetary defense is an international security problem. But at the moment NASA (and America, by expansion) is the forefront. The planetary defenders are the viewers on the wall, the people who are most responsible for finding these potentially dangerous asteroids before they find us, but also those who are most able to develop and use technology to effect prevent. America is the only nation with (for now!) A well-financed near-earth object show, and is the only nation that has tested a planetary defense technique. It is a film cliché that America is the only nation that can save the world from cosmic threats. But for the time being – even with amazing planetary defense mistakes from ESA and Jaxa – that cliché will remain true.
