[X] My Assistant

[helvick]

Pedantic time again - the numbers don't make sense.

Assume that the meteor is "only" travelling at 11.1km/sec when it exits the atmosphere (ie it is just fast enough to escape at the end), anything faster than this will be even less likely to be seen\heard at the same time so this is a good number to base the argument on.

Assume that the "listener" is directly below the point of closest approach to the surface - this must be the point at which the shock will reach the ground soonest. If this was 56km it will take at least 165 seconds to reach the ground at which point in time the meteor will be (at least) 1700km downrange, although probably quite a lot more as it has been decelerating at some rate all the way to its exit velocity. Again the speed of sound is not constant at altitude but I used a sea level speed of 340m/sec here which is higher than the speed at sound throughout most of the atmosphere below 60km.

As it exits the atmosphere the object is at ~96km altitude and some distance downrange (this may or may not be under 1700km, it doesn't matter for the purposes of this exercise). The horizon distance at 96km is 1110km and this is the absolute maximum range from the point of exit that anything on the surface could see this object while it was still in the atmosphere. It's highly unlikely that anything that wasn't in the process of destroying the planet would actually be visible at that range through air but in any case the object's velocity means that it will be much further than this by the time anyone on the surface could has heard the shock wave from the primary object.

I'm not disputing that nprev and others both heard and saw something at the same time and that means that either ugordan's point applies (the point of closest approach has to be much lower for the numbers to work out) or there were secondary objects involved that trailed the primary by some considerable distance (hundreds of km by the time of closest approach).

[tedstryk]

I know a Swedish team once succeeded in recording the sounds, though I have never heard the recordings. One popular explanation is that the sounds are actually a result of Very Long Frequency radio waves, which would explain the time lag. Here is a good article on the topic - http://science.nasa.gov/headlines/y2001/ast26nov_1.htm

[nprev]

FWIW, the sound was the only reason that I saw it at all. It was around 2PM, broad daylight, double boom (very similar to that heard during a Shuttle reentry, BTW), and I looked up expecting to see a fighter contrail...surprise!

[remcook]

steve-you're right of course. other things must be important in this case too, changing the angle at which you see the thing. might be secondaries,as noted, but that doesn't work for e.g. the space shuttle.

found an interesting link:
http://www.rsnz.org/publish/nzjgg/2004/021-lo.pdf
wind plays a role too apparently.

so maybe refraction and wind can do the trick

[helvick]

The Shuttle boom being audible while it's visible is not the same problem - it's decelerating all the way down to subsonic so for some section of its final approach it must be visible at the same time as people on the ground can hear it.

[remcook]

ah,..sorry about that.me being thick again

[SteveM]

Hubble will be covering the 2007 WD5/Mars close approach. From the schedule:

11497 A6C Noll Observations of Mars and 2007 WD5
2008.030 10:54:00 11:05:00 1149705 Noll 05-001 MARS WFPC2 IMAGE PC1-FIX F410M 1.20 05 01 02
2008.030 10:54:00 11:05:00 1149705 Noll 05-002 MARS WFPC2 IMAGE PC1-FIX F502N 1.80 05 01 03
2008.030 10:54:00 11:05:00 1149705 Noll 05-003 MARS WFPC2 IMAGE PC1 F673N 0.40 05 01 04

FWIW, the Hubble observations have not yet made their way into the major asteroid tracking sites. Both JPL and NEODys are reporting their most recent observations as 2008-01-09. Assuming they can do good astrometry from a moving telescope, observations so close to the asteroid's pass by Mars should refine the subsequent trajectory.

Steve M