<p>Your &quot;it would smash into the earth just like any other asteroid&quot; intuition is basically correct. This hypothetical comet would have less <em>mass</em> than an asteroid of the same size (because ice is less dense than rock), so it would do somewhat less damage, but it would still be quite devastating.</p>
<p><a href="https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/" rel="noreferrer">This site</a> provides a web form you can use to estimate the effect of different impactors. If you put in the size (1 mile), velocity (16 km/s), density (there's a pop-up menu that lets you select &quot;ice&quot;), and impact angle (90 degrees), you get an impact (not an airburst) that produces a fireball with a radius of about 12 km, leaving behind a crater with a diameter of roughly 16 km. You can also put in your distance from the impact site and get fun things like thermal damage from the fireball and damage from the shock wave (plus tsunami wave height if you select an ocean impact).... It also notes that the total kinetic energy before it hits the atmosphere is around 60,000 megatons (TNT equivalent).</p>
<p>(There's an <a href="https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/effects.pdf" rel="noreferrer">accompanying paper</a> with the gory details of how they calculate the estimated effects.)</p>
<p>This wouldn't be an extinction-level event -- keep in mind that the estimated size of the Chicxulub impactor that <a href="https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event" rel="noreferrer">killed the dinosaurs and other creatures</a> was almost ten times larger and was made of rock, and so would have had several thousand times the mass -- but it would be much worse than, for example, the <a href="https://en.wikipedia.org/wiki/Tunguska_event" rel="noreferrer">Tunguska event</a>.</p>

CC BY-SA 4.0
2024-11-20T15:15:36.367Z
I believe this is correct provided it's a solid block of ice, like an ice cube.  If it's more like a comet (a mixture of gravel and fluffy ice) then you'd get something more like the Chelyabinsk meteor where it'll break up into a buckshot spray of debris pretty high up, increasing the exposed surface area, which is where you'd get the fireball air burst.
Darth Pseudonym
CC BY-SA 4.0
2024-11-20T17:38:00.870Z
@DarthPseudonym I'd not count on that. 1km is big enough and the thermal conductivity of granular matter low and latent heat of water high enough that it would arrive virtually unscathed at the surface regardless (except superficial erosion / ablation like most meteors).
planetmaker
CC BY-SA 4.0
2024-11-20T17:53:27.410Z
If the atmospheric stress breaks apart a 'dirty snowball' into pieces that are smaller than a meter, they're definitely going to disintegrate before they hit the ground. There might be some impacting material but if it undergoes a total breakup I'd expect each piece to ablate in a fairly short distance.
Darth Pseudonym
CC BY-SA 4.0
2024-11-20T18:58:53.133Z
@DarthPseudonym I don't think that's true when the comet is that big. If you play around with the calculation form I linked to, you'll find that small ice-density objects *do* air-burst like Chelyabinsk. Objects of $\sim$1000-1500 m don't; they start to break up, but still hit the ground, creating a fireball and a crater. For 1500m or more,  you mainly just get a ground impact.
Peter Erwin
CC BY-SA 4.0
2024-11-20T19:09:15.437Z
[Toon et al. (1997)](https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/96RG03038) note: "Only a rather large comet will reach the surface with a considerable fraction of its energy left, because comets are relatively weak and easily fragment." But "relatively large" turns out to mean "$r >$ 200 m" (from their Table 4, for short-period or long-period comets; this is radius at the object will reach ground with half its initial energy).
Peter Erwin
CC BY-SA 4.0
2024-11-26T13:16:53.533Z
The question's *sphere of water ice* sounds pretty solid to me, not just a dirty snow ball. I wouldn't expect any significant breaking up when it crosses the atmosphere in a few seconds.
Zac67
CC BY-SA 4.0
2024-11-20T09:34:10.447Z
2024-11-20T09:34:10.447Z
Peter Erwin
<p>Although the question sounds silly to me, my friend says it's a hot question on Quora, so I told him I'd ask what the smart people think.</p>
<p>It seems various factions say it will:</p>
<ul>
<li>Melt and rain</li>
<li>Burst into a fine mist</li>
<li>Hit the ground and shatter</li>
<li>Explode in the air like a bolide</li>
<li>Have the same effect as a rock</li>
<li>Whatever idle minds can come up with</li>
</ul>
<p>Other than the fact that It would have about five seconds between hitting the atmosphere and the ground, and that the ice would be super hard after being near absolute zero, the only thing I can say is that I guess it would smash into the earth just like any other asteroid. The damage is done by the momentum of the mass, and that can't change, whether it's water or solid.</p>
<p>The phase don't enter into it.</p>

<asteroids><near-earth-object><collision>
What would happen to a mile-wide sphere of water ice that slammed into the Earth at 90 degrees straight down, at 35,000 MPH?
Miss Understands
# What would happen to a mile-wide sphere of water ice that slammed into the Earth at 90 degrees straight down, at 35,000 MPH?

(Asked by: username_0 on 2024-11-20)

Although the question sounds silly to me, my friend says it's a hot question on Quora, so I told him I'd ask what the smart people think.

It seems various factions say it will:

* Melt and rain
* Burst into a fine mist
* Hit the ground and shatter
* Explode in the air like a bolide
* Have the same effect as a rock
* Whatever idle minds can come up with

Other than the fact that It would have about five seconds between hitting the atmosphere and the ground, and that the ice would be super hard after being near absolute zero, the only thing I can say is that I guess it would smash into the earth just like any other asteroid. The damage is done by the momentum of the mass, and that can't change, whether it's water or solid.

The phase don't enter into it.

**Tags:** asteroids, near-earth-object, collision


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## 1 answers

### Answer by username_1 on 2024-11-20. Score: 7 | Accepted answer

Your "it would smash into the earth just like any other asteroid" intuition is basically correct. This hypothetical comet would have less *mass* than an asteroid of the same size (because ice is less dense than rock), so it would do somewhat less damage, but it would still be quite devastating.

[This site](https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/) provides a web form you can use to estimate the effect of different impactors. If you put in the size (1 mile), velocity (16 km/s), density (there's a pop-up menu that lets you select "ice"), and impact angle (90 degrees), you get an impact (not an airburst) that produces a fireball with a radius of about 12 km, leaving behind a crater with a diameter of roughly 16 km. You can also put in your distance from the impact site and get fun things like thermal damage from the fireball and damage from the shock wave (plus tsunami wave height if you select an ocean impact).... It also notes that the total kinetic energy before it hits the atmosphere is around 60,000 megatons (TNT equivalent).

(There's an [accompanying paper](https://impact.ese.ic.ac.uk/ImpactEarth/ImpactEffects/effects.pdf) with the gory details of how they calculate the estimated effects.)

This wouldn't be an extinction-level event -- keep in mind that the estimated size of the Chicxulub impactor that [killed the dinosaurs and other creatures](https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event) was almost ten times larger and was made of rock, and so would have had several thousand times the mass -- but it would be much worse than, for example, the [Tunguska event](https://en.wikipedia.org/wiki/Tunguska_event).

> **Comments:**
> **username_2**: I believe this is correct provided it's a solid block of ice, like an ice cube.  If it's more like a comet (a mixture of gravel and fluffy ice) then you'd get something more like the Chelyabinsk meteor where it'll break up into a buckshot spray of debris pretty high up, increasing the exposed surface area, which is where you'd get the fireball air burst.  
> **username_3**: @username_2 I'd not count on that. 1km is big enough and the thermal conductivity of granular matter low and latent heat of water high enough that it would arrive virtually unscathed at the surface regardless (except superficial erosion / ablation like most meteors).  
> **username_2**: If the atmospheric stress breaks apart a 'dirty snowball' into pieces that are smaller than a meter, they're definitely going to disintegrate before they hit the ground. There might be some impacting material but if it undergoes a total breakup I'd expect each piece to ablate in a fairly short distance.  
> **username_1**: @username_2 I don't think that's true when the comet is that big. If you play around with the calculation form I linked to, you'll find that small ice-density objects *do* air-burst like Chelyabinsk. Objects of $\sim$1000-1500 m don't; they start to break up, but still hit the ground, creating a fireball and a crater. For 1500m or more,  you mainly just get a ground impact.  
> **username_1**: [Toon et al. (1997)](https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/96RG03038) note: "Only a rather large comet will reach the surface with a considerable fraction of its energy left, because comets are relatively weak and easily fragment." But "relatively large" turns out to mean "$r >$ 200 m" (from their Table 4, for short-period or long-period comets; this is radius at the object will reach ground with half its initial energy).  
> **username_4**: The question's *sphere of water ice* sounds pretty solid to me, not just a dirty snow ball. I wouldn't expect any significant breaking up when it crosses the atmosphere in a few seconds.  

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