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Comment by marcosdumay | Hacker News Reader

On movies those are usually very large objects. Very large objects do hoover menacingly for a very long time before they finally appear to fall for a few seconds.

The larger it is, the longer it will stay hoovering and only slowly growing in size.

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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.

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

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.

Or a grain of TNT

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

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.

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.

For the ultimate nerd snipe, how far would the light from that explosion be visible?

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?

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?

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.

> 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

>stay hoovering

Hehe, I too love vacuuming.

No, that's Hoovering. case sensitivity is important /s

Edit: added /s for those unable

I've never seen anyone capitalize googling or astroturfing, though my dictionary seems to want to capitalize Xeroxing.

No one does that. Except perhaps the PR and legal departments of the Hoover company.

I suppose you capitalise aspirin as well.

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