[X] My Assistant

[edstrick]

"Here's a very near miss..."

<sigh> I WAS THERE at the time... In the vicinity of the Old Faithful visitor's center or driving around Yellowstone avoiding spot showers and grumbling at @#$#@ idiots who stop in the middle of the 2-lane road to take photos of the @#$# bears.

NO, I didn't see it. We found about it the next day in the paper's headlines. But I've been as close to an asteroid as they person who took that pic from east of the Grand Tetons.

[nprev]

I was there, too, and outside up in Butte; heard the double sonic boom, looked up, saw this thing streaking across the sky...first thought was that it was an ICBM headed for Malmstrom AFB!

[ugordan]

heard the double sonic boom, looked up, saw this thing streaking across the sky

I never could quite figure out how you can hear the sonic boom while the object's still flying overhead? That had to be a pretty low pass. Say the meteor's flying at 3 km/s overhead at 20 km altitude. That's one minute before the sound hits you. By that time the object's travelled almost 200 km downrange (assuming level flight) so how can it be readily visible? It ought to be a mere 5 degrees in elevation at most at that time, far into the distance.

I'm not accounting for the deceleration here, but the 3 km/s velocity could be regarded as average and is not very high, implying significant breaking has already occured by the time it flies above your head.

[tasp]

nprev:

thanx so very much for providing the 'classic' picture of the Jackson Lake Bolide I mentioned in my post in this thread 12/30. I neglected uploading the picture, and it certainly is an awsome thing to look at.

[remcook]

ugordon - I think the shock wave that reaches you is actually created earlier and the shock wave itself moves quite quick horizontally. need to check though

[tedstryk]

Whatever the explanation is, this is an area in which using those numbers would be sort of like using Thomas Gold's radar and infrared data to show that one couldn't land on the moon without sinking, even post Apollo. The result is a known - the existence of bolides is not controversial. In fact, I once experienced a large bolide that I would have missed, except that I heard it and looked up.

[ugordan]

The result is a known - the existence of bolides is not controversial. In fact, I once experienced a large bolide that I would have missed, except that I heard it and looked up.

I'm not denying that people have heard bolides (or the Space Shuttle reentering overhead for that matter), I'm just puzzled by how the sound can get to you almost simultaneously from a distant object.

[djellison]

Ahh - I see the question. It's not a sound from the vehicle that gets to you - it's the shockwave passing you.

Imagine a car driving past you - with a massive long stick sticking out the window. The stick could hit you before the sound of the car. Sort of.


Bad analogy - but I think that's the point.

Doug

[ugordan]

The problem with that is the "shockwave" is only a true shock (as in "really energetic") in front of the bolide, it quickly transitions to a regular acoustic wave (as it loses energy, it quickly slows down) travelling at the speed of sound. It's just that it has a peculiar shape - not concentric sound waves, but rather a sound front which is the thing making the loud boom. If you have a fast object, it will make a conical sound front (Mach cone) which will appear to drag behind it. The faster the object travels, the narrower the cone. While both the object and the cone appear to move rapidly forward, it's only the transverse velocity of the sound that gets to you. And that one, I *think* is the regular sound speed.

There's a nice web page showing some animations too: http://www.kettering.edu/~drussell/Demos/d...er/doppler.html

Notice that, for supersonic shocks, the wave is still travelling outward at the speed of sound.

This is what makes hearing bolides so early counterintuitive to me. The most plausible explanation would be those things pass by a LOT nearer than I thought they do when you hear them.

[helvick]

Your explanation makes perfect sense to me - if something is audible on the ground and visible then it can't be from something that is moving more or less overhead at multiple km/sec speeds _or_ it has to be very low. In this case perhaps it was much lower and smaller than we're assuming? If not then I wonder would it be possible for an obect with a fairly dense solid core but covered in rubble to shed the rubble right at the start of the "graze" and have that slow down and trail behind the dense core in such a way that certain points along the ground track could get the sound from the shock wave from the high speed core reaching the ground at the same time that the fireball from the slower debris was coming into view?

I saw a very bright bolide one night sometime in the early 80's. It was bright enough to cast noticable shadows and my very dodgy memory of it was that it took many seconds to pass. I was with a group of friends and we thought it had to be some sort of flare but we learned later that it was a meteor\bolide and had also been seen in the midlands (approximately 80 miles north of us). One thing that I am certain of was there was no sound from it certainly nothing as noticable as a sonic boom or even distant thunder.

[nprev]

The 10 Aug 1972 object was apparently 5-10 m across, doing something like 10 km/sec relative to the Earth. Closest approach was 35 miles (56 km) over the Montana-Idaho border, or about 60 miles (96 km) west of where I was...and, let me tell you, that double boom was loud. IIRC, some windows in town were broken. F-106s used to cause booms fairly often back then, so I was familiar with that sound, but this was much, much more intense. (Just offering up an eyewitness account as data, here).

Apparently I wasn't the only one who initally thought this was an ICBM, either. I remember a local news story a few days afterward that said that NORAD was something like thirty seconds from making a launch decision, mostly because the ground track was coming uncomfortably close to some of the Malmstrom AFB Minuteman sites (they must have thought at first that it was a submarine launched missile)--brr. Significant & ultimately useful event in this context, though; think everyone can tell a meteor from a missile now.

EDIT: The best estimate I could find for the encounter velocity seems to be 14.7 km/sec, which makes sense; it was obviously above escape velocity. Also, sonic booms were only heard in Montana during C/A; critical altitude threshold for producing booms in Earth's atmosphere seems to be around 60 km.

[mchan]

14.7 km/s would be something like a "rods from God" hypervelocity ICBM RV.

The track in the video I saw appeared roughly parallel to the horizon. An ICBM RV comes in a much steeper angle than say a shuttle re-entry track with one advantage being minimizing the time in the atmosphere for an endo-interceptor. Still, I might be having a "Depends" moment if I had been there and knowing that Malmstrom was right next door.

[nprev]

Still, I might be having a "Depends" moment if I had been there and knowing that Malmstrom was right next door.

I came close to that! Actually, I was 9 years old at the time, and was with my 7-year old brother; we ran like hell home & got in the basement. When the world didn't blow up after half an hour or so, we came back up.

FYI, from my viewpoint the fireball & its track were at least 50 deg above the horizon (closer to the zenith than the horizon, anyhow) so it was not apparent that it wasn't on an impact trajectory.

[remcook]

My point was that the shock you hear was made when it was over the horizon behind you. The shockwave then travels with the meteor until you see the meteor at the angle of the shockwave, so in front of you - I think.... does this make sense??

edit - even better - the object MUST be in your field of view when you hear a boom, just by the geometry of the shockwave (cone with moderate angles).

[SteveM]

My point was that the shock you hear was made when it was over the horizon behind you. The shockwave then travels with the meteor until you see the meteor at the angle of the shockwave, so in front of you - I think.... does this make sense??

edit - even better - the object MUST be in your field of view when you hear a boom, just by the geometry of the shockwave (cone with moderate angles).

It would be in your field of view, but almost to the horizon. At 14.7 km/sec entry velocity and a speed of sound of about 350 m/sec, the sides of the cone are almost parallel to the line of flight (the difference is only 1.4°). Looked at another way, when you hear the shock wave, the bolide is about 88.6° from the point of closest approach.

BTW, before anyone comments, I'm assuming sea level temperature and pressure here. I think that the shock front should refract to those conditions as it moves from the high altitude where it's generated to the region near sea level where it's heard. I'm ready to be corrected by anyone whose physics is more recent than mine.

Steve M