Figuring out how to fit a 16-meter mirror into a four-meter hole was the easy part.
Two years before a NASA-led collaboration is set to launch the James Webb Space Telescope — with a collecting mirror 6.5 meters in diameter — Mel Ulmer, physics and astronomy, is researching the feasibility of putting that mirror into space.
Reflecting telescopes rely on concave mirrors to collect and focus light in a manner that has produced brilliant images, including those from the Hubble, which launched in 1990 with a 2.4-meter mirror.
“A major limitation to increasing the size of these telescopes is that the rockets used to launch them can only carry a solid monolithic mirror up to about four meters in diameter,” says Ulmer, a Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) faculty member. “That means we are left to design a system that’s deployable, meaning it can change shape once it reaches space.”
While the Webb Telescope solves the launch conundrum by folding 18 hexagonal mirrors upon itself like the leaves on a dining table, Ulmer is exploring the use of a magnetic smart material (MSM). The MSM will allow engineers to later correct the shape of the monolithic-membrane mirror, which can be folded like an umbrella to fit into a rocket’s nose cone or fairing.
“The benefit of this concept is clear,” says Ulmer. “Coating the mirror’s non-reflective side with an MSM allows us to open it up and then fine-tune the optics using magnets positioned on booms behind the mirror.”
MSMs are designed to contract or expand when exposed to a magnetic field. The research team led by Ulmer, which includes Jian Cao, mechanical engineering; Yip-Wah Chung, materials science and engineering; sophomore Ally O’Donnell; and faculty at Texas Tech University, would like to exploit the MSM membrane’s inherent characteristics to refine the telescope’s viewing capabilities.
“We’re working to characterize exactly how magnetostrictive — or magnetic smart — material behaves under the influence of a magnetic field, measuring the amount of deformation and how long the deformation can hold,” says Cao, director of the Northwestern Initiative for Manufacturing Science and Innovation (NIMSI). “The idea is pretty interesting, as it eliminates the need for thousands of electrical wires needed to power other deployable options.”
Ulmer’s APERTURE — A Precise Extremely-large Reflective Telescope Using Re-configurable Elements — project is being funded by a two-year, $500,000 phase-2 grant from NASA’s Innovative Advanced Concepts (NIAC) program. NIAC exists to nurture visionary ideas that could transform future NASA missions with the creation of breakthroughs that are radically better or represent an entirely new aerospace concept. While bold, such efforts are not sure things.
“If our project were guaranteed to work, we probably wouldn’t have received NIAC funding,” says Ulmer. “The idea is not that complicated but the application is.”
The development of a 16-meter diameter mirror is meant to one-up the Webb Telescope — an $8 billion project — and image the most distant galaxies in the Universe. The technology would also be a boon for Earth space observations because the large mirror would allow the telescope to be positioned far beyond low-Earth orbit (LEO). LEO is the sphere between 99 and 1,200 miles above Earth where the International Space Station and many observational satellites orbit the planet about every 90 minutes.
“At 20,000 or 40,000 feet, a telescope with enhanced magnification capabilities could be positioned in geostationary orbit (GEO), looking back at Earth in great detail,” says Ulmer. “An object in GEO can be pointed permanently at a specific position on Earth, rotating at the same speed as the planet. This makes our work extremely interesting to the weather and surveillance communities as well.”
Researchers at Northwestern and Texas Tech have completed small-scale proof of concept experiments, verifying that their material responds as expected when exposed to magnetic forces. The next steps at Northwestern will be to demonstrate that deflections of the size necessary can be produced when the membrane is tightened. In parallel, collaborators at Texas Tech will work on a deployment design that can produce a membrane mirror whose deviations from the perfect shape are within the range that the Northwestern group can correct.
“Our collaborators are working to evaluate what will give us the best shot at opening our mirror and then adjusting its shape in space,” says Ulmer. “Applying an MSM and using magnetics to unfurl a deployable space mirror hasn’t been explored before, but that doesn’t mean it’s impossible.”