How I Killed Pluto and Why It Had It Coming

by Mike Brown

  The time it took to get from the top of the trail to the bottom, where my cabin was, depended almost entirely on the phase of the moon. When the moon was full, it felt almost like walking in daylight, and I practically skipped down the trail. The darker quarter moon slowed me a bit, but my mind seemed to be able to reconstruct my surroundings from the few glints and outlines that the weak moonlight revealed. I could almost walk the trail with my eyes closed. I had memorized the positions of nearly all the rocks that stuck up and all the trees and branches that hung down. I knew where to avoid the right side of the trail so that I wouldn’t brush against the poison oak bush. I knew where to hug the left side of the trail so that I wouldn’t fall off the twenty-foot embankment called “refrigerator hill,” named after a legendary incident when some previous inhabitants of the same cabin had hauled a refrigerator most of the way down the trail before losing it over the edge and into the creek at that very spot.

  I had almost memorized the trail, but every twenty-nine days I was reminded that there is quite a big difference between memorization and near memorization.

  Every twenty-nine days the moon became new and entirely disappeared from the sky, and I was almost lost. If by luck there were clouds that night, I might be able to get enough illumination from the reflected lights of Los Angeles, just a few miles away, to help me on my way. But on days with no moon and no clouds and only the stars and planets to light the way, I would shuffle slowly down the trail knowing that over here—somewhere—was a rock that stuck out—there!—and over here I had to reach out to feel a branch—here! It was a good thing that my skin does not react strongly to the touch of poison oak.

  These days I live in a more normal suburban setting and drive my car right up to my house. I even have indoor plumbing. The moon has almost no direct effect on my day-to-day life, but still, I consciously track its phases and its location in the sky and try to show my daughter every month when it comes around full. All of this, though, is just because I like the moon and find its motions and shapes fascinating. If I get busy, I can go for weeks without really noticing where it is in the sky. Back when I lived in the cabin, though, the moon mattered, and I couldn’t help but feel its monthly absences and the dark skies and my own slow shuffling down the trail.

  Contrary to the way it might sound, however, back then the moon was not my friend. The two-and-a-half-year-old daughter of one of my best friends—a girl who would, a few years later, be the flower girl at my wedding—would say, when asked about the bright object nearly full in the night sky: “That’s the moon. The moon is Mike’s nemesis.” And indeed, the moon was my nemesis, because I was looking for planets. Astronomers build telescopes in the most remote places they can find—the mountains of Chile, volcanoes in Hawaii, the plains of Antarctica, even in outer space—partially in the hope of escaping the city glare that increasingly permeates the skies. For all that effort, though, we can’t hide from the brightest light that illuminates the night skies and washes out the faintest stars: the full moon.

  As a new graduate student in astronomy at Berkeley, I had never previously considered the moon to be an obstacle. It was still the world that people had walked on early in my childhood, the scene that I’d drawn picture books of, the thing I’d tried to reproduce in my muddy, rock-splattered backyard; it was not a menace to be avoided. But I soon learned the lingo: Nights when the moon was full or nearly full were called “bright time” and were to be avoided by serious astronomers looking for faint objects in the sky. Times when the quarter moon was out for half of the night were “gray time.” But the coveted nights were those when the moon was new and didn’t disturb the dark sky at all. Only on those nights—“dark time”—do astronomers have a hope of detecting the very faintest blips of light that their telescopes can possibly see. I was now looking for planets, and a distant planet would indeed be a faint blip of light that the full moon would thoroughly overwhelm. So the moon became my nemesis.

  I had started looking for planets by accident. In 1997 I began working as an assistant professor at Caltech, and I realized that I didn’t really know what I was doing. Caltech is one of the best places in the world to be an astronomer. The university owns an inordinate number of the largest and best telescopes in the world, so Caltech astronomers are always expected to be—and often are—the leaders in their fields. When I started at Caltech, at the age of thirty-two, I suddenly had access to all of these premier telescopes, and I was told, essentially: Go forth! Use these telescopes to lead your field to new great things!

  I had spent most of the six years of my Ph.D. studying Jupiter and its volcanic moon, but it was time to start something new, and here was my chance. Go forth! I thought. Okay. But where? Sure, I knew how to use the telescopes and the instruments and how to point them at the region of the sky in which I was interested, and I knew how to collect and analyze the data. But figuring out where to point the telescopes in the first place and why you’re doing it is much harder. I was thoroughly overwhelmed. But I would not last long as an assistant professor if I didn’t discover something big soon. I took out the list of all of the telescopes with all their capabilities and thought and stared.

  It had been five years since that afternoon when Jane Luu had first told me about the Kuiper belt, and by this point almost a hundred small bodies were known in distant orbits beyond Neptune. It was becoming increasingly clear that the study of these very distant, very faint objects was going to be a major new field in astronomy. Big telescopes are particularly good for studying very distant, very faint objects, and I suddenly had big telescopes at my disposal. Go forth! I thought.

  I didn’t quite boldly go forth; instead I took a tiny step. I set out to test one of the hypotheses that was floating around in the scientific community at that time: that the objects in the Kuiper belt have mottled surfaces owing to the effects of craters formed by giant impacts, just like those that I could see on the moon. Proving or disproving this hypothesis would not be considered by anyone to be a major scientific advancement, but it was a start, and I needed a start. To test the hypothesis, I was going to spend three nights at the 200-inch Hale Telescope carefully studying a few objects out in the Kuiper belt to see if their surfaces were indeed mottled. The three nights I was scheduled to be at the telescope happened to fall over Thanksgiving, a fate that often befalls the youngest astronomer on the block. But the three nights were dark time. There would be no moon to disturb my view.

  A day before Thanksgiving, I took the three-hour drive south from Pasadena, across the farmland (now housing developments) of the Chino Hills, through the dusty Pala reservation (now a multistory casino), and into the forested road (now a road through burned stumps) leading to Mount Palomar. The drive gives ample opportunity to stare at the sky and fret about occasional clouds and potential bad weather. This day there were no occasional clouds or potential bad weather: There was total cloud cover and continuous bad weather. The forecast was bleak. Astronomy is not always about bad weather at telescopes, but when you are young and eager for discovery or even just a few small steps, the nights of bad weather are the ones that seem to stick most in the mind.

  The fog settled in thickly around the mountain as I arrived at the top and drove to the ornate old two-story heavy stucco dining and sleeping area known as the Monastery (which was an appropriate image for the earlier days of astronomy, when women were not allowed to stay). I went to the telescope to set up the instruments for the night of work; I spent hours in the windowless dome testing and calibrating and double-checking all of the settings I was going to use. As I finally stepped outside to walk to dinner, a light snow began to fall. After dinner the snow stopped, but a dense fog remained for the night. I stayed awake the whole time, hoping that somehow the fog would lift and I could start working. But it never did. I finally left the telescope to head back to the Monastery as the sun was rising and turning the fog from thick and black to thick and vaguely gray. At the Monastery, I closed the blackout curtains in my tiny room
and slept until 2:00 p.m.

  Opening the blackout curtains, I was greeted with more fog and now a heavy covering of wet snow. I was informed that the snow meant that there was no chance the telescope would be working that night; the dome enclosing it was frozen shut and would require direct sunlight to get unstuck. The snow also meant that the roads up and down the mountain were impassable in my two-wheel-drive truck. Instead of a quick meal before sunset with the other astronomers so that we could all run to our different telescopes when darkness arrived, we were all stuck at the Monastery for Thanksgiving. There was no television and no Internet connection, so after dinner, the other astronomers and I built a fire and caught up on our scientific reading. I was still scouring everything I could find to help me come up with ideas of what I might do. Every time I had a thought, I would ask the others around the fireplace questions about the local telescopes and how I could use them to help with this problem or that.

  “How well does the infrared camera at the Hale Telescope work?” Very well, was the answer. A general conversation would follow. We would all drift back to our reading.

  “Is there a long-slit mode for the echelle spectrograph?” I would pipe out. No, was the answer, but we all speculated about how a quick modification would make one possible.

  “Does anyone know anything about the new thermal imager that is coming next year?” Yes, indeed.

  During the course of the evening, I covered, I thought, every combination of telescope and camera and spectrograph and instrument that was available at Palomar.

  Eventually one of the other astronomers asked: “Have you ever thought about the 48-inch Schmidt Telescope?”

  No. I hadn’t. In fact, I only vaguely knew where it was. Down one of those side roads I never drove down? That little dome over by the water tower, maybe?

  I did know, though, that when astronomers were building the huge 200-inch Hale Telescope more than fifty years ago, they realized that having the biggest telescope in the world didn’t do you much good if you didn’t know where to point it (a dilemma with which I am quite familiar). They decided that they needed to make a detailed atlas of the entire sky—a road map for the big telescope. So they built a smaller telescope, then known simply as the 48-inch Schmidt (after the size of the mirror and the general type of telescope), just down the road. The 48-inch Schmidt took pictures of the sky night after night until finally—for the first time in history—every patch had been photographed. The resulting maps of the skies—the Palomar Observatory Sky Survey—are famous throughout the astronomical world. At one time, all astronomy libraries had a wall full of cabinets containing fourteen-inch-square prints that together made up the complete Palomar Observatory Sky Survey. Each print, when pulled out of its special protective envelope, shows an area of the sky that looks about as big as your fully outstretched hand held at arm’s length. It takes 1,200 of those prints to cover the whole sky, from Polaris, the North Star, all the way down to the Southern Cross.

  As a graduate student, I had been instructed in the arcane mysteries of the correct use of the Palomar Observatory Sky Survey, which was simply called POSS by the cognoscenti. First, you go to the astronomy library and open the big cabinets; then, based on the sky coordinates of where you want to be looking, either you find the library ladder and climb to the top (if you’re looking in the far north), or you sit on the floor (for the farthest southern objects), or, if you are fortunate enough to be looking for something directly overhead, you can stand comfortably and look straight ahead. With luck, you will find that the prints are stacked in the order they are supposed to be in, from pictures of the sky farthest to the east to pictures of the sky farthest west. If you’re unlucky, the one picture you’re looking for will be the only one out of place, and your search might take an hour. When you find the picture you want, you pull it out, set it on the large library table, put your face down close to the picture to see the millions and millions of stars and galaxies, and use the jeweler’s loupe to find the precise area of the sky you’re looking for. Finally, you pull out a custom-built Polaroid camera from its case, point it at the spot you’ve identified, and take an instant picture of a postcard-sized section of the sky survey print. That Polaroid print is now your personal road map.

  For decades, astronomers carried these Polaroids with them to telescopes all around the world. When you commanded your large telescope to point to the spot in space in which you were interested and you looked at the TV screen, you were usually greeted by a fairly unremarkable field of stars. The Polaroid pictures were the only way to know that the unremarkable field you were looking at was the one in which you could find the galaxy or the nebula or the neutron star you were looking for. In the control room of any telescope at any night of the year, you could find an astronomer or a group of astronomers holding a Polaroid print and staring at the TV screen. Often the actual image of the sky from the telescope was flipped or upside down and no one could ever remember which particular way this combination of instrument and telescope flipped images, so there would always be a time in the night when three or four astronomers would be squinting at a little screen full of stars, holding a little Polaroid picture full of stars, and turning the picture sideways and upside down until someone exclaimed, “Ah ha! This star is here, and that little triangle of stars is here and we’re in just the right place.” These days the technique is mostly simpler—the Palomar Observatory Sky Survey pictures are all quickly available over the Internet, and the cabinets full of prints are gathering dust; but because you can’t take the computer screen and turn it sideways or flip it over, the little group of three or four astronomers is now more often than not standing with their heads cocked in all possible combinations of directions until the lucky one exclaims, “Ah ha!” and then all heads immediately tilt to that direction.

  Though the 48-inch Palomar Schmidt was famous to astronomers the world over, I had not even considered it worthy of thought, for one good reason: The telescope still used relatively primitive photographic technology to take pictures. Astronomers a generation before me all learned photographic astronomy: how to load film in the dark, how to ride in a tiny cage suspended at the top of the telescope, how to carefully move the telescope through the sky, how to develop and print. My generation was the first entirely digital generation of astronomers. All telescopes today have digital cameras that use, in only slightly fancier form, the same technology used in everyone’s handheld digital cameras around the world. The change in astronomy is as dramatic as it has been in photography. The ease and speed with which images can be obtained and examined and manipulated and shared has transformed the way that astronomy is done today. So when I overlooked the 48-inch Schmidt Telescope, it was mostly because I considered it a relic from the days of prehistoric astronomy.

  But on that snowy, foggy Thanksgiving night at Palomar, I decided that visiting this relic to see how ancient astronomy used to be performed would be an entertaining way to spend a few of the nighttime hours. After making sure I knew exactly which way to go, I walked down the dark, snowy road through the piney woods, past the largest telescope, down a road I had never taken, to where the 48-inch Schmidt resided. Someone was inside, tidying up in the cramped control room that sat underneath the telescope. I introduced myself and met Jean Mueller. She was tidying, in lieu of her usual nighttime job, which was to use the 48-inch Schmidt to once again make a new map of the full sky to compare to the first.

  Using the 48-inch Schmidt? It was a fossil. Why would anyone still want to use it and its messy and cumbersome photographic plates? The answer is relatively simple. Even though astronomy has progressed greatly since the days of photographic plates, and even though digital cameras make astronomers’ lives incomparably easier and better, one thing had gotten worse. A Schmidt telescope is designed to look at a huge swath of sky at once. Every time a fourteen-inch-square photographic plate—literally just a piece of glass with photographic emulsion painted on one side of it—is placed at the back of the teles
cope and exposed to the night sky, an enormous piece of the sky is photographed. Digital cameras on telescopes, in contrast, are much better at seeing faint detail but much worse at seeing large swaths of sky. A typical telescope equipped with a digital camera could, at the time, only see an area of the sky more than one thousand times smaller. The obvious solution would simply be to build a bigger digital camera, but to see as much sky as you could see with the photographic plate you would need a five-hundred-megapixel digital camera. Even today that is a daunting number. At the time, when only high-end photographers had a single megapixel to their name, if you wanted to make a map of the sky, just as the 48-inch Schmidt had done in the 1950s, it made much more sense to accept the hardships of the photographic plate for its unparalleled ability to sweep up the night sky at a fast pace.

  Jean explained this latest survey and described how the photographic plates were taken and developed. She talked about how she had come to be working there at Palomar after a few years at another observatory. She then wistfully told me that the days of the 48-inch Schmidt were almost over. This second Palomar Observatory Sky Survey was almost complete, and she didn’t anticipate that anyone else would be using the telescope and its photographic plates after that. All of the fall sky had already been photographed, and no one planned on using the telescope at all during the fall season the following year.

  All major telescopes around the world are scheduled to be used every single night of the year, with the occasional exception being Christmas, though I’ve worked at telescopes on Christmas Day plenty of times myself. I find the idea of a telescope not being used almost viscerally painful. It’s bad enough when the reason is technical or simply weather related, but when a telescope is not being used for simple lack of interest it feels worse. Yes, the photographic technology was old and clunky, but clearly the 48-inch Schmidt was one of the best telescopes in the world, at least for imaging wide areas of the sky.