Many times here on ATS we see the same question brought up: "Why can't the Hubble Space Telescope see (insert object here) in detail?"
It's a good question to ask if you do not know a lot about telescopes and optics in general.
Many have pointed out the beautiful, detailed images that Hubble takes of say Orion's Nebula or another galaxy, but wonder why it cannot see detailed
images of say, the surface of Mars, or the surface of Pluto.
Hubble Image Of Orion's Nebula:
Hubble Image Of Mars:
Hubble Image Of Pluto that's been computer enhanced.
The answer to this question has two parts to it. The reasons are: Angular Resolution and Size / Distance of the Object.
Let's take a look at Angular Resolution
, to see what it is.
Angular Resolution is the ability of an imaging device (telescope, camera, microscope, your eye) to be able to resolve the detail of another
Angular Resolution is normally measured in Arc Seconds.
To give you an idea of what an Arc Second is: Look at our sky. If you measure your horizon from East to West (or North to South), you have 180 degrees
1 degree = 60 arc minutes. Or 1 degree = 3600 arcseconds
The full moon in the sky covers about 1/2 a degree, or 1800 arc seconds.
The way we find Angular Resolution is by dividing the wavelength of the light emitted (or reflected) off an object by the diameter of the aperture of
the imaging device. For a telescope we use something called
, which is basically a formula for finding out the resolving power of a
Basically it is R=4.56/D where R is Arcseconds, and D is the diameter of the aperture in inches.
For metric you use R=11.6/D where D is the diameter of the aperture in centimeters.
The human eye has a resolution of about 60 arcseconds. This is why we can see the full moon quite clearly since it covers 1,800 arcseconds and why we
can even make out features on its surface with just our eyes.
But now this is where the second part of this discussion takes over: Size / Distance of that object.
Both cars in this image have the same width, but the one further away has a smaller Angular Diameter:
The Moon's diameter is 3476 kilometers. Its average distance from us is 384,000 kilometers. As I said above, a full moon covers about 1/2 a degree of
our sky, or 1,800 arcseconds. Knowing this, and knowing both the size and distance of the moon, we can figure out what the smallest detail on the moon
is that your eyes can see:
Take the diameter of the moon, 3476 km, and divide it by the amount of arcseconds a full moon covers, 1800 arcseconds, and you get 1.93, which is how
many arcseconds 1 km is when seeing the moon from the Earth's surface. Now multiply that by the resolution of your eyeball, which is 60 arcseconds,
and that means the smallest object your naked eye can see on the moon's surface is: 115.86 km wide.
So as you can see, it's not only the size of the aperture that matters, but also how big an object is AND how far away it is.
Hubble's primary mirror is 2.4 meters in diameter, or 240 centimeters. Using Dawes' Limit, that means it has an angular resolution of 0.05 arcseconds.
Much better than our eyeballs of course.
But how does that stack up to seeing something on the Moon with Hubble? Again, all we have to do is the math. We already determined that due to the
Moon's size and distance, 1.93 kilometers is how big 1 arcsecond is (but again, that's based on the Moon's size and distance). Multiply Hubble's
Angular Resolution to 1.93, and we get:
So even as powerful as Hubble's Angular Resolution is, it can still only see something as just under a kilometer in size on the Moon using the Hubble
Hubble Image of Tycho Crater On The Moon:
Picture provided by ATS member Saint Exupery, as the picture I had was wrong.
What about Mars?
Well, to find that out, you already have Hubble's Angular Resolution: 0.05 arcseconds. So now you only need to find out how much sky Mars covers.
Mars' diameter is 6792 km. The closest it gets to Earth is 54.6 million kilometers. That means that Mars takes up 25.08 arcseconds in our skies when
it is as close as it can get to us. Now you know why it only looks like a orange point of light to just your eyeball, because its angular diameter
even at its closest distance is too small to make out any details at all using just your unaided eye.
Let's have Hubble look at it with its Angular Resolution of 0.05 arc seconds. First, how many kilometers is an arcsecond for Mars? Based upon the
numbers we have, when Mars is at its closest to Earth, 1 arcsecond is 270.81 kilometers. So that means Hubble can only see things that are at least
13.54 kilometers big on the surface of Mars.
That's actually pretty good considering how small Mars is and how far away it is. What about a telescope you might own?
Let's go big and say you have a 25.4 centimeter telescope (10 inch reflector). The Angular Resolution of a telescope that big is 0.45 arcseconds. So,
not as good as Hubble (not to mention you're at the mercy of the Earth's atmosphere, etc).
But that means that with your home telescope, the biggest object you can see in detail on Mars would be 121.86 kilometers.
So what about Pluto?
Pluto's Angular Diameter when it's closest to Earth is only 0.115 arcseconds. That's small. VERY small! With a diameter of only 2368 kilometers and
the closest it gets to Earth being 2.66 BILLION miles, that's very small, and very far away. So it's no wonder its largest angular diameter is only
So....what's the biggest thing Hubble can see on Pluto?
2368 km / 0.115 arcseconds = 20594 km. That's how big 1 arc second is looking at the distance of Pluto and its size. So:
20594 km x 0.05 arcseconds = 1029.56 km. That's almost half the size of the planet itself!
So, as you can see, even as powerful as Hubble is with its Angular Resolution, Pluto is just too small and at such a great distance for Hubble to make
out anything of real detail.
So why can Hubble give us such detailed images of say, the Andromeda Galaxy? It's around 2.5 million light-years away!
Hubble Image Of Andromeda:
Yes, it is that far away. But it's big. Very big! It's 220,000 light-years wide! As a matter of fact, its angular diameter in our sky is 3 degrees!
It's actually 6 times bigger in our sky than the full moon!
So now you know: the reason that Hubble can show us great detail of objects much further away such as nebulae and galaxies compared to the planets in
our own solar system, is because those objects are huge. Their details are huge compared to planets (even Jupiter), which are much smaller than they
are. The smaller the object is and the further away it is, the harder it is to see the detail of it.
edit on 6/25/2015 by eriktheawful because: Corrections made on errors pointed out by other members
edit on 6/25/2015 by
eriktheawful because: (no reason given)