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Headshot of Emily Lakdawalla

Why can Hubble get detailed views of distant galaxies but not of Pluto?

Posted by Emily Lakdawalla

14-02-2013 12:37 CST

Topics: pretty pictures, explaining science, spacecraft, trans-neptunian objects, Pluto, dwarf planets beyond Neptune, New Horizons, stars and galaxies, Hubble Space Telescope

How come Hubble's pictures of galaxies billions of light years away are so beautifully detailed, yet the pictures of Pluto, which is so much closer, are just little blobs? I get asked this question, or variations of it, a lot. Indeed, it's hard to fathom; in our heads the distance to Pluto is "far" and the distance to other galaxies "very far" but I doubt that very many people have any intuitive understanding for how far either one of those distances is, much less their relative sizes. (I certainly don't.)

The good news is, some fairly basic math will help you understand why Hubble's pictures of galaxies look like this:

Galaxy NGC 5584 from Hubble WFC3

NASA, ESA, A. Riess (STScI/JHU), L. Macri (Texas A&M University), and the Hubble Heritage Team (STScI/AURA)

Galaxy NGC 5584 from Hubble WFC3
The brilliant, blue glow of young stars traces the graceful spiral arms of galaxy NGC 5584. Thin, dark dust lanes appear to be flowing from the yellowish core, where older stars reside. The reddish dots sprinkled throughout the image are largely background galaxies.

While the pictures of Pluto and its moons look like this:

A fifth moon for Pluto

NASA, ESA, M. Showalter (SETI Institute) and L. Frattare (STScI)

A fifth moon for Pluto
An image taken on July 7, 2012 by the Wide Field Camera 3 on the Hubble Space Telescope shows the recently discovered fifth moon of Pluto. Moons P4 and P5 are now known as Kerberos and Styx, respectively.

The basic question that we're asking here is, how large do galaxies and Pluto appear in the sky? To answer that question, we need to know their sizes and distances.

  • According to the "Fast Facts" published with that Hubble image of galaxy NGC 5584, it is about 72 million light-years away, and the photo spans 50,000 light-years.
  • On the date of the Pluto-and-moons image (July 7, 2012), the Solar System Simulator tells us that Pluto was 4.675 billion kilometers from Earth. Pluto is about 2400 kilometers across.

To gauge how large these things appear in our sky, we can take the ratio of these things' sizes to their distances. But don't take out your calculators yet. Before you start punching in numbers with lots of zeros, you should first do a mental reality check on their order-of-magnitude proportions.

  • The galaxy is like a hundred thousand wide divided by a hundred million away; that ratio should be around a thousandth.
  • Pluto is like a thousand wide divided by a billion away; that ratio should be around a millionth.
  • So we already know that the galaxy should appear about a thousand times bigger in the sky than Pluto does!

It's important to do a reality check like this first, because when you're dealing with very large or very small numbers, forgetting to punch in one zero in your calculator can majorly affect the outcome of your calculations. Now that we've done that, we can plug in the actual numbers.

  • For the galaxy, 50,000 light-years / 72 million light-years = 0.00069
  • For Pluto, 2400 km / 4675 million km = 0.00000051
  • Take the ratio of those two and you'll see that the galaxy appears 1300 times bigger than Pluto. (See, our earlier order-of-magnitude estimate of a thousand times bigger was pretty close.)

How well should Hubble see either the galaxy or Pluto? To answer this question, you need to know the angular resolution of Hubble's camera. The Wide Field Camera 3 (WFC3) was used for both of these photos. Look up its angular resolution and you'll find out that it is 0.04 arcseconds. (An arcsecond is 1/3600 of a degree.) That is, a single Hubble pixel spans an angle of 0.04 arcseconds.

I like to convert this number into radians: you get 0.04 arcsec * 1/3600 degree/arcsec * 2pi radians / 360 degrees = 0.00000019 radians, or 0.19 microradians. Why would I want to do this conversion? It's because of a trick you can do with angular measurements when you're dealing with very small angles. When the angle is very small, the tangent of that angle is approximately the same as the angle, when you're expressing the angle in radians. Remember how you calculate the tangent of an angle? SOHCAHTOA? It's the length of the side of a right triangle opposite the angle divided by the length of the side adjacent to the angle. Our triangle is so skinny that it's a fair approximation to call it a right triangle. Looking at our galaxy on the sky, we're talking about the galaxy's width divided by its distance from the observer -- the same ratio we calculated before.

So the ratio of the galaxy's width to its distance from us, 0.00069, tells us the angle that the galaxy subtends on the sky, in radians. Divide that by WFC3's angular resolution in radians per pixel, 0.00000019, and you get 3600 pixels. We calculated earlier that Pluto appears only 1/1300th as large -- so it's not even 3 pixels across in Hubble's view.

Pluto is a rather small world, but even if you put Jupiter (which is 60 times bigger) at Pluto's distance from the Sun, it'd only be about 150 pixels across -- still way smaller than the galaxy appears.

Galaxies are far away, it's true, but many of them are bigger than they are far, at least by comparison to the worlds within our solar system! Think about that for a moment -- how something that is millions of light-years away can still appear a thousand times bigger than something that's inside our own solar system.

Homework problem (yes, homework!):

Here's a related question that I get asked a lot. There is a spacecraft on its way to Pluto right now, with a very sharp-eyed camera. New Horizons is beyond Uranus' orbit now, and is closer to Pluto than it is to Earth. In fact, Pluto is just about a billion kilometers away right now, while Earth is about 4 billion kilometers away; so New Horizons is 4 times closer to Pluto than Earth is. Shouldn't New Horizons be getting pictures of Pluto that are better than Hubble's by now?

Obviously we're not seeing great New Horizons pictures of Pluto yet, so the answer must be no, but you should try calculating it for yourself. And then answer the question: when will New Horizons' pictures of Pluto be better than what we can get from Hubble? Some key information to get you going: New Horizons' highest-resolution camera, LORRI, has an angular resolution of about 5 microradians. And if you want to see how far New Horizons will be from Pluto on future dates, use JPL's Solar System Simulator.

I'll post the answer on Monday!

The Hubble Space Telescope


The Hubble Space Telescope
View of Hubble captured by astronauts from the space shuttle Columbia during mission STS-109, March 2002. The astronauts installed new solar arrays as well as the Advanced Camera for Surveys and repaired the NICMOS instrument.
See other posts from February 2013


Or read more blog entries about: pretty pictures, explaining science, spacecraft, trans-neptunian objects, Pluto, dwarf planets beyond Neptune, New Horizons, stars and galaxies, Hubble Space Telescope


@caseiokey: 02/14/2013 01:30 CST

I keep thinking that the amount of light that comes from Pluto, resp. the galaxy, is another important factor.

Jorn Elfving: 02/14/2013 02:45 CST

It's also a question of what you mean by "detailed". The picture of NGC 5584 doesn't even come near to showing individual planets (sorry: dwarf planets, of course!), whereas the Pluto picture shows even the tiny moons.

Mark Brown: 02/14/2013 03:15 CST

Wow! Thanks SO much for this blog post! I just asked this question in the comments of Dr. Betts Intro to Astronomy the other day! Many kudos.

Emily Lakdawalla: 02/14/2013 04:23 CST

@Mark: I know, that's what prompted me to write this :) @caseiokey: Hubble can see things with quite a range of inherent brightnesses. That's not the problem with Pluto; it's really all about size. @Jorn: I think that most people know what I mean! The people who are asking this question are thinking, the galaxy picture is pretty; the Pluto one is not, without the context of knowing what you are looking at.

Jeff Olmstead: 02/14/2013 10:17 CST

Emily, The way I read it, the Solar System Calculator you linked to this article doesn't show the distance from New Horizons to Pluto at a future date, but only shows a simulated picture of Pluto from New Horizons at a future date. Am I missing something?

Ian McNee: 02/15/2013 09:24 CST

Long time since I exercised my brain on this sort of thing! Would we start seeing nicer images around the end of February 2015? Definitely in time for my birthday in mid-March! :-)

Alexander McLin: 02/15/2013 11:27 CST

If Pluto only covers little less than 3 pixels on the WFC3, how can we actually see the little moons? Won't they be smaller than the individual pixel? And thanks for the homework, I love trigonometry and enjoy exercising those rusty brain cells! Feel free to censor the below paragraph since it contains my answer. I see that New Horizon would need to be approximately 160 million kilometers away before its camera gets better pictures than Hubble as opposed to its current position where Pluto would only cover half of a pixel on the camera.

Ted Hartley: 02/15/2013 03:14 CST

13,333 Km provided I got all the 0's in place

David Gash: 02/17/2013 12:35 CST

Lo: this way over my head, but still very intersting

Anonymous: 02/17/2013 01:11 CST

Something else, but relevant: I remember when being a kid interested in Astronomy I thought that galaxies was dim almost point light sources. I guess that was because all the star atlases I had a chance to look at showed galaxies with the same symbol. Only after I got access to newer atlases which shows the actual extent of the galaxies did I realize just how large they were on the sky. But of course with very low surface brightness. I think many people will be surprised to hear the a lot of galaxies actually seems the size of a quarter of the full moon or even larger (the Andromeda Galaxy is six full moons long, if my memory is correct).

Emily Lakdawalla: 02/18/2013 02:48 CST

@Jeff: If you look next to Pluto on the image produced by SSS, you'll see the a figure telling you the range to Pluto in billions of km. I'll try to get the homework problem answer posted soon!

robertwb : 02/20/2013 12:09 CST

@Alexander We can see the little moons because, despite being less than "1 pixel" wide, they light up the entire pixel (and, as this is pushing the limits of the resolution of the Hubble's optical system, the light from these moons gets smeared out to neighboring pixels as well).

Jim Rugg: 03/08/2013 04:48 CST

What a beautiful article and helpful discussion following.

Vladimir Inzunza: 05/12/2013 04:29 CDT

Thank you so much for this, Glad to be here, I'm an amateur from Mexico and I've been wondering, who takes the pictures of our milky Galaxy and what is the estimated distance to make that shooting? Can you help my curiosity please

Emily Lakdawalla: 05/17/2013 10:45 CDT

Hi Vladimir, A perceptive question! We of course don't have any photos of the Milky Way taken from outside it. We are on an outer arm, so we can take pictures of *most* of the galaxy. But photos that claim to show the Milky Way as a spiral, looking down on it, are either not actually of the Milky Way, or they are artworks.

Trena Holsten: 06/17/2014 01:03 CDT

This exact question has been tumbling around my brain recently, I'm so glad to have found the answer! I'm not too great with math, but I do generally understand the explanation there. Thinking about the vastness of space and distances never ceases to blow my mind.

Deborahbogan: 06/29/2014 12:35 CDT

I love discussions that get people thinking and talking. Thanks for this. To me the picture of Pluto (one little planet) (I know it's not "really" a planet because of its size but that only further brings home the point) shows a large-ish (relative to the blobs of light in the picture of the galaxy) blob of light which is one planet and blobs of light for it's moons and stuff. And picture of the galaxy doesn't even show individual planets or suns. Just the blobs of light of clumps of things that shine. So from that perspective the picture of Pluto is much clearer than the galaxy. The difference is that the galaxy is millions of Pluto-ish objects, suns, solar systems, clusters of gas. It would be like saying why does my blurry picture of my friend standing far away not look as clear as the satellite pictures of the United States with all it's pretty lights. It is clearer. It's just your perspective.

thereaverofdarkness: 10/06/2014 05:10 CDT

If Pluto spans between 2 and 3 Hubble "pixels", could we not get an image of it with an extended exposure? I would think if Hubble were to make an extended exposure of Pluto for a few minutes at a time each time it had the same side facing us, eventually we would have a very clear image of Pluto. This same trick could be used for other KBOs of which we know the sidereal rotation period. Am I wrong?

NorthernSoulOuterLimits : 07/03/2015 02:09 CDT

Why Hubble cannot see Pluto I guess will become apparent. With all the major Volcanoes Dark organic lakes which absorbs light. Making Pluto the enigmatic Planet difficult to observe. A galaxy has light illuminating gases. Simple answer. Neither is Pluto a perfect spherical as suggested. Looking at the major impact crater. I think it throws aside the theory of Asteroid Impacts threatening all life. A very diminutive and interesting Planet Pluto has turned out to be.

acepublic: 07/09/2015 09:59 CDT

You could have simply said for the same reason you only see an airplane in the sky as a tiny dot when it's 238 miles away whereas you can see the moon in detail which is 238,900 miles away :/

macfifty2: 07/14/2015 11:19 CDT

Where are the stars, how is it that the lens is wide open to catch what little there is of Pluto which by the are extraordinary, but can not catch a faint glimmer of light in the vicinity. Please explain.

alienobserver: 07/21/2015 02:57 CDT

@macfifty2, they would've processed the image so you could make out Pluto. See all the many dots on the left and right third of the photo? I'm guessing that's your stars. The horizontal stripes are probably processing artefacts.

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