Recent comments in /f/space

rocketsocks t1_jajtuvr wrote

Reply to comment by Embite in NASA’s DART data validates kinetic impact as planetary defense method | DART altered the orbit of the asteroid moonlet Dimorphos by 33 minutes by mepper

It's not about pessimism it's about the difficulty of simulation, and lack of detailed knowledge.

And no, this is not a simple inelastic collision problem, it's not simply a matter of the final momentum of the asteroid being its starting momentum plus the probe's momentum, if it were we wouldn't have sent the probe. The asteroid is a rubble pile, which means that the impact ejected a huge plume of debris out of a crater. Because it sends a debris plume backwards (and that momentum needs to be balanced) you get greater than 1:1 momentum transfer, essentially turning the crater into a rocket engine powered by the probe's kinetic energy. The details of that plume depend greatly on the compositional and structure details of the asteroid, something we have very little firm data on up until now. We have literally fewer than five data points to go on for this sort of thing. So folks put together some simulations with variations according to the knowledge we have. As it turned out in this one instance the result was on the high end of all of the simulations, well above the average.

One thing worth pointing out here is that this is still just one data point. It may be that dimorphos is actually an outlier in terms of its compositional structure. Or it may be that the particular spot we hit was unusual. The average could be lower or higher than what we achieved with this specific instance. That's why we need a lot more studies like this one to collect enough data to be actually of practical usefulness.

23

coriolis7 t1_jajr75p wrote

Reply to comment by Kear_Bear_3747 in NASA’s DART data validates kinetic impact as planetary defense method | DART altered the orbit of the asteroid moonlet Dimorphos by 33 minutes by mepper

Not exactly. Momentum is always conserved, but the kinetic energy is not. A fully elastic collision preserves kinetic energy, while a partially inelastic collision does not.

In both cases m1 x v1(initial) + m2 x v2(initial) = m1 x v1(final) + m2 x v2(final).

However, only in the fully elastic collision does the following hold: [m1 x v1(initial) + m2 x v2(initial)] / 2 = [m1 x v1(final) + m2 x v2(final)] / 2

It doesn’t matter if it’s in space, in a lab, or wherever.

I think what the above redditor was saying is that because the outer material was more loosely held, more of the material could be ejected. That ejected material has additional momentum. Even though the probe never bounced off (ie elastic collision) the ejected material made the collision act as partially elastic.

19

Tuna-Fish2 t1_jajnj3b wrote

Reply to comment by 12edDawn in NASA’s DART data validates kinetic impact as planetary defense method | DART altered the orbit of the asteroid moonlet Dimorphos by 33 minutes by mepper

In an inelastic collision, momentum is conserved. So Sum(momentum of all bodies) is the same before and after the collision.

If Dimorphos was a rigid body, the spacecraft would only add it's own momentum to it.

However, because the impact by the spacecraft caused just loads of material to be thrown off, the end momentum of Dimorphos is old_momentum + dart_momentum - sum(momentum of all the ejecta). Since the ejecta is going the other way, the net effect is that dimorphos was accelerated more.

(Remember, kinetic energy = ½mv^2, momentum = mv, so spreading out the energy over more, heavier objects increases efficiency.)

29

danielravennest t1_jajhve6 wrote

Reply to comment by the_fungible_man in NASA: Several large asteroids projected to zoom past Earth this week by 1080krld

Risk is based on size and impact probability. The asteroid with the highest risk ranking is 101955 Bennu. But it won't have a chance of impact until 2178.

We already sent a probe to it, and grabbed a sample that is due to land on Sept. 24th.

Everything else that shows up in news stories is either lower chance of impact, too small to do much damage, or so far in the future we're not worried yet.

On April 13th, 2029 the asteroid 99942 Apophis will pass only 31,600 km from the Earth's surface, but we know its orbit with 3 km uncertainty, so it will definitely miss us. The same probe that visited Bennu is being retasked to meet up with Apophis. There will probably be a lot of hype about that asteroid before it arrives.

Any other news story about asteroids approaching Earth are clickbait. If one that was a real hazard is discovered, it would be the top story in all the news media.

3

Kear_Bear_3747 t1_jajfhid wrote

Reply to comment by lagavulinski in NASA’s DART data validates kinetic impact as planetary defense method | DART altered the orbit of the asteroid moonlet Dimorphos by 33 minutes by mepper

In space, objects will absorb all of the momentum of the object. That’s basic Newtonian Physics, dealing with Inertia.

On Earth it matters because there are other forces in play like Gravity and Friction so kinetic energy can dissipate in different ways, whereas in space there’s nothing to arrest that energy, it will impart itself on whatever object it collides with.

−4

Djasdalabala t1_jaje456 wrote

Reply to NASA’s DART data validates kinetic impact as planetary defense method | DART altered the orbit of the asteroid moonlet Dimorphos by 33 minutes by mepper

Note that while this is an effective method to steer an asteroid, the difficulty scales with its mass and delay before impact. This would not be practical in a (very unlikely) "don't look up" scenario, where the asteroid is huge and discovered only a couple of months from impact.

1