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Comment by cubefox | Hacker News Reader

As important as the DART mission was, we are still quite far from actually defending against a mass extinction sized asteroid or even comet. The Dimorphos asteroid, which was hit by the DART craft, has a mass of only about 5 * 10^9 kg, while the Chicxulub asteroid (which caused the Dinosaur mass extinction) had a mass of 1 * 10^15 to 5 * 10^17 kg. That's six to eight orders of magnitude difference, or a factor of one million to 100 million.

Somebody has probably calculated the details, but I suspect if we have to hit such a large impactor, we have to launch as many hydrogen bomb equipped Starships at it as we can. And our time window might be quite short. The worst case is if it's actually a comet, as these are hard to detect in advance, since they aren't aligned with the plane of the solar system.

By the way, if you Google for "DART mission" or "Chicxulub impactor", be careful, something might hit your search results...

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> if you Google for "DART mission" or "Chicxulub impactor", be careful, something might hit your search results...

That was cool, thanks.

I enjoy the traces of whimsical-not-evil old Google that still resurface every now and then.

But if detected early enough it might not take much more than a DART sized nudge of even a Chicxulub sized asteroid to change it’s course enough to miss the Earth. The whole reason DART targeted a two asteroid system was so we could measure how much we changed Dimorphos orbit around Didymos. If we have to blow these things into rubble shortly before impact we might be in big trouble.

DART nudged it's target about 2.7mm/sec. To ensure a miss you need a sideways deflection of up to 4000 km (depends on the geometry.) To generate a miss by Dimorphos assuming the worst case you're looking at 1,481,481,481 seconds of lead time. That's about 47 years. Against the dinosaur killer you're looking at a lead time far beyond our ability to predict orbits.

Note, also, that if you're throwing kinetic stuff at your target you get little choice in how it hits, you very well might not be able to shove it to the side.

Despite the refusal to consider the option there really is only one choice against a major threat: Orion. We know it works (it has been flight tested with conventional charges, it has been tested with single impulses with nuclear charges), while there are some serious question marks about whether you could actually build an Orion capable of launching form Earth (how do you keep the pusher plate from getting too hot??) that does not apply if you're using it against an asteroid. You don't care what shape the asteroid ends up in, just that it get out of the way. Your only limit is that you don't want to make any given shove big enough to risk breaking the asteroid.

By a crude approximation you can easily get 10% of the bomb energy to show up as impulse, this can be increased by putting the bomb closer (remember, you don't care about damage short of destruction) and some still-classified data says that it's possible to build a shaped charge that directs the majority of it's energy at the asteroid. Obviously, exact numbers are not known to us mere mortals.

Gravity tractors would also work, with just a couple of years of lead time.

I think Orion requires SLS rockets, which are very slow and expensive to produce.

Wrong Orion. This is the one with the nuclear bombs and pusher plate.

With a ten kilometer Chicxulub class asteroid, blowing it into rubble is not on the table anyway, even with rather large hydrogen bombs. We can only hope to deflect them.

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