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Comment by scottshea | Hacker News Reader

And that ladies and gentlemen is why having an atmosphere is a very good thing.

I wonder if the meteorite that exploded in the atmosphere over Russia did more or less damage than if it had struck the earth intact.

Clearly Atmosphere makes a good shield for burning up the small meteorites - but I wonder if it exacerbates the damage from medium sized ones?

Had that impact hit the ground it would have released over 100 kilotons of energy. It would have been far, far more destructive and deadly.

But without an atmosphere the shockwave from that 100 kiloton explosion should be pretty minimal (through the ground only), and any debris flung into the atmosphere would fall right back down on parabolic trajectories (or go into orbit), not linger in the sky for months/years.

The danger posed by such an explosion, off the top of my head, would be getting hit by the object itself, getting hit by falling debris ejected by the explosion, structural damage from the shockwave that traveled through the ground... I'm not sure what else.

Of course without an atmosphere everyone would be dead anyway, so I suppose this is kind of a silly thing to think about.

Of course without an atmosphere everyone would be dead anyway, so I suppose this is kind of a silly thing to think about.

Just never walk out of your front-door without wearing your space suit.

Hmm. Living in pressure vessels makes structural damage to buildings a great deal more dangerous.

Sort of, there's still a blast wave from the vaporized rock, and there's still the ejecta blanket (which could easily ruin your day if any of it hit you).

How could it release more energy if it hits the ground? The total kinetic energy released must be the same whether it is released in the atmosphere or on impact with the ground.

The toal energy released is the same. The total energy released on the impact is quite different.

100 kilotons of energy released along a several hundred kilometer long tube in the upper atmosphere, at altitudes of tens of kilometers poses much less of a threat to people who live on the surface of the Earth than if all that energy is released at once on the ground.

Define "damage", I guess. It's spread over a larger area, but there's a qualitative difference between widespread broken windows and a local 440 kiloton (per wikipedia) explosion on the ground that will level the surrounding terrain.

People tend to focus on the size of projectiles from space, but their great speed is often overlooked - from a simulator at: http://impact.ese.ic.ac.uk/ImpactEffects/

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s. Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets. The maximum Earth impact velocity for objects orbiting the sun is 72 km/s

For comparison, the velocity of the ISS is about 7.7 km/s, about equal to the speed of detonating cord.

The minimum impact velocity on Earth is 11km/s??? I beg to differ -- bodies can impact the Earth at arbitrarily low velocities. It depends on where they originate. There are clearly some assumptions being made there that aren't spelled out.

Starting from rest at a distance infinity, any object (regardless of it's own mass) necessarily undergoes a minimum increase in kinetic energy due to the gravitational potential of the mass of the target object (in this case the Earth, which has a known mass and so this is calculable). So I think the assumption is that the object isn't propelling itself away from the earth, and the object hasn't experienced any major competing gravitational forces inside the radius of the Moon's orbit.

Also, 11km/s isn't terribly quick on the celestial scale, I'd imagine.

What you say is true but most objects are not starting from rest at infinity. Things thrown up from the Earth, for example.

It also neglects that there is an atmosphere...

>Impact Velocity - This is the velocity of the projectile before it enters the atmosphere. The minimum impact velocity on Earth is 11 km/s.

I think it's implied that the impacting objects come from outside the Earth-Moon System. Otherwise there would obviously be no minimum impact velocity.

No kidding. Shoot an 8-foot 2x4 at the Earth at 100km/sec and guess how fast the carbon-fluff will be going when it hits. The pieces of anything that breaks up as it descends will quickly slow to their terminal velocity at that height.

NASA says the February Russian fireball was going about 11.6 miles/sec when it blew up (14 miles up) but there's a lot of denser atmo in that 14 miles.

http://neo.jpl.nasa.gov/news/fireball_130301.html

That, and breathing is nice.

Don't forget cosmic rays!

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