Taking Pluto’s Portrait

Even the mighty Hubble Space Telescope has to strain to see this tiny, distant planet.

Hubble Pluto PIA00825.jpg
Images of Pluto taken with Hubble’s Faint Object Camera in June and July 1994 were enhanced and processed to make a global map of the planet at different longitudes. The tile pattern is an artifact of the processing.

On a hot summer day in 1994, six years after he first asked for it, Pluto finally arrived on Alan Stern’s desk. The planet—digital images of it, to be exact—came by express mail, on two cassette tapes. The meager handful of photons captured in those images had traveled nearly three billion miles through space before bouncing off the main mirror of the Hubble Space Telescope, which is orbiting 330 miles above Earth.

From there the photons converged at the Hubble’s focal plane, striking a detector in the European-built Faint Object Camera. The resulting electrical signal was then relayed from the telescope to a NASA satellite, down to an antenna in New Mexico, over to NASA’s Goddard Space Flight Center in Maryland, and then to the Space Telescope Science Institute (STScI) in Baltimore, where the data was calibrated, cleaned up, and shipped off to Stern’s office at the Southwest Research Institute in San Antonio. The whole journey, from Pluto to Texas, took just a few days.

Almost two years later, when Stern and his collaborator, Marc Buie, finally released the images at a NASA press conference in Washington, D.C., all the major television networks carried the news on their evening broadcasts. The photos—the most detailed look yet at a planet discovered in 1930—made the New York Times, USA Today, and dozens of other newspapers and magazines around the world.

The success of the press conference was in some measure due to Stern’s skill at explaining science. Eminently quote-worthy, he spoke of “knock-your-socks-off” images, “a tantalizing first look,” and the need to send a spacecraft to Pluto to take even better pictures.

But the cautious scientist in him knew that as good—even historic—as these pictures were, they represented, to him and his colleagues anyway, only one small step on the path to fully exploring Pluto. “What usually comes out of Hubble or any other telescope,” he had mused back in 1994, before he ever saw the Pluto pictures, “are little advances, this whole edifice that you build up, brick by brick.”

In 1988, when he first proposed using the world’s most powerful telescope to study the solar system’s last uncharted planet, Stern was still in graduate school at the University of Colorado. Today, at the age of 38, he is one of the country’s top planetary scientists. The Pluto observation was his fourth turn on the Hubble; previously, he had looked at Jupiter’s aurora and Neptune’s largest moon, Triton (twice). But it’s Pluto that really holds his interest. At the time of the Hubble observation, he was deeply involved in planning a NASA spacecraft mission then called the Pluto Fast Fly-By. Until such a project materializes (now renamed the Pluto Express, the concept is sti1l awaiting funding), the Hubble telescope will provide our best look. And that appealed to the explorer in Stern.

He began the project by assembling a team of experts. Laurence Trafton of the University of Texas helped with the detailed planning that might make the difference between a failed observation and a winner. Marc Buie of the Lowell Observatory in Flagstaff, Arizona, who joined the team later, was also a Pluto aficionado. When the planet and its moon Charon had gone through a rare series of “mutual events” in the late 1980s, repeatedly eclipsing each other as seen from Earth, it was Buie who had done the most sophisticated analysis of the changing pattern of shadows cast by the eclipses. Careful study of this data told him which parts of the planet’s surface had a higher albedo, or brightness.

In fact, Pluto had already been crudely mapped with ground-based telescopes before the Hubble came on the scene. Buie and others had used the mutual events and more than 30 years of data on the planet’s variable brightness (which is caused by different viewing angles from Earth, as well as the planet’s 6.4-day rotation period) to infer where the light and dark areas were on its surface. These techniques had a lot of built-in uncertainty, however, and the results depended on how the numbers were crunched. The two best maps both showed a bright south polar cap, for example, but disagreed on whether the north had a similar feature.

The Hubble observation, if the team could pull it off, would replace shaky inference with direct photographic evidence, and would help determine which indirect mapping technique had been the most accurate. Even with the Hubble, it wouldn’t be easy. Pluto is so small and distant that ground-based instruments can’t clearly separate it from Charon, much less show any detail. The planet is only about 1/10 of an arc-second, or 1/36,000 of a degree across, about the limit of Hubble’s resolution. That makes viewing details on Pluto akin to reading the print on a golf ball from 33 miles away, or counting the spots on a soccer ball from 400 miles, or distinguishing between two headlights… well, you get the idea.

Buie would be invaluable not only for his Pluto expertise but for his familiarity with the arcana of Hubble data processing. He had worked at the STScI and had helped to write the first programs for tracking planets with Hubble, and he knew how to get the most out of a meager amount of space telescope data. The Pluto pictures, in all their glory, would have only about eight picture elements (pixels) across the whole disk of the planet. Because each pixel represented more than 170 miles, the scientists knew they would have to wring every last bit of information from each.

Other teams had already used the Hubble to observe Pluto, including a German group that photographed the planet after the telescope was repaired in 1993. But Stern hoped—somewhat audaciously—to extract enough information to map its surface. By taking photographs at four different longitudes as Pluto slowly turned on its axis over the course of two six-day rotations, he would gain almost full global coverage. The proposal also called for recording images in two types of electromagnetic radiation: visible light, at a wavelength of 410 nanometers, and ultraviolet light, at a wavelength of 280 nanometers. The shorter-wavelength, higher-frequency ultraviolet radiation emitted by Pluto would provide an image with finer resolution and more information about the surface. (To understand the relationship between wavelength and resolution, think of a pair of calipers: A pair with a finer scale of markings can measure more detail than a pair with coarser markings.) Measuring in the UV was a clever way to get twice the resolution from the telescope’s Faint Object Camera (FOC), which would better differentiate variations in Pluto’s icy surface. And by comparing the UV and visible-light maps, Stern and Buie would have a powerful tool for modeling the composition of the surface.

It was this proposal that the STScl accepted back in 1988, shortly before the telescope was launched. Because of the subsequent problem with the Hubble’s mirror, though, all observations requiring high resolution—the Pluto pictures among them—had to be put off. Worse yet, after the telescope was repaired, anyone who wanted to use it had to enter into a whole new competition for viewing time. To improve his chances of being accepted, Stern dropped his original plan to use both the telescope’s Wide Field Planetary Camera and the FOC, settling for just the FOC. His proposal was selected again, and in the summer of 1994, Observation #5330, “High Resolution Mapping of Pluto’s Albedo Distribution,” finally came off as planned.

The telescope took the pictures on four days in late June and early July—a set of ultraviolet and visible observations on each day, three exposures for each observation, for a total of 24 pictures. STScl then did a standard computerized “pipeline processing” (which includes factoring out the handful of known dead spots in Hubble’s field of view), placed the data in the permanent archive, and shipped off copies to Stern. One of the tapes got lost in the mail (its images having traveled across the solar system!), so the STScI had to send another copy.

Once the image files were loaded onto computers at Southwest Research, the work could begin in earnest. The raw data looked promising—it was obvious that some squares in the checkerboard-like images were bright and some were dark. But it was way too early to start jumping to conclusions about whether these were real features.

Stern and Brian Flynn, a postdoctoral scientist working with him at Southwest Research, first did a few simple reality checks, like making sure the same features showed up in different exposures taken on the same day, or checking to see if a spot that appeared on one day had moved on the next day’s image, when the planet had rotated a quarter-turn. If it did, Stern and Flynn would have more confidence that they were seeing something real.

Still, the best anyone could hope for at this resolution was to see gross provinces of light and dark, which was precisely the point of the experiment. As Buie would explain two years later at the NASA press conference, “You can’t do geology in these images,”  meaning you could forget about distinguishing mountain ranges from smooth plains.

The albedo variations did tell you something, though. The light areas are thought to be regions where fresh nitrogen “snow” has fallen out of the planet’s thin atmosphere. The darker areas are what passes for bare ground on Pluto—methane ice darkened by the effects of the scant sunlight that reaches the planet. Even though these pictures ultimately revealed Pluto to be the most “contrasty” object in the solar system (with the exception of Earth), the variation in brightness only amounts to the difference between clean Colorado snow and dirty Boston snow.

Before nailing down where these provinces were on a map, though, there was still a lot of hard image processing ahead. Stern compares it to “twiddling knobs”—adjusting the picture on a TV set, but with a dozen or more variables to tune exactly right. With each step lurked the prospect of making a mistake. Stern still remembers with a “sinking feeling” the time he published a result that turned out to be dead wrong. He had made an observation using a brand-new telescope, and the processing software the observatory sent along with his data had a bug in it. That experience taught him that you can’t be too cautious. ‘‘Take it from a guy who’s been wrong, “ he says. “You have to be wrong once or twice to appreciate that.”

For an object larger than Pluto, the data processing would have been fairly straightforward, and it wouldn’t have mattered much if a few pixels here and there were out of alignment. But in this case, a few pixels were all they had, and the computer processing was everything. “The data have so much subtlety to them,” says Stern, “that you really have to get in there and have the bits almost talking to you to really be sure of what you’re seeing.”

The first task was to sharpen each of the images as much as possible, using a computerized process known as deconvolution. But it didn’t go very well. The technique involves applying a mathematical formula characterizing a particular instrument’s known degree of blurring (every telescope has some) to reconstruct what a perfect image would have looked like. Deconvolution, in effect, corrals the blurred light from an object back into a nice, tight circle.

It fell to Buie, the person with the most experience massaging Hubble data, to do much of this nitty-gritty work. But after applying three different types of deconvolution and thousands of computer iterations, the images weren’t coming out exactly right. In some cases they even got worse. Features that had shown up clearly in the raw images would disappear. Or, if the computer happened to sharpen the image’s noise instead of real light from Pluto, some spurious feature would pop up out of nowhere. The problem with deconvolution, particularly when you are working with only eight pixels, is that “you don’t know when to quit,” says Buie. “You can just keep iterating and iterating and sharpening and sharpening.”

So they decided to drop deconvolution and go to Plan B. First Buie used a computer to generate an artificial image exactly the size and shape of the ones of Pluto, with a grid overlaying its surface just like the grid of pixels on the Hubble image. Then he tuned the brightness of each pixel in the artificial image to roughly match what appeared in the real images. Next came deliberate blurring—to duplicate the blurring effects of Hubble. The final step was to carefully align, or register, the two pictures (fake and real) and subtract out the difference, leaving—voilà—an idealized but noise-free version of what the telescope actually saw.

Buie repeated this process for each of the 24 images. It was time- and computer-intensive, rewarding in its own way but frustrating too. Every little thing had to be taken into account. At one point he found that even a slight jitter in the telescope at the time the exposures were taken—only 1/10,000 of a degree or so—had degraded some of the images.

Even more frustrating was never being able to work on the problem for more than a short stretch of time. Buie had to fit the Pluto work into an already hectic schedule: observatory visits, meetings, proposal writing, and all the other hassles of the working scientist.

Stern was even busier. In fact, he was in the middle of one of the most frantic years of his life. Shortly after receiving the Pluto data, he and his wife had a third child, and the family moved from San Antonio back to Colorado. During one stretch in the spring of 1995, which was not at all atypical, Stern traveled to the McDonald Observatory in Texas to observe the moon’s atmosphere, made a quick pit stop at home, then flew to NASA’s Marshall Space Flight Center in Alabama to work on an experiment that was flying on the space shuttle. After another guest appearance at home, it was off to Toulouse, France, for a scientific meeting on ices in the solar system, then home for one night, then into the field with a sounding rocket experiment for two and a half weeks, back home briefly, then to California for another meeting. The day after he returned home from nearly six weeks of continuous travel, his taxes were due.

“If I could shut out the world and go to Antarctica, we would do this whole project in two months, “ he lamented.

It wasn’t as if they had forever, either. By STScI rules, any scientist who uses the Hubble gets exclusive access to the data for exactly one year from the day the observation is made. After that, anyone can go into the archives and help himself or herself to the original tapes. At first Stern felt some pressure—what could be worse than another scientist scooping you with your own data before you could publish the results? As the months wore on, though, and it became obvious that the team wouldn’t be able to publish within a year, it started to seem less of a worry. For one thing, it was a damn difficult task. If anyone else thought they could do it better or faster and still get it right, they were welcome to try.

By the time of the annual Lunar and Planetary Science Conference in Houston in March 1995, Stern trusted the pictures enough to begin presenting them to other scientists. In his talk he said that the images showed roughly a dozen albedo regions on Pluto’s surface. One intriguing linear feature might even be a crater ray (later he felt less confident about that interpretation and dropped it). It was always “dangerous to overinterpret” the pictures, announced Stern, who underplayed how far along he and Buie had taken the image processing. This was a “progress report” only.

For months, NASA’s press people had been bugging him to release the pictures. Not until they’re ready, was his standard reply, and NASA always backed off. But magazines were starting to ask too: When are we going to see the pictures?

In the end, it was an external event—an educational project in which schoolchildren got to make their own Hubble observations of Pluto—that forced the team to wrap up the first phase of their work. The original plan had been to publish the Pluto images first as a short paper in a scientific journal, then do the NASA press conference, then follow with a more comprehensive paper comparing the new photos to the old maps. But to avoid having the schoolchildren steal their thunder by releasing Pluto images first, Stern and Buie would have to go public, then publish.

It wasn’t a matter of resenting the schoolkids (Buie was in fact supervising the observation for them). But it did force them to hurry up the remaining work to meet the deadline. For one thing, the images—each of which covered only part of Pluto’s surface—still had to be “unwrapped” and laid down in strips to create a flat map showing where the light and dark regions were.

On March 7, 1996, Stern and Buie stood on a stage in Washington, D.C., and unveiled images from two of the four days of Pluto observations, along with flat maps they had made from the full set of pictures, plus global maps made from those. A month later they turned in their scientific paper to the Astronomical Journal, closing out the first phase of the project.

Both scientists continue working with the images, and already they’ve improved their maps since the NASA press conference. And what do the pictures show that’s new? Not much, at least in the first analysis. Buie is pleased that his old, indirect maps turn out to be pretty accurate, except for one or two details. A bright spot near Pluto’s equator, for example, appears in the wrong place in the old maps. The Hubble pictures also reveal two bright regions just above that. And the jury’s still out on the north pole—Buie and another group had concluded it was dark, but the new Hubble pictures and another older map say it’s bright. Unfortunately, this is an area where the Hubble data is less trustworthy, due to the blurring effects.

So Pluto is still a mystery. What Stern and Buie would really like is a regular monitoring program to look for changes as the planet gets colder and more snow falls on its surface. Stern may even put in a proposal to use a new Hubble camera scheduled for installation next year. Maybe he’ll suggest taking pictures at more longitudes, or with different filters. But that’s a proposal to be written another day.

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