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The HabEx team offered a 4-meter telescope combined with a coronograph and a star shadow with a diameter of 52 meters. (“It’s good to have both a belt and suspenders,” said Bertrand Maneson, chief scientist at NASA’s JPL and co-chair of HabEx.) In addition to providing the potential for 10-10 suppression, the star shadow can depict a wide frequency band of light spectra, checking for ozone, oxygen and water vapor wavelengths in one image. (The LUVOIR coronograph will need to take many images to capture the full light spectrum for clues to these characteristics.) It may also allow an exoplanet to be imaged at a shorter distance from its host star, helping to capture planets. which “hide” closer in orbit to their suns.
Still, the shadow of the star, which must fly apart from the telescope, poses some challenges that the coronograph does not. The need for a separate energy source would limit the ship’s use to about 100 observations before it has to be scrapped or refueled. It would also require both ships to participate in a delicate, coordinated flight.
And then, of course, the question is to unfold like origami. Arya and others have worked on this task, producing several large-scale test star shades made from blanket-like polymer sheets from Kapton and an unfolding carbon fiber frame. (The “blanket” is made of many layers of kapton, so that any holes drilled in the shadow by micrometeorite impacts do not compromise its shadow.) It’s not easy. The edge of the petals of the star shadow must be extremely sharp to reflect as little sunlight as possible in the telescope, and any interference can affect the image of exoplanets. “We create an optical precision structure that has to be folded and unfolded robotically, and that’s a lot of a challenge,” says Arya. “We are gradually approaching these issues, and there is still a list of things that still need to be done to prove this technology.”
Perhaps because the task is so difficult, some astrophysicists believe it is a coronograph plus a stellar hue can be the perfect hit with one or two. “I really see the benefits of the hybrid system,” says Meneson. By redirecting from star to star, the coronograph could depict a large number of potentially habitable exoplanets, and then the star shadow can provide a high-resolution, high-bandwidth view and light transmission of each planet – great for a deep characterization of its habitability . The HabEx and LUVOIR teams have worked closely together and all future teams are likely to benefit from their members.
Starry nuances can be useful for more than deep space missions. NASA has provided funding to the Mater team for research using an orbital star shadow to spot exoplanets from Earth. ORCAS, or orbiting a configurable artificial star, will be the first hybrid space observatory to use a laser beacon in space to help focus the ground-based telescope, cutting out the distortion caused by looking through the atmosphere. The next step in the proposal will be the 100-meter RemoteOcculter star shadow in Earth orbit, where it will cast its shadow on the telescope. “The shadow of the orbital star is much more difficult, but it may be the best system for observing exoplanets,” Mater wrote in an email. “Using it, we could see the Earth in orbit around a nearby star for one minute of exposure, and in an hour we could see if there is water and oxygen like ours.
The decision on which of these projects will continue for many years to come. The instructions for HabEx and LUVOIR may come during the NASA City Hall at the American Astronomical Society meeting on January 11, and proposals for the ORCAS and RemoteOcculter missions are still being studied. But the James Webb Space Telescope, which launches in December, will soon be rebroadcasting images made with its lower-contrast star hue. This telescope will be fully operational in mid-2022 and is expected to be the new leader in exoplanet hunting – until even more powerful casting shadows appear.
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