Hubble telescope & Pluto?

Question

Can someone answer this question please. I'm wondering why if the Hubble telescope can take excellent pictures of stars & galaxies forming millions of light years away, then why can't it take a good picture of Pluto?

Answer 1

Pluto can be seen by Hubble, but not by much. Another way to ask this question is: if a person can easily see mountains in the distance, why can't he read a newspaper held in front of him at a distance of 1/2 mile?

It's not a dumb question, but in fact, a very interesting one, which relates to fundamental theories of OPTICS which are used in making telescopes and also filling prescriptions for your glasses. Resolution of the instrument viewing, size of the object, and distance of the object, are all key variables. These relationships have been worked out by a lot of people working over four centuries. So your question can be answered in terms of 1. Resolution of the instrument. 2. Size of the object. and 3. Distance of the object. And we can put numbers on each:

A galaxy that is 100,000 light years long at a distance of 13 billion light years subtends about 1.5 seconds of arc.

Pluto at it's CLOSEST distance of 2.8 billion miles subtends .108 seconds of arc.

The resolution of the Hubble telescope is 0.048 seconds of arc. (Dawes' limit)

And to answer the inevitable follow up a moon lander on the moon subtends .0018 seconds of arc.

1. A standard galaxy at the edge of the universe is about 30 times over Hubble's minimum resolution limit.

2. Pluto is about 2 times *over* Hubble's minimum resolution limit. The light can be detected and so can the moon, but there is precious little detail.

3. A moon lander is about 27 times *under* Hubble's minimum resolution limit.

So:

Hubble *can* see Pluto's disk and Charon, but not enough to squeeze out a lot of detial.
Hubble can't see moon landers.
Hubble *can* see galaxies at the edge of the universe.
Hubble can't see individual stars at the edge of the universe.
Hubble *can* see individual stars in some nearby galaxies.
Hubble *can* detect individual stars at the edge of the universe if they do us the courtesy of blowing up.

Incidentally, the same optical principles go into the calculations about whether it makes more sense to send a six inch telescope to Mars to take pictures from Mars orbit versus using a 100 inch telescope in Earth orbit. As you may guess, it usually makes more sense to send a small telescope and a radio transmitter billions of miles away rather than attempt to use a larger one close to home.

Hope that helps,
GN

ps. In a first version of this post, I made a mistake with the decimal placement on the galaxy/distance calculation and gave a value for pluto in a fraction of a degree rather than arc seconds. Both errors have been fixed about four hours later.

Answer 2

Pluto is only 1,400 miles across and so far away, it's a starlike point of light even to the Hubble, whereas extra-galactic objects have a measurable size because they're thousands or tens of thousands of light years across. Hubble can resolve objects .1 arc second apart, and even the most distant galaxies are much larger than that in apparent size. Because Pluto is also .1 arc second across in apparent size, there's no way for the Hubble to resolve surface features directly. Somone looking at Pluto through a telescope 10 times larger in aperture would see some vague surface features, but the only way to get a good look at Pluto is by a spacecraft making a close approach to it. In 2015, NASA's New Horizons will reach the planet and give us a close up look at a large Kuiper Belt Object. In fact, the Hubble can just separate Pluto and Charon, showing both as points of light, along with it's other moons Nix and Hydra. An arc-second is 1/3600th of a degree, and in contrast to Pluto's apparent diameter of .1 arc second, the Sun and Moon are each about 1,800 arc seconds in apparent size.

Answer 3

It's not distance so much as size that is the key point here. Galazies are not only vast on an absolute scale, they are actually quite large in the sky as seen from here too. Pluto is tiny. Seen fromhere it is visibly indistinguishable from a star through any telescope, and is only identifiable as a planet because of its motion. The hubble telescope, above atmospheric interference, can resolve the disc of pluto, but it is still tiny. If you look at a galaxy through a telescope, however, you will clearly see that it is vast, spreading over a much wider area of sky than any star or planet. The reason you can't see it with the unaided eye is because it is so faint. Some galaxies an nebulae even cover a wider area of the sky than the full Moon, but are simply too dim to see. Using a telescope, however, you can capture good images of them.

Answer 4

A perfect analogy is why you can see a skyscraper from 40 miles away, but not see a bug from 40 feet. It's a matter of simple geometry. While Pluto is indeed much closer than the galaxies Hubble views, it is smaller by a very large magnitude. Pluto's angular size is far too small for the Hubble to get a clear picture of.

Answer 5

The galaxies that Hubble photographs are usually many thousands of light years across, with billions of glowing stars within them. Compared to tiny Pluto those are huge targets pumping out immense amounts of light. The same is true for many of the nebulae Hubble has targeted, but you'll notice that all the stars it photographs are still just points of light (..none of them reveal a disc..)

Answer 6

Pluto is both small and far away. In relative terms, it occupies a very small angle of the sky. A galaxy, although it its much further away, actually composes a larger angle in the sky, plus it directly gives off the light that you see. The light arriving at the telescope comes directly from the source; while with pluto, the telescope recieves pluto's reflection of the sun's light.

Answer 7

it all has to do with lighting. all a camera does is capture light. while stars and galaxies are much farther away, they are far brighter and larger than pluto. Pluto is much too dark to get a decent picture.

imagine trying to photograph a person standing in front of you, at night, with no flash, and no light except a streetlamp somewhere outside, your picture probably would not come out very well. But because stars are emitting such a huge amount of light, they are much easier for the camera to photograph.

Answer 8

Galaxies are... well... somewhat bigger than asteroids such as Pluto, even though they are far away.

If you had a newspaper a mile away, you could not read it. However, you can still clearly see the moon. The moon is bigger than the newspaper and has a larger apparent size in the sky. Similarly, galaxies take up far more of the sky than Pluto, and thus have larger apparent sizes.

Answer 9

Because although Pluto is billions of times closer than those galaxies, it is trillions and trillions of times smaller. It is just like how you can clearly see the Moon 240,000 miles away with just your eyes, but you cannot clearly see an ant 20 feet away.

Answer 10

Maybe the sun is too far away to shine light on Pluto