The HabEx team proposed a 4-meter telescope paired with a vertebra of the crown and the shadow of a star 52 meters in diameter. (“Having both a harness and suspenders is a good thing,” says Bertrand Minison, a principal scientist at NASA’s Jet Propulsion Laboratory and co-chair of HabEx.) In addition to providing the capabilities for 10-10 Funnel, the stellar shadow can image a wide range of light spectra, checking the wavelengths of ozone, oxygen and water vapor in a single image. (LUVOIR’s coronal vertebra would need to take several images to capture the entire spectrum of light for evidence of these features.) It might also allow an exoplanet to be imaged at a smaller distance from its host star, helping to capture “hiding” planets near it. revolve around their suns.
However, the stellar shadow, which must fly separately from the telescope, poses some challenges that the coronagraph does not. The need for a separate power supply will limit vehicle uses to about 100 or so observations before it needs to be disposed of or refueled. It would also require the two aircraft to engage in a precise and coordinated flight.
And then, of course, there’s a matter of unfolding like origami. Arya and others worked on this task, creating several large-scale experimental star shades made from blanket-like Kapton polymer panels and a carbon fiber frame. (The ‘blanket’ is made of so many layers of Kapton that no holes in the shadows by minute meteor strikes would affect its shadow.) It’s not easy. The edge of the stellar shadow’s petals must be very sharp to reflect as little sunlight as possible into the telescope, and any disturbances that could affect the imaging of exoplanets. “We’re making a precise light structure that has to fold and open automatically, and that presents a lot of challenges,” Arya says. “We are approaching these issues incrementally, and there is still a list of things that need to be done to demonstrate this technology.”
Perhaps because the task at hand is so difficult, some astrophysicists believe there is a vertebra from the crown Plus A star shade can be a perfect one or two punch. “I really see the benefit of a hybrid system,” Minison says. Remapping from star to star, the crown can image a large number of potentially habitable exoplanets, then the star’s shadow can provide a high-resolution appearance with the broad bandwidth and light throughput of each planet—great for a deep characterization of its habitability. The HabEx and LUVOIR teams have worked closely together, and any future teams are likely to draw from their members.
Star shadows may also be useful for more than just deep space missions. NASA has granted funding to Mather’s team to study the use of a stellar shadow orbiting the discovery of exoplanets from Earth. ORCAS, or Orbital Configurable Artificial Star, will be the first hybrid Earth-space observatory, using a laser beacon in space to help focus a ground-based telescope, thus eliminating distortion caused by looking through the atmosphere. The next step in the proposal will see the “RemoteOcculter” star 100 meters long into near-Earth orbit, where it will cast a shadow over the telescope. “The star’s shadow is much tougher to orbit, but it could be the ultimate exoplanet observation system,” Mather wrote in an email. “Using it, we can see the Earth orbiting a nearby star in a one-minute exposure, and within an hour we can tell if it has water and oxygen like ours.”
The decision on which of these projects to move forward is still many years away. Guidance for HabEx and LUVOIR may come during NASA City Hall at the January 11 American Astronomical Society meeting, and proposals for the ORCAS and RemoteOcculter mission are still being considered. But the James Webb Space Telescope, which was launched in December, will soon return images taken with the help of a low-contrast stellar shadow. This telescope will become fully operational in mid-2022 and is expected to be the new leader in the hunt for exoplanets – until the more powerful shadow ejector comes.
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