Four flagship missions are proposed in the 2020 Decadal Survey
Every ten years, astronomers from around the world gather together to decide the future of their field. A committee of scientists, project managers, and tech experts from a range of backgrounds consider what topics in their field are most pressing, and how they should be studied. It's known as the Astronomy and Astrophysics Decadal Survey, and for the 2020 survey, they're debating four proposed projects in the running for NASA's next flagship mission.
"Decadal surveys have tremendous impact because they make tough decisions," said Fiona Harrison, co-chair for the 2020 decadal survey's committee and astronomer at Caltech. "What ends up getting recommended has broad support [in the community]."
This decade's proposals - Lynx, LUVOIR, HabEx, and Origins Space Telescope - showcase the best new advancements in astronomy. The space-based observatories, which were first identified several years ago, are exceptionally well thought through and budgeted. "I think we learned a lot of hard lessons from James Webb overblowing the budget," said Asantha Cooray, co-chair on Origins and astronomer at University of California, Irvine. The James Webb Space Telescope, which was selected by the 2000 decadal survey, is now over two years delayed from its initial launch target and close to $1 billion overbudget.
The newly proposed missions rely heavily on well-tested technology and designs that can be leveraged and enhanced in new ways. Many also propose budgets for immediate technology development while things are being finalized. One estimate for the LUVOIR mission finds that spending just under $700 million in the next five years could save several billion dollars over the project's lifetime.
With these budget-minded plans, some astronomers hope that decadal survey will recommend several of the proposals, though others caution that it's possible none will be recommended.
"What many of us have in mind is we would like to see all of them operating at once, as we did in the 90s when we had Hubble, Chandra, and Spitzer all operating at the same time," said Bradley Peterson, co-chair on LUVOIR and astronomer at Ohio State University. "We had that suite of great observatories, and people are referring to these as the greater observatories, you know the natural successors."
In late 2019, each project submitted its final report, offering their comprehensive and refined plans for their telescopes. Now, they're awaiting news on who will be selected, which should be announced spring 2021. Here is a look at the telescopes under consideration and how their ground-breaking technology could revolutionize astronomy.
At just three meters, Lynx has the smallest primary mirror of all the proposed projects. But that doesn't mean it won't make big advancements in science. In fact, its main target is some of the biggest things out there - supermassive black holes lurking at the dawn of the universe.
The Lynx X-Ray Observatory aims to follow in the footprints of Chandra, one of NASA's Great Observatories launched in 1999. Observing the universe at x-ray wavelengths, Lynx hopes to see some of the earliest black holes and probe the mechanisms driving galactic and stellar evolution. Since x-rays effectively can't be observed from the ground, Lynx offers a rare chance at advancing this field.
Lynx concept overlaid on a Chandra image of M51. Credit: NASA/MSFC
These advancements are due in large part to recent breakthroughs in x-ray mirror technology. Lynx will use 457 concentric mirror shells constructed from lightweight mirror segments made of single-crystal silicon - an inexpensive material used in semiconductors. This material is lightweight, durable, and has a low coefficient of thermal expansion, which makes it ideal for a space telescope. Additionally, the design allows for easy construction at minimal cost.
"Another huge development is in the direction of instruments," said Feryal Özel, co-chair for Lynx and astronomer at the University of Arizona. "The way we've been able to shrink the pixel sizes and scale up arrays, is again a breakthrough in just the last two or three years. If you're building such an exquisite optical system, obviously you want the instruments behind it to match that."
To keep up with the new mirror, Lynx will have three instruments developed to capture the high angular resolution and increased sensitivity. The proposed microcalorimeter will have 100,000 pixels - dramatically higher than previous models - and an x-ray grating spectrometer will have five times higher spectral resolution than Chandra. Advances in CMOS and CCD technology have enabled reduced noise and high read-out speeds for the third instrument, a high-definition imager.
All together, these advances endow Lynx with a field of view 20 times greater, a factor of 50 times higher throughput, and the ability to operate at 1,000 times the speed of Chandra. This will allow it to detect sources 100 times fainter - the same as going from a 1-meter optical telescope to an 8-meter telescope.
Opposite of Lynx, the Habitable Exoplanet Observatory - HabEx - wants to explore much smaller targets. This telescope, covering from ultraviolet to near -infrared wavelengths, will help astronomers answer the question, are we alone?
"We're at this kind of very special time in human history, where for the first time we think we're capable of launching a mission that can directly image Earth-like planets around sun-like stars, take their spectra and look for signatures of habitability and perhaps life," said Scott Gaudi, HabEx co-chair and astronomer at Ohio State University.
An artist's concept (not to scale) of the HabEx mission shows the telescope and the separate starshade that will fly with it. Credit: HabEx/NASA
HabEx will use onboard instruments to directly image Earth-like planets and probe their lower atmospheres for chemical signatures that resemble early-Earth atmospheres for the first time.
But the real star of the show is a state-of-the-art coronagraph and starshade used to block out the light of stars to reveal their planetary companions. This 52-meter starshade will be flown around 76,600 kilometers ahead of the telescope to allow the imaging and spectroscopy of planets close to their host star. The flower-petal patterned edges of the starshade have to be fabricated to within a micrometer and designed to precisely fold and unfurl upon deployment. Onboard the spacecraft, a state-of-the-art vector vortex coronagraph, combined with systems designed to provide exceptional pointing stability, will allow contrast levels necessary for directly seeing exo-Earths.
While mapping planetary systems and searching for life is HabEx's main goal, it will also be capable of observing other astrophysical systems from bodies in the solar system to distant galaxies. With this breadth, the HabEx team envisions the telescope as Hubble's successor. Currently, Hubble is the only large space-based observatory that can operate in the ultraviolet range, and at thirty years and counting, no one knows how long it will continue to be operational.
"[HabEx] is a telescope definitely for the masses of the astronomical community," Gaudi said. "It would capture the public''s interest in multiple different ways - in all the ways Hubble has, but also in actually going out searching for life."
While HabEx may seem like an ambitious mission for observing exoplanets, it is relatively small in comparison to LUVOIR, the Large UV Optical Infrared Surveyor. In fact, LUVOIR was developed as two options, LUVOIR-A and LUVOIR-B, which those involved hope will increase their chances of selection. Either way, LUVOIR is expected to do big things.
"I've been doing astronomy for nearly 50 years now, and there was a culture change that occurred in about 1990 with the launch of the Hubble Space Telescope," Peterson said. "I think LUVOIR is going to be similar. It's going to be so enormously capable. It will change astronomy. If we do detect life, it can change civilization."
A preliminary concept for the 15-m LUVOIR telescope. The sunshield is rendered transparent so that the spacecraft is visible. The front view shows the primary mirror and secondary support structure, while the inset shows the rear view and highlights the support frame holding four instruments. Credit: LUVOIR/NASA
While similar to Hubble, LUVOIR will have 100 to 1,000 times the sensitivity, which it hopes to use to find the next "pale blue dot" and survey the birth and evolution of the cosmos. To achieve this range of science goals, LUVOIR will be well equipped with a range of instruments including a coronagraph, a high-definition imager, spectrograph, and spectropolarimeter.
The mirror itself will depend on the version selected. LUVOIR-A would have a 15-meter primary mirror, and LUVOIR-B an 8-meter primary mirror. New, advanced mirror coatings will allow observations at wavelengths shorter than can be observed with Hubble, which opens up observations of highly ionized oxygen - -an important probe of hot gas across the universe. The team is still working on developing actuators to keep the mirror segments aligned as well as a way to isolate any vibrations, so they don't affect the instruments' measurements.
Notably, LUVOIR-B would be an off-axis telescope, meaning the secondary mirror isn't centered. This significantly increases the engineering challenges but allows the throughput to remain high - making it possible to still image faint exoplanets close to their bright host stars.
Origins Space Telescope
At the far end of the spectrum from the mid to far infrared, the Origins Space Telescope will study the cool, faint universe, probing planet-forming regions and galactic gas clouds. At 5.9 meters, Origin's primary mirror will be much larger than previous infrared telescopes, but its biggest advancement will be a solar-powered mechanical cooling system that will drop the telescope's temperature to just four degrees above absolute zero, with certain instruments sinking even lower. This will ensure Origins is able to see faint infrared targets without inadvertently covering up the signal with its own thermal emissions.
Artist concept of the Origins Space Telescope. Credit: NASA
While cooling has been done in the past, it has only been temporary. The Spitzer Space Telescope employed a cryostat system that used a tank of liquid helium to cool its instruments to 1.4 degrees Kelvin, but its helium reserves only lasted five years. By setting up a closed mechanical system with helium, Origins will be able to cool the entire telescope for its expected lifetime.
"These design elements in the telescope provide a factor of 1,000 sensitivity improvement relative to past infrared telescopes," Cooray said. "At this wavelength, we haven''t even touched the low hanging fruit yet. We'd be opening up a new window to the universe."
By following the collapse of gas clouds into stars and planetary disks, Origins can observe the development of habitable worlds right from their dusty origins. While Origins won't be able to observe Earth-like exoplanets, it will be able to study possible companions to the most common stars in our nearby solar neighborhood - the cool M dwarf stars - for which there has not yet been a comprehensive survey. Origins will also trace larger events, such as how galaxies and black holes co-evolve.
The largest remaining barrier for Origins is detectors at infrared wavelengths, which aren't currently able to take advantage of the telescope's heightened sensitivity over its whole wavelength range. To account for this, Origins team members are already working on developing bolometers, spectrometers, and polarimeters to capture infrared light with low levels of noise. If this hurdle is cleared, Origins could be a relatively low-cost mission with the potential to reveal unknowns about the universe that aren't even on astronomers' radars yet.
"If the decadal survey wants something safe, and something reliable, and something they know can get done quickly, Origins is the shortest path," Cooray said. "We have shown in our report a path to build this thing in 10 years."
From giant, high-tech mirrors, to creative new cooling systems, this decade's proposals are sure to see new things in our universe - likely even some astronomers can't yet predict.
"When you open a new window to an instrument capability, or a new telescope in a part of the electromagnetic spectrum that no one''s explored before, everything will be different from what we expect today," Cooray said. "The science will be driven by the new discoveries we will make."
The involved scientists see the proposed telescopes and instruments as key to these new discoveries and the next great era of astronomy. Whether it's discovering inhabited planets or the exploring the evolution of galaxies, there certainly will be plenty of exciting new discoveries in the coming decades.
Attend the free Astronomical Telescopes + Instrumentation Digital Forum in December and learn more about Lynx, HabEx, LUVIOR, the Origins Space Telescope, and other exciting developments in astronomical engineering.
Related SPIE content:
Lynx X-Ray Observatory: an overview
The Habitable Exoplanet Observatory (HabEx)
Telling the story of life in the cosmos: the LUVOIR telescope concepts
Origins Space Telescope May Answer the Big Questions
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