Searching For New Planets
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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THE MIRROR of the Hubble Space Telescope (the first space telescope, which has been in orbit since 1990)
is 2.4 metres in diameter and weighs 250 kilograms per square metre.
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Nickel-titanium shape memory alloy could allow astronomers to build larger
telescopes By Dean Jobb
Nickel Magazine, July 2006 --The next generation of optical space telescopes could feature huge mirrors made of a lightweight membrane that are deployed using a nickel-titanium alloy.
Aerospace firm Lockheed Martin Corp. has been issued a patent for "a lightweight active mirror" made of thin layers of composite material bonded to ultra-thin strips of a shape-memory alloy. The alloy of choice is Nitinol ( N01555), a roughly equal mixture of nickel and titanium.
"One of the real interests (in astronomy) is looking for planets that are orbiting distant stars," says the mirror’s inventor, Lockheed Martin Fellow Stephen Winzer of the company’s Space Systems Advanced Technology Centre in Palo Alto, California.
"As you might expect, to do that you need very large apertures, big telescopes. And to put something that big into space is a real issue … there’s been a lot of research and development driven towards making very lightweight mirrors so that you can lift them into space and use them for these kinds of observations."
The payload capacity and boosting power of rockets limit the size and weight of any telescope put into orbit. The mirror of the Hubble – the first space telescope, which has been in orbit since 1990 – is 2.4 metres in diameter and weighs 250 kilograms per square metre.
But Winzer envisions space telescopes with giant membrane mirrors of up to 300 metres in diameter that weigh less than a kilogram per square metre. In comparison, the largest earth-based optical telescope is only about 30 metres in diameter.
His patent, issued by the United States Patent Office in June, sets out the specifications for a mirror that can operate in the cryogenic temperatures (below -150 Celsius) of space. A reflective surface of aluminum, gold or chromium is sandwiched between layers of Kapton, vinyl or nylon. The back of the mirror is a framework of flexible carbon fibre rods that give the mirror its shape once it is deployed.
The mirror would be stowed for launch and unfurled in orbit, and that’s when the nickel-titanium alloy comes into play. "Its purpose is to give the membrane its initial shape," Winzer explains. "It serves to tighten the membrane in the structure." Heat is applied to strips of Nitinol attached to the membrane – each strip would be a micron thick and a few centimetres wide – causing both to change shape until the membrane assumes its proper position. Winzer likens the process to stretching plastic wrap over the rim of a bowl, creating a tight drum-like surface.
The mirror of a space-based telescope must withstand vibration, heating, cooling, solar wind and other forces that can affect image quality. To enable operators to correct such aberrations in real time, the specifications for the mirror’s backing include electroactive elements.
Besides aiding in the search for planets orbiting other stars, a telescope with a large membrane mirror would be able to peer inside distant galaxies that are too far away for the Hubble to explore in detail.
Winzer’s design has not reached the prototype stage but antennas have been deployed in space using shape-memory materials. His mirror will be difficult to manufacture, he points out, since the strips of nickel-titanium must be applied to the membrane in a vacuum. NASA is assessing the technical requirements for the lightweight, low-cost space telescopes of the future, but Winzer has no idea if or when his mirror will leave the drawing board. "This kind of a mirror," he admits, "is a pretty far-out concept."
Dean Jobb is a Halifax, N.S.-based freelance writer.
PHOTOS: NASA
U.S. Patent Office |
