The HabEx team has proposed a 4-meter telescope with a chronograph and a 52-meter diameter star shadow. (“It’s better to have both a belt and a suspender,” says Bertrand Menason, a chief scientist at NASA JPL and vice president of HabEx.)-10 Suppression, the shadow of a star can illustrate a wide bandwidth of the light spectrum, examining the wavelengths of ozone, oxygen and water vapor in a single image. (Louvre’s coronagraph would need to take a number of images to capture the entire spectrum of light for the source of those features.) They orbit the sun.
Yet a star shadow, which must fly apart from the telescope, creates some challenges that the coronagraph does not. The need for a separate power source will limit the use of the craft to about 100 observations or it will need to be scraped or refueled beforehand. It will require two crafts to engage in a subtle, integrated flight.
And then, of course, it’s a matter of origami. Aryan and others are working on that, building several large-sized test star sheds from blanket-like Captain Polymer sheets and an exposed carbon fiber frame. (The “blanket” is made up of several layers of Captain’s so that any hole pierced in the shadow by a micro-meteor strike will not compromise its shadow.) This is not easy. The edge of a star shadow petal must be extremely sharp to reflect as little sunlight as possible into the telescope, and any disturbance can affect exoplanet imaging. “We’re building an optical precision structure that needs to be robotically folded and unfolded, and that presents a lot of challenges,” says Arya. “We’re dealing with these issues step by step, and there’s still work to be done to prove this technology.”
Some astronomers may believe the coronagraph to be a very difficult hand task Plus A star shade can be the perfect one or two punches. “I really see the benefits of a hybrid system,” Menason said. Redirecting from star to star, a coronagraph can give a wide range of images of potential habitable exoplanets, then a star shadow can provide a high-resolution view with a wide bandwidth and light throughput of each planet – great for its deep habitation feature. The HabEx and LUVOIR teams have worked closely together, and any future teams will probably draw from their members.
Star shades can be more useful than deep space missions. NASA has funded Mather’s team to study the use of star shadows in an orbit to find exoplanets from Earth. ORCAS, Or orbital configurable artificial star, will be the first hybrid ground-space observatory, where using a laser beacon to help focus a terrestrial telescope, can cut through distortions caused by looking through the atmosphere. The next step in the proposal is to see the shadow of a 100-meter “remote occultator” star in orbit near Earth, where it will cast its shadow through a telescope. “The shadow of the orbiting star is much harder, but it could be the ultimate exoplanet observation system,” Mather wrote in an email. “Using it, we can see a star orbiting a nearby Earth with one minute of exposure and within an hour we can see if it has water and oxygen like us.”
The decision as to which of these projects will go ahead is still many years away. Instructions for the HABX and Louvre may come at NASA Town Hall at the January 11 meeting of the American Astronomical Society, and ORCAS and RemoteOcculter mission proposals are still being studied. But the James Webb Space Telescope, which launched in December, will soon beam back images of its low-contrast star shadows. That telescope will be fully operational by mid-2022, and is expected to be the new leader in the Exoplanet Hunt – until a more powerful shade-thrower arrives.
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