Developing a Giant Space Mirror

Scientists have figured out how to make mirrors that are so light that it should be possible to put colossal telescopes in orbit that will dwarf the Hubble Space Telescope.

The Hubble is the pride of the astronomical community, and it has thrilled us with crystal-sharp images of distant wonders, but it is already an antique. Its primary mirror was designed to withstand the rigors of launch and the demands of space, but it's a solid chunk of glass nearly 8 feet (2.4 meters) in diameter.

It isn't possible to make a bigger Hubble and launch it into space using that technology, which is essentially the same technology that is used to build ground-based telescopic mirrors. So scientists have known for a long time that if they wanted to build bigger space telescopes, they would have to get the weight down. Way down.

Reflecting Ourselves From Space

They've done just that at the University of Arizona in Tucson, and they feel so confident they're even talking about building mirrors larger than a football field that could be placed in orbit high above the Earth. But instead of facing out, these mirrors would face in, looking at the Earth instead of into space. A mirror that large could provide high-resolution images of half of the globe, allowing scientists to keep track of weather systems, fires, and all sorts of things.

The mirror would be in geosynchronous orbit 22,241 miles above the Earth, so it would travel at just the right speed to remain in a fixed position relative to the ground.

Of course, scientists are a long ways away from building such a monster mirror, but they're a lot closer now to building a large telescope that will pick up where the Hubble leaves off. To get there, they had to throw out the rule book.

Instead of mirrors that would be so rigid they could withstand all sorts of stresses, scientists had to learn how to make mirrors that are so flimsy they could be flexed into the right shape while the mirror is in space.

The challenge to come up with the technology to do that was given to the Optical Sciences Center and the Steward Observatory Mirror Lab at the University of Arizona. The lab's James H. Burge is in charge of developing a 2-meter prototype mirror for NASA's "Next Generation Space Telescope," which is scheduled to be launched in 2009.

Burge and his team drew on the lessons they learned while designing flexible secondary mirrors for ground-based telescopes. These mirrors can be warped many times each minute to remove distortion caused by the Earth's atmosphere, thus greatly improving images from observatories on mountain tops around the world.

Atmospheric distortion in space isn't a problem, of course, but the idea is to use that technology to adjust a space-based mirror to offset such forces as thermal expansion, caused by heating and cooling from the sun, as well as bumps and bruises encountered during the bumpy ride up from the ground.

Huge, But Feather-Light

Burge's team is now putting together the demonstration mirror for the Hubble's successor, but even it is too heavy for the kind of mega-mirrors that scientists envision. So they began trimming wherever they could.

"We're taking a technology we're already familiar with and then pushing it, making a mirror as lightweight as possible," Burge says.

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.