Well, space is big and empty, and not only did they hit an asteroid, they hit the smaller of the only two asteroids in the area. It is terrible navigation like that that makes getting out of bed hazardous. Aren’t we glad we aren’t flying on their Starship?

On second thought, this mission was a kinetic test, which gives us an idea of what we can do with Newtonian physics without the fancy explosive equipment. Fragmenting the asteroid with nuclear weapons may not be necessary, but even if we did, it would reduce the mass of the pieces that could collide with the Earth, reducing the damage they cause.

As a kinetic test, we could analyze using the law of conservation of energy ( conserve energy; it is more than just a good idea, it is the law), but we don’t know how much energy was lost due to the DART probe and surrounding rocky material vaporizing or melting. Adding the energy of a nuclear weapon does not help, because we don’t know how much energy would be lost and how much energy used to change the asteroid’s orbit.

We could use the law of conservation of momentum (conserve momentum; it is more than just a good idea, it is the law), which does not suffer from unknown heating losses. Whether it vaporizes, melts, heats up, or does not gain heat energy, the system’s total mass remains the same (the law of conservation of mass: conserve mass; it is more than just a good idea, etc.), and momentum is merely the mass and its velocity (which need not be conserved).

Dimorphos is 5 billion kg ( https://en.wikipedia.org/wiki/Double_Asteroid_Redirection_Test#Target_asteroid ) and we can set it as the frame of reference, zero velocity. DART is 500 kg traveling at 6.6 km per second ( https://en.wikipedia.org/wiki/Double_Asteroid_Redirection_Test#Course_of_the_impact ) Assuming an inelastic collision, in which all the mass ends up as part of Dimorphos, if I have the decimal in the right place, the final velocity of Dimorphos becomes 0.7 millimeters per second. This comes out to 20,000 km per year.* Quite a change, over time.

The reality is that DART knocked off plenty of rock from the asteroid. Since this mass ended up with a velocity in the negative direction, more momentum was transferred to the remaining part of the asteroid, the main part, than was calculated in the previous paragraph. https://www.youtube.com/watch?v=hg4DiOmKlVM#t=316 (Scott Manley, “Asteroid Smashing”). We should get more change in velocity than expected due to a beta factor, as Manley called it in the video. DART will give us one data point for determining that factor, without the use of a nuclear weapon.

Who knew that Newton would one day save the world? We previously went on the assumption that it would be Einstein (or Oppenheimer or Teller).

Then there is the Yarkovsky effect. https://en.wikipedia.org/wiki/Yarkovsky_effect This changes orbits of certain sized asteroids. Making it harder to analyze and predict their danger to us. The pull of nearby planets, including the Earth, adds to this uncertainty. Oh, the pesky uncertainty of science and of models. It can bite you if you aren’t careful, just as happened with the Wuhan flu panic and is happening with the global climate warming change panic: we have less than eight years left, so conserve them wisely. With the climate-caused end of the world being only a few years away, why are we worried about killer asteroids and changing California to all-electric kitchens — I mean — all-electric cars by 2035? It isn’t as though the state is going to make more power plants in order to feed those cars.

Conserve energy! We will need it for the cars.

* You all should now be pointing out that this assumes DART’s velocity vector goes through Dimorphos’s center of mass, because any deviation from this would impart a change in angular momentum, which must be subtracted from the change in orbital velocity due to conservation of momentum. Oh, those pesky assumptions. Essentially, my analysis is only valid for a spherical asteroid in a vacuum. ( https://www.youtube.com/watch?v=Id0Ppz4OBKE 1 minute, Big Bang Theory: spherical assumption explained). As we saw in the images as DART approached Dimorphos, there was a bright spot jutting up around the horizon of the asteroid, suggesting that there was much more mass of the asteroid hidden in shadow in the lower left. Since the probe was programmed to aim for the center of the illuminated portion, it certainly missed the center of mass, possibly by quite a bit.

I thought about your question, and considered some things.

With that type of device used planet side, you have several effects.
1. Massive Heat and energy expansion.
2. The sudden expansion creates a shock wave outward, through the medium of the atmosphere. The air.
3. A second shock wave back, as the air then rushes back in to fill the space it just left.
4. Radiation.

In space, the first energetic release will still happen, with lots of heat.
But there will be no expanding shock wave, as there is no medium (air/atmosphere) to transport it.
Radiation will dissipate. We do not care, really, compared to the possibility of a strike on the planet.
In a vacuum, how quickly with the energy be dissipated? It is an expanding sphere.
And unless is it very close to the target rock, it will have nothing to push against.

The result is that the warhead’s effective radius is vastly diminished.

In the case of DART, they aimed for center mass, and let it impact.
I am thinking, do not let such a device impact. Detonate above the surface, so it pushes, rather then penetrates.
It would still probably crack it.
And in the case of “rubble pile” asteroids, scatter parts of it, as you suggested.

But the more rapid expansion of energy between us the the rock(s), the more those pieces are deflected into an angle (however small) away from us.

And if the resulting pieces are small enough for the atmo to burn up most of the mass on entry, that works too.

In short, your wanting a “nudge” or a push, rather than trying to penetrate it and blow it up.

If the asteroid is discovered too late then perhaps a short series of these detonations, in a timed sequence might do it. But that gets even trickier. You want that close detonation, but the follow on device(s) must already be aimed at where you THINK the rock will be on it’s new path, post nudge.

I am not much of a writer, but I wrote a short story with this idea in mind. It is terrible, and I will not subject you to it. But it was a fun mental exercise.

I would imagine that it all depends on the size of the asteroid, and the time before impact we blow the things to bits…. That bugger that hit Jupiter some years ago split up due to gravity, but the bits still hit bullseye enough to leave planet sized scars on Jupiter’s atmosphere, which started the planetary defense movement.

I guess a distributed rain of space rocks could be better than one big hit, but no one really knows, and given the uncertainty added with my question, is it even possible to know?

I can see an argument that a close-proximity explosion would create many fragments that would no longer be on a collision course, and also that some portion of the threatening mass would simply be vaporized into a gas or a dust cloud incapable of penetrating the Earth’s atmosphere. Thus the overall damage could be reduced.

Any other thoughts on this?

I have a small iron meteorite from the Campo del Cielo strike in Brazil, and there seems to be many small pieces available for purchase from reputable dealers, so iron meteors obviously can break up in our atmosphere, but the thing I have been pondering is what the difference would be of an impactor on either type.

I can imagine both that an impactor would move a metallic astroid more because of the sold nature, or less than a rubble pile because of the mass difference…

Any thoughts?

By the way, the picture from Webb is the first I have seen with the directions of impactor flight and solar radiant indicated, although I am sure that data was consistently captured.