Why do pictures of distant galaxies have higher resolution than those of nearby planets? (Intermediate)

Hello. This question is mostly about the Hubble Space Telescope. Well you know if you look in an astronomy book you'll see all kinds of pictures and planets and galaxies, the pictures of the galaxies are crystal clear and very detailed. How come whenever you see a picture of Pluto and Charon you can never see any details and it just looks like a big blob of nothing?

Whats going on here? All of those giant galaxies are a lot farther away than one of our planets inside our little Milky Way. I hope you can answer my question.

The problem that you address below is a matter of the relative size scales of the features the Hubble Space Telescope (HST) images when it looks at galaxies and planets, respectively. For example, take a look at these two HST images: The Antennae Galaxies | Pluto

The first is a picture of a merging galaxy system called the Antennae, and the second is a picture of Pluto. In the first, the bright spots in the Antennae seem very sharp, whereas the picture of Pluto shows a blobby, fuzzy mess. Is the resolution of the HST poorer when it looks at planets, then? The answer is no; you just have to keep in mind the size of the objects that the HST is imaging in each case.

Let's say HST is able to resolve small clusters of stars in nearby galaxies (that's what the blue dots in the Antennae picture are). These star clusters are about 5 parsecs in size, or about 1.5×1014 (1.5 times ten to the fourteen) kilometers (km) across. Pluto, on the other hand, is only 2400 km across. So, the open clusters are 60 billion times bigger than Pluto! (And each cluster is just a small part of a galaxy!) Compare this to the relative distances of the Antennae and of Pluto: Pluto is about 6×109 km away, whereas the Antennae are about 6×1020 km away. So, the Antennae are about 100 billion times farther away than Pluto is from us.

Now, compare the numbers that we got above: the Antennae are a hundred billion times farther away than Pluto, but the objects that we image in the Antennae are sixty billion times bigger than Pluto. The conclusion: it is about as easy for HST to image open clusters of distant galaxies as it is for it to image Pluto as a blob, because Pluto is so much smaller than an open star cluster in real life. And, imaging the entire galaxy is much easier than imaging just a small part of the galaxy.

The best way to get detailed information about a planet's surface, then, is not to use HST, but to send spacecraft to the planet for a closer look.

For more discussion of this concept, please see this page from Emily Lakdawalla of the Planetary Society.

This page was last updated on February 10, 2016.

About the Author

Kristine Spekkens

Kristine Spekkens

Kristine studies the dynamics of galaxies and what they can teach us about dark matter in the universe. She got her Ph.D from Cornell in August 2005, was a Jansky post-doctoral fellow at Rutgers University from 2005-2008, and is now a faculty member at the Royal Military College of Canada and at Queen's University.

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Kristine's websites: 
http://www.astro.queensu.ca/people/Kristine_Spekkens/main.php
http://www.rmc.ca/aca/phy/per/spekkens-k-eng.php

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