Planning for NASA's Habitable World Observatory Underway In early August, scientists and engineers gathered in a small auditorium at Caltech to discuss building the first space telescope capable of detecting life on planets similar to Earth. The proposed mission concept, called the Habitable World Observatory (HWO), would be a powerful astrophysics observatory following NASA's James Webb Space Telescope (JWST).
This technique is used to obscure starlight to reveal the presence of planets orbiting the star.
It will have the ability to study stars, galaxies and a host of other cosmic objects, including planets outside our solar system, known as exoplanets. While discovering life on exoplanets may be a distant goal, the purpose of the Caltech workshop was to assess the level of technology BGI would need to search for life elsewhere.
"We need to develop key technologies as much as possible before designing a mission," said Dimitri Mawet, a member of the HWO Technology Assessment Group (TAG), the David Morrisroe Professor of Astronomy and a senior research scientist at the Jet Propulsion Laboratory (JPL), which is managed by Caltech for NASA. We are in the technological maturity stage. The idea is to further advance technologies that will enable a Habitable World Observatory to deliver revolutionary scientific results while minimizing the risk of cost overruns."
An artist's impression of the rocky exoplanet Kepler-186f, one of the most promising candidates to become a potentially habitable planet, but how similar or different would it be to Earth to support life? Image credit: NASA/Ames/SETI Institute/JPL-Caltech.
HWO will launch in the late 1930s or early 2040s as part of the National Academy of Sciences' 2020 Decadal Survey of Astronomy and Astrophysics (Astro2020). The mission's observation time will be divided between general astrophysics and exoplanet research.
"The Tenth Anniversary Survey recommends this mission as a top priority because of the transformative capabilities it will bring to astrophysics while also understanding the entire solar system beyond our own," said Fiona Harrison, Caltech's Harold A. Rosen Professor of Physics and the Kent and Joyce Cresa Leadership Chair in the Division of Physics, Mathematics, and Astronomy and one of the two co-chairs of the Astro2020 Tenth Anniversary Report.
Technological progress and challenges
The ability of space telescopes to characterize the atmospheres of exoplanets and thus search for possible signs of life depends on technology that blocks the glare from distant stars. There are two main ways to block star light: one is a small baffle inside the telescope, called a coronagraph; the other is a large baffle on the outside of the telescope, called a starshade. In space, the star shield expands into a giant sunflower-shaped structure, as shown below.
The animation shows a prototype of a star shield, a giant structure designed to block a star's glare so that future space telescopes can take photos of planets. Source: NASA
In both cases, the star's light is blocked, allowing the faint starlight reflected from nearby planets to show through. The process is similar to blocking the sun with your hand when photographing a smiling friend. By capturing the planet's light directly, researchers can use other instruments called spectrometers to scrutinize those lights, looking for chemical signatures. If there is any life on planets orbiting distant stars, the collective inhale and exhale of that life might be detectable in the form of biosignatures.
"We estimate that there are as many as billions of Earth-sized planets in the habitable zone of our galaxy alone," said Nick Siegler, chief technologist for NASA's exoplanet exploration program at JPL. "The habitable zone is the area around a star where temperatures are suitable for liquid water." Regions of growth. We want to probe the atmospheres of these exoplanets, looking for oxygen, methane, water vapor and other chemicals that may indicate the presence of life. Instead of looking for little green men, we want to see spectral signatures of these key chemicals, which we call biosignatures."
According to Siegler, NASA has decided to focus on the coronagraph route of the HWO concept, which builds on recent investments in NASA's Nancy Grace Roman Space Telescope, which will use advanced coronagraphs to image gas giant exoplanets. (Caltech's IPAC is home to the Roman Science Support Center). Today, coronagraphs are used on several other telescopes, including the orbiting JWST, Hubble, and ground-based observatories.
Sara Seager of MIT gave a talk at the Caltech Symposium entitled "Starlight Suppression for a Habitable World Observatory." Source: Caltech
Innovation and future prospects
Mavitt developed the coronagraph, an instrument used at the W.M. Keck Observatory atop Mauna Kea on the Big Island of Hawaii. The latest version of the coronagraph, known as a vortex coronagraph, was invented by Mavitt and housed in the Keck Planetary Imaging and Characterization Instrument (KPIC), which allows researchers to directly image and study the thermal radiation of young and warm gas giant exoplanets. A coronagraph cancels out the star's light, allowing it to take photos of planets that are a million times dimmer than the star. This allows researchers to describe in detail the atmospheres, orbits and rotational characteristics of young gas giant exoplanets, helping answer questions about the formation and evolution of other solar systems.
But directly imaging a Twin Earth planet - the planet where life as we know it is most likely to thrive - would require massive improvements in existing technology. Planets like Earth orbiting sun-like stars in the habitable zone are easily obscured by the star's glare. For example, our sun is 10 billion times more powerful than Earth. To achieve this level of starlight suppression in a coronagraph, researchers had to push their technology to the extreme. "As we get closer to the required levels of starlight suppression, the challenge increases exponentially," Marvitt said.
Through the explanation of Dr. Nick Siegler, technical manager of NASA's exoplanet exploration program, he introduced in detail the working principle of the coronagraph and how it can help directly image exoplanets. Source: NASA
Attendees at the Caltech workshop discussed coronagraph technology, which involves controlling light waves with ultra-precise deformable lenses inside the instrument (see video above). Although a coronagraph blocks most of a star's light, stray light still makes its way into the final image, appearing as spots. By using thousands of push rods to push and pull on the reflective surface of the deformable mirror, the researchers were able to eliminate the spots of residual starlight.
The upcoming Nancy Grace Roman Space Telescope will be the first to use this type of coronagraph, which is called an "active" type because its mirrors actively deform. After more testing at JPL, the Roman coronagraph will eventually be integrated into the final telescope at NASA's Goddard Space Flight Center and launched into space no later than 2027. The Roman coronagraph will allow astronomers to take images of exoplanets that are a billion times dimmer than their stars. This includes mature and young gas giants, as well as disks of debris left over from planet formation.
Vanessa Bailey, JPL's Roman coronagraph technical expert, said: "The Roman coronagraph is NASA's next step in the search for life outside the solar system. The performance gap between today's telescopes and habitable world observatories is too large to be bridged all at once. The purpose of the Roman coronagraph is to be a stepping stone in the middle. It will demonstrate several necessary technologies, including coronagraph masks and deformable mirrors, at a level of performance that has never been achieved outside the laboratory."
Aiming to directly image Earth's twin around a sun-like star means pushing the technology behind Roman's coronagraph even further. "We need to be able to deform the mirrors down to the picometer level," explains Marvitt. "We need to suppress the starlight about 100 times more than the Roman coronagraph. This workshop helps guide us in figuring out where our technology gaps are and where we need more development over the next decade."
Other topics discussed at the workshop included the best primary mirror types for use with coronagraphs, mirror coatings, dealing with damage to mirrors from micrometeoroids, deformable mirror technology, and detectors and advanced tools for integrated modeling and design. Engineers also provided an update on the star shield and its technical readiness status.
Discovering the Road to Earth's Twins
At the same time, as technology continues to advance, other scientists are also turning their attention to stars, looking for Earth-like planets that HWO can image. To date, more than 5,500 exoplanets have been discovered, but none of them are truly Earth-like planets. Planet-hunting tools, like the new Keck Planet Probe (KPF) at the Caltech-led Keck Observatory, are already better at discovering planets by looking for the pull they exert on stars as they orbit them. Heavier planets exert a greater tug, as do planets closer to their stars. KPF is designed to find Earth-sized planets in the habitable zones of small red stars, which are closer to their stars. Over the next few years, KPF will continue to improve and may be able to detect Earth twins.
By the time HWO launches in the late 1930s or early 1940s, scientists hope to have a catalog of at least 25 Earth-like planets to explore.
Despite the long road ahead, scientists at the symposium were eager to discuss these challenges with colleagues who traveled to Pasadena from across the country. JPL Director Laurie Leshin (M.A. ’89, Ph.D. ’95) delivered an inspiring speech at the beginning of the meeting. "This is an exciting and difficult challenge. But that's exactly what we're about. We're not fighting this alone. We need to work together," she said.