The HabEx team proposed a 4-meter telescope paired with a coronograph and a 52-meter-diameter star shade. (“It’s good to have both a belt and suspenders,” says Bertrand Mennesson, NASA’s chief scientist JPL and co-chair of HabEx.) In addition to providing the potential for 10-10 suppression, a stellar hue could image a wide range of the light spectrum, checking ozone, oxygen, and water vapor wavelengths in a single image. (The LUVOIR coronograph should take many images to capture the full light spectrum to indicate these characteristics.) It could also allow exoplanets to be captured at a smaller distance than the host star, helping to capture planets “hiding” closer in orbit to theirs. the sun.
However, the hue of the star, which must fly apart from the telescope, presents some challenges that the coronagraph cannot. The need for a separate energy source would limit the use of the spacecraft to about 100 observations before it should be scrapped or refueled. It would also require two aircraft to engage in a delicate, coordinated flight.
And then, of course, there’s the thing of developing like origami. Arya and others worked on the task, creating several large trial star shades made of kapton polymer sheets resembling a blanket and a carbon fiber frame that opens. (The “cover” is made of many layers of Kapton so that any holes in the shade pierced by micrometeorite impacts do not endanger its shadow.) It is not easy. The edge of the petals of the star’s shadow must be extremely sharp to reflect as little sunlight as possible into the telescope, and any perturbations could affect the image of the exoplanet. “We create an optical precision structure that has to be robotically bent and unfolded, and that’s a lot of a challenge,” says Arya. “We are approaching these problems gradually, and there is still a list of things that still need to be done to prove this technology.”
Perhaps because the task at hand is so difficult, some astrophysicists trust the coronographer plus a star shade could be the perfect one-two stroke. “I really see the benefit of a hybrid system,” says Mannesson. Redirecting from star to star, the coronograph could capture a large number of potentially habitable exoplanets, and then the stellar hue could provide a high-resolution appearance with wide bandwidth and light transmission of each planet – great for in-depth characterization of its habitability. The HabEx and LUVOIR teams have worked closely together and all future teams are likely to draw from their members.
Star shades can also be useful for more than deep space missions. NASA has given Mather’s team the means to study using the shadows of stars in orbit to spot exoplanets from Earth. ORCAS, or an orbiting configurable artificial star, would be the first hybrid Earth-to-space observatory to use a laser beacon in space to help focus the Earth’s telescope, reducing distortion caused by looking through the atmosphere. The next step in the proposal would be to see the 100-meter star “RemoteOcculter” in orbit near Earth, where it would cast its shadow on a telescope. “The shadow of a star in orbit is much heavier, but it could be a top-notch system for observing exoplanets,” Mather wrote in an email. “Using it, we could see the Earth orbiting a nearby star in a one-minute exposure, and in an hour we could know if there was water and oxygen like ours.”
The decision on which of these projects will move forward remains for many years to come. Guidelines for HabEx and LUVOIR could come during NASA City Hall at a meeting of the American Astronomical Society on January 11, and the proposals of the ORCAS and RemoteOcculter missions are still being studied. But the James Webb Space Telescope, launched in December, will soon broadcast images taken with the help of its lower-contrast star. The telescope will become fully operational in mid-2022, and it is expected to be the new leader in the hunt for exoplanets – until even more powerful shadow throwers arrive.
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