There are several reasons why it may not be so hot when it reaches the ground. Ablation obviously occurs and heat build may be counterbalanced by heat transfer back into the air.

I’m not satisfied with glib.

Chris P

That’s what I mean! There was nothing wrong with the URL, but my post still got spammed!

Are we back to mangling the URL’s once more? Sigh! Moan! Sigh!

I was amused later to hear that people were ascribing the Ponsonby fire to this! Personally I would describe the meteor fireball as originating in the West and travelling towards the ESE. I live West of the suburb of Ponsonby in Auckland about 20 km out. It’s “apparent” trajectory was therefore “towards” the city. But it was also apparent to me that it was at a very high altitude. I have also heard of eye witnesses to this event as far south as Motueka in the South Island of New Zealand. Which I think confirms the high altitude. I’m obviously no expert but I would think these things rule out any possibility of a landfall in or around Auckland city.

Fortunately, I wrote it out on WordPad and saved it on my computer’s hard-drive. So, it’s not a problem for me to resubmit it, if you’re interested;
otherwise, you can live in ignorance.

Remember too, that left a nice crater in the ground. There was nothing like that with this warehouse fire. I am still pretty sure this was unrelated to the fireball. We still have no reports of the direction of the fireball, and whether it was anywhere near the actual location of the warehouse. People are in general TERRIBLE witnesses for meteorite impacts; they cannot judge distance at all in these cases.

Mind you, those reports were mostly in newspapers, and they notoriously cannot be trusted for accuracy, especially right after an event like this.

I think what we can gather from all this is:
1) The meteorite heats the air due to compression.
2) Compression makes the air hot enough to glow because of conservation of energy: the same amount of energy suddenly occupies a much smaller space, which equates to increasing “hotness”.
3) If friction causes the meteorite to slow down sufficiently before impact, we get a cold spacerock that plumps down “gently” at terminal velocity.
4) If friction isn’t sufficient to slow it down, we get a massive rock surrounded by burning air slamming into the planet at many times the speed of sound.

Neat! Refresher in thinking critically about physics! Thanks guys!

I still think we should ask the astrologers, though

Forget the Star of Bethlehem! What physical explanation can anyone come up with for this miracle?

http://en.wikipedia.org/wiki/2007_Peruvian_meteorite_event

At 11:45 local time (16:45 GMT) on September 15, 2007, a chondritic meteorite crashed near the village of Carancas in the Puno Region, Peru, near the Bolivian border and Lake Titicaca (see map box on right). The impact created a crater larger than 4.5 m (15 ft) deep, 13 m (43 ft) wide, with VISIBLY SCORCHED earth around the impact site. A local official, Marco Limache, said that “BOILING WATER started coming out of the crater, and particles of rock and cinders were found nearby”, as “fetid, noxious” gases spewed from the crater. The crater size was given as 13.80 by 13.30 meters (45.28 by 43.64 feet), with its greatest dimensions in an east-west direction. The fireball had been observed by the locals as strongly luminous with a SMOKY TAIL, AND SEEN FROM JUST 1000 METERS (3280.84 FT) ABOVE THE GROUND. The object moved in a direction toward N030E. The strong explosion at impact shattered the windows of the local health center 1 kilometer (0.62 miles) away. A SMOKE COLUMN was formed at the site that lasted several minutes, and BOILING WATER was seen in the crater.

It’s obviously an incomplete picture being presented so far. The compression and increase in temperature of the air (called adiabatic heating) in the shockwave just in front of the meteor does NOT account for the meteor itself heating up or ablating. There are several possible heat transfer mechanisms for getting the heat from the air shockwave to the meteoroid – thermal conduction, thermal convection, radiative transfer, etc. but I think friction is probably a very efficient mechanism. Thermal conduction and thermal convection are probably very minor influences at the high altitudes and low air densities where ablation is taking place. So I wouldn’t be so quick to kick friction out of the picture entirely.

A book on the threat of meteoroids–what a great idea.

Maybe someone we know here might try writing one. Something with a catchy title like: “Doom From Above” or “Comet Catastrophies.”

Seriously, I was amazed Phil didn’t add a sentence or two. Maybe he figures we all got the message and book sales are high enough.

And I still have to wait until a week from Wednesday to unwrap and begin reading my copy!

Bah. Humbug. I spent the whole hour trying to explain to my friend that by the time meteors hit the ground, they’re nowhere near hot enough to cause fires, and that the only way I could imagine that happening would be if (as mentioned above) the thing hit a gas tank hard enough to rupture it and struck a spark. He wasn’t buying it.

I’ve heard from lots of people that the DC seems to exist primarily to confoozle people about all things scientific. More and more, I find myself agreeing with them.

I don’t have my copy of Hoerner at hand to check the numbers but there are fundamentally two types of drag associated with a body moving through a fluid. One is called pressure drag which is what is causing the compression heating and the other is friction drag caused by the passage of fluid over the skin.

Just because you can break the tiles with your bare hands doesn’t make them weak – it may mean they are brittle. The frictional forces are low otherwise the shuttle wouldn’t fly well.

“So if there is no friction then why does it slow down?”

Wait, who said there is no friction? Phil said that compression causes the heat, but he didn’t say friction does not exist.

Objects large enough to survive the heating from compression will certainly slow down, due to the increase in atmospheric density. As others have said, the object will slow down to its terminal velocity, which is certainly caused by air resitance.

Perhaps, but it would be a very small portion. The shockwave that forms in front of the meteor is itself in front of a layer of slower-moving air that surrounds the meteor.

The best example I’ve heard explaining this is the heat tiles on the Space Shuttle. They are fragile and can easily be broken using bare hands. If there were enough friction to create the heat necessary to make them incandescent, they would all be immediately stripped off.

The explanation isn’t that thorough in this article (it was a digression!), but it’s not exactly difficult to look up elsewhere.

Chris P

The answer given is half assed. Obviously part of the heating is due to friction. It just sounds better to make the radical claim when it is not strictly true.

If there is an atmosphere – there is friction – simple fluid dynamics.

Chris P