66 million years ago, something brought an end to the long-established dinosaur empire. The pendulum just kept shifting between an asteroid and a comet, astronomical bolides that actually have different chemistries, hence the distinctions.

Bolides that explode in midair are not unheard of. As of 2020, it remains the likeliest explanation for what happened in the remote Siberian land of Tunguska in 1908. And, of course, there is the numerous video evidence of such a thing at Chelyabinsk in 2013.

In an alternate Earth, the Chicxulub impactor breached the planet's atmosphere, but exploded at only five miles above sea level. With this specified detail, which kind of bolide impactor would detonate in a bigger explosion (therefore cause greater damage to life on Earth) at five miles above sea level--an asteroid or a comet?

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    $\begingroup$ The answer might depend on why it detonated as an airburst in the first place instead of hitting the ground. I suspect that bolides need to be a certain size to reach the ground without detonating; smaller bolides might break up at higher altitudes. The problem is, small breakable bolides might not lend themselves to large booms. $\endgroup$
    – Zxyrra
    Feb 3, 2020 at 1:22
  • $\begingroup$ Would you accept a swarm of smaller bodies which simultaneously exploded in generally the same area? $\endgroup$
    – Muuski
    Feb 3, 2020 at 20:12
  • $\begingroup$ @Muuski I could. $\endgroup$ Feb 4, 2020 at 0:20

4 Answers 4


Frame Challenge: An Air Burst could not do as much damage as Chicxulub

Air bursts make nukes do more damage because they allow you to spread the energy of the initial blast over a larger surface area, but when considering impactors the size of Chicxulub, that stops being the major source of lethality. For an explosion to cause global extinctions, it needs to carry a debris field around the globe. For this you get a better outcome hitting the ground than you do with an air detonation because you aerosolize more solid matter.

Another reason is that objects larger than a certain size are too big to air-burst. For a comet or asteroid to be able to do that, it would need time for it to build up enough potential energy from resistance with the atmosphere to cause it to structurally fail. Larger bolides don't have time during thier decent to do this so they impact the Earth directly. Comets have less integrity; so, the maximum size of a comet that might air-burst is theoretically bigger. The largest recorded air-burst in history was from a bolide currently believed to be an S-Type somewhere between 50-80 meters across which exploded at an altitude of 10-14km (roughly 85-90% of the way into the atmosphere). Using https://impact.ese.ic.ac.uk/ImpactEarth/cgi-bin/crater.cgi, we can estimated that the largest bolide capable of bursting would be a comet about 200m across striking the Earth at an angle just barely obtuse enough to prevent deflection and the smallest thing that could impact without bursting would be a 25 meter M-type asteroid hitting the atmosphere dead on. So, when you are talking about an impact with a body that is over 10km across, even a comet would not be able to air-burst. Instead it's outer layers would ablate from friction while the vast majority of the mass would impact as a solid object.

So, one air-burst won't work, but many air-bursts caused by multiple bolides of the same total mass as Chicxulub might happen. The problem here though is that asteroid debris fields the size of Chicxulub consolidate over time due to thier own gravity. This means for asteroids to be an option, the shower needs to be from a recent collision between massive bodies. (Which is really unlikely)

The most plausible way for this to happen would be if the Earth were to pass through the tail of a very large comet such as Hyakutake. I doubt you would get nearly the total mass of Chicxulub, but you could potentially get hundreds of bolides large enough to cause a dangerous air-burst.


Asteroid - assuming they're the same size

The way a bolide works is pretty cool - when a meteor screams down from space into Earth, it develops a high-pressure field of air in front of it and a vacuum of air behind it. The high-pressure air tries to go to the vacuum, perfectly normal behavior for air, and decides, it additional to the conventional wisdom of going around the giant space rock, it decides to go through it. The high-pressure air goes into the rock and starts blowing chunks of it off - hence the midair detonation. (At least that's our current understanding.)

The explosion isn't reliant on the meteor - it's reliant on the atmosphere. Therefore, when it comes to a question of 'what makes a bigger boom', the answer is clear: Asteroids. They're more dense, have more kinetic energy, and thus will make a bigger explosion. Consider comets, made of ice, which have a density of 1 g/cm^3 to asteroid, which are said to have an average of 2 g/cm^3, although a more accurate representation is to divide them into three categories (C,S,M) which have three sets of weights (1.38 g/cm^3, 2.71 g/cm^3, 5.32 g/cm^3).

  • $\begingroup$ Higher density means that it has less surface area for its mass. If we are talking two same diameter bolides that are small enough to explode, then this is correct, but I think the OP is asking about the biggest possible explosion. In this case, you can get a bigger/more massive comet to reach its explosion point before impacting than you can with an asteroid. $\endgroup$
    – Nosajimiki
    Jun 25, 2020 at 13:36
  • $\begingroup$ @Nosajimiki I did make the assumption that both objects are the same size. It's possible that the higher surface are to mass ratio that comets have would allow for a theoretical larger explosion because at certain levels of masses a comet can turn into a bolide and an asteroid can't, but I'm not certain I'm convinced of that. It's entirely possible that bolides can still happen regardless of asteroid size - after all, it's a chain reaction and all it needs to do is start somewhere on the asteroid to continue and detonate the whole thing. $\endgroup$
    – Halfthawed
    Jun 25, 2020 at 15:06
  • $\begingroup$ It's not exactly a chain reaction so much as a breaking point. Picture holding a stick in your hand. When you apply pressure you are storing potential energy in the molecular bonds that hold it together, but if you apply enough pressure, it will release all that energy all at once as it breaks apart. The bigger the stick, and the stronger the type of wood you use, the more energy it can store before it cracks. But if your stick is too big and strong, you are not able to break it in your hands at all. $\endgroup$
    – Nosajimiki
    Jun 25, 2020 at 16:20
  • $\begingroup$ Small asteroids/comets break up higher in the atmosphere, larger ones get closer to the ground before they break up. M-type asteroids as small as ~50 meters are able to survive reentry mostly intact as is evidenced by Barringer Crater. Larger, less dense C-types and comets break up more easily resulting in things like the Tunguska event. Larger asteroids instead only ablate thier outer layers. This still generates a fair amount of heat, but not an explosion $\endgroup$
    – Nosajimiki
    Jun 25, 2020 at 16:20

Your options are basically limited to;

  1. Changing the composition of the impactor.
  2. Changing its velocity (higher equals more friction) .
  3. Changing its size/volume.
  4. Changing the angle of entry to the atmosphere

Or in reality a combination of any these factors.

The kinetic energy released by an object hitting the Earth is basically governed by its mass and velocity. However density/structure is also an issue.

Comets are icy bodies originating from the fringes of the solar system and composed of light volatile frozen gasses and dust. Consequently the don't have a lot of structural integrity.

Asteroids on the other hand are left over rubble from the formation of the inner solar system and come in three basic types based on their composition. One type consists of mostly light elements like silicon and carbon, the second is a mixture of the first type with metals added and the last is mostly metallic (Nickel/iron). Obviously this last is your worst choice in terms of a mid air explosion unless it is much smaller that the Chicxulub object. Think armour piecing bullet vs hollow point bullet.

Assuming they approach at the same speed and angle as the original impactor you would want a smaller and/or lighter object. A comet (or the remains of one) might be your best bet because it should have less structural integrity than a denser metallic asteroid of the same size which will retain its integrity longer and therefore is more likely to hit the Earth.

Angle of decent is important because a steeper angle increases the energy (shock of impact) on the object as it enters the atmosphere while at shallower angles the time it takes to travel though it before hitting the surface is marginally longer. Depending what the object is made of fractions of seconds would count.

So if you want it blow up before hitting the earth, make it smaller or alternatively less dense than the original and/or have it come in a slightly shallower angle so that it has more time to 'burn' on its way down.


Doesn't Matter - as Long as there's Two of Them

File this one under "extremely unlikely but not actually physically impossible."

Two bolides enter Earths atmosphere at the same time coming from different directions and collide mid-air.

A non-trivial portion of both bolides are vaporized by the energy of the collision, which causes a rapid increase in heat and pressure... otherwise known as an explosion.

This neatly side-steps the issues raised by Nosajimiki, which are all reasonable problems that make the original scenario infeasible.


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