Some time later this summer, it is predicted, the National Academy of Sciences will release its long-awaited Decadal Survey for astrophysics, which is expected to recommend the science and architecture that NASA should embrace for its next “Great Observatory.”
Many Worlds earlier featured one of the four concepts in the running — LUVOIR or the Large UV/Optical/IR Surveyor. With a segmented mirror potentially as wide as 50 feet in diameter, it would revolutionize the search for habitable exoplanets and potentially could detect one (or many) distant planets likely to support life.
Proposed as a “Great Observatory” for the 2030s in the tradition of the Hubble Space Telescope and the James Webb Space Telescope (scheduled to launch later this year), LUVOIR would allow for transformative science of not only exoplanets but many other fields of astronomy as well.
Also under serious consideration is the Habitable Exoplanet Observatory, HabEx, which would also bring unprecedented capabilities to the search for life beyond Earth. Its mirror would be considerably smaller than that proposed for LUVOIR and it would have fewer chances to find an inhabited world.
But it is nonetheless revolutionary in terms of what it potentially can do for exoplanet science and it could come with a second spacecraft that seems to be out of science fiction, designed to block out starlight so exoplanets nearby can be observed. That 52-meter (or 170-foot) petal-rimmed, light-blocking disc is called a starshade or an occulter, and it would fly 76,600 kilometers (or 47,000 miles) away from the HabEx spacecraft and would work in tandem with the telescope to make those close-in exoplanet observations possible.
While the capabilities of HabEx are fewer compared to LUVOIR and the potential harvest of habitable or inhabited planets is less, HabEx nonetheless would be cutting edge and significantly more capable than the Hubble Space Telescope in nearly every way, while also being less expensive than LUVOIR and requiring less of a technology reach.
Scott Gaudi, an Ohio State University astronomer, was co-chair of the NASA-created team that spent three years studying, engineering and then proposing the HabEx concept. He put the potential choice between HabEx and LUVOIR this way: “Do you want to take a first step or a first leap? HabEx is a major step; LUVOIR is a huge leap.”
“There are important pros and cons to each of the two concepts, but the teams have worked together to some extent to present a suite of telescope possibilities that will have exoplanet science as a priority. Our hope is that the National Academy recommends, and NASA adopts, a plan for a Great Observatory capable of telling us whether some exoplanets can and do support life.”
These goals are significantly more ambitious than those of any current or planned space or ground telescope.
While exoplanet research and discovery has exploded since the first one was definitively identified in 1995, most of the 4,000-plus distant planets known so far were discovered using indirect methods of detection. The primary techniques have been to find stars that “wobble” because of the gravitational pull of an orbiting planet (the radial velocity method) or detecting tiny, regular dips in the amount of light coming from stars as a result of planets orbiting in front of them (the transit method.)
A small number of large planets, usually quite distant from their host stars, have been directly imaged by ground or space telescopes. But the Earth-like planets most likely to be habitable are small, rocky and usually close in to their host star. The revolution in astronomy that HabEx (or LUVOIR) would usher in is the ability to directly image those much smaller exoplanets and to read the chemical composition of their atmospheres via spectroscopy.
Of course, it’s possible that the National Academy will recommend one of the two other concepts in the running — the far infrared Origins Space Telescope and the Lynx X-ray Surveyor of the unseen cosmos — or will make no grand observatory recommendation at all. NASA has a very full plate right now with the Artemis mission to send American astronauts back to the moon, missions to Jupiter’s moon Europa, to Venus and Mars and to better understand the changing Earth from orbit.
But the very fact that not one but two major exoplanet and habitability concepts have been extensively studied and could be selected reflects both the perceived importance of the science and how far along both that science and the engineering that could produce a breakthrough spacecraft have progressed. (Both HabEx and LUVOIR also offer enormous possibilities for a wide range of other astrophysics endeavors, but we’ll focus here on the exoplanet and habitability questions.)
In the final report produced by the HabEx Science and Technology Definition Team (STDT), stood up by NASA in 2015 and including more than 100 scientists and engineers, these are described as the top two mission goals:
- HabEx will search for Habitable Zone Earth-like planets around sunlike stars using direct imaging and will spectrally characterize promising candidates for signs of habitability and life.
- HabEx will take the first “family portrait” of nearby planetary systems, detecting and characterizing both inner and outer planets, as well as searching for dust and debris disks.
The goals are similar to those of LUVOIR and have been a focus for many astrophysicists and exoplanet scientists for almost two decades. The first formal effort came in 2002 with the not-yet-mature Terrestrial Planet Finder concept, which would be designed to search for small, rocky exoplanets. The TPF concept was the subject of two NASA study efforts, but they ended without producing a mission plan deemed feasible.
The central technological challenge of the next phase of exoplanet research is daunting: to directly imaging distant and relatively tiny exoplanets near their much brighter star. Finding and then studying such directly-imaged exoplanets has been likened to looking from Washington, D.C. for a firefly alongside a searchlight in San Francisco.
To succeed, a space telescope has to collect many more photons of light than anything flying now, and that means the mirror has to be larger or the instruments more stable and precise. Or the observatory needs a new way to block out that blinding light of a host star.
The LUVOIR mirror would be much larger than anything flying now, while HabEx is proposed to have a 4-meter mirror (slightly less than twice the size of the Hubble Space Telescope.) ) But it would have more advanced capabilities in terms of blocking out the light of host stars and would have far greater stability than any space telescope so far — a capability that improves precision seeing greatly. With its cutting-edge microthrusters controlling its pointing, the HabEx study concludes that it would be three times as stable as the Hubble Space Telescope.
Based on data from the Kepler Space Telescope mission, the percentage of sunlike stars likely to have rocky exoplanets in orbit is substantial — about 24 percent. Taking that estimate along with other uncertainties into account, the HabEx final report concludes that a 5-year HabEx mission with 50% of its time dedicated to exoplanets would be expected to find, measure and characterize 8 habitable zone rocky planets (and many other planets, stars and solar systems.). The study reports “a 98.6% chance of detecting and characterizing at least one rocky planet in the habitable zone of a sunlike star.”
In contrast, the LUVOIR study team estimates the largest version of the telescope also has a very high percentage probability of find and study 54 potentially Earth-like planets orbiting sun-like stars over a two-year observing period.
Because HabEx and LUVOIR have similar goals, the two study teams have often worked together and have offered a range of possible architecture to the National Academy.
The starshade, which unfurls and operates as shown in the JPL animation above, is an idea that has been around for a long time. Astronomers going back to Lyman Spitzer of Princeton in 1962 have proposed the external starshade idea as a way to block most of the starlight from reaching the telescope, thus enabling the direct imaging of small planets around nearby stars. Internal coronagraphs — which block light as well — have been used inside ground-based telescopes, but a starshade has never been deployed.
Sara Seager of the Massachusetts Institute of Technology, also co-chair of the HabEx STDT, has been a long-time advocate for large, external starshades.
She says the external starshade allows for a significantly deeper analysis of an exoplanet because of the amount of starlight that is blocks, making the light from the planet that becomes accessible. A starshade can also block light in many wavelengths at once, while a coronagraph has to piece that blockage together through numerous viewings.
But a starshade is large and it needs fuel and time to move from one viewing spot to another. And that helps explain why the HabEx proposal calls for both an internal coronagraph — to identify potentially interesting exoplanets — and then the starshade to stare at it with far more intensity.
Remarkably, Seager said, keeping the telescope and starshade properly aligned despite being almost 50,000 miles apart “is actually not that hard to do”
“At first it seems nearly it’s impossible to have two spacecraft — a starshade and a telescope — that fly tens of thousands of kilometers apart and to line them up just so,” she said.” But our team dug down and found all the different formation flying activities we have already mastered, like how we can dock a capsule arriving at the (International Space Station),” she said. “And with no gravity in deep space where the starshade would be, lining the two up with precision is quite possible.”
“What we found was most challenging was the sensing, how the telescope would know where the starshade is using lasers or LED,” she said. “The light source expands over distance and so that’s a problem. But we took it to the lab and figured it out.”
The petal-shaped edges of the proposed starshade are needed to limit light diffraction — the bending of waves when they hit a surface. Each petal would be covered in a high-performance plastic film that resembles gold foil.
Seager said the starshade idea has been proposed and found too difficult many times, and was often left for dead. But now with NASA support and lots of study and the engineering input of the Jet Propulsion Lab, it is seen as a possible and necessary addition to a mission — be it HabEx or the Roman Space Telescope or something further in the future.
Indeed, HabEx co-chair Gaudi said that his fondest dream is for an observatory with a LUVOIR sized mirror and a HabEx starshade.