American Scientist

NASA has a long and rich tradition of designing, developing, and flying space telescopes that serve the international astronomy and astrophysics community. Especially important have been the Great Observatories—the Compton Gamma-Ray Observatory, the Chandra X-ray Observatory, the Hubble Space Telescope, and the Spitzer Space Telescope—which gave us cosmic eyes in the gamma-ray, x-ray, ultraviolet, visible, and infrared ranges of wavelengths. The Great Observatories have a brilliant track record of delivering paradigm-shifting answers to age-old questions about our cosmos.

Conceived in the 1970s, Hubble was originally envisioned as a means to study a broad range of astrophysics, including measuring distances to other galaxies and characterizing the matter between them (the intergalactic medium). What was not anticipated was that Hubble would be used to study the atmospheres of exoplanets. The discovery that light fluctuates as an exoplanet moves in front of the star it orbits opened unforeseen potential for measuring the abundances of atoms and molecules in its atmosphere. Because Hubble was not designed for this purpose, astronomers had to develop clever data procurement and analysis techniques to accomplish those aims.

The key lesson learned from Hubble is that there is exciting science that one cannot plan for—the unknown unknowns. The best one can do is to develop a Great Observatory that is sufficiently capable of addressing the cutting-edge scientific questions of the present and yet versatile enough to adapt to future paradigm shifts. This important lesson has propagated into the building of the James Webb Space Telescope (JWST), which is the latest of the NASA flagship observatories. The explosive developments in exoplanet science have influenced the design of the instruments on board JWST. (See Perspective, March–April 2017.)

By the 2030s and beyond, with even more capable space observatories available, the exoplanet community aspires to routinely detect, identify, and study habitable worlds and ultimately hunt for telltale signs of life. (See Perspective, May–June 2016.) LUVOIR, which is short for the Large Ultraviolet/Optical/Infrared Surveyor, continues NASA’s tradition of observing a wide range of electromagnetic wavelengths in order to push the limits of our understanding within and beyond the Solar System. For the first time in history, we are designing a space telescope with the main intention of detecting and characterizing habitable worlds in unprecedented detail.

To 2021 and Beyond

With JWST now scheduled to launch in 2021, NASA is looking further ahead, studying four space telescope concepts for its next flagship mission, to be launched in the 2030s: LUVOIR, the Habitable Exoplanet Observatory (HabEx), the Origins Space Telescope (OST, or Far-Infrared Surveyor), and the Lynx X-ray Observatory. The fate of these four mission concepts lies with the Astronomy and Astrophysics Decadal Survey, which is conducted by the National Academy of Sciences to identify the priorities of the field for the next decade.

Three out of four of these mission concepts aim to sharpen humanity’s view of habitable worlds, as the pace of discovery in exoplanet science accelerates in the coming decades. From 2018 onward, the American-led Transiting Exoplanet Survey Satellite (TESS; see "What's Next for Finding Other Earth-Like Worlds") and the Swiss-led Characterizing Exoplanet Satellite (CHEOPS) are poised to deliver a bonanza of prized exoplanet targets orbiting nearby bright stars, a few of which will be amenable to the characterization of their atmospheres by JWST. (See “The Next Great Exoplanet Hunt,” May–June 2015.)

NASA’s next hunt for signs of life will use one of the three proposed missions. For the LUVOIR team, the challenge lies in envisioning the exact form the space telescope would take to balance pragmatism with our big ambitions to gather as much useful information as possible.

Designing the Next Great Observatory

Part of the strategy for designing a space telescope is to leverage existing technology and the experience gained from building earlier space telescopes. LUVOIR has a rich heritage to draw from. With a mirror size that is expected to dwarf even the 6.5-meter diameter of JWST, the primary mirror of LUVOIR, like that of JWST, will be segmented and deployed in orbit.

The technology for building segmented mirrors has been carefully developed and honed during the building of both JWST and giant ground-based telescopes. The development of coronagraphs for space is part of NASA’s upcoming Wide Field Infrared Space Telescope (WFIRST; see "Mapping the Distant Universe" in this issue), which aims to study both exoplanets and dark energy. The technology for building spectrographs that record light from the ultraviolet to the infrared has been improved with each generation of space telescopes.

In anticipation of a complex budgetary landscape, the LUVOIR Science and Technology Definition Team (STDT) has designed two versions of the mission: LUVOIR-A (with a mirror that is 15 meters in diameter) and LUVOIR-B (with a mirror diameter of about 8 or 9 meters). There is a running joke among team members that we have “large” and “really large” versions of LUVOIR. Both versions are intended to be serviceable and to operate at the Sun-Earth L2 Lagrangian point (about 1.5 million kilometers from Earth), where LUVOIR may maintain a stable orbit in the long term.

A suite of four instruments on board LUVOIR will give it ultraviolet, visible, and infrared eyes that can see wavelengths of 0.1 to 2.2 micrometers. The range of wavelengths covered influences our ability to identify atoms and molecules in the recorded spectra. The wider the wavelength range, the more confidently one can confirm the identity of an atom or molecule.

However, recording spectra at wavelengths beyond the near-infrared requires active cooling of the spectrograph in space, which drives up the cost. LUVOIR has therefore enforced a 2.2-micrometer cutoff on its spectrograph. Earth analogs—twins of Earth orbiting a close sibling of the Sun—that are detected by direct imaging are generally not expected to transit their stars, which means that their true sizes are unknown. This limitation means that Neptune-sized exoplanets may masquerade as Earth analogs—false positives—if only limited information about their atmospheric chemistry is recorded.

Instruments on LUVOIR will be able to take sharp images of faraway objects with unprecedented resolution and also will allow the gathered light to be split into colors, thus decoding valuable information about the chemistry and physics of these objects. The ECLIPS (Extreme Coronagraph for Living Planetary Systems) instrument is a telescopic attachment capable of blocking out starlight and isolating the light from exoplanets and their atmospheres.

The key lesson learned from Hubble is that there is exciting science that one cannot plan for—the unknown unknowns.

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Two imagers can distinguish colors: The high-definition imager covers the near-ultraviolet (NUV) to the near-infrared and is capable of performing precision astrometry. The LUVOIR Ultraviolet Multi-Object Spectrograph (LUMOS) is a far-ultraviolet (FUV) imager and a FUV-NUV-optical spectrograph. It features a microshutter array to allow for simultaneous observations of hundreds of sources. European astronomers are contributing an ultraviolet spectropolarimeter, which allows for the study of how light from exoplanets and other sources is polarized.

The LUVOIR team is currently studying several complex relationships between mirror size and configurations of the instruments. As a prime example, we expect the yield of Earth analogs to be strongly tied to the size of the primary mirror: Only a few or as many as a hundred Earth analogs could be detectable. Different versions of LUVOIR-B, in which the primary mirror may be located either on- or off-axis relative to the secondary mirror, are under consideration. The off-axis design requires a slightly smaller primary mirror but is riskier in terms of engineering. As with all space missions, a delicate balance must be struck among scientific rewards, hardware legacy, risk, and cost.

Delivery and Assembly in Space?

The choice of a rocket to fly LUVOIR to its L2 orbit is another point of discussion in designing this mission, requiring us to weigh cost against ambitions. Specifically, we are limited by the size of the rocket fairing, which is the cone-shaped casing that houses the telescope and protects it from high pressures and heating as it departs Earth’s atmosphere.

LUVOIR-A and LUVOIR-B assume 8.4-meter and 5.0-meter fairings, respectively. The 8.4-meter fairing works with the planned NASA Space Launch System (SLS) and its Block 2 design. Conceived as a replacement for the Space Shuttle heavy-lift capability, the SLS is envisioned to be the most powerful rocket ever built and to enable NASA to routinely deliver heavy payloads to the Moon and Mars. The 5.0-meter rocket fairing needed for LUVOIR-B will work with currently available commercial rockets or the SLS Block 1 design.

The final choice of rocket depends on a careful analysis of cost versus performance. It comes at a particularly interesting time in history, when several immensely wealthy individuals are attempting to privatize space flight. Leading the way is Elon Musk’s Space Exploration Technologies (SpaceX), which is pioneering the development of reusable rockets to drive down the costs of payload delivery to space. By adopting a “fail and learn fast” strategy, SpaceX has achieved a number of firsts in rocket flight, including being the first privately funded company to launch and recover previously expendable parts of a rocket.

The current workhorse for SpaceX is the company’s Falcon 9 rocket, which has delivered payloads to the International Space Station. The Falcon Heavy rocket essentially straps together three Falcon 9s to achieve superheavy lift. The planned Big Falcon Rocket (BFR) is expected to dwarf the Falcon Heavy, both in terms of size and thrust, and bring us a step closer to Musk’s dream of routine human flights to Mars.

To achieve its scientific goals, LUVOIR needs a primary mirror of at least 8 meters in diameter. This size constraint translates into a launch vehicle with a fairing that is 5 meters in diameter and 19.8 meters in length. With all of its instruments on board, we estimate that the launch vehicle will need to deliver at least 14 metric tons to the L2 orbit.

These considerations exclude the Falcon Heavy as a launch vehicle, especially because SpaceX has publicly ruled out making bigger fairings for the Falcon series of rockets. The BFR is an intriguing possibility, but given Musk’s serial optimism, it is difficult to make precise plans for using this launch vehicle.

Another private company, Blue Origin, is also pushing the envelope on rocket development. Founded by Amazon.com titan Jeff Bezos, the company aims to dramatically reduce costs by incrementally building from suborbital to orbital space flight. So far, Blue Origin has focused on the New Shepard series of suborbital rockets. If and when the company’s New Glenn rocket is successfully implemented, it will be a viable competitor as a launch vehicle for LUVOIR.

Currently, both versions of LUVOIR are designed to be launched on a single rocket and to autonomously deploy, as JWST will. It is not inconceivable that a LUVOIR-class telescope may adopt a modular design and be assembled in space. Such an approach has several clear advantages. By delivering a space telescope as several separate payloads, it relaxes the constraints on the choice of launch vehicle. It also puts the burden of “ruggedization” against launch stresses on subsystems rather than on the entire space telescope.

Modularization would allow an evolutionary approach toward building a large telescope in space: One might first construct a “bare-bones” version with the essential, “must-have” instruments on board, with the mind-set that it can be upgraded and augmented in the future. That is not a preposterous notion, because two large-scale projects in the past have, in principle, taught us the lessons needed to execute such a plan: The International Space Station demonstrated that assembly in space is possible, and Hubble has been successfully serviced on-orbit by astronauts on five occasions.

The main challenge of in-space assembly and servicing is that the costs will be beyond what the NASA Science Mission Directorate (SMD) could afford at current funding levels. This plan could only work if it were an agencywide priority for NASA, because cooperation between SMD and the Human Exploration and Operations Directorate would be required for servicing and assembly by some suitable combination of humans and robots. In other words, we would need the NASA scientists and astronauts to work closely together.

Such a collaboration would be an opportunity to develop tools and techniques that can be applied more broadly to leverage a more permanent human presence beyond the Earth and low-Earth orbit. From a political standpoint, there would be the advantage that the various upgrades, repairs, and enhancements to the telescope could be considered as distinct, periodic budgetary requests.

Although assembly in space may seem like a pipe dream, NASA has long developed the idea of the Deep Space Gateway, which is essentially a mini–space station in low orbit around our Moon. It is one embodiment of NASA’s quest to extend human operations to cislunar space. In NASA’s 2019 budget request to Congress, the Deep Space Gateway was renamed the Lunar Orbital Platform-Gateway (LOP-G) and approved for funding. The existence of the SLS/LOP-G infrastructure enables servicing and assembly of large telescopes in cislunar space. Once a telescope is assembled, tested, and commissioned there, transfer to the Sun-Earth L2 Lagrangian point requires little energy.

What excites us about our discussions as we design LUVOIR is the anticipation of the paradigm-shifting scientific results that it will deliver. LUVOIR will have the sensitivity to detect biosignatures encoded in the spectra of dozens of nearby exoplanets and thus to perform the first census of Earth analogs in our cosmic neighborhood. If no biosignatures are detected, that will tell us definitively that life is extremely rare, and therefore extremely precious. Either a detection or a nondetection will be a civilization-changing discovery: Regardless of the outcome, our view of humankind will be fundamentally altered.