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!

Science Clock Series: Part II

In part two of this series, we look at a subject from nuclear physics. Or chemistry. It's kind of at the point where the two overlap.

Today's number is two, and it is approximately equal to:

\[2\approx\text{T}_{1/2}\,^{237}\text{Np}\,(\times10^6\,\text{y})\] T\(_{1/2}\) refers to the half-life of a substance, which means the amount of time, on average, that it takes for half of a sample of a radioactive substance to decay into something else. \(^{237}\)Np is the chemical symbol for the element neptunium (specifically, the isotope neptunium-237), and “\(\times10^6\)” is scientific notation for “multiply this number by 1,000,000”. So the whole expression means “approximately equal to the half-life of neptunium-237 when multiplied by two million years,” referring to two.

Neptunium is the element with atomic number 93 and the first transuranic element. This means it is the first element after uranium (atomic number 92), and is thus only found in nature in extremely tiny amounts (after uranium no element is found in nature in anything other than trace amounts). Neptunium has at least nineteen known isotopes, of which the most stable is neptunium-237 (also written \(^{237}\)Np) with 93 protons, 144 neutrons, and a half-life of 2.144 million years.

So the full expression can be read as “two (million years) is approximately equal to the half-life of neptunium-237.” And now you know where it comes from. Check back next time for something from cosmology! Click here to jump directly to it.

As an aside, the name neptunium comes from the planet Neptune which follows the planet Uranus out from the Sun, just as neptunium follows uranium in the periodic table. (Plutonium also follows neptunium just as Pluto follows Neptune [most of the time, anyway].) Uranium was named after the seventh planet from the Sun, which is now known as Uranus, but which was not always the case. When it (the planet) was originally discovered there was some controversy over what it should called: Herschel, the discoverer, wanted “Georgium Sidus” (“George's Star” in Latin), after his patron King George III of England. Astronomers from other countries were (understandably) a bit miffed at a celestial object bearing the name of a foreign monarch, and several alternate names were proposed, including "Uranus" by the German astronomer Johann Bode, who first determined Uranus' orbit. A few years later when Bode's colleague Martin Klaproth discovered a new metal (in 1789) he named it uranium in support of Bode's proposed name (which eventually beat the competition to become the standard today). By the time neptunium was discovered (officially in 1940) the name Uranus was long the standard, so neptunium and later plutonium were simply nice additions. 

Project Orion

Today I came across what is certainly one of the most intriguing and arguably one of the best ideas the human mind has ever come up with: Project Orion.

Project Orion was a serious research program in the late 40's and 50's dedicated to achieving rocket propulsion in a...slightly unorthodox manner. In essence, it boiled down to chucking atomic bombs out the back of the rocket and detonating them to provide thrust. Mull on that for a few seconds. This was a dead-serious project by some top physicists. The explosion from the bomb would push on a large pusher plate at the back of the vehicle that would be attached with some heavy-duty inertial dampeners causing the whole thing to act like a spring, bouncing back after each explosion only to be greeted with a fresh one for the maximum impulse. Serious calculations showed that it would take about the same about of bombs to get a wide range of payload masses, from 2,000 tons (on the order of the size of the Saturn V rockets that went to the moon) all the way up to 8,000,000 tons (on the order of the size of a small city[!]) into orbit, meaning you could launch much greater masses with the same amount of "fuel" than you could using conventional measures. How many bombs does it take to launch a spaceship into orbit, you ask? The guys in charge of the project have you covered. It would take about 800 nukes at the rate of about one per second to launch something into orbit in a manner akin to "an atomic pogo stick."

Imagine that, if you will.

Imagine being among the first crew to experience one of these. The whole point of the inertial dampener is to decrease the acceleration when the bomb goes off from a lethal 100g to a more human-survivable 2 to 4g, but that's still some significant shock. And every second for over ten minutes you'd get another one. Another interesting fact: the guys in charge of the project were worried that the random nature of the explosive fireballs might send a spacecraft off course, but were reassured when they found that the effects tended to cancel out. Well, that sounds nice in theory, but in reality it means that in addition to the jerky forward motion you'd be feeling, there'd be tiny, random, deviations each time which you know aren't going to knock you off course, but which I'm sure would be pretty unsettling anyway.

Plans for launching such space vehicles included -- I am not kidding here -- covering the landing pad in a layer of conventional explosives and literally blowing the ship far enough up into the sky that it could drop its first nuclear charge without risk of shrapnel from the nuke interacting with the landing pad damaging the ship itself. I mean, how cool is that?? The project ended up being shelved not because of the extreme amounts of fallout it would generate (the idea was never actually flight tested), but because no one at the time could think of a need to launch thousands of tons of payload into space (the signing of the 1963 Partial Test Ban Treaty also made it impossible to test. The U.S. tried to get an exemption for nuclear propulsion into the treaty, but the Russians were [understandably] reluctant).

In the aftermath of bans on open-air nuclear explosions, some people have proposed a similar idea, but one where the spaceship would be launched by more conventional means (or assembled in space) before igniting its nuke drive far enough away from Earth to be non-hazardous. It was calculated that a mission to Pluto and back could be completed in a single year using such a drive. In contrast, New Horizons, the probe currently heading to Pluto, is the fastest man-made object ever, and it's still going to require nearly 15 years to get to Pluto.

And now, perhaps, you can begin to see why I find this idea so fascinating, so captivating, so wonderfully and uniquely "out there" that I could not sleep without writing this post about it. Because when you get right down to it, really, all worries aside, who wouldn't want to ride a nuclear-powered rocket? ("Prime the nuke drive, Scotty!")