Question regarding the Hubble

Can anybody explain why the Hubble would be unable to see the
equipment left behind by the lunar landings in the late 60's early
70's?

I have heard that some intelligence agencies have satellites that can
see objects as small as cigarette cartons.

I'm sure the answer has to do with optics, and perhaps distance from
the object being observed.

If anybody could speak on this topic, I would be very interested to
read.

Thanks much!

In article ,
arteec wrote:
Can anybody explain why the Hubble would be unable to see the
equipment left behind by the lunar landings in the late 60's early
70's?

I have heard that some intelligence agencies have satellites that can
see objects as small as cigarette cartons.

I'm sure the answer has to do with optics, and perhaps distance from
the object being observed.

That's right, it does. The Moon is considerably further away from
Hubble than spy satellites are from the Earth (by roughly a factor of
1000). If I've done the arithmetic correctly, the finest details
Hubble could see on the surfact of the Moon are a few hundred meters.
If you want to work out the details, look up "angular resolution" and
"small-angle formula" in an astronomy textbook.

I'd bet that the Hubble can't take high-resolution pictures of the
Moon's surface anyway, simply because the Moon's too bright. Hubble's
instruments are designed to look at much fainter things. I'd bet that
pointing them at the Moon would lead at best to overexposed images and
probably to frying the electronics. But I don't observe with Hubble,
so I'm guessing about this.

-Ted


--
[E-mail me at , as opposed to .]

Can anybody explain why the Hubble would be unable to see the
equipment left behind by the lunar landings in the late 60's early
70's?

I have heard that some intelligence agencies have satellites that can
see objects as small as cigarette cartons.

I'm sure the answer has to do with optics, and perhaps distance from
the object being observed.

Correct both times. The angular resolution of a telescope
depends on its diameter, D, and the wavelength of the
light it collects, lambda. To a rough approximation, the
smallest angle a telescope can resolve is

angle (radians) = lambda / D

where lambda and D are measured in the same units.
For HST, D = 2.3 meters, and a representative wavelength
is lambda = 400 nm = 400 x 10^(-9) meters. Thus,
the angle is 1.7 x 10^(-7) radians, or about 1 x 10^(-5)
degrees, or about 0.04 arcseconds.

If an object of length L is placed a distance M away
from you, its angular size will be (again in radians)
roughly (when M L)

angle (radians) = L / M

The distance to the Moon is roughly
M = 384,000,000 meters. The smallest object HST
could resolve at the Moon's distance is roughly

L = M * (lambda / D)

which is about 67 meters in size.

The only overhead pictures we have of hardware on the Moon
come from the Lunar Orbiter. You can see a few
examples at

http://stupendous.rit.edu/richmond/a...ar_lander.html

I'd bet that the Hubble can't take high-resolution pictures of the
Moon's surface anyway, simply because the Moon's too bright. Hubble's
instruments are designed to look at much fainter things. I'd bet that
pointing them at the Moon would lead at best to overexposed images and
probably to frying the electronics. But I don't observe with Hubble,
so I'm guessing about this.

Good guess, and a common one, but wrong. Only one of
HST's instruments was really too sensitive to observe
the Moon lest it be damaged. Some of the others tried
using the Moon -- or the Earth! -- for flatfield exposures.

As an example, look at

http://hubblesite.org/gallery/album/...on/pr1999014c/