Origami shades of planet hunting

A giant folded starshade could uncover alien worlds

by Mary Blake

Now that Kepler has found more than 2,000 planets orbiting other stars and the K2 mission keeps adding to the tally, scientists are eager for a closer look. They’re exploring a number of ways to find Earth’s twin and maybe discover where E.T. lives.

Artist's concept, New World's Observer. Credit NASA GSFCBuilding a giant, origami-folded starshade that unfurls and flies thousands of miles in front of a telescope is one idea that continues to gain traction. NASA has long been exploring mission concepts to directly image exoplanets and is looking at what it will take to field a cost-effective exoplanet imaging mission that meets science requirements.
Recently NASA’s Exoplanet Exploration Program created a Star Shade Readiness Working Group (SSWG) and invited Chip Barnes, chief engineer, Ball Aerospace Civil Space, to participate.
“It’s all about the big questions humankind needs to answer: Are we alone? How did we get here? Where are we going?” said Barnes. “Kepler dramatically changed our view of the universe, but it doesn’t provide actual pictures of the exoplanets.” 

Testing the concept
The SSWG is charged with recommending a technology development path for a 50 meter (164 ft. diameter) starshade capable of enabling a telescope to directly image exoplanets.  The group will make a recommendation whether the concept can be tested on the ground or will need a tech demo mission in space. Based on knowledge of how tech demos are used to create a path to an operational system, Ball provided the working group with a historical perspective of space-based demo missions.
NASA tapped Ball for its expertise in systems engineering, wavefront sensing and control, optical work on the James Webb Space Telescope, tech development and the ability to solve tough problems. In addition, Ball had prior experience with New Worlds Observer, an early NASA starshade concept, designed with the Webb Telescope in mind. The company’s experience with telescopes, testing expertise and studies on formation flying were also a factor.

Two different challenges
New World's Observer deployment. Credit NASAScientists want to be able to characterize the exoplanet’s atmospheric conditions, internal structure, mass, and signs of life. That requires suppressing the blaze of light from the target star so that a telescope can image the exoplanet.  One way to do this is by using a coronagraph with an internal mask to suppress the starlight.
This method is extremely challenging since even very small aberrations in the telescope and optics make it difficult to find the planet in the resulting background noise from the scattered star light. “The coronagraph must be aligned very accurately and the precision and stability of the optics are far more difficult than the starshade approach,” Barnes explained.
Flying a starshade and a telescope on two different spacecraft presents simpler requirements for the telescope, but the big drawback is moving the starshade around to different target stars, which takes a lot of fuel and time. The two spacecraft fly in formation, with the starshade positioned around 80,000 km (nearly 50,000 miles) in front of the telescope. “Starshades are better suited to get the full spectrum of light that the telescope can break into wavelengths to characterize the planet,” said Barnes.

Questions and choices
Starshade concept. Credit JPLThe working group will determine what is needed to get the starshade Technology Readiness Level to TRL 6, which is a full functional prototype or representational model; or TRL 7, which requires that the working model be demonstrated in a space environment. The group will tackle two big questions: How do you test on the ground? Do you have to go to space?
Studies underway on the Wide Field Infrared Survey Telescope (WFIRST) are  presenting an opportunity to consider adding a formation flying capability to WFIRST so that a starshade could be flown in front of the WFIRST telescope with a coronagraph on board.

This concept, called the Starshade Rendezvous Mission, was proposed by the JPL Exoplanet Exploration Program in March 2015. The proposal details how leveraging an existing telescope can easily achieve starshade science more economically and allows for the use of a larger, more capable telescope.

NWO's starshade shadows the telescope from the star, while light from a terrestrial exoplanet passes the edge of the starshade. Credit NASAFor more information on the WFIRST/starshade concept, see the white paper.

Said Barnes, “Ball built Kepler and we found that every star has at least one planet. The next step is to characterize those planets.”