Besides the stars, there are seven objects that everyone can see with the naked eye: the Sun, Mercury, Venus, the Moon, Mars, Jupiter, Saturn. (Well, don't look at the Sun, but you know it's there.) You might notice that the seven days of the week are named after these same objects. It's obvious that Monday is for the moon and Saturday is for Saturn, at least---less obvious that Tuesday is for Mars (unless you use another language, then it's obvious).
OK, but what about the other planets? What about Neptune and Uranus? Uranus was discovered in 1781 and Neptune was discovered in 1846 (both were discovered much later than the discovery of the Sun). And what about Pluto? Of course you know that Pluto isn't classified as a planet---but it will always still be Pluto. Pluto was discovered in 1930 by Clyde Tombaugh.
We don't know much about Pluto. We know its orbital path and we have an estimate for its mass. But what about surface features? What does it look like? It turns out that it's just damn difficult to see Pluto. Even with the Hubble Space Telescope, this is about the best we can do.
So, why is it so difficult to see Pluto? Three reasons.
Here is a simple experiment you can try. Take a red apple (or any colored object will do). Now bring your red apple into a room with no windows and no lights (no lights at all). In this dark room, what color does the apple appear? If you answer "you can't see that apple," I will give you partial credit. The correct answer is that the apple appears to be black. Of course the rest of the room is also black so that you can't really tell what part is the black room and what part is the red apple.
This simple experiment shows that in order for you to see this apple, you need light. Light from a lamp would reflect off the apple and then enter your eye. This is how we see most things---but not all. Some other things create their own light so that they are their own light source (like the Sun). However, Pluto is like the apple. In order to see it, you need light to reflect off the surface of the planetoid and enter your eye.
Where does this light come from that reflects off Pluto? It comes from the Sun. But there is a small problem. The Sun shines light that is essentially uniform in all directions. This means that you can think of light as an expanding sphere centered on the Sun. The light from the Sun is then spread over the surface area of this sphere. Since the area of a sphere is proportional to the square of the radius of the sphere, doubling the distance from the Sun decreases the intensity of light by a factor of 4.
Pluto is very far from the Sun. In fact it is about 30 to 50 times farther from the Sun than the Earth. So, there is significantly less light from the Sun at the location of Pluto. But wait! It gets worse. When the sunlight hits the surface of Pluto some of it is absorbed and some is reflected. Of the light that is reflected, it also expands outward from the surface of Pluto much like the Sun. By the time the light has gone from the Sun to Pluto to Earth, the reflected light intensity is just super small (not a scientific term).
If you look up the brightness for Pluto, it will be listed as an apparent magnitude of 13.64 to 16.3. What is apparent magnitude? This is an archaic system of reporting the brightness of stars and planets that was created by Greek astronomers a long time ago. The magnitude system breaks visible stars into 6 groups with magnitude 1 being the brightest and 6 being the faintest. Modern adjustments to the original classification says that each level of magnitude decreases the apparent brightness by a factor of 2.512. This means that a magnitude 1 star appears 100 times brighter than a magnitude 6. Note that Pluto is at BEST at magnitude 13.64. You just can't see this planetoid with the naked eye.
Is there a way to fix this brightness problem? Yes. The best way to create an image of very faint objects is to gather more light from that object. This can be accomplished with a larger diameter optical instrument like a telescope with a large mirror as the primary optical piece. Bigger telescopes are better.
You can probably do a simple experiment. Hopefully you have a pair of binoculars that you can use. If so, take them outside at night. First, look at some section of the sky where you can see some stars. Now look through the binoculars at the same section. You should be able to see many more stars with the binoculars than you could with just your eyes. Why? Because the lens of the binoculars are much larger than your pupils. This gathers more light so you can see dimmer objects.
There is one more problem, light pollution. Humans tend to have artificial lights on during the night time. These artificial lights illuminate the ground the sky as well. Light scatters off the air and makes it difficult to see dimmer stars. There are three solutions to light pollution. 1) Turn off the lights. 2) Move to a higher elevation with less air (like on a mountain top). 3) Move to where there is no air---like in space (Hubble Space Telescope).
Maybe you can see Pluto with your super awesome and huge telescope. Also, you are out in the middle of no where so that there's no light pollution. What next? Well, you probably want to see some details about the planet. This is where magnification comes into play. If you have used a pair of binoculars you know that when you look through them, things look bigger.
Actually, I'm not going to say anything else about magnification. You probably already have a good feeling for this and it usually isn't the problem.
If you make a tiny hole in a sheet of metal, light can pass through this hole and make a spot on a nearby screen. With a single light as the source, it might look like the spot on the screen is a perfect circle, but it's not. Light doesn't pass through openings in a clean manner but instead it is more fuzzy. This fuzziness is due to the diffraction of light.
Imagine a similar (but easier to visualize) situation. You are sitting on the beach watching the waves come in. Next you move to another location that has a breaker wall a little bit off shore. If this wall has an opening, the waves can pass through. And here you can see diffraction. The waves don't pass straight through, they bend as the pass through the opening. It would look something like this.
Yes, the waves in the water bend as they pass through the opening. But wouldn't this mean that we could see around corners? Yes and no. Visible light does indeed bend when it passes through a doorway. However, the amount of diffraction bending depends on the wavelength of light. Visible light has a wavelength around 500 nanometers (5 x 10-7 m). In order to get noticeable diffraction with visible light, you either need a tiny opening or you need to be looking really close. Guess what, a telescope has a large opening but you are looking very close (high magnification).
Again, you can fix the diffraction problem with a bigger telescope. The size of the opening is proportional to the smallest angular size that you can resolve (called the Rayleigh Criterion). If telescope has a diameter of d and looking at light with a wavelength of λ then we can write the following for the smallest angle it can resolve (θR):
Let's use this to calculate the diameter of a telescope we could use to look at Pluto. Let's say we want to get a nice view of the surface with details down to 1 kilometer. If we say that Pluto is 35 AU away from Earth then we can use this 1 km sized feature on the surface to calculate the angular size of this feature. Now put this angular size into the Rayleigh Criterion and we get a telescope diameter of over 3,000 meters. Yes, that's a problem. Oh sure, there are ways around actually building a telescope this big---but still it's a problem.
Perhaps you can see the solution to the Pluto image problem already. The best way to get a nice image of the surface of Pluto is to get closer. That is the only way we are going to get a more detailed picture of the surface of Pluto. This is the exact goal of the NASA New Horizons spacecraft.
The New Horizons spacecraft is still on its way towards Pluto. However, it has already passed the point where it is close enough to Pluto to get a better image than the Hubble Space Telescope. The spacecraft is estimated to have its closest approach to Pluto on July 14 (2015) to be within a distance of just 27,000 km. Yes, that's pretty close.
What will we see when New Horizons gets to Pluto? Who knows? That's why this is so exciting.
