Oh, Pluto. Beloved Pluto. The endearing, scrappy runt of our solar system litter, demoted but never defeated. We heart you, you crazy little Kuiper Belt Object, with your icy surface and newfound moons and multiple references to Disney films, what with Mickey Mouse’s dog and now the dwarf planet thing. Hi-ho, Pluto. Hi-ho.
So today we’re going to discuss a question I encounter on occasion. “Hey,” it goes, roughly, “How come we can see all this detail in a galaxy 46 million light-years away, but your pictures of Pluto look like someone airbrushed a basketball and photographed it through a Vaseline lens at the bottom of a swimming pool? Pluto’s right next door, while this galaxy is clearly at least as far away as the mall.”
Well, aggrieved person suspiciously intent on invading Pluto’s privacy, first understand that these images are actually really neat. This is the most detailed view ever of the entire surface of Pluto. You’re watching the seasons occur on an object so far away that it takes light from the Sun over five hours to reach it. We’re honestly a little hurt. Do you know how much effort that took? How much manpower? Do you think it’s easy to smear that much Vaseline on a lens in space? It is not.
You are right, though – Hubble’s pictures of Pluto do lack the detail people have come to expect from the telescope. Basically, it’s an issue of size. To understand it, you have to understand something called “angular resolution,” which – where are you going? Get back here right now, Pluto e-mailer, and listen while I explain angular resolution to you in excruciating detail! That’s right. We’re all in this together, now.
So, angular resolution. Draw two lines on a piece of paper and have a friend hold that paper up and start to walk backwards. Eventually, you’ll only see one line. That’s angular resolution in action – the ability of a picture-taking device – in this case, your eye – to distinguish between details. If you use a much bigger piece of paper, and draw the lines farther apart, your friend will get farther before you stop seeing two distinct lines. Both size and distance matter when it comes to angular resolution.
Angular resolution is a set thing – for eyes, for cameras, and for telescopes. The human eye has an angular resolution of about one arcminute. Hubble has an angular resolution of .05 arcseconds. That’s good enough to stand on the East Coast of the US and distinguish between lines spaced about three feet apart on a piece of paper that your friend is holding up on the West Coast. Also, that’s one really good friend you have there. Does he owe you money, or a kidney, or something?
Now, galaxies are huge. When Hubble looks at a galaxy, it’s observing a collection of immense objects at incredible distances from one another. (Even then, you’re usually seeing clusters of stars, not individual stars in these galaxies – Andromeda is about the farthest galaxy in which Hubble can actually resolve individual stars.) The beautiful spiral galaxy NGC 2841 is a bit over two arcminutes in size.
Little Pluto? He’s about .06 to one arcseconds wide in the sky.
The angular resolution of a telescope, just like the angular resolution of your eyes, is fixed. Hubble is resolving both the galaxy and Pluto the exact same way. But one has a lot more space and size to work with than the other.
These images of Pluto are painstakingly compiled from lots of Hubble data with the help of years of computer processing. As one astronomer put it, they’re really maps, not pictures.
But it’s not over, saddened, would-be Pluto observer. You’ve got 2015 to look forward to, when the New Horizons spacecraft will complete its decade-long, 3-billion-mile journey to the smallest sort-of member of the solar system and give us a spectacular, up-close view of this Kuiper Belt object … for six months. And then it’s back to fuzzy basketballs as seen through multiple layers of gauze on a foggy day with debilitating nearsightedness.
So time will be, as they say, of the essence. And that’s when those seemingly dull Hubble maps will prove their worth, helping astronomers decide the most interesting areas on Pluto for the probe to target while it can.