Monday, May 2, 2016

How NASA's Next Big Telescope Could Take Pictures of Another Earth

A “starshade” flying alongside the WFIRST observatory could deliver images of potentially habitable worlds decades ahead of schedule

Can NASA’s next big space telescope take a picture of an alien Earth-like planet orbiting another star? Astronomers have long dreamed of such pictures, which would allow them to study worlds beyond our solar system for signs of habitability and life. But for as long as astronomers have dreamed, the technology to make those dreams a reality has seemed decades away. Now, however, a growing number of experts believe NASA’s Wide-Field Infrared Survey Telescope (WFIRST) could take snapshots of “other Earths”—and soon. The agency formally started work on WFIRST in February of this year and plans to launch the observatory in 2025.

WFIRST was conceived in 2010 as the top-ranked priority of the National Academy of Sciences’ Decadal Survey, a report from U.S. astronomers that proposes a wish list of future missions for NASA and other federal science agencies. The telescope’s heart is a 2.4-meter mirror that, although the same size and quality as the Hubble Space Telescope’s, promises panoramic views of the heavens a hundred times larger than anything Hubble could manage. Using a camera called the Wide Field Instrument, WFIRST’s primary objective will be to study dark energy, the mysterious force driving the universe’s accelerating expansion. But another hot topic—the existential quest to know whether we are alone in the universe—is already influencing the mission.

Researchers have discovered more than a thousand exoplanets—planets around other stars—since the Decadal Survey’s crucial recommendation of WFIRST as NASA’s top-priority next-generation astrophysics mission. They expect to find tens of thousands more within the next 10 years. Many will be discovered by WFIRST itself when it surveys the Milky Way’s galactic bulge for stars that briefly brighten as planets cross in front of them, acting as gravitational lenses to magnify their light. That survey could yield at least as many worlds as NASA’s wildly successful planet-hunting Kepler space telescope, which used different techniques to net about 5,000 probable planets before hardware failures ended its primary mission in 2013.

Already, rough statistics from the entirety of known planets suggest that every star in the sky is accompanied by at least one, and that perhaps one in five sunlike stars bears a rocky orb in a not-too-hot, not-too-cold “habitable zone” where liquid water can exist. The best way to learn whether any of these worlds are Earth-like is to see them—but taking a planet’s picture from light-years away is far from easy. A habitable world would be a faint dot lost in the overpowering glare of its larger, 10-billion-times-brighter star. Glimpsing it would be like seeing a firefly fluttering next to a searchlight or a wisp of bioluminescent algae on a wave crashing against a lighthouse.

FIGHTING THE LIGHT

The Earth’s turbulent, starlight-blurring atmosphere is a severe obstacle to imaging faint planets from ground-based observatories, and most experts agree that the solution is to use space telescopes. But neither Hubble nor its supersize successor, the James Webb Space Telescope launching in 2018, comes anywhere close to achieving the high contrast needed for such observations. Both telescopes’ hardware was mostly designed before the massive surge in planetary discovery—when imaging worlds around other stars was still considered a fringe research topic. In addition to carrying a Wide Field Instrument for general astrophysics surveys and dark-energy studies, WFIRST is also planned to use an advanced planet-imaging coronagraph, an instrument inside the telescope that filters out starlight using a complex series of masks, mirrors and lenses. But this second instrument is a late addition, and WFIRST’s mirror and other optical components are not optimized for a coronagraph. Consequently, most experts predict that WFIRST’s coronagraph will fall short of the contrast required to image Earths. Instead, it will focus on imaging gas-giant planets and, in the most favorable circumstances, a few so-called “super-Earths” or “mini-Neptunes” that are roughly twice the size of our own planet and thought to offer poor prospects for life.

WFIRST’s coronagraph is officially only a technology demonstrator meant to accelerate the development of more sophisticated coronagraphs that could collect images of alien Earths—someday. Snapping such pictures is so challenging that NASA’s tentative plans call for putting it off for perhaps 20 years or more as the agency develops the technology and budgetary breathing room to build an entirely new space telescope after WFIRST. And during that time, astronomers will continue discovering astronomical numbers of tantalizing planets.

WFIRST, however, may offer a shortcut via technology called a starshade—a sunflower-shaped, paper-thin screen half as big as a football field that would float tens of thousands of kilometers directly ahead of the telescope, blocking out a target star’s light in much the same way one might blot out the sun in the sky with an extended thumb. The starshade’s feathered shape is designed to prevent waves of light flowing around it like water over a rock, something that would ruin its ultradark shadow. Unlike coronagraphs, which must be custom-built for any given telescope’s optics and function best with very large ones, starshades work with practically any size space telescope.


A SHORTCUT TO EARTHS

Last year a NASA study found that a functional starshade could be built and flown as an independent mission to rendezvous with WFIRST for an estimated half billion to a billion dollars. Working in tandem, the starshade and the telescope could snap pictures of perhaps 40 planets, including a few that in size and orbit would mirror Earth. “If and only if it has a starshade, WFIRST could give us images of a few true-blue Earths late next decade rather than waiting for another 20 years,” says Jeremy Kasdin, a Princeton University professor and lead scientist for WFIRST’s coronagraph who co-authored the NASA starshade study. “This is a real opportunity to find another Earth sooner and for less money before making a huge investment in NASA’s next giant space telescope.”

In the basement of Princeton’s sprawling Frick Chemistry Lab, Kasdin is feverishly working on a test bed: a meter-wide, 75-meter-long tube with a camera at one end, a laser at the other and a scaled down starshade in between. By the end of the summer, he predicts, the test bed will have demonstrated the necessary contrast ratio that, scaled up to full size, could enable the imaging of Earth-like planets.

Kasdin is not alone in his basement labors; a burgeoning “starshade community” is now performing additional work. The aerospace company Northrop Grumman has tested miniaturized starshades at a dry lakebed in Nevada and at a giant solar telescope in Arizona. And at NASA Jet Propulsion Laboratory (JPL), researchers are demonstrating how to fabricate a larger-scale starshade’s delicate petals, fold the entire structure up inside a rocket, and deploy and unfurl it to the size of a baseball diamond.

Despite WFIRST being nearly a decade away from launch, the decision to move forward with preparations for a starshade rendezvous must come soon, because WFIRST must receive minor modifications to allow it to sync up with a starshade across tens of thousands of kilometers of empty space. “We call this being ‘starshade ready,’” Kasdin says. “This is not in WFIRST’s current design, and we can’t just put these changes off indefinitely…. By next year we pretty much need to have decided whether we’re going to make it starshade-ready or not.”

A “STARSHADE TECHNOLOGY PROJECT”

But so far, senior NASA officials have expressed caution about committing to such a mission. “In principle, adding a starshade would enable WFIRST to image and study exoplanets in more detail than possible with the internal coronagraph,” says John Grunsfeld, outgoing associate administrator of NASA’s Science Mission Directorate. “However, starshade technology is still in its infancy, so we have a ways to go before we could build a starshade mission to fly with WFIRST.”

A key impediment is money. WFIRST’s budget of $2.3 billion is relatively svelte as far as major NASA space telescopes go, constrained by the extreme cost growth of its more ambitious yet-to-launch predecessor, the $8.8-billion Webb. With NASA’s Astrophysics Division prepping Webb for launch and ramping up work on WFIRST, its present budget cannot support the full-throttle development of a starshade at the same time.

Politics and timing are other obstacles. NASA’s professed budgetary limitations dovetail with the agency’s aversion to unilaterally choosing major astrophysics missions. Instead, NASA intends to let U.S. astronomers chart its course via the next Decadal Survey in 2020, in which a starshade rendezvous with WFIRST could formally compete for prioritization with a large and diverse menu of other possible space missions. “Right now there are no plans to put a starshade on WFIRST,” says Paul Hertz, director of NASA’s Astrophysics Division. Instead, he says, the agency is “in a ‘don’t-preclude-a-starshade’ mode.” As it happens, though, not precluding a starshade closely resembles a concerted effort to build and launch one.

When NASA announced the formal start of WFIRST in February, it also confirmed that the telescope would be launched into an orbit 1.5 million kilometers from Earth, where conditions are tranquil enough for a starshade to function undisturbed. In many earlier plans the mission was instead bound for high Earth orbit, where sunlight bouncing off our planet would have scuttled a starshade’s delicate work. In January NASA formed a Starshade Readiness Working Group to devise plans for validating the necessary technology for a starshade in time for a rendezvous with WFIRST. Finally, last month NASA formally designated the starshade as a “technology development activity,” a move that integrates all the agency’s disparate related projects into a more cohesive whole. “Internally, we are calling this the ‘Starshade Technology Project,’” says Nick Siegler, chief technologist of NASA’s Exoplanet Exploration Program based at the JPL. “The goal is to mature all the starshade’s technology before the end of the decade so we can submit it with high confidence for endorsement by the Decadal Survey.”

RISKY BUSINESS

Although politically prudent, NASA’s current plan to defer a decision on the starshade–WFIRST rendezvous until the 2020s carries significant risks. It would almost certainly delay the launch of any starshade until after WFIRST’s six-year primary mission had ended. Most of NASA’s spacecraft outlive their primary missions and enter “extended” phases after achieving their key objectives, but mission-ending accidents or hardware failures can and do occur. “As a scientist, I want to see a starshade fly with the primary mission of WFIRST,” says Sara Seager, a professor at Massachusetts Institute of Technology who chaired NASA’s starshade rendezvous study, “because we can’t guarantee WFIRST will live beyond its primary mission.” She adds, “The Kepler mission had a four-year primary mission, and guess how long the spacecraft lasted? Four years. I’d hate to develop a starshade only to have no WFIRST to use it with. What will NASA do then—build us another telescope? That doesn’t seem likely.”

According to Siegler and others, an additional risk to deferring starshade development is that it could hinder planning for a notional space telescope to come years if not decades after WFIRST, one with a monumental mirror eight or even 16 meters wide. That would be big enough to search for and image Earths around thousands of the sun’s neighboring stars—big enough to at last provide a statistically meaningful answer to the question of whether or not we are cosmically alone. Just last month NASA chartered two teams to study and recommend possible designs for such telescopes, ordering each team to submit their findings back to the agency by 2019—well before the Decadal Survey would decide about a starshade rendezvous. “The big question is which starlight-suppression technology are these studies going to pick?” Siegler says. “You might think the coronagraph is in pole position because it is easier to build and to test, and you would probably be right. That’s one reason it is already baselined on WFIRST. The starshade could be a better option, but it calls for both a telescope and a separate spacecraft, with something like 50,000 kilometers between them. Where on Earth can you really test something like that? Wouldn’t it be a great use of taxpayer dollars to test a starshade on a telescope that is already in space?”

In their observational capabilities, the two technologies are complementary. As just another instrument on a telescope, coronagraphs can nimbly dart between targets on the sky and excel at planetary discovery. Starshades, by contrast, take days or weeks to drift between targets—but they make up for their slow pace with better broadband sensitivity that makes them the superior choice for studying planets in greater detail.

Aki Roberge, an astrophysicist at NASA Goddard Space Flight Center and lead scientist for one of the agency’s studies of future telescopes, says the correct path to finding another Earth may be to choose and use both. “In a perfect world the perfect mission probably has both a coronagraph and a starshade. Anything that can be done to keep both options open is desirable for the long-term future.”