Question

If the Hubble telescope can take excellent pictures of stars and galaxies forming millions of light years away, why can't it take a good picture of Pluto?

Answer 1

The Hubble telescope has an angular resolution of `.05 " arc-seconds"`. Galaxies are much larger than that, but Pluto is not.

Explanation:

While researching this answer, I came across this article which provides a more in depth answer to the question.

Hubble uses a special type of camera called a CCD, or charged couple device. These cameras use a grid of "bins" which collect and count photons. These "bins" correspond to pixels in the resulting image. If we look at a Hubble image of Pluto from 1996, we can see these pixels.

Currently, Hubble's resolution is about `.05 " arc-seconds"`. That means that each of these pixels is `.05 " arc-seconds"` wide. A single arc-second is `1"/"3600`th of a degree. We measure resolution in arc seconds because it tells us how much of our view of the sky an object takes up. To put this in perspective, the entire sky is a `360^o` field of view, or `1,296,000 " arc-seconds"`.

Pluto's angular size changes as it orbits the sun. The closer it is to the Earth, the larger it appears. At its closest, Pluto is `4.28 " billion km"`. Comparing that with Pluto's diameter of `2372 " km"` tells us that Pluto's largest angular size is;

`"Angular Size" = "Size"/"Distance" = (2372 "km")/(4.28 * 10^9 "km") = 5.54*10^-7`

This is the angular size in radians. If we convert to arc-seconds, we get;

`.11 " arc-seconds"`

Which is about `22` pixels across for Hubble. Now lets look at a galaxy. NGC 1300 is a spiral galaxy `23.7 " million light years"` away, which has an angular size of `1116 " arc-seconds"`. That's about `10,000` times the angular size as Pluto, meaning that we can see much more detail.