11 ms·

> By what mechanism. Impact spallation. When the strong shock wave generated in rock by an impact hits a rock/atmosphere (or rock/vacuum) surface, it is refle

by pfdietz 4mo ago

> By what mechanism. Impact spallation. When the strong shock wave generated in rock by an impact hits a rock/atmosphere (or rock/vacuum) surface, it is reflected, producing strong tensile stress. This causes very large acceleration while not subjecting the spalled layer to extreme temperature or pressure. This is the same process that HESH (High Explosive Squash Head) antitank rounds use to defeat tank armor without penetration. The explosive squashes against the armor before detonating, launching a shock through it causing material to spall off the inner surface at high speed. The shock here is weaker than in the impact case, however.

red-iron-pine 4mo ago
while that's possible, it's also likely to burn the hell out of anything in / on / around. any would-be hitchhikers would need to be able to survive extinction-ish level impacts on Mars, survive through the vacuum (e.g. months), and then survive atmospheric re-entry and landing. and then be able to survive in the new ecosystem
- pfdietz 4mo ago
  Certainly a great deal of energy is released. However, on the time scale of an impact, heat doesn't diffuse in rock very far. Even on the time scale of reentry to an atmosphere heat doesn't diffuse far. It's common for a meteorite that has fallen to Earth, despite the surface having been melted and ablated, to have an inside that is still freezing cold.
  - stackghost 4mo ago
    Ah, this was the piece I was missing. I assumed the heat of impact and/or re-entry would sterilize just about anything that wasn't an engineered heat shield.
    - pfdietz 4mo ago
      The concern in the impact is how much heat is produced in the material itself, by the shock wave. A shock wave is dissipative, producing entropy and heat as it propagates, but near the spalled surface it cancels itself out and the heating becomes small.
    - pfdietz 4mo ago
      You might also be interested in the story of Lew Allen's Balls (which is much less prurient than the name might suggest). These were pumpkin-sized steel spheres, coated in a thin layer of graphite, that were suspended as close as 10 meters from one of the Teapot nuclear explosions (22 kT) in Nevada. The explosion vaporized about 1 mm of graphite but otherwise didn't damage the spheres much at all. https://www.youtube.com/watch?v=by1xpy8ob8E https://www.youtube.com/watch?v=by1xpy8ob8E (starting around 5:50).

by pfdietz 4mo ago

red-iron-pine 4mo ago

while that's possible, it's also likely to burn the hell out of anything in / on / around. any would-be hitchhikers would need to be able to survive extinction-ish level impacts on Mars, survive through the vacuum (e.g. months), and then survive atmospheric re-entry and landing. and then be able to survive in the new ecosystem

pfdietz 4mo ago
Certainly a great deal of energy is released. However, on the time scale of an impact, heat doesn't diffuse in rock very far. Even on the time scale of reentry to an atmosphere heat doesn't diffuse far. It's common for a meteorite that has fallen to Earth, despite the surface having been melted and ablated, to have an inside that is still freezing cold.
- stackghost 4mo ago
  Ah, this was the piece I was missing. I assumed the heat of impact and/or re-entry would sterilize just about anything that wasn't an engineered heat shield.
  - pfdietz 4mo ago
    The concern in the impact is how much heat is produced in the material itself, by the shock wave. A shock wave is dissipative, producing entropy and heat as it propagates, but near the spalled surface it cancels itself out and the heating becomes small.
  - pfdietz 4mo ago
    You might also be interested in the story of Lew Allen's Balls (which is much less prurient than the name might suggest). These were pumpkin-sized steel spheres, coated in a thin layer of graphite, that were suspended as close as 10 meters from one of the Teapot nuclear explosions (22 kT) in Nevada. The explosion vaporized about 1 mm of graphite but otherwise didn't damage the spheres much at all. https://www.youtube.com/watch?v=by1xpy8ob8E https://www.youtube.com/watch?v=by1xpy8ob8E (starting around 5:50).