Posted on 02/07/2003 4:30:37 AM PST by The Magical Mischief Tour
Uh oh, after reading part of the thread, I'm afraid this may have a chance to have legs. :^)
I disagree completely. The shuttle was doing, what, around mach 4. That's ~3000 mph. That means the "wind" was going by at 3000 mph. The external tank is 154 ft. long so let's say the foam/ice was accelerated by this 3000 mph wind for 100 ft. It would be going much faster relative to the shuttle wing than 100 miles per hour, especially if it only weighed 2.5 lbs. and had all that surface area. Whether ice or foam, the impact would be significant. If it was ice then just a few mph would be enough to cause enough damage to bring down the shuttle. If it was foam, then the high relative speed may have been enough to cause significant damage.
OK, I read it, now what's your point?
If you don't know who you were debating, how the heck do you expect me to know?
Very interesting article. Not to be too skeptical, but I'd like to see that photo first, sil vous plais, before making any judgment.
Overcoming the problems of making an optical system that could perform (as alluded to in the thread article) are daunting, to say the least. Photographing low contrast surface features on an object traveling at 12,500 mph and 200,000 feet away through a turbulent atmosphere with high resolution, is nothing short of spectacular.
Absent atmospheric affects, the absolute bare minimum resolution (the diffraction limit to the so-called "airy disk") is given as (approx.) 120/D (where D is the diameter of the telescope mirror in millimeters). And that limit is only valid when viewing high contrast objects, which definitely would not be the case here.
While it would at first appear that all you need, in order to get the needed resolution, is to increase mirror size, the atmospheric turbulence that causes "speckle", or spatial aberration, increases with mirror diameter. The result is that in mirror diameters greater than about 250 mm, any increase in resolution is offset by an increase in speckle. And thus, your theoretical resolution, under the very best circumstances, of high contrast features, 200,00 feet away, using a telescope with a 250 mm (10 inch) mirror, would be approximately 1/2 foot.
In the last 20-30 years, astronomers (and especially the military) have developed and use adaptive optical systems to over come the effects of speckle. But these systems are quite complex and require either multiple (hundreds of) identical exposures and/or a tracking laser in order to successfully overcome speckle. Whether such optical systems could have worked in the present case is just pure speculation, without seeing the actual photos that this article claims exist.
(Just guessing here, but the actual sensor to keep the tracking system pointed at the orbiter was probably radar or a transponder, rather than the more common optical sensors.)
There are other serious problems that make we wonder about the true resolution of these photos. In order to maintain such spectacular angular resolution, the tracking system (in this case) would have had to be able to track the object smoothly at tracking rates exceeding 5° per second. When trying to imagine such a tracking system, keep in mind that there is no such thing as an electrical motor that does not produce torque ripple. Torque ripple is not a big deal until you are trying to push the limits of diffraction and speckle correction.
Additionally, since the orbiter vehicle is reported to have an angle of attack of approximately 35° during re-entry, the telescope in question would have only a limited angular window during which to view the wing in a manner that would reveal the damage they reported.
Could they really have an optical system capable of such a stupendous feat and overcoming all those problems? Apparently they do, but damn, that has really got to be one hell of a state-of-the-art system and one that only the government could afford to build! I would have given my eye teeth to have been involved in the design and development of that system.
Regards,
Boot Hill
Boot
Boot
The other points are the the normal sequencing of the RCS thrusters. Is all that beyond you?
They don't have to thermally stabilize the system! They're the government, they can afford a mirror made of zero temp-co ZerodurTM, Cer-VitTM, or ULE. (It's just tax money, don't you know, there's always more where that came from!)
From the blurring around the orbiter tail in the photo of the silhouette, you can calculate the approximate effective shutter speed of the photo as being about 1 ms. This is a resolution limiting problem for that USAF system. IMO, they need something on the order of 1-10us and that might raise a small problem for the CCD manufacturers.
I sure hope that the low pixel resolution of the posted picture above is an artifact of the jpeg copies they made for public distribution and not an indication of the actual pixel resolution of the CCD they used.
Boot
--Boot Hill
Same Here! I got a elbow in the ribs from Mrs. Doomonyou in the quiet movie theater for that outburst.
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