Why New Horizons Can See Pluto, and Hubble Can't

About a day ago the New Horizons space probe finally reached the end of its nine-year journey through space and accomplished its mission: a fly-by of Pluto, marking the first time humanity has gotten to see the surface of this mysterious minor planet.

And what a surface it is!

Pluto, as imaged by New Horizons. Credit: NASA/JHUAPL/SWRI

Just look at all those surface features! There's a large icy vaguely heart-shaped region in the middle (which reminds me of Antarctica for some reason). It's flanked on two sides by extremely dark patches. On the right side of the picture long shadows betray the presence of fierce mountain ranges jutting from the smooth plains around them. What looks like a long canyon sits on the left side of the image, while vast smooth plains fill the top half. Speaking of which, there's a noticeable dearth of obvious impact craters—I can spot a few, but it's nothing like, say, Mercury, or our Moon.

When I was growing up in the 90's, from as early as I could remember I was fascinated by other planets. This was the beginning of my lifelong journey to become an astronomer, as I devoured every bit of reading material I could get my hands on pertaining to the solar system. This was right after the two Voyager probes had completed their missions to the outer planets (Voyager 2 flew by Neptune the year I was born, 1989), so there was an eclectic mixture of information in the books I read, depending on how old they were and how up-to-date their information was. (Looking back, I realize this was excellent training for my young self in sifting multiple conflicting sources of information and piecing together a coherent narrative from them. Huh.)

The newer books had pictures of the outer planets and their moons from the Voyager probes that were of resoundingly better quality than the ones before it. Those two probes taught us so much about the planets that we simply couldn't see from our vantage point on Earth. The point to this rather rambling divergence is that I know now what people must have felt like when those first pictures of each new planet were coming back. If you're not familiar with our previous best images of Pluto, let me show you one (courtesy of the Hubble Space Telescope):

Credit: NASA/STScI

To be clear, the actual photos of Pluto are those two small pictures at the top; the larger ones are computer models extrapolating from those pictures. These were among the best images of Pluto we had until yesterday. And yes, that's a photo from 1996, but we didn't really get any better ones in the intervening time period; here's another one from 2012:

Credit: NASA/STScI

The letters WFC3 at the top of this image stand for Wide Field Camera 3, the last and most technologically advanced camera installed on the Hubble Space Telescope, so this is as good it's possible for Hubble to get. With that in mind it's easier to appreciate just how amazing the pictures from New Horizons are.

“But hang on,” you may be saying, “why can't Hubble get better pictures of Pluto? It gets all those amazing pictures of galaxies, and they're a lot further away than Pluto is!”

If you're asking this, then you're in luck, because I asked myself the same thing driving home from work today. The apparent discrepancy comes about due to us humans not having a good intuitive sense about sizes and distances so far outside our everyday experiences. To really get a feel for why things are the way they are, we need to use math.

My idea for this was find the diameters and distances to Pluto and a nice galaxy that Hubble had photographed, take their ratios, and see just how much bigger the galaxy would appear on the sky. Then while researching these bits of information in order to write this post I discovered that an astronomer named Emily Lakdawalla had already done exactly that. So rather than write up another post that would say pretty much the exact same thing, you get to go read her blog post. (She also already has an excellent image showing the relative sizes of a lot of Pluto-sized bodies in the solar system using the newest images of Pluto and Charon!)

I had an idea to take a picture of a galaxy and a picture of Pluto and shrink the Pluto picture down and stick it on the galaxy picture to see how they compare, but I did a quick back-of-the-envelope calculation with a galaxy picture I picked out and discovered that Pluto would be about two pixels across (which agrees quite well with the conclusion in Emily's blog post that Pluto would theoretically cover less than two pixels of Hubble's WFC3). I tried sticking a little 2×2 bright green square into the image, and could barely make it out at 100% resolution even knowing where to look. So I figured it wouldn't be especially interesting to show given that putting the picture up on this blog would further shrink it. Sorry.

But to come back to the point I was trying to convey originally, this is a historic day (well, yesterday technically) for planetary science, unmanned space probes, and Pluto. If you come across any of the doubtlessly many more images to come back from New Horizons I hope you now better appreciate them for just what a huge leap forward they represent for our understanding of this fascinating little ice-and-rock-ball out on the outskirts of our solar system. A hui hou!

P.S. Also, New Horizons' mission isn't quite as over as made it sound in the opening sentence. It will continue to observe Pluto and its moons for about another month or so as it whips on past, and will probably continue to send back observations about anything else it can see way out there for a long time to come after that. Exciting!