[meteorite-list] Fireball dynamics questions:

From: Rob McCafferty <rob_mccafferty_at_meteoritecentral.com>
Date: Thu, 19 Feb 2009 18:33:39 -0800 (PST)
Message-ID: <625094.22938.qm_at_web55201.mail.re4.yahoo.com>

Here's what I know


>
> 1a. Speed of sound at sea level? 1500 meters per second?
      

no, it's 340m/s


>
> 2. What is the approximate maximum distance a ground
> observer can see fireballs over the horizon as limited by
> the curvature of the earth? my memory also says 250 miles

I remember deriving an solution for "how far can an arial observer see across the ocean from different heights" equation for someone around 4 years back. Light travelling in straight lines and all, the same derivation would apply in the opposite direction, for a ground observer looking at an arial object.

My calculations suggest that an object incandescent at an altitude of 100km could be visible from as much as 600km away. A fireball 6 miles up could be seen from as much as 160km. It'd be right on the horizon though so in reality, probably much closer if there are any hills or buildings. If it stops glowing while above the horizon, it will be closer. Directly overhead being around 15km or so. (see below)

250 miles fits in between both of these values.

> 3. What are the approximate maximum and minimum altitudes
> where a meteoroid can incandesce? 130 down to 5 miles asl?

Atmospheric drag isn't really appreciable above 100km.
Incandescence of a actual meteorite stops at altitudes of 30km - 5km. They will still be travelling at some speed at this height but not fast enough to ionise the air around them. Friction will continue to slow the meteorite




> 4. What is free fall maximum velocity assuming 0 cosmic
> energy remaining? 120 or 400mph?

Again, depends on the mass of the final object and shape of the meteorite. Stoke's Law can be used if modified (air being a compressible fluid) but you'll need to approximate the meteorite fragemts to a sphere. Not always practical.
Terminal velocity will depend on the mass of type of meteorite as well. It's between 120m/s and 250m/s for a fist sized rock of stony and iron meteorites, respectively
It's worth pointing out that unlike ordinary falling objects that start with a vertical component of zero and then increase in speed, a meteorite will start from high speed then reduce to terminal velocity at 6 miles and actually slow down further as it falls into thicker air. Considering the uncertainties in the other factors, though, careful consideration is unlikely to improve the accuracy of your predictions.


Rob Mc






      
Received on Thu 19 Feb 2009 09:33:39 PM PST