A new ultra-black coating, designed for use on starshade spacecraft to enable direct imaging of exoplanets, has been developed and tested through a collaboration between NASA and ZeCoat Corporation. The coating reduces reflected sunlight from starshade edges by a factor of approximately 20, meeting the necessary requirements for blocking starlight during exoplanet observations.
Purpose and Challenge
A starshade is a flower-shaped spacecraft, roughly half the size of a football field. In operation, it is positioned between a space telescope and a distant star in order to cast a shadow on the telescope. This technique blocks less than one part per billion of starlight, allowing the telescope to detect the fainter light from orbiting exoplanets.
A known challenge for this method was stray sunlight scattering from the starshade's edges. Previous attempted solutions included razor-sharp edges made from amorphous metals with a thickness of 300 nanometers and the application of black coatings, such as carbon nanotubes. These approaches were deemed insufficient, as the coatings were several microns thick, which increased light scatter.
Coating Technology
The ultra-black coating was developed by David Sheikh at ZeCoat Corporation. It is produced using a physical vapor deposition process. The coating consists of extremely thin, partially transparent metal layers separated by dielectric glass layers. These layers create nanoscale cavities that trap and absorb light through resonance, functioning similarly to a Fabry-Perot cavity.
The coating is 100 times thinner than previously tested black coatings for starshades.
Testing and Results
In 2020, NASA's Exoplanet Exploration Program at the Jet Propulsion Laboratory (JPL) chartered the Starshade Science and Industry Partnership (SIP) to advance starshade technology. As part of this effort, ZeCoat's coating was applied to prototype starshade edges, specifically 50-cm long amorphous metal blades. Tests conducted at JPL using a laser scatterometer showed that the coating reduced reflected light by a factor of approximately 20. These results were reported as sufficient for exoplanet detection and were published in the SPIE digital library.
Further Development
In 2021, supported by a NASA Small Business Innovation Research (SBIR) contract, ZeCoat developed a roll-to-roll coating process for application to large polyimide membranes. These coated membranes, approximately one meter wide, could be used for a starshade's central disk and petal surfaces.
The work was carried out in part at the Jet Propulsion Laboratory, which is managed by the California Institute of Technology for NASA under contract 80NM0018D0004.
Additional Applications
The coating technology has potential applications beyond starshades. These include:
- Darkening the surfaces of satellite constellations to reduce their visibility from the ground.
- Darkening surfaces near cell phone cameras.
- Providing thermal control coatings for space vehicles.
ZeCoat has also received a NASA SBIR Phase I contract to develop micrometeorite-resistant thermal control coatings for spacecraft such as the proposed Habitable Worlds Observatory.