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

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 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 (the HST picture is the one on the right), 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 5x10^13 (five times ten to the thirteen) kilometres (km) across. Pluto, on the other hand, is only 2500 kilometres across. So, the open clusters are 10 billion times bigger than Pluto! Compare this to the relative distances of the Antennae and of Pluto: Pluto is about 6x10^9 km away, whereas the Antennae are about 6x10^20 km away. So, the Antennae are about 10 billion times farther away than Pluto is from us.

Now, compare the numbers that we got above: the Antennae are ten billion times farther away than Pluto, but the objects that we image in the Antennae are ten billion times bigger than Pluto. The conclusion: it is just 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.

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. This has been done for Mars in particular (Pluto is too far away from the Earth to send spacecraft to as of yet), so that we now know the topography of the Red Planet better than we do that of the Earth itself! For an example of the topography, check out this map.

June 2002, Kristine Spekkens (more by Kristine Spekkens) (Like this Answer)

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