Metorite crashes into Doctors office building !

[Letum]

Originally Posted by UnderseaLcpl

Aren't most meteorites travelling at thousands of kilometers per hour relative to the Earth anyways?

Yes, but the energy carried by a small object, even one going very,
fast is always going to be small. Further more, small objects have
large surface area to mass ratios and that means a lot of drag per
unit of mass. It doesn't take much to slow them down and it will
happen very quickly.

Aren't the ones that actually hit the ground necessarily large or dense because they are the only ones that can survive the heat when they enter the atmosphere?

No. The vast majority that reach earth are very small. This is no
coincidence. Large rocks will usually burn and break up into small
rocks. they have lots of energy to loose. Small fast rocks burn much
slower and for less time because they have less energy to lose.

"What kind of mass would a rock have to have for gravity to be a significant factor in its acceleration towards the surface?

It's a big factor before the rock reaches the atmosphere, but inside
the atmosphere all rocks below a certain size will quickly loose any
kinetic energy they gained through gravitational acceleration.

What mass/density is so big that it is likely to continue to accelerate in the atmosphere?
That can be to worked out and even objects as big as 100,000tonnes
have terminal velocities significantly below 2km/s (according to my
very rough calculations). That means that gravity will not stop the
rock slowing down a lot in the atmosphere. Although not necciceraly
all the way down to TV.

A 200lb metallic rock might impact at some 200mph. Anything under
200 grams is going to be doing well under 100mph.

To put it simply; if it won't kill when you when dropped it out of a 4th
floor window; it won't kill you falling from space either. In both cases
small rocks are likely to accelerate to, or slow down to, terminal
velocity.


Only rocks so big that they could kill you where ever they fell from will
not have enough atmosphere to pass to in order to slow down to TV.

[UnderseaLcpl]

@ Letum

Wow, I had no idea that air resistance had such an effect on objects entering the atmosphere. I knew it was significant, but not to such a degree.

I guess I shouldv'e known, given how quickly small bullets can lose speed, but I never put 2 and 2 together, it seems.

Originally Posted by Letum

Large rocks will usually burn and break up into small
rocks. they have lots of energy to loose.

Well at least I was half-right about this. I didn't elaborate much on the point but I meant that any large rock entering the Earth's atmosphere would likely be very small by the time it hit the ground.



Okay, one more question, if you'll bear with me; The Tunguska blast was supposedly caused by a meteorite. How fast must that rock have been traveling to instantly explosively vaporize its entire mass, assuming it was made of the usual combination of porous but high-density rock and iron?
You don't have to do any calculations or anything, I'm just wondering if you know off the top of your head.

[Letum]

Originally Posted by UnderseaLcpl

Wow, I had no idea that air resistance had such an effect on objects entering the atmosphere. I knew it was significant, but not to such a degree.

It's no so surprising when you think of those intense re-entry
temperatures objects entering the atmosphere build up. All that
heat comes from the drag causing them to expel their kinetic energy.

The air gets highly pressurized in front of the object and becomes a
dense, hot soup. Not easy to push through.

Okay, one more question, if you'll bear with me; The Tunguska blast was supposedly caused by a meteorite. How fast must that rock have been traveling to instantly explosively vaporize its entire mass, assuming it was made of the usual combination of porous but high-density rock and iron?
You don't have to do any calculations or anything, I'm just wondering if you know off the top of your head.

Haha! I'm no physicist. A bit of googleing tells me that most
extraterrestrial objects enter the atmosphere at 5 - 20 km/s.
I would guess that objects that meet the atmosphere at an acute
angle would be more likely to explode and that ice would play a big
factor, but I really have no idea.