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Gyrations Help Studies : Pluto Teams With Its Moon for Rare Celestial Sideshow

November 14, 1987|LEE DYE | Times Science Writer

The most distant planet in the solar system, so far away and so tiny it is 1,600 times too faint to be seen with the unaided eye, is putting on a show for astronomers that comes only once in a lifetime.

Pluto, the oddest ball in the solar system, lies on its side as it orbits the sun, and it is so aligned with the Earth right now that its moon passes in front of and directly behind Pluto as it spins around the planet every 6.4 days.

By measuring such things as reductions in luminosity as part of the planet or its moon is obscured by the other, astronomers are able to learn much about the size, density and composition of both bodies. And they won't have a chance like this again for 124 years.

Not until Pluto, now about 3 billion miles away from Earth, travels halfway through its bizarre orbit around the sun will it once again position itself so that similar measurements can be made. And the next time it happens, it will be much farther from the sun than it is now, so it will be about 10 times as faint, according to David Tholen of the University of Hawaii's Institute for Astronomy.

"So this is really a chance of three lifetimes," he said in an interview. Tholen and a host of astronomers discussed their research this week during the annual convention of the American Astronomical Society's division of planetary sciences, which is meeting in Pasadena.

Seen as Tiny Pinpoint of Light

Pluto is so distant and so dim that even the most powerful telescopes on Earth record only a tiny pinpoint of light when focused on the planet. And Pluto missed out on the grand tour of the planets by the Voyager spacecraft. Voyager is to visit Neptune in 1989, but Pluto will be in the wrong place to encounter the robotic probe, leaving it as the only planet unvisited by a man-made device.

And there are no plans to send a spacecraft to Pluto.

So for astronomers like Tholen, this is the best shot they are likely to get.

"This is the next best thing to a grand tour," he said.

Pluto is the only planet in the solar system that does not orbit in the flat plane shared by all the other planets. Pluto, instead, travels in a highly eccentric orbit that carries it far above and below the orbits of the other planets.

All of the planets, with the exception of Uranus and Pluto, rotate about an axis that is nearly perpendicular with the sun. The Earth's axis is inclined only 23 degrees, which, incidentally, is what gives it its four seasons. But both Uranus and Pluto are tipped over so that they lie on their sides as they revolve around the sun.

Thus parts of Pluto could also be known as the land of the midnight sun, because when one pole is pointed at the sun, the other remains in darkness for years.

Alternate Roles in Eclipses

Twice during each orbit, Pluto is positioned so that its equator faces the Earth. Since its only moon, Charon, orbits around the equator, that means that during those periods Pluto and Charon take turns eclipsing each other as viewed from the Earth.

That process started late in 1984, when Charon just began clipping the edge of Pluto, and it will continue through October, 1990. But right now, and through most of next year, the two bodies are nearly perfectly aligned, so Charon is completely eclipsed by Pluto when it passes on the far side.

"It's just like a total eclipse of the sun," Tholen said.

Tholen has been using the University of Hawaii's 88-inch telescope on Mauna Kea for his research. The telescope is equipped with a photoelectric photometer, which he said is a "fancy name for a light meter."

Like a meter in a camera, the photometer measures the light from Pluto and Charon as they go through their gyrations. The meter is so sensitive that the amount of light begins to drop just three or four minutes after one body begins to eclipse the other.

When Charon is completely behind Pluto, only the light from the planet is visible, so the meter records a much dimmer level. By subtracting that amount from the total received when both are visible, the brightness of each can be determined.

That is a considerable milestone, Tholen noted, considering that before 1978, no one even knew Pluto had a moon. Pluto itself was discovered just half a century ago.

Tholen plots the readings on a chart, which shows a rapid decrease in light at the beginning of the eclipse, followed by a considerable period when the amount of light remains constant while Charon is completely hidden. The length of the dimmest period tells him how long it takes the moon to pass behind the planet, and thus its size.

Diameter of 1,396 Miles

Long a matter of debate among astronomers, Tholen told the Pasadena meeting that his research shows Pluto's diameter is 1,396 miles, making it nearly twice the size of Charon, which is 735 miles in diameter. Both figures are down slightly from previous estimates.

With Pluto's size known precisely, it is easier to determine its density, and subsequently, its composition.

It turns out, Tholen said, that Pluto is far denser than scientists had expected.

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