4 min read
John Mather
NASA Goddard Space Flight Center
We will design the first family of ISEE's (Inflatable Starshade for Earthlike Exoplanets) with sizes from 35 to 100 m diameter. A starshade would enable any telescope to observe exoplanets, a top priority for astronomy worldwide. Compared with other starshade concepts, we aim for a lower mass, cost and complexity, while still providing high performance and science yield (>100 targets). Our starshades would be compatible with the 6 m diameter Habitable Worlds Observatory (HWO) now being planned, as well as the world's largest telescope, the 39 m diameter European Extremely Large Telescope now being built in Chile, working as part of the HOEE, (Hybrid Observatory for Earthlike Exoplanets), and other future telescopes. We need to observe oxygen at visible wavelengths and ozone at UV.
An ISEE, positioned between a target star and the telescope, would block the starlight without blocking the exoplanets. Starshades have perfect optical efficiency, they work with any telescope, and they can block the starlight much better than the requirement, for a star >1010 times brighter than the target.
The competing technology uses a nearly perfect and perfectly stable space telescope like HWO, with an internal coronagraph, to keep the starlight away from the image of the planet. Coronagraphs have the key advantages that they are compact, testable, and have instant availability. However, tested coronagraphs have not yet met the contrast requirement. Moreover, there is no possibility of an ultraviolet coronagraph. If the extreme picometer stability and optical perfection requirements on HWO and its coronagraph could be relaxed by using it with a starshade, then HWO itself could be built at much lower cost and risk. If UV observations of exoplanets are essential, then a 35 m starshade with HWO is the only possible solution.
The HWO will be NASA's next great observatory, and it will include a high performance coronagraph to observe exoplanets. This choice changed the landscape for the competing starshade technology. A starshade mission could still become necessary if: A. The HWO and its coronagraph cannot be built and tested as required; B. The HWO must observe exoplanets at UV wavelengths, or a 6 m HWO is not large enough to observe the desired targets; C. HWO does not achieve adequate performance after launch, and planned servicing and instrument replacement cannot be implemented; D. HWO observations show us that interesting exoplanets are rare, distant, or are hidden by thick dust clouds around the host star, or cannot be fully characterized by an upgraded HWO; or E. HWO observations show that the next step requires UV data, or a much larger telescope, beyond the capability of conceivable HWO coronagraph upgrades.
An inflatable starshade would overcome the main obstacle to starshades: their mechanical design. Starshades have never been flown, they have strict shape and edge requirements, and they must be propelled and precisely positioned. Prior designs based on discrete elements can be scaled up to the size required for HWO (35-60 m) and HOEE (100 m), but they are massive and hard to test leading to high cost and risk. Our mass budget aims for 250 kg for the 35 m HWO case, 650 kg for the 60 m case and 1700 kg for the 100 m HOEE case.We will extend our ideas and produce detailed designs and finite element models, suitable for strength, stiffness, stability, and thermal analysis. We will develop small-scale laboratory test equipment and verify solutions to issues like bonding large sheets of high-strength material into inflatable systems. Deliverable items would include mass/power budgets, strength and stiffness, and lab tests of critical items. We will update mission concepts for HWO and HOEE based on the starshade parameters.
Depending on progress with the HWO mission, starshades could be required to complete our knowledge of exoplanets. An inflatable starshade could make them possible.
