tellumFS

So NPR Science Friday is having an episode about all-life-on-Earth-killing comets and asteroids.

I caught that certain sized objects do damage. OK. But, does it matter what the object is made out of? Would an ice ball burn up more than an asteroid/comet made up of rock?

Coppertop

It has more to do with mass. Anything entering the atmosphere is going to lose mass as it heats up if improperly/non shielded.

tellumFS

So the material has nothing to do with it? Say a 1km square rock vs. a 1km square ice hunk. Would the ice hunk burn up more, or at that size, does it matter anymore?

Yakk

I don't know if this helps, but it is some neat information. =)

At that size, energy goes from the orbital thingy to the earth.

1 km square ice hunk weighs about 1 billion tonnes. Solar escape velocity is about 43 kilometers a second. Assuming that comets impact the earth at about solar escape velocity seems reasonable. (The Earth orbits the sun at about 30 km/sec. Comets hitting the Earth would be moving faster than the Earth. Their sum coming to solar escape velocity seems reasonable.)

E = 1/2 m v^2
E =~ 1.9 * 10^18 kg m^2/s^2

This is roughly the same amount of energy in the earthquake that triggered the Indian ocean tsunami. Or, the same energy as about 1000 hydrogen bombs (1 H-bomb could wipe out greater NYC -- every building flattened). The tsunami only contained the energy from about 5 megatonnes of TNT (5 H-bombs) -- the tsunami was a secondary effect.

The earthquake, however, was mainly about shaking large chunks of ground and moving rock around. A comet would instead vapourize water and rock where it hit, sending large quantities of dust into the atmosphere, and a plasma plume into space that would land on the far side of the world.

How much would the comet slow down?

The amount of mass in a column of air 1 km square, to the edge of the atmosphere, is 100 KPa * 1 km^2 / 9.8 m/s^2 =~ 10^10 kg. This is 100 times lighter than your 1 km cube of ice: I doubt it would get in the way much. The comet is coming in supersonically, so all that air would build up under the incoming body without having time to move sideways much. It only takes the comet a second to go from the ozone layer to the surface of the Earth, and shockwaves don't move that quickly through air.

Take a car, and ram it into something weighing 100 times less than the car. The car doesn't even slow down noticeably -- I suspect the same would happen to the 1 km^3 comet.

A rock comet coming down would have 2 to 4 times the density of the ice cube, which means 2 to 4 times the mass. This would make the air even more trivial, and make the impact that much more impressive.

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Last edited by JHVH : 10-29-4004 BC at 09:00 PM. Reason: Time for a rest.

MageB420666

Does it matter if the 1 m^3 object falling on you is made out of lead or aluminum? No, cause either way your gonna die.

What if it's made out of lots of little copper BB's? Then it matters.

It mainly depends on whether the object is going to stay in one peice one entry into the atmosphere or shatter into thousands of smaller pieces. Once the object reaches a certain mass it doesn't matter what it's made out of, if it hits the Earth, say goodbye to humanity. But if the object breaks up into thousands of smaller pieces, it may devastate a large area, but it won't have the same effect as one huge piece.

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flstf

It is certainly unsettling that there are extinction sized comets and asteroids out there with our name on it. It's just a matter of time. Hopefully a long long time.

tellumFS

Yakk - Thanks! Very interesting post!

Now, probably a pretty stupid question here, but say a five megatonne bomb were to be detonated in the ocean...would it have the same kind of effect in creating a tsunami as an earthquake, or would the earthquake's effects be different because it's spread out more on a fault?

MageB420666

If the bomb is detonated on the surface it would do little in the way of creating a tsunami type wave.

Tsunamis are created when a large volume of ocean water is suddenly displaced, causing the wave to actually extend all the way down to the bottom of the ocean, normal ocean waves only travel along the surface or just below it. Earth quakes would generally cause a larger tsunami than a bomb because they can displace a larger quantity of water than a bomb, even though a bomb may release more energy than the earthquake.

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Yakk

That, and while there was 5 megatonnes of energy in the tsunami, there was over 1000 megatonnes of energy in the earthquake.

Less than 0.5% of the energy of the earthquake was turned into the tsunami.

Nuclear bombs have simular problems: they release alot of energy as heat and heavy radiation. Heavy radiation doesn't make a wave. A bunch of the energy would be wasted turning water into steam (or plasma) and escape. Another bunch would be absorbed by the sea floor. Only the parts that pushed water would form a tsunami.

If you wanted to make a really big tsunami with a nuclear weapon or two, I'd be tempted to find some geologically unstable rocks and knock them into the ocean. Las Palmas, in the Canary Islands. http://www.bbc.co.uk/science/horizon..._tsunami.shtml Just push 500 millon tonnes of rock into the water, and you get a super-sonic wave crossing the Atlantic Ocean, destroying everything within 20 km of the coast.

Sort of cute really. Using chemical explosives to trigger a fission explosive to trigger a fusion explosives, which causes alot of rocks to drop, which pushes water, which crosses a quarter of the world and kills millions. Hmm, maybe cute isn't the right word.

As an aside: a computer simulation of a 1.1 km rock hitting the ocean http://www.space.com/scienceastronom...id_030602.html
Big wave! They got a 60 megaton impact-blast and a 400 foot tall wave from a 61 000 km/hour impact.

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Last edited by JHVH : 10-29-4004 BC at 09:00 PM. Reason: Time for a rest.