3 ms·

Nah, even the big ones are shown sauntering through the atmosphere at maybe Mach 3, trailing smoke and flames like lit pitch. A city-sized asteroid wouldn't be

Nah, even the big ones are shown sauntering through the atmosphere at maybe Mach 3, trailing smoke and flames like lit pitch. A city-sized asteroid wouldn't be more than a bright dot until the last seconds before entry, and at interstellar speeds it would take about three seconds to go from the vacuum of space to ground impact unless it had a very shallow vector.

pfdietz 3y ago
At relativistic speeds it would take about 1 millisecond to go from LEO altitude to the ground.
- buildbot 3y ago
  A relativistic asteroid would not be a good time for earth. Apparently according to gpt4 (so big grain of salt), a 1km asteroid going 0.9c is roughly 4.37×10^16 megatons of TNT worth of energy.
  - szundi 3y ago
    Or a grain of TNT
  - dylan604 3y ago
    that's one of those too big to comprehend type numbers. that's fine when talking about number of atoms, or the distances to another galaxy. trying to have my brain apply that to explosion just immediately goes to BSOD
    - buildbot 3y ago
      Yeah same haha - for a slightly more relevant number, if every nuclear weapon yielded 50MT of TNT, the asteroid impact is approx 63 Billion times the entire worlds nuclear arsenal in terms of power.
      - short_sells_poo 3y ago
        And that is still impossible to comprehend, so how about this, the Sun's energy output is about 10^10 megatons of tnt per second, so this impact would release 1 million second's worth of the Sun's output, or about 12 days. In other words, it would instantly vaporize the entire planet.
        buildbot 3y ago
        For the ultimate nerd snipe, how far would the light from that explosion be visible?
        dylan604 3y ago
        using what to "see"? naked eye? binocular aided? telescopes? also, seen by who/what? for the only known living things that could see it, it would be the last thing they did see. so maybe as far away as a couple of inches?
  - pineaux 3y ago
    This is in the correct ballpark. But 0.9c is crazy fast. [Expanse spoiler alert] I love how they use this mechanic for storytelling in "The Expanse". The rebels of the belt threaten earth by slinging rocks at it at really high speed. Seems a very realistic way of interplanetary war. Why not bend some fast moving rocks to your enemy?
    - aurelwu 3y ago
      I don't know if it's been the first time in fiction where that concept was described but in Heinleins "The Moon is a harsh Mistress" this was what the Moon Colonists did.
  - Terr_ 3y ago
    > according to gpt4 (so big grain of salt) [...] 4.37×10^16 megatons I decided to challenge the glorified-autocorrect machine with some more-manual napkin math, and I get 5.56×10^13 megatons, which is smaller by a factor of ~786x. That discrepancy is too big to explain just in terms of asteroid composition: Even an impactor of pure Osmium would only be ~12x more energetic. Here's my work, if anyone wants to check for errors: Asteroid volume 1 km³ Asteroid density 1 2 grams / cm³ https://en.wikipedia.org/wiki/Standard_asteroid_physical_characteristics Volume conversion 1E+15 cm³ / km³ Asteroid density 2 2E+15 grams / km3 Asteroid mass 2E+15 grams Asteroid velocity 0.9 c Relativistic kinetic energy 2.33E+29 joules https://www.omnicalculator.com/physics/relativistic-ke TNT energy 4.18E+03 joules/gram TNT mass equivalent 1 5.56E+25 grams Mass conversion 1.00E+12 grams per megaton TNT mass equivalent 2 5.56E+13 megatons

pfdietz 3y ago

At relativistic speeds it would take about 1 millisecond to go from LEO altitude to the ground.

buildbot 3y ago
A relativistic asteroid would not be a good time for earth. Apparently according to gpt4 (so big grain of salt), a 1km asteroid going 0.9c is roughly 4.37×10^16 megatons of TNT worth of energy.
- szundi 3y ago
  Or a grain of TNT
- dylan604 3y ago
  that's one of those too big to comprehend type numbers. that's fine when talking about number of atoms, or the distances to another galaxy. trying to have my brain apply that to explosion just immediately goes to BSOD
  - buildbot 3y ago
    Yeah same haha - for a slightly more relevant number, if every nuclear weapon yielded 50MT of TNT, the asteroid impact is approx 63 Billion times the entire worlds nuclear arsenal in terms of power.
    - short_sells_poo 3y ago
      And that is still impossible to comprehend, so how about this, the Sun's energy output is about 10^10 megatons of tnt per second, so this impact would release 1 million second's worth of the Sun's output, or about 12 days. In other words, it would instantly vaporize the entire planet.
      - buildbot 3y ago
        For the ultimate nerd snipe, how far would the light from that explosion be visible?
        dylan604 3y ago
        using what to "see"? naked eye? binocular aided? telescopes? also, seen by who/what? for the only known living things that could see it, it would be the last thing they did see. so maybe as far away as a couple of inches?
- pineaux 3y ago
  This is in the correct ballpark. But 0.9c is crazy fast. [Expanse spoiler alert] I love how they use this mechanic for storytelling in "The Expanse". The rebels of the belt threaten earth by slinging rocks at it at really high speed. Seems a very realistic way of interplanetary war. Why not bend some fast moving rocks to your enemy?
  - aurelwu 3y ago
    I don't know if it's been the first time in fiction where that concept was described but in Heinleins "The Moon is a harsh Mistress" this was what the Moon Colonists did.
- Terr_ 3y ago
  > according to gpt4 (so big grain of salt) [...] 4.37×10^16 megatons I decided to challenge the glorified-autocorrect machine with some more-manual napkin math, and I get 5.56×10^13 megatons, which is smaller by a factor of ~786x. That discrepancy is too big to explain just in terms of asteroid composition: Even an impactor of pure Osmium would only be ~12x more energetic. Here's my work, if anyone wants to check for errors: Asteroid volume 1 km³ Asteroid density 1 2 grams / cm³ https://en.wikipedia.org/wiki/Standard_asteroid_physical_characteristics Volume conversion 1E+15 cm³ / km³ Asteroid density 2 2E+15 grams / km3 Asteroid mass 2E+15 grams Asteroid velocity 0.9 c Relativistic kinetic energy 2.33E+29 joules https://www.omnicalculator.com/physics/relativistic-ke TNT energy 4.18E+03 joules/gram TNT mass equivalent 1 5.56E+25 grams Mass conversion 1.00E+12 grams per megaton TNT mass equivalent 2 5.56E+13 megatons