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Comment by Zimahl | Hacker News Reader

Can someone explain to me why this would be bright? I understand that this should send up a fair amount of ejecta but wouldn't most of the light be just from a reflection of the sun off of the particles?

If so, this isn't much of an explosion - it's just the most significant impact we've seen so far. I guess explosion is more interesting to the every-man.

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The massive kinetic energy must be dissipated on impact. Some is consumed in displacing the material on the lunar surface. Some is converted into thermal energy, resulting in hot enough material to radiate energy in the visible spectrum.

Read rkaplan's comment above. In short, it's hitting with so much energy that the force of the impact alone is enough to leave the rock glowing-hot.

And even vaporize some of the material, thus an explosion.

Consider the absolutely insane energies involved - the report says "The 40 kg meteoroid measuring 0.3 to 0.4 meters wide hit the Moon traveling 56,000 mph."

That's a thing roughly half the size of a fridge. Traveling at a speed of about 25 kilometers, per second, coming to a full stop.

The ejecta aren't tossed about due to recoil they are lifted out of the ground by an expanding sphere of vaporized rock. And that vaporized rock glows because it is superheated to thousands of degrees, that's what creates the bright flash.

Do you have any reference for this? It seems like you're saying the vaporized rock is imparting force to the impacted surface, which is in turn ejected. I had not understood that the vaporized rock played a central role.

I thought the impactor imparted force directly to the surface. Basically there is a shock wave in the underlying rock which pushes ejecta out of a region underneath the impact site.

Best easy source: http://www.impact-structures.com/understanding-the-impact-cr...

Let me backtrack a bit and say, more accurately that "the ejecta aren't just tossed about due to recoil", both effects play a role. My point was that the flash is caused by superheated/vaporized rock, which you will pretty much always get at such speeds due to the amount of energy released.

And this response, which is quite typical for laypeople like myself, shows why the site put that footnote on that article.

Apparently, the impact would be so hard that the rock heats up quite significantly.

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