Re: How come we cannot get detailed pictures of Pluto?

Aside from the fact that galaxies and nebulae put off their own bright light and seeing Pluto relies entirely on reflected light, the main reason we can obtain such stunning detail of galaxies and nebulae and not of Pluto has to do with the fact that the angular size, or “apparent size” of Pluto is much smaller than the other objects.

As you may know, the "apparent size" or “width” of objects in the sky is measured in terms of their “degrees of arc”, with 360 degrees in a circle, 60 arc- minutes in a degree, 60 arc-seconds in an arc-minute. So, an arc-second is 1 / 1296000th of a circle.

If you divide the width of an object by the distance you get a ratio that is approximately equal to the sine of an angle, the angle being the apparent angle of arc, or size the object appears in the sky. So, the angle = a = arcsin( w / d ).

Pluto is about 2274 kilometers in diameter and 5.8 billion kilometers from us. The ratio is 2274 / 5,800,000,000 = 3.9x10^-7 = 1 / 2550571. This is a very small number. The Arcsine of this ratio is 2.2x10^-5 degrees = 1.3x10^-3 arc minutes = 0.08 arc seconds. (This calculation is approximate. The average observed “apparent width” of Pluto is about 0.13 arc seconds.)

By comparison, the Andromeda galaxy is 200,000 light years across and 2.9 million light years away. The ratio here is 200,000 / 2,900,000 = 0.069 = 2 / 29. The Arcsine of this ratio is 3.95 degrees. (Again, this calculation is approximate. The average observed “apparent width” of Andromeda is about 2.97 degrees.)

The resolution of the Hubble Telescope is about 0.1 arc seconds, and so the image of Pluto from Hubble will only be a few digital pixels in size (a pixel is like one dot of light on your computer screen), thus the lack of detail. The image of Andromeda, on the other hand, will take up hundreds of millions of digital pixels. (The average computer screen has close to 1 million pixels on it.) This obviously provides much better detail.

It's a lot like asking, "If I can read a sign along side the highway from one kilometer away, why can't I read a book only 50 meters away?" It all has to do with "apparent size."

I hope that answers your question.

(Thanks to Susan Strosahl and Charlie Coburn.)