>Less than you'd think. I am in favor of numbers in a discussion like this, even if wrong, (and I am not a physicist) they're more interesting than without. -

>Less than you'd think. I am in favor of numbers in a discussion like this, even if wrong, (and I am not a physicist) they're more interesting than without. - The asteroids considered a threat to the Earth start at about 460 feet in diameter. This is about an order of magnitude less (in diameter, not mass) than the impact that got the dinosaurs but would still probably be a lot worse than anything in recorded history, if it hit land or sea. It's said to be about a 10,000 year level. - Density is uncertain, but I found a figure of 2.7 g / cm^3 as a mid range. - This equates to 3.9 million metric tons or more being probably what we would want to do something about. - Freeman Dyson wrote a paper in 1968 or so, about nuclear pulse rocket design, that described a hypothetical spacecraft that would mass forty million metric tons. - It would, however, accelerate very slowly; at 0.00003 g (supposedly). - But that's really not so slow. - Ignoring orbital mechanics, because I just can't right now, if something is headed for Earth, it should be enough to move it by one radius in some direction...right? - Earth is something like 7926 miles in diameter, and 6.4 megameters is ~half that. - d = 1/2at^2, 1/2a = 0.000147 m/s^2, so divide the radius by 0.000147, take the square root, and I get ~209,000 seconds. - That is...2.4 days! - However, it presumes a one megaton bomb going off every 100 seconds, for a total of around 2,100. - That sounds like a lot. On the other hand, it's less than 3% of the nuclear bombs the US has produced since 1945. - However they would have to be custom designed, is my impression, as "nuclear shaped charges" and not just recycled warheads. - The asteroid might have to be reinforced on the size that was being pushed. The spacecraft design included a 1mm thick sheet of copper to absorb thermal energy. - However, if the asteroid was four and not forty million tonnes, then less time and fewer explosions would be needed, I would assume. Would it be ten times less? I'm leaving that as an exercise for the reader. https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls... https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_States https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_...