An astronomer searching for alien worlds and an outfielder tracking a fly ball on a sunny afternoon face a common challenge. They both run into the problem of “contrast.” Simply put, the object they seek is not only small but exceedingly faint compared with the blazing star nearby. Baseball players have learned to get around this issue by blocking the sun’s light with an outstretched hand. Astronomers might want to try a similar trick.
Of course, it is much harder for planet hunters. In our solar system, for instance, Earth is 10 billion times fainter than the sun in terms of visible light. Trying to see an Earth twin around another star “is like trying to see a firefly fluttering less than a foot from a huge searchlight — when the searchlight is 2,600 miles away,” writes MIT astronomer Sara Seager in her book Is There Life Out There? It would take a perfectly placed screen, of just the right proportions, to block out that searchlight, but not the firefly.
More than 10 years ago, University of Colorado astronomer Webster Cash designed a bit of space apparatus that might pull off such a feat. Akin to a dark, flower-shaped kite minus the string, this rocket-powered screen would fly into position in orbit where it would obstruct light from a target star. An accompanying telescope could then sit in the shadow, inspecting the star’s environment without being blinded. Ever since, Cash has been obsessed with getting his so-called “starshade” off the ground, but he has garnered little encouragement from NASA. (Although Cash coined the term starshade and patented the first realistic design, he did not invent the idea. In a 1962 paper, astronomer Lyman Spitzer suggested the use of an “occulting disk” to aid in the detection of planets around other stars.)
Between 2008 and 2014, NASA turned down 12 of Cash’s proposals in a row. But things are finally starting to look up, he says. In recent months, the agency’s attitude has begun to change, and there’s a good chance an upcoming orbital observatory will be equipped with a starshade accessory. Until the space shields are incorporated into an official planet-hunting mission, though, Cash is determined to test them by any means necessary — whether they’re mounted on the crest of a rocky peak, dangling from a high-altitude balloon or even suspended from a floating zeppelin. “Twenty-first century astronomy will be about exoplanets,” he says. “NASA says they don’t have the money to do it. I say they don’t have the money not to do it.”
Hunting the Right Light
Cash got hooked on astronomy as an 8-year-old, casting aside his earlier interests in dinosaurs and medieval history after hearing a lecture about the search for extraterrestrial intelligence. “It was a splendid talk,” he recalls. “I came out of there wanting to be an astronomer, and that never changed.” Before Cash became entangled with starshades, the 63-year-old scientist had a productive career designing equipment for the Hubble Space Telescope, the Far Ultraviolet Spectroscopic Explorer, the XMM-Newton observatory and other big-ticket astronomy missions. However, when he sensed that funding in his original field, X-ray astronomy, was drying up, he started thinking about new tools for studying planets around other stars.
The starshade entered the mainstream in 2013 when NASA commissioned a study, chaired by MIT’s Sara Seager, of a hypothetical, billion-dollar mission called Exo-S (a contraction of Exoplanet-Starshade). At the centerpiece of this mission would be a 30-meter-diameter starshade, working in tandem with a 1.1-meter telescope to look for, and investigate, exoplanets.
Although Cash is part of the science team that submitted a final report of Exo-S in March 2015, he is dismayed that NASA set aside money only for a study, not for an actual mission. But Seager emphasizes the importance of the analysis, which established scientific goals for the mission, compiled a tentative list of target stars and evaluated starshade designs and implementation strategies in space. “We have to show the world that the starshade is part of the astronomer’s toolkit,” she says.
With NASA possibly including a starshade in the upcoming Wide-Field Infrared Survey Telescope mission, Cash is pleased that he might see a NASA-sanctioned starshade mission take flight within a decade or so, “while I’m still in my 70s, rather than in my 80s or beyond.”
That interest led him to a late 2004 meeting in Boulder, Colo., with representatives from the aerospace firm Northrop Grumman. The company, which Cash considers “the world leader in deployable structures in space,” was eager to develop technology for future exoplanet missions. They sought creative input from Cash, whose contributions to similar space missions were well known. Northrop Grumman specifically wanted his advice regarding the use of external screens in the search for Earth twins. At first, Cash wasn’t enthusiastic.
The deal-breaker for him was the problem of diffraction — light’s tendency to bend around the edge of an optical barrier, thus ruining any hopes of getting unobstructed high-resolution views of a planet. “You can see waves going around a breaker in the ocean,” Cash explains, just as sound waves bend around corners in a building. “Light waves do that, too.”
But Northrop Grumman engineer Jon Arenberg pressed Cash on this point, asking whether there might be a way around the problem. Although he had initially discounted this possibility, Cash soon came around to thinking that Arenberg might be on to something. So far as Cash knew, no one had seriously examined this issue for many years. The more he thought about it, the more he became convinced that it might be worth “trying to crack the problem.” He decided to give it a shot.
How the Shade Was Made
Starting in November 2004, Cash spent six months exploring theoretical possibilities, testing out various shapes in the hopes of keeping diffraction in check — an effort supported by a modest award from NASA’s Institute for Advanced Concepts (NIAC). Finding the optimal shape, Cash explains, boils down to a mathematics problem.
He experimented with 100 different equations, each associated with a different flower-like shape, to see how they did in combating diffraction. He wrote elaborate computer codes and ran lengthy calculations, but none of his results bore fruit. “It’s hard to spend all that time when you don’t know if there’s a better solution to be had,” Cash says.
Inspiration finally struck in April 2005. In all of his prior designs, the flower-like petals fanned out directly from the center of a circle, like a lily. This time he tried something different, placing a dark, round disk in the middle of his shade and having 12 to 16 petals stick out from there — a pattern more like a daisy. Cash showed that this arrangement could, in principle, cut down a star’s glare by a factor of 10 billion, precisely the reduction needed to observe sister Earths near their host star. And a shade made of thin, black plastic, roughly 50 meters across, could do the job. A structure that size could be light enough to be launched and reliably deployed in space, maintaining the “well-defined edge” needed to block a star while laying bare its neighboring planet. “Web is the first, so far as I know, to turn the starshade into a practical idea by showing that it could be smaller, lighter and therefore much more feasible,” says NIAC head Jay Falker.
To Observe New Worlds
Later in 2005, NIAC funded a two-year architecture study of the starshade concept by the University of Colorado, Northrop Grumman Aerospace Systems and Princeton University. The resulting plan called for two spacecraft — the starshade and an orbital telescope — to launch together and detach in space, or to go up separately. Either way, the starshade would use thrusters to maneuver to a spot, about 50,000 kilometers from the telescope, from which it could screen out the star.
After examining a planetary system, the telescope would turn to the next target, with the starshade flying to a new, strategically chosen locale — a feat that would require exquisite, though technologically plausible, coordination between the two spacecraft.
Trials in the Desert
Until a starshade makes it into outer space, researchers will have to settle for experiments closer to home. On nights in May and September 2014, a Northrop Grumman-led team — including Steven Warwick, Cash’s graduate student Anthony Harness and Keith Patterson of the Jet Propulsion Laboratory — conducted tests in a dry lakebed in Nevada.
They placed small (1 to 2 feet wide) starshades midway between a telescope mounted on a tripod and five light sources (LEDs) lined up 2 kilometers away. The brightest LED simulated the star, whereas the fainter ones represented planets. With the help of a starshade, they could see a “planet” 100 million times fainter than the pseudostar.
Subsequent tests allowed them to pick out a target a billion times fainter than the pseudostar. The current limitation is the dusty desert air, which obscures their views. Higher sensitivity to such faint objects should be possible in space, where dust is not a factor. In the meantime, Cash says, the mountains of Colorado would also do the trick.
Cash presented the concept in a 2006 paper, and later that year the team proposed that NASA send a starshade to fly in concert with the upcoming James Webb Space Telescope, a 6.5-meter instrument currently scheduled for launch in 2018. Such a powerful telescope, together with a 50-meter-diameter starshade, would allow astronomers to study Earth-like planets out to 30 light-years, taking direct images of exoplanets and analyzing their light.
In the process, they might identify a planet’s surface features — such as oceans, continents, ice caps and even cloudbanks — and detect the presence of biomarkers like oxygen, methane and water. No one has been able to do this yet, says Cash. “The starshade, so far as I know, is the only viable, affordable technology that shows, at least on paper, that we can get this kind of look at other Earths.”
Unfortunately, Cash says, NASA officials declined the proposal for reasons that he still doesn’t understand. At the heart of the problem, he suspects, is a tendency among agency decision-makers to shun innovation because they are risk averse, “so they end up building bigger versions of observatories they have already flown.” Despite the meager assistance from NASA, Cash has gotten by with help from Northrop Grumman. But rather than receiving money, he gets “in-kind support,” with the company providing the lab resources — including engineering analyses and tests — needed to turn the starshade idea into reality. NIAC’s Falker has long been disappointed by NASA’s tepid backing of the starshade, especially given the priority the agency has attached to exoplanet surveys. Cash did a great job conceptualizing his starshade mission a decade ago, Falker says, “but he didn’t get any opportunity for follow-on development, which was frustrating.”
The Drive to Observe
Cash and Anthony Harness hoped to take starshades into the realm of astronomy in a trial planned for Utah’s Bonneville Salt Flats — famous for its dark skies and flat, open terrain ringed with mountains. After planting a starshade atop a small mountain, they would have mounted a telescope in a truckbed and waited for the star to duck behind the shade. By moving the telescope a few miles per hour on the truck, they could theoretically get good views continuously for about half an hour.
Unfortunately, they never got the funds. While it may still happen in the future, Cash has found another alternative: Northrop Grumman’s Steven Warwick and Harness have been using the McMath Solar Observatory on Arizona’s Kitt Peak to create a non-moving image of a star, and have placed a starshade in that beam. They have detected new stars, previously not cataloged because of their proximity to bright Vega.
The situation seems to have improved in the past year, with serious consideration now being given to incorporating starshades into the proposed Wide-Field Infrared Survey Telescope mission next decade. And Cash says NASA plans to create a “Starshade Project Office” this year. “That’s proper recognition of the value of starshades at long last.”
“People say that if you hang in there long enough, things will change,” says Cash. “I don’t normally believe that, but it looks like there may be light at the end of the frustration tunnel after all.” Or, if he’s truly lucky, maybe that light will be blocked by a starshade instead.
Alpha Centauri or Bust
Cash dubs a particularly ambitious starshade experiment the “Mission to Alpha Centauri,” which aims to get unprecedented looks at Earth’s nearest star system, just 4.3 light-years away.
The original plan was to hang a starshade from a 246-foot-long zeppelin stored at the NASA Ames Research Center in California. At night, when the zeppelin reached the right spot in the sky, a telescope on the ground could begin to explore the neighborhood of a target star. Cash’s student Anthony Harness spent most of 2012 devising ways to precisely control the airship’s position. But the plan fell through in November 2012 when the zeppelin company folded its U.S. operations and took the airship back to Germany.
Cash has also been authorized by NASA to launch several starshade-bearing rockets out of the Mojave Desert. The first rocket flight took place last summer, and showed the system to be quite stable, but not enough to be scientifically useful.
Undeterred, Cash and Harness are now pursuing a loftier strategy: suspending a starshade from a high-altitude balloon and observing from an airplane (with a telescope strapped to its wing) that flies in the starshade’s shadow. If this kind of experiment proves successful, Cash says, it will open up an entirely new kind of science. “We can get into the habitable zones of Alpha Centauri A and B and get a clean view of both planetary systems. This is unexplored, undiscovered country.”