Taking Pluto’s Portrait

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

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. (Alan Stern (Southwest Research Institute), Marc Buie (Lowell Observatory), NASA and ESA)
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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.

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