Let There Be Light

Inventive / by Lee Billings /

Astronomers will soon find scores of Earth-sized exoplanets, but imaging them may be decades away. That is, unless NASA decides to build a starshade.

Two support engineers inspect the starshade prototype testbed at Northrop Grumman’s facilities in Redondo Beach, California. Courtesy of Northrop Grumman.

During a January 1996 speech at the American Astronomical Society in San Antonio, Texas, NASA’s then-administrator, Daniel Goldin, was brimming with excitement. Three months earlier astronomers had announced the first discovery of a planet orbiting another Sun-like star, though nothing could live there—the “exoplanet” was perhaps half the mass of Jupiter and orbited hellishly close to its sun. Yet even in those early days, with only a handful of known exoplanets, a trend was becoming clear: Scientists were quickly discovering progressively smaller exoplanets in more clement orbits, making the eventual detections of potentially habitable Earth-sized exoplanets seem inevitable. The coming decades, Goldin said, would bring an amazing era of discovery in astronomy. In his speech, he speculated that “perhaps in 25 years” NASA would study any nearby Earth-like exoplanets for signs of life and be able to obtain images of them “with a resolution to see oceans, clouds, continents, and mountain ranges.”

Goldin envisioned an initial flotilla of large “terrestrial planet finder” telescopes (“TPF” for short), capable of locating Earth-sized exoplanets and even imaging them—but as no more than smudgy dots. If the TPFs found promising worlds, more ambitious “planet imager” telescopes could then be built, which would operate in unison to deliver Goldin’s promised maps of alien earths.

Viewed across the light-years, a world like Earth would appear extremely close to its star; distinguishing the former from the latter would be very difficult. Marc Postman, an astronomer at the Space Telescope Science Institute, compares the feat to reading the date stamped on a dime at a distance of 5 kilometers. Worse yet, the star may outshine the exoplanet by a factor of 10 billion. To get even a meager clump of exoplanetary pixels, a TPF telescope must somehow eliminate the star’s glare so that the faint exoplanet’s light can be seen. It would be like photographing a lit match on the cusp of a detonating hydrogen bomb.

Under Goldin’s leadership, NASA intensively studied two potential solutions to these problems. One, TPF-C, called for using an internal coronagraph, a small light-blocking apparatus placed inside a telescope—you can see the same principle in action by raising your thumb to blot out the Sun or Moon in the sky. The other, TPF-I, relied on a technique called interferometry, where starlight is split into separate beams and recombined so that it annihilates itself, like two out-of-phase waves colliding in a pond to form a placid surface.

While both TPF-C and TPF-I could in theory nullify starlight to image nearby Earth-like exoplanets, neither would work properly beneath Earth’s interfering atmosphere. They’d need to be lofted into the harsh vacuum of outer space, adding billions of dollars to their already high costs. 

In 2001, the incoming Bush administration replaced Goldin as NASA administrator. Then, in 2004, Bush announced a major change for NASA, a plan to return humans to the Moon and then on to Mars. Unfortunately, the bold new vision was not accompanied by bold new funding, and in 2006 NASA raided its science budget in support of Bush’s spaceflight program. TPF’s budget plummeted to zero, and the project is now “deferred indefinitely.” The only major new telescope NASA can afford in the near future is the James Webb Space Telescope (JWST), the agency’s replacement for the aging Hubble. JWST is slated to launch in 2014—and it isn’t designed to image exoplanets.

In hindsight, like many things from the 1990s, Goldin’s prediction seems the product of irrational exuberance. But he was eminently correct in at least one thing: During the years that NASA scientists toiled to define how Earth-like exoplanets could be imaged, other experts focused on the significantly simpler and cheaper task of merely detecting such worlds—propelling the field to amazing heights. Today more than 400 exoplanets are known, some only a few times the mass of Earth. A handful even orbit like the Earth in “habitable zones” around their star where liquid water may exist. Now, with the March 2009 launch of NASA’s powerful Kepler spacecraft, the hunt for habitable planets beyond our solar system—Earth-like exoplanets—has begun in earnest.

Kepler has the sensitivity and precision to detect Earth-like worlds by the shadows they cast earthward as they cross in front of their stars. If all goes according to plan, Kepler’s first potentially habitable finds are no further than three years in our future—and several ground-based searches may find additional promising exoplanets even sooner. But without something like a TPF to image them, it will be nearly impossible to study such exoplanets for signs of life.

Because designing and building a space telescope typically takes decades of planning, many astronomers estimate a TPF-like mission won’t be realized until the late 2020s or 2030s, and that Goldin’s planet imagers lie beyond their lifetimes. The runaway success of exoplanet detection techniques combined with faltering support for exoplanet imaging has created a situation where we are rushing headlong toward discovering scores of worlds that could quite possibly harbor alien life—only to then be forced to wait a generation before properly investigating them.

But it just may be possible to shrink the gap to less than a decade. Besides TPF-C and TPF-I, there is another possible tool for imaging Earth-like exoplanets. It’s called an external occulter, or a starshade, and functions similarly to a coronagraph but lies far outside the telescope rather than within it. By keeping the coronagraph outside the telescope, the telescope itself can be simplified, potentially reducing costs.

Though first conceived by Princeton University astronomers Lyman Spitzer and Robert Danielson in 1962, planet-imaging starshades have only recently come into vogue, largely due to a proposal by University of Colorado astronomer Webster Cash. Cash has spent the last five years designing a starshade that could augment JWST. At an estimated cost of $700 million, Cash’s starshade could allow JWST to provide images of alien earths around a handful of nearby stars, sooner than a TPF and at a fraction of the price.

“If you don’t mind waiting 20 or 30 years, there are other ways to do it,” Cash says. “But right now, starshades are the only identified approach that could possibly lead to a spectrum of an Earth-like exoplanet in the next 10 years.” A spectrum can tell astronomers about an exoplanet’s surface and atmosphere—for instance, whether a world has water oceans. A spectrum can even reveal signs of life, such as the presence of oxygen from photosynthetic plants and methane from anaerobic bacteria.

Cash’s starshade would resemble a many-petaled sunflower—if sunflowers were matte-black and about half a football field in diameter. Its special shape is designed so that waves of starlight will diffract around it, lapping against and nullifying each other to cast an ultra-dark shadow, ensuring that only an exoplanet’s light falls on the JWST’s huge mirror. Equipped with small thrusters, the starshade would fly some 70,000 kilometers in front of the JWST, precisely aligning to block light from a target star so that its accompanying planets could be seen.

The question of whether or not Cash’s starshade will ever be built won’t be answered until next summer at the earliest, when Astro2010, a committee of top US astrophysicists gathered by the National Research Council, releases its report selecting the next decade’s research priorities. NASA typically hews as closely as it can to the decadal recommendations. Cash has submitted his starshade concept to the committee; other researchers, notably David Spergel and Jeremy Kasdin at Princeton University, are promoting a competing starshade-related proposal. Spergel and Kasdin laid many of the foundations for Cash’s work, and collaborated closely with him on his concept until 2007, when they split off to pursue their own starshade design.

Kasdin says his team agrees with Cash’s about the basic concept of an external occulter, but points out that trying to integrate a starshade into JWST’s established program could introduce unnecessary risks: JWST’s launch date could slip, pushing up costs on the starshade, and the telescope simply isn’t optimized to work with a starshade or to image planets. “It turns out flying an occulter with JWST may be harder rather than easier,” Kasdin says. “JWST is big and more efficient at gathering photons, but you’d only get maybe 7 percent of mission time. We’re looking at flying a smaller occulter with a smaller dedicated telescope. You’d get much more observing time, so in the end the science returns turn out to be about the same.”

Though a starshade could significantly complicate JWST’s mission, JWST’s project scientist, Nobel laureate John Mather, is tentatively supportive of the idea. “This is the least expensive method that I know of [for imaging Earth-like exoplanets],” Mather says. “It seems fairly clear to me that if we had the resources now to get started on the occulter study, we should.”

In the absence of new funding from NASA, Cash has been validating his concept by collaborating with a team led by Ron Polidan, a former top technologist at NASA who left the agency to become chief architect of the civil space division at Northrop Grumman, the aerospace company. Northrop Grumman, as well as Ball Aerospace, began supporting Cash’s work in 2005 after an initial round of NASA funding was depleted. Kasdin and Spergel have a similar arrangement with another aerospace giant, Lockheed Martin. It may be that the technology required to deploy a starshade in space also proves useful for classified defense-related applications. Imaging of habitable exoplanets could have a bright future indeed if private companies can devise ways to generate profits from the required technology development.

Polidan’s team has built a scale model of Cash’s starshade and tested its light-suppression capabilities in a long vacuum chamber, but most of the work remains theoretical, locked inside relatively inexpensive computer simulations. According to Charles Beichman, an astronomer at Caltech and NASA’s Jet Propulsion Laboratory who formerly headed TPF development, lingering uncertainties will make securing more funding difficult. Even deviations as small as a tenth of a millimeter in the vast surface of a starshade could severely compromise its performance, and designing against these possibilities could drive up costs. “It’s a matter of a lot of study right now,” Beichman says. “And anyone who claims to know these answers doesn’t.”

Cash and Polidan ruefully admit that they, along with Spergel and Kasdin, will have a hard time satisfying critics without more support from NASA for technology development—support that may not materialize even with a strong recommendation from the upcoming decadal review.

“The problem here is not the technology, but the lack of money to demonstrate it one way or another—and there’s something wrong with that,” Cash says. “This wouldn’t be just helping me, it would be NASA helping itself. NASA has a unique opportunity to conduct an experiment whose results, if positive, will never be forgotten.”

Every subfield of astronomy has its favorite unsolved mysteries, and these reasonably compete against each other for a slice of NASA’s limited resources. But Polidan makes the case that, unlike most competing questions, the search for habitable worlds offers social resonances rarely found in the space sciences.

“Knowing that there are islands of life out there besides our own is, to me, a major discovery for this century. We’re stuck now with this pre-Copernican view that we’re the center, that we’re all there is for life,” Polidan says. “If I could in five or six years show you the image of a planet around another star and tell you that it has water and oxygen, that it can support us, that it may support others like us, this wouldn’t only stimulate space exploration—it could cause tremendous change in our culture.”

Originally published November 17, 2009

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