11.01.2023

The HabEx concept included a starshade, but the Habitable Worlds Observatory may only include an internal starlight blocker called a coronagraph.SCOTT GAUDI

Astronomers are always looking to the next big thing. This week, at a meeting of the American Astronomical Society, researchers packed into a standing-room-only conference room to hear about a successor to JWST, the 6.5-meter space telescope that began operations last year. Flush with JWST’s success, NASA is now planning an optical telescope that would be just as big as JWST and have a grand new goal: looking for signs of life on Earth-like planets, perhaps by the early 2040s.

Mark Clampin, NASA’s astrophysics division director, told the audience that little about the telescope has been settled. But what he did say tantalized them: The telescope will, like JWST, be perched at L2, a gravitational balance point 1.5 million kilometers from Earth. Unlike JWST, it will be designed for robotic servicing and upgrades, which could enable it to operate for decades, getting better with age. Without a dedicated budget, Clampin says he can’t yet make much headway on the design and technology. But he does have a working name for the telescope: the Habitable Worlds Observatory (HWO).

“I’m really, really excited to see it actually happening,” says John O’Meara, chief scientist of the W.M. Keck Observatory. “Serviceability will be huge,” says Aki Roberge of NASA’s Goddard Space Flight Center. It essentially creates a “mountaintop observatory at L2,” she says. Like a telescope on Earth, the mirrors and structure can remain while increasingly sophisticated instruments are swapped in. “It’s the instruments that make a difference,” she says.  

The HWO won’t be NASA’s next flagship space telescope after JWST. The agency plans in 2027 to launch the Nancy Grace Roman Observatory, a 2.4-meter survey telescope that will hunt for dark energy and exoplanets. But with the HWO, NASA is following through on the top priority of astronomy’s decadal survey, a community-led wish list that guides funding agencies and lawmakers. The survey’s final report, published in November 2021, called for NASA to resurrect its Great Observatories program, which launched the Hubble Space Telescope and several others in the 1990s and early 2000s. The report said an $11 billion, 6-meter telescope sensitive to ultraviolet, optical, and near-infrared wavelengths should kick off the new Great Observatories program. It specified that the telescope, in addition to doing general astrophysics, must be capable of detecting signs of life on 25 nearby Earth-like exoplanets—the minimum needed to confirm statistically whether life is common in the Galaxy.

NASA had proposed to the decadal survey several options for this next big thing in space, but the decadal report called for something between two of NASA’s proposals, HabEx and LUVOIR. HabEx would have relied on a 4-meter monolithic mirror, as well as a robotic starshade, floating more than 100,000 kilometers away, to screen out the light of an exoplanet’s star so the planet could be seen. LUVOIR, as big as 15 meters across in one configuration, was designed more as a multipurpose observatory, and would build on JWST’s segmented mirror technology. Although segmented mirrors can’t produce images quite as crisp as those from monolithic mirrors, they can be folded up, making it possible to pack a far bigger telescope into a rocket fairing.

As described, the HWO “contains no technology that has not already been thought about for HabEx or LUVOIR,” says Scott Gaudi of Ohio State University, Columbus, one of the designers of HabEx. But Clampin said the agency will take a conservative approach to the HWO, to avoid the cost overruns and delays that plagued JWST. That project required many unproven technologies, which took longer to refine than expected. For the new telescope, NASA will take advantage of technologies already developed or in development, including segmented mirrors such as the one used in JWST and the Roman observatory’s coronagraph, an optical device within the telescope that blocks the light of a star so that faint exoplanets nearby can be seen. It will also set up a Great Observatories Technology Maturation Program (GOMaP) to refine those technologies for the HWO and do similar prep work for subsequent great observatories.

For example, because the HWO will work with optical light, which has shorter wavelengths than the infrared light JWST captures, the HWO will need much tighter control over the shape of the mirror. It will need to be perfectly shaped down to a level of 1 picometer—one-millionth of one-millionth of 1 meter—compared with billionths of a meter for JWST. The HWO will also have to improve on the Roman telescope’s coronagraph, which can block out the light of a star 100 million times brighter than its planet. The HWO’s coronagraph will need to cope with stars that are 10 billion times brighter. One key will be suppressing stray light, which may require a cylindrical baffle around the HWO, similar to the one surrounding the Hubble telescope. That would protect its mirror from micrometeorites of the sort that have already struck JWST. Every pit in the mirror from a meteorite strike causes stray light.

Some astronomers argue that a monolithic mirror, which has fewer edges than a segmented one, would scatter less light—which might push NASA toward a design more like HabEx. But Clampin says recent research suggests coronagraphs can also work with segmented mirrors. “None of these are impossible problems,” says O’Meara, who was a member of the LUVOIR team. He prefers a segmented design, which allows engineers to make the mirror bigger if the science requires it, without running into the space constraints of a rocket fairing.

Making it possible to send service and repair missions to a flagship telescope also marks a change for NASA. The Hubble telescope was serviceable—at huge expense—by space shuttle astronauts in low-Earth orbit. Future missions, Clampin says, will exploit the wealth of private companies that are developing robots to help service NASA’s Artemis program of lunar exploration. “Robotic servicing is part of the architecture and philosophy” of the HWO, he says, adding that the extra distance of L2 “is not a great challenge.” As well as extending the life of a mission by installing new instruments—as was done for Hubble—servicing also allows flexibility in development. If, say, one instrument proved tricky to get ready for launch, it could be added later. Extending the life of a mission also looks good to funders. “It makes it more palatable to Congress,” O’Meara says.

Congress is perhaps Clampin’s first big challenge. Last month, lawmakers allocated $1.51 billion to NASA astrophysics for this year, a decline of 4% from the previous year. Astrophysics was the only one of NASA’s four science divisions to lose funding. Without funds to start GOMaP, Clampin is repurposing some existing tech development funding to support small studies of the trade-offs of different designs. After that, he says, he will “work with stakeholders to align funding.” That’s a polite way of saying that for the HWO to succeed, NASA—and the astronomical community—needs to get Congress onboard with the idea.

Quelle: AAAS