maximum speed in atmosphere without triggering atomic fireball

[dstarfire]

In an xkcd what-if article, they stated that if you could somehow throw a baseball at near the speed of light, the air molecules wouldn’t be able to move out of the way and would be smashed apart by the baseball, inducing a nuke-style chain reaction.
*
So, what’s the fastest speed an object could travel in atmosphere **without **causing that? *

*What factors would affect this maximum speed and how much? *(I assume the objects shape would have an effect, but I’m not sure).

[Leo_Bloom]

Umm…

IANAPhysicist, but

1. It’s not going to happen. Something happened at Los Alamos when a military person worried that the atmosphere might catch fire, or something like that, but not that the whole thing would go nuke.

2. Air. The material and structure of the object. Propulsion.

If you could, cite the cartoon.

[Hail_Ants]

It wouldn’t matter how fast the baseball went, air is not fissionable. IOW its atomic structure is such that it could never even start (let alone sustain) a nuclear chain reaction. Either the ball would quickly (and violently) disintegrate, or if it was made of some theoretical ‘Unobtanium’ the air would simply get converted to plasma in a massively growing shock wave. Same thing that happens to re-entering spacecraft only a lot fasting & bigger…

[lazybratsche]

I’ll just drop off this (obligatory) link: “What would happen if you tried to hit a baseball pitched at 90% the speed of light?” Randall’s answer is that there will be a *fusion *reaction between the baseball and the air, creating a rather large explosion.

[chorpler]

Yeah, it’s not that the air molecules are being smashed apart, they’re being smashed together hard enough to create a zone of nuclear fusion.

[dstarfire]

So, terminology aside, my question still stands: What’s the maximum speed an object can travel in earth’s atmosphere without creating a massive fireball (assume at sea-level, 70’ Fahrenheit temp, and relative humidity of 30-70%, if those matter)?

[Chronos]

There are actually two questions: How fast can you go before you get a massive fireball (and just how massive must it be before it counts?), and how fast can you go before you start triggering fusion. For an object the size of a baseball and any reasonable definition of “fireball”, the former will happen long before the latter.

[Leo_Bloom]

Anything can be initiated, caused to fuse, be involved in a fusion reaction (don’t know how to say it)?

[Sunspace]

@Chronos

There are actually two questions: How fast can you go before you get a massive fireball (and just how massive must it be before it counts?), and how fast can you go before you start triggering fusion. For an object the size of a baseball and any reasonable definition of “fireball”, the former will happen long before the latter.

Well, we know from Chelyabinsk that an object travelling at around 18 km/s in the atmosphere will produce a dandy fireball…

[Machine_Elf]

@Leo_Bloom

Anything can be initiated, caused to fuse, be involved in a fusion reaction (don’t know how to say it)?

Under the right conditions, yes. Maybe an astrophysicist will reply, but my understanding is that all the heavier elements exist because of fusion: young stars fuse hydrogen into helium, but at later stages, helium gets fused into even heavier elements. see stellar nucleosynthesis.

So Randall@XKCD may be right: if you move an object through the atmosphere at truly ludricous speed, it may be possible to fuse (that is, to join) atoms of oxygen and/or nitrogen into heavier atoms just by virtue of slamming them into the object (or each other) at high enough speeds.

[dstarfire]

@Chronos

There are actually two questions: How fast can you go before you get a massive fireball (and just how massive must it be before it counts?), and how fast can you go before you start triggering fusion. For an object the size of a baseball and any reasonable definition of “fireball”, the former will happen long before the latter.

I’m really only interested in the second question.

From the original article:

The ball is going so fast that everything else is practically stationary. (…) The ideas of aerodynamics don’t apply here. Normally, air would flow around anything moving through it. But the air molecules in front of this ball don’t have time to be jostled out of the way. The ball smacks into them so hard that the atoms in the air molecules actually fuse with the atoms in the ball’s surface. Each collision releases a burst of gamma rays and scattered particles. (…) They start to tear apart the molecules in the air, ripping the electrons from the nuclei and turning the air in the stadium into an expanding bubble of incandescent plasma.

Compared to that, meteor-like effects are fairly boring.