ciroluiro t1_iqy8evy wrote on October 3, 2022 at 11:39 PM

Reply to comment by jamin_g in After DART: Using the first full-scale test of a kinetic impactor to inform a future planetary defense mission by EricFromOuterSpace

I would imagine that the composition would be less of a concern than knowing the exact mass of the asteroid.
A collision in space is as good of a scenario you could get in terms of applying conservation of momentum. Then, knowing that the worst case scenario is a plastic collision (because the probe going through the asteroid completely is very much impossible), you could know with very good certainty the final momentum (assuming we have fairly precise and accurate information on the velocity of both objects). I'm purely speculating as I haven't looked into the mission, but I also doubt the scientist expected a best case scenario of a bounce from the asteroid.

VolvoRacerNumber5 t1_iqywdop wrote on October 4, 2022 at 2:42 AM

I think everyone here is ignoring how material gets ejected by the impact. They know the momentum of both the spacecraft and asteroid very well. There's nothing new the impact will show in regards to that very basic momentum conservation problem.

What they are actually trying to characterize is the way debris is ejected by the impact. This ejecta carries off its own momentum and can impart a significant additional momentum to the asteroid.

ciroluiro t1_iqz47cw wrote on October 4, 2022 at 3:48 AM

But that's kinda my point. Anything ejected would only "push off" the asteroid and help nudge it off course, given that they mostly get ejected either in the opposite direction to the spacecraft, or perpendicularly in a roughly even manner (this last one is speculation on my part since I imagine it depends heavily on the angle of impact and possibly more things I don't know, but I don't think it's a wild assumption)

Impiryo t1_iqywgoc wrote on October 4, 2022 at 2:42 AM

There was a huge amount of ejected mass from the collision - that is likely the majority of the momentum change, and what we are measuring.

Plastic vs elastic collisions fail when explodey stuff happens.

ciroluiro t1_iqz3jul wrote on October 4, 2022 at 3:42 AM

Unless the ejected mass went somehow forward, I don't see how that would steal momentum from the asteroid. Maybe if the ejected mass went perpendicular in a manner was not even in all directions? Kinda stretching it.

Anyway, explodey stuff would only help, not hurt.