What we can learn from Pluto

LAKE TEKAPO, New Zealand — Twenty-four hours before showtime, our prospects looked bleak. Halfway around the world from our homes in Massachusetts, our team of astronomers had traveled to this lake's barren shores for a two-minute encounter with Pluto. But the wind was at gale force, so we weren't even allowed to open the telescope dome.

As luck would have it, the next night — the small hours of June 30 in New Zealand — the wind died down, the sky was free of clouds, and the 1,500-mile-wide shadow of Pluto fell upon Earth, cast by a star trillions of miles away and in precise line with the Mount John Observatory. It was just as we had hoped.

Our team — scientists and students from Williams College, M.I.T. and Lowell Observatory — had won a gamble only astronomers would have made: placing bets, months in advance, on good weather over one remote spot on Earth for two minutes on the calendar. Only under those conditions could we use Earthbound telescopes to study starlight filtering through Pluto's atmosphere as the planet obscured (we say occulted) a star millions of miles behind it.

We had done this before, but this occasion was special. The occultation kicked off a two-week period that promises to change forever the way we see Pluto, perhaps the most mysterious body in our solar system.

On Tuesday, we astronomers are anticipating humanity's first up-close look at Pluto, during a flyby by the NASA spacecraft New Horizons, which was launched in 2006. More than 4 billion miles from Earth, Pluto was discovered only 85 years ago, and is so small it was eventually reclassified to be the chief "dwarf planet," rather than a minuscule ninth planet, in our solar system.

Until now, studying an occultation, with its risks of failure, has been among the few good methods to learn about this celestial body.

But if all went well, as New Horizons passed Pluto on Tuesday about 8 a.m. Eastern Daylight Time, one of its cameras captured details of the surface as clearly as a satellite photo shows a lake in Central Park.

Interestingly, though, it is Pluto's atmosphere that has provided some of the best clues so far about what the dwarf planet may be like.

Consider one discovery during an occultation a dozen years ago: We knew Pluto took 248 Earth years to circle the sun in a huge elliptical orbit, and it had passed the orbit's closest point to the sun in 1989. By 2002, we were assuming Pluto had been cooling ever since. Some computer models even predicted that its atmosphere would have disappeared, having fallen as snow.

Its passage in front of a star told us otherwise.

If Pluto had no atmosphere, we had reasoned, the starlight would wink out instantly when Pluto passed. Instead, it faded over several seconds, a clear sign that an atmosphere had survived. There were signs, as well, that it had warmed a bit and increased in pressure. This told us that Pluto experiences temperature lags, as does Earth. (The warmest point of an ordinary day in New York City occurs at 3 or 4 p.m., rather than noon.)

Such observations can have practical implications. For example, Pluto's atmosphere, like Earth's, is mostly nitrogen. Methane and other ice deposits on Pluto's surface can change into gas as sunlight hits them. So studying how those gases change Pluto's atmosphere over time may help us refine computer models used to study our own atmosphere, and the role played by gases like carbon dioxide when released.

We have expected a few preliminary pictures of wonderful, if limited, quality as the spacecraft approaches Pluto. But then come dangers: The craft must pass undamaged through the plane of Pluto's moons, and it might collide with the space dust that orbits Pluto. We won't be sure it has reached its destination above Pluto for hours after it does so; only after 12 hours of examining the dwarf planet will it be turned to point its antenna toward Earth to send data, and those signals will take 4½ to reach Earth. And because so much data must travel so far, it will be 16 months before we have received all of its highest-resolution images.

Even then, those pictures will not replace the need to monitor Pluto from Earth.

While New Horizons' view of Pluto will be intimate, it will be just a snapshot of a single visit. So it must be read in the context of time passing — what we have learned and will continue to learn from much more distant observations.

Until this month, the realm of Pluto and its moons has been too far away to ever observe it from Earth with the detail we would like to have. That is now changing. And if we can draw conclusions from Pluto's shadow in starlight about how bodies in orbit warm (or cool), just imagine how much further we can now go with more close-up probes and ever more sophisticated long-range observations, as we seek insight about our own rapidly warming home planet.