Developing a Giant Space Mirror

What came out of that effort is a 21-inch mirror that weights only a little over 2 pounds. "The Hubble's primary mirror weighs 180 kilograms (about 400 pounds) per square meter," says David Baiocchi, a researcher working on the project. "Ours weighs 5."

Instead of a sheet of glass several inches thick, the mirror has a "facesheet" that is less than four-hundredths of an inch thick. Some 31 tiny, computer-driven actuators, each weighing less than one-fifth of an ounce, attach the mirror to a rigid carbon fiber support.

The actuators flex the mirror to produce the exact shape, or "figure," and they do so with astounding accuracy. The scientists say that if their 21-inch prototype were scaled up to the size of a football field, the difference between the highest and lowest points (the flaws) would be the thickness of two human hairs.

The system would allow the mirror to be tweaked whenever necessary, but that probably wouldn't be very often.

"The idea is that once it's up there and in operation we would only be correcting the figure once a week or once a month," Baiocchi says.

Theoretically, if they can shield it enough from the sun to eliminate thermal expansion and contraction, it may not be necessary to adjust it at all, once compensations have been made for the ride into space.

The next step is to send a demonstration mirror into space, possibly aboard the shuttle, to prove that the necessary adjustments can be made and the mirror can be whipped into perfect shape. Then, it will be onward and upward, the scientists hope.

Space Assembly Required

But, about that huge mirror that could someday be looking down on us …

It would have to be at least 100 times larger than those used for Earth-observing satellites that are in low Earth orbit, because it would be farther out in space. That means it would need to be at least 100 meters in diameter, or bigger than a football field.

No rocket could carry that large a mirror into space, regardless of how light scientists could make it, so that presents a problem. The solution, Baiocchi says, is to build the mirror out of smaller segments, like the Keck Telescope in Hawaii, and assemble the segments on orbit into a single mirror.

That would be a pretty big job, and it would probably have to be done by robots, but one mirror that size could do the work of many satellites in lower orbit. And it could "see" nearly half the planet.

By the way, we should be able to see it as well.

"It would probably look like a very bright star," Baiocchi says.

Not your run-of-the-mill eye in the sky, to be sure.

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