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Why don't they take a photo of the lunar landing sites with Hubble? It can image galaxies on the other side of the universe, so why can't it see the LM on the moon?
This is a common question that people ask me. I wasn't sure, but I had a few ideas. Principally because I thought that Hubble really wasn't designed to take lunar pictures. I was wrong. Hubble has taken photos of the moon. But they're still no-where good enough to see the LM on the moon's surface.
WARNING! Mathematics coming up. Most people may just want to take my word for it rather than read on!
Size of Lunar Lander. Let's be generous and say 10m square.
Distance between Hubble and Moon. About 350, 000km.
This works out as an visual angle of (10m)/(3.5 x 10^8m) * (180/PI) = 1.6 x 10^-6 degrees = 6 milliarcseconds.
The WFPC2 'telescope' on Hubble has the following resolution: 800x800 pixels of a 35 arcseconds field of view with a pixel scale of 46 milliarcseconds. Actually resolution in practice is a little below this.
So what does this all mean? Well, roughly speaking, it means that the LM would have to be 15 times larger before it would even cause a dot on a Hubble picture
But doesn't this same Hubble take photos of things billions of light years away? Yup. Makes you feel very very very small, doesn't it?
I have to thank Terry Hancock for helping me out with this info. You didn't think I worked it out myself, did you? Any errors on this page probably lie with my interpretation of his explanation.
I stole the following off a NASA discussion board. I would usually just link to it, but discussion messages have a habit of expiring and this was too good to lose. In it Ed Cheng explains there's a law of physics that would prevent Hubble seeing the Lunar Module, and it's to do with the size of its light collecting mirror.
The wavelength of visible light is around 550x10^-9m (i.e. very very small).
The diameter of Hubble's mirror is 2.4m.
Highest ever possible resolution = 1.4 x 550 x 10^-9 /2.4 m = 3.2 x 10^-7 radians
At a distance of 350,000km this works out as about 124 metres. As Ed says, roughly the size of a football field.
So even if Hubble's camera had a greater resolution, it still couldn't see the Lunar Module.
The problem with the satellites photographs, even the extra top secret military ones that no-one will admit to being there, is that they are a lot closer to the Earth than the moon. By a way big margin. The satellites that take these photos are not in a constant orbit, they swing in and out of a parking orbit as required, just skimming the upper reaches of the atmosphere above the desired location. But there is simply no way they could get as close to the moon, or get anything like as clear a photo.