A STARSHADE TO SEE THROUGH THE STARLIGHT
JPL JUST DESIGNED A WAY TO FIND EXOPLANETS BY STARING AT THE SUN, WITH A LITTLE HELP FROM SOME CUTTING EDGE ORIGAMI
STORY: DAN CRANE
ILLUSTRATION AND DESIGN: ANDREW HALL
PHOTOGRAPHY: AARON SILVERSTEIN
“HOW DOES ONE TAKE SOMETHING THAT LARGE, THAT NEEDS TO BE SPREAD OUT FLAT WHEN DEPLOYED, AND STOW IT INTO A CYLINDER IN ORDER TO LAUNCH IT INTO SPACE?
Imagine standing outside on a cloudless afternoon. You suddenly hear a plane flying overhead, somewhere in the direction of the sun. You tilt your head back to look at it, and instinctively raise your hand to block out the sun for a better view of the plane. Now, imagine you’re a telescope in deep space trying to see a planet orbiting a distant star; unfortunately, that star is so bright it renders the orbiting planet nearly invisible. If only you had a giant hand that could reach up and block that star. Well, that’s where Starshade comes in. Only, instead of a giant hand, it’s a giant flower. Sort of.
Starshade, a technology currently in development at NASA’s Jet Propulsion Lab in Pasadena, CA, resembles a giant sunflower minus the stem. The idea is that it will fly in tandem with a telescope, at a distance of roughly 50,000 kilometers (about the distance of four earths). The structure and the shadow that it casts will block the light from a star, while allowing the light from a distant planet orbiting that star to fall through to the focal plane of the telescope.
Ultimately, the goal is to capitalize on the current “exoplanet gold rush,” the period that’s taken place since 1988 during which time thousands of exoplanets (or planets orbiting stars beyond our solar system) have been discovered. Even as recently as May of this year, NASA announced the discovery of 1,284 additional exoplanets. “Before the Kepler space telescope launched, we did not know whether exoplanets were rare or common in the galaxy,” said Paul Hertz, Astrophysics Division director at NASA Headquarters. “Thanks to Kepler and the research community, we now know there could be more planets than stars.” It’s been suggested that, on average, there might be a planet for every star you can see in the night sky. Unfortunately, most of these are tens of light years or more away from earth.
With Starshade, scientists are hoping to hunt for planets in “Goldilocks zones,” says Case Bradford, the lanky, loquacious Technical Group Supervisor in Advanced Deployable Structures, who gave me a tour of the Advanced Large Precision Structures (or ALPS) lab at JPL which supports a variety of technology development needs for precision space structures. These are earth-like planets on which life may be possible: planets that are “not too far, or too close to the star; not too hot, and not too cold; not too fast in orbit, and not too slow in orbit.”
Currently, exoplanets are discovered primarily via the “transit” method. Telescopes like Hubble stare at a star and wait for an exoplanet to come between the star and it. So, very little detail aside from the planet’s mass can be detected. Not only would Starshade allow telescopes to capture heretofore seen visuals of an exoplanet, but other important details may be recorded as well. “We can find if there’s liquid water. We can find if there’s oxygen. We can find if there’s methane, because the way that the light would land on our telescope would tell us something about the different chemical compositions of those distant planets.” If Starshade works as intended, it may help locate planets that have oceans and could harbor life, either now or sometime in the future—planets that may become, as Bradford puts it, “our vacation home in 500 years.”
One of Starshade’s biggest challenges from a mechanical engineering standpoint is deployment. “How do I take something 34 meters tip to tip—the size of a baseball diamond, it’s enormous—package it into something that can launch in a rocket, survive the launch, go out and deploy, and be just where you need it to be at optical tolerances? That’s just a fun problem by itself,” muses Bradford, who got his Ph.D. in Civil Engineering at Caltech and wrote his thesis on “Time-frequency analysis of systems with changing dynamic properties.” In other words, he might just have a different definition of “fun” than you or I.
How does one take something that large, that needs to spread out flat when deployed, and stow it into a cylinder in order to launch it into space? One potential answer unfolds (if you will) from an unlikely source: origami. “No joke—we’ve hired origami Ph.D.’s to come in and help us with our fold patterns,” says Bradford. “A lot of origami loves zero thickness paper, but we need to fold things that are not zero thickness. We need to fold things that are two layers of foam, or two layers of kapton [a polyimide film] with a piece of foam in the middle.” Later, when asked to explain zero thickness paper, Bradford offers with amusement, “Oh, that’s just a mathematical abstraction!”
The current deployment plan is that Starshade would be a series of petals folded and wrapped around a central hub. As the petals unfurl one by one (or bloom, in a sense), then a perimeter truss unfolds front to back, front to back, as the petals go flat.
Why the flower shape, rather than say, a perfect circle? Ironically, a solid opaque circle creates a shadow with diffused edges. By feathering the edges of the Starshade, says Bradford, they can “tune” the shadow such that it’s cleanly blocking out all the unwanted light while still allowing the light reflected off the distant planet to go past the Starshade and land on the telescope. Bradford contrasts the effect of the petals to what we see when we view an eclipse from space. “You watch the shadow of the moon go across North America and it’s not a solid black line marching across the continent. That edge is due to diffraction.” To avoid a fuzzy, gradient shadow due to diffraction, they add the petals, which, thanks to “very complicated math equations that some other folks develop,” the light from the star gets evenly scattered away from the telescope. “If you could feather the edge of the moon, and, stay with me this is getting weird,” he waxes, continuing with his eclipse example, “but if you could feather the edge of the moon, maybe by putting zigzag mountains and valleys on the edge of the moon, you could crisp up the shadow of the eclipse. That’s the sort of thing that we’re trying to do with the Starshade activity.” (JPL is precise in their wording—“activity” implies a different level of funding and commitment than say, “program”).
Does that mean that the ideal place for the telescope to look for a planet is between the valleys of the petals? No, says Bradford. The Starshade, which has its own on-board propulsion system to get it into position, will actually be spinning as well, in order to mechanically average out any imperfections. “Let’s say we’ve got one petal that’s a bit crooked, but if you spin it, then it makes the whole shadow just a little bit bigger, but not lopsided,” explains Bradford.
Starshade’s petals also come with their own set of engineering challenges. Petal-to-petal tolerance is roughly a centimeter, but the edges are incredibly sharp, “like tens of microns thinner than a hair,” says Bradford. “You’d cut yourself while assembling them, and it’s going to be a 4-meter long razor blade.” The edges of the petals have to come to a really, really sharp point because the petals themselves are casting the shadow and they want to avoid scattering light from the distant star in to the focal plane of the telescope; instead, they want to scatter that light away. “We care about mechanical tolerances far tighter than you typically would for a mechanical structure because it’s scattering light.”
To test Starshade on earth, the team went to the desert and waited until the air was clear after a rainstorm, drove a few kilometers away in a truck, and put a Starshade in front of two light bulbs—one much brighter than the other—so that they could block the bright light bulb while allowing the light from the dim light to pass. Their results were pretty good. “We achieved 10 to the 7 [light suppression],” says Bradford. “Not 10 to the 10, but something that showed that we were are on the right track, and we were able to suppress that light to the mathematically calculated amount that we thought we’d be able to.”
Though it could still be a decade or more before Starshade blossoms to fruition, it’s slated to be compatible with the Wide Field Infrared Survey Telescope (WFIRST) currently in development at NASA. “We’re just going to keep charging forward,” acknowledges Bradford. “So far we feel like this is a very exciting opportunity to be here where we are in the exoplanet gold rush, to be coming up with the technology that could be the one that helps us find oceans. I think that’s exciting.” Once deployed, this strange man-made flower floating through space could very well provide the key to finding life beyond our solar system. Exciting, indeed.