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Missile Defense

Russian military leaders have expressed concern about US plans for a national missile defense system. Will defense technology be limited by possibilities for a strategic imbalance? Is this just SDI all over again?

gisterme - 06:03pm Jul 2, 2001 EST (#6424 of 6428)

rshowaler wrote ( rshowalter 7/2/01 4:53pm ): So resolution for detection of the existence of an object, within a range of angles, is not the same as resolution of detection sufficient to hit the object, even if everything else was perfect.

That's just more nay-saying, Robert, apparently based on a lack of knowledge about what's been done already.

We're not talking about a distant star here, Robert nor are we talking about firing a laser at interstellar distances (no need for infinite light speed). Think way back to the Apollo program...there was a reflector placed on the moon, carried there by the teeny-tiny Apollo space ship and set up by astronauts on the lunar surface. How large do you suppose that reflector could be? The purpose of the reflector is to reflect a laser beam sent from earth, through earth's atmosphere, to the reflector on the moon, back to earth, through the atmosphere again, to have the reflection detected. The purpose of the experiment is gather very precise data about the lunar orbit, rotation and composition...

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html

Now look at the photo of the reflector, and consider hitting that at a distance of about 240,000 miles. Judging by the relative size of the footprints around the reflector array I'd estimate that it's about 24 inches square. Fair enough? So given that, angular incidence of the 24" square relfector at a distance of 240,000 miles (1.52e10 Inches) is about 9e-8 degrees or 1.58e-9 radians. NASA was able to aim at and hit that tiny target with a laser, on the orbiting moon, from the rotating earth and detect the reflection with 3 cm resolution, back in the early '70s. Gee, Robert, how do you suppose the laser control system worked for that experiment? Hmmm.

How do you reconcile that evidence of laser aiming performance with your cogitations intended to prove that a laser can't hit a 50 ft tall blazing rocket at a few thousand miles using 2001 technology? Would you estimate that technology was better then than it is now? I doubt it.

Incidently, the reflector is still in use.

gisterme - 06:05pm Jul 2, 2001 EST (#6425 of 6428)

Great rail-guage post, Robert!

gisterme - 06:41pm Jul 2, 2001 EST (#6426 of 6428)

All your rhetoric about laser performance sounds a bit like the North Carolina whisky add you mentioned. :-) At first, a lot of folks didn't believe in vitamins either. And some brilliant British (I think) mathemetician who was trying to prove that railroads were a terrible idea (back around the time that they were invented) "proved" mathematically that the breath would be sucked out of a human at something like 15 mph, causing suffocaion. I wish I could remember THAT guy's name. Today his only claim to fame is due to the laughable magnitude of his computational blunder.

rshowalter - 06:43pm Jul 2, 2001 EST (#6427 of 6428)
Robert Showalter showalte@macc.wisc.edu

Great example, gisterme !

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/ApolloLaser.html

Measuring the Moon's Distance Apollo Laser Ranging Experiments Yield Results (from LPI Bulletin, No. 72, August, 1994)

"The reflectors are too small to be seen from Earth, so even when the beam is precisely aligned in the telescope, actually hitting a lunar retroreflector array is technically challenging. At the Moon's surface the beam is roughly four miles wide. Scientists liken the task of aiming the beam to using a rifle to hit a moving dime two miles away.

Note the spreading.

(By the way, a friend of mine, thirty years ago, just for fun, bounced a radar beam off the moon -- got distance to 9" (a nanosecond) or thereabouts. But the attentuation, the ratio between radar beam intensity, and return signal, was of the order of 10 billion to one.

That's OK for detection. But for burning holes in things , the attenuation is not so good.

Gisterme, I think the example you chose, to support your position, actually argues strongly against it.

rshowalter - 06:51pm Jul 2, 2001 EST (#6428 of 6428)
Robert Showalter showalte@macc.wisc.edu

Spreading angle for the moon lasar distance experiment about 8x 10e-6 radians. For missile defense, 1 x 10e-7 radians would be excessive.

Do you really think that collinearity has improved THAT MUCH?

(I'll grant it can be better - but more than 100 times better -- which would still make the lasar weapons terribly marginal, even at 200 miles? Under field conditions?

I respectfully doubt it.

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