Astronomy crossed a threshold three decades ago with the discovery of planets around other stars. Compared to scientists' previous expectations set by the Solar System, exoplanets are wonderfully abundant and varied. Indirect planet discovery techniques have shown that small rocky planets residing in stellar habitable zones, where such planets may have liquid water on their surfaces, are not rare. This revelation drives us to ask more ambitious and fundamental questions, that fascinate scientists and the public alike: are there other truly Earth-like planets out there and do any of them harbour life? Today, exoplanets are largely small black shadows' to us, with measurements of orbits, sizes and masses (all three in the best cases).The upcoming James Webb Space Telescope and future 30-m-class ground-based telescopes will characterize the atmospheres of habitable planet candidates orbit in glow-mass M dwarf stars. However, deeply probing atmospheres of the exoplanets most similar to the Earth, those around Sun-like stars, remains out of reach for currently planned observatories. Bringing them within our grasp is a primary motivation for the Large UV/Optical/Infrared Surveyor(LUVOIR) mission concept, currently the focus of a three-year NASA study

Moustakas, Leonidas A.

Roberge, Aki

NASA Technical Reports Server

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The Large Ultraviolet/Optical/Infrared Surveyor
LUVOIR is a concept for a powerful, flexible space observatory to enable the first survey for exoplanets most  
similar to the Earth, search for signs of life in our Solar System and beyond, and revolutionize astrophysics in the 
twenty-first century.
Aki Roberge and Leonidas A. Moustakas
Astronomy crossed a threshold three decades ago with the discovery of planets around other 
stars1,2. Compared to scientists’ previous 
expectations set by the Solar System, 
exoplanets are wonderfully abundant and 
varied. Indirect planet discovery techniques 
have shown that small rocky planets residing 
in stellar habitable zones, where such planets 
may have liquid water on their surfaces, are 
not rare. This revelation drives us to ask 
more ambitious and fundamental questions 
that fascinate scientists and the public alike: 
are there other truly Earth-like planets out 
there and do any of them harbour life?
Today, exoplanets are largely ‘small black 
shadows’ to us, with measurements of orbits, 
sizes and masses (all three in the best cases). 
The upcoming James Webb Space Telescope 
and future 30-m-class ground-based 
telescopes will characterize the atmospheres 
of habitable planet candidates orbiting 
low-mass M dwarf stars3,4. However, deeply 
probing atmospheres of the exoplanets most 
similar to the Earth, those around Sun-like 
stars, remains out of reach for currently 
planned observatories. Bringing them 
within our grasp is a primary motivation 
for the Large UV/Optical/Infrared Surveyor 
(LUVOIR) mission concept, currently the 
focus of a three-year NASA study.
LUVOIR is a space telescope in the 
tradition of NASA’s Great Observatories, 
with diverse and powerful capabilities to 
revolutionize the astrophysics landscape. 
The view of the Science and Technology 
Definition Team (STDT) directing the study 
is that a broad, community-driven observing 
programme would make best scientific use 
of LUVOIR’s capabilities. LUVOIR is being 
designed to be serviceable and upgradable, 
features that greatly contributed to the 
unparalleled success of the Hubble Space 
Telescope (HST). LUVOIR aims to return 
the highest benefit-to-cost ratio from a 
large mission with a long development 
timescale, with the flexibility to respond to 
future discoveries and answer questions the 
scientific community hasn’t yet conceived.
The two LUVOIR designs being studied 
have five-year primary missions and a possible 
launch date in the late 2030s. The first design 
(LUVOIR-A) has a 15-m diameter primary 
telescope mirror designed for launch on 
NASA’s future Space Launch System rocket. 
The second (LUVOIR-B) has a roughly 
8-m diameter primary mirror intended for 
launch in a heavy-lift rocket similar to those 
in use today. The two architectures explore 
the scalability of the LUVOIR segmented 
telescope design in science return, design 
complexity, technical risk and cost.
Four instruments are being designed 
for LUVOIR-A. The Extreme Coronagraph 
for Living Planetary Systems (ECLIPS) 
is designed for ultrahigh-contrast (10–10) 
observations: imaging (from 200 nm to 
1,030 nm), integral field spectroscopy  
(R = 140 from 515 nm to 1,030 nm, R = 70 
from 1,000 nm to 2,000 nm), and point-
source spectroscopy (R = 200 from 1,000 
nm to 2,000 nm). The LUVOIR Ultraviolet 
Multi-Object Spectrograph (LUMOS) will 
 
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Potentially
Earth-like
planets
Rocky
planets
Super
Earths
Sub-
Neptunes
Neptune-like
planets
Jupiter-like
planets
Fig. 1 | Expected exoplanet detection yields with the LUVOIR-A concept. Planet classes from left to 
right are: habitable planet candidates (green bar), rocky planets, super-Earths, sub-Neptunes, Neptunes 
and Jupiters. Red, blue and ice blue bars indicate hot, warm and cold planets, respectively. The habitable 
planet candidates are a subset of the warm rocky and super-Earth planets. All these different exoplanets 
are expected to be detected during the habitable planet search campaign. Credit: C. Stark (STScI); 
Earth-like, NASA; rocky, NASA/Ames/JPL-Caltech; super-Earth, ESO/M. Kornmesser/Nick Risinger 
(http://skysurvey.org/); sub-Neptunes, NASA/JPL-Caltech/R. Hurt (SSC); Neptune-like, NASA/JPL; and 
Jupiter-like, NASA/ESA/A. Simon (NASA Goddard).
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provide imaging from 100 nm to 200 nm 
and multi-object, multi-resolution  
(R = 500–60,000) spectroscopy from  
100 nm to 400 nm. The High Definition 
Imager (HDI) is a wide field (2ʹ × 3ʹ) camera 
covering 200 nm to 2,500 nm, Nyquist 
sampled at 400 nm. Finally, POLLUX is a 
high-resolution (R = 120,000) point-source 
spectropolarimeter covering 97 nm  
to 390 nm; this instrument is being 
designed by a consortium of European 
institutions, with support from the French 
space agency (CNES). Variants of the first 
three instruments are being studied for 
LUVOIR-B.
LUVOIR’s science cases stretch from 
the dawn of the Universe to observations 
of Solar System worlds. Below we highlight 
just three of LUVOIR’s ‘signature science 
cases’. The LUVOIR Interim Report 
showcases a dozen signature science 
cases that represent compelling observing 
programmes scientists might carry out with 
LUVOIR at the limits of its performance. 
As exciting as these are, they are surely 
an incomplete vision of LUVOIR’s future 
scientific potential.
Exoplanets and the search for life
To determine which exoplanets are 
actually habitable, we must measure the 
molecular constituents of rocky planet 
atmospheres, including water vapour and 
other greenhouse gases. Making these 
measurements for Earth-size planets around 
Sun-like stars demands analysing light from 
the planets themselves via direct spectra5,6, 
which is challenging due to the faintness 
of the planets and their close proximity to 
their host stars. Direct observations of rocky 
exoplanets around Sun-like stars demand 
the ultrahigh contrast possible only with a 
space-based telescope coupled to a high-
performance starlight suppression system.
Measuring the frequency of Earth-
like conditions on rocky worlds requires 
surveying hundreds of stars to find dozens 
of candidate exoplanets for study. This 
goal is a strong driver of telescope aperture 
size but guarantees that whatever is found 
will have a major scientific impact. In the 
absence of water detections, we would know 
that global-scale surface oceans are rare on 
rocky worlds in the habitable zone.  
On the other hand, if we find that Earth-like 
conditions are common on rocky worlds 
near our Sun, then a stunning vista of 
hospitable new worlds will be unveiled.
While LUVOIR delivers the habitable-
planet census, it will study hundreds of  
other types of planets, vastly expanding the 
scope of comparative planetology  
(Fig. 1). It will also probe the atmospheres 
of habitable planets for biosignature gases 
and might find the first signs of life outside 
the Solar System. The spectrum of the 
Earth contains features arising from gases 
of biological origin, including oxygen and 
methane. By targeting exoplanets with sizes 
and orbits similar to Earth’s and host stars 
similar to the Sun, we hope to increase our 
chances of finding — and recognizing — 
extraterrestrial life.
the births and deaths of galaxies
Throughout their lives, material flows into 
and out of galaxies as they interact with 
neighbouring galaxies and the intergalactic 
medium7. Some galaxies are born small but 
become the building blocks of giant spirals 
like our own Milky Way8. Some galaxies are 
actively forming new stars, while others have 
undergone slow stagnation as their stars 
age and no new ones are born. Our current 
understanding of this complex interplay is 
highly incomplete.
Much of the cycling of matter to, from 
and within galaxies is currently unobserved, 
as the gas flows are hot and tenuous. 
Ultraviolet spectroscopy, which must be 
done from space given the opacity of Earth’s 
atmosphere below 300 nm, can reveal these 
processes. Furthermore, galaxy mergers 
result in bursts of star formation but may 
also be responsible for the ultimate ‘death’ of 
merged galaxies. LUVOIR can illuminate the 
role mergers play in forming today’s massive 
galaxies by enabling deep, high-resolution 
imaging of all types of galaxies across 
cosmic time. The smallest building blocks 
of galaxies become visible to LUVOIR at a 
time when the Universe was only 3% of its 
current age (Fig. 2).
monitoring the solar system
The search for life also takes place closer 
to home. Some moons of the outer Solar 
System have liquid water beneath their icy 
surfaces. These sub-surface oceans must be 
heated from below, which may also provide 
the energy needed for life. Ultraviolet 
imaging of Europa with Hubble has revealed 
aurorae produced by geysers emanating 
from the surface, possibly allowing glimpses 
into the deep ocean9 (Fig. 3). LUVOIR 
can provide unique high-resolution UV 
spectral imaging of icy moons with high 
temporal cadence over several years. This 
will reveal the currently unknown strength 
and frequency of geyser activity, valuable 
supporting information for future spacecraft 
2.4 m
4.0 m
6.5 m
9.3 m
15.1 m
8 9 10 11 12 13 14 2 4 6 8 10
log (Mvir/M⊙)
lo
g 
(M
*
/M
⊙
)
Redshift z
2.4 m
4.0 m
6.5 m
9.3 m
15.1 m
Bright dwarfs
Classical dwarfs
Ultra-faint dwarfs
2
3
4
5
6
7
8
9
10
11
12
Fig. 2 | LUVOIR’s view of the least luminous galaxies, the building blocks of galaxy formation, extends 
to eras when the Universe was just 3% of its current age. This corresponds to the z = 10 point in the 
right panel, which shows the galaxy mass detection limits for LUVOIR-type architectures, including the 
15.1-m LUVOIR-A. M* is the galaxy mass in stars and Mvir is the galaxy halo (virial) mass. Credit: left 
panel adapted from ref. 8, Annual Reviews; right panel M. Postman (STScI).
HST LUVOIR
Fig. 3 | spectroscopic imaging of Europa and 
its geysers. The left panel shows an aurora on 
Europa observed with HST7. This UV hydrogen 
emission (Lyman-α ) comes from dissociation 
of water vapour in geysers emanating from the 
surface. The right panel shows a simulation of 
how hydrogen emission from Europa would look 
to LUVOIR-A. The moon’s surface is bright due to 
reflected solar Ly-α emission. Credit: G. Ballester 
(LPL).
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visiting these other ocean worlds to search 
for signs of life.
the future of astrophysics
The USA astrophysics community 
determines its future scientific priorities 
for both ground- and space-based research 
via Decadal Surveys managed by the USA’s 
National Research Council. Unfortunately, 
the top priorities in the large space mission 
class from the last two Astrophysics 
Decadal Surveys are still on the ground: 
the James Webb Space Telescope and the 
Wide Field Infrared Survey Telescope 
(WFIRST). The reasons for this state of 
affairs are far beyond the scope of this 
Comment, but the consequences for 
astrophysics, as well as LUVOIR and other 
future large space observatory concepts, 
must be addressed.
It is important to acknowledge the 
vital role large missions play in a balanced 
scientific portfolio. Powerful, general-
purpose observatories are by their very 
nature especially well-suited to enabling 
unexpected scientific advances, which are 
often the most revolutionary. Furthermore, 
they serve the largest number of scientists 
across the widest range of fields. For 
many astronomers in the USA, NASA’s 
Great Observatories have been their 
most important community facilities for 
decades10.
The argument has been made that long 
development timescales and cost overruns 
on large missions harm NASA’s ability to 
execute smaller, more focused missions, 
which would also be damaging to a balanced 
scientific portfolio. Examination of historical 
NASA Astrophysics budget data suggests 
that ‘flagships eat flagships’, not smaller 
missions (https://go.nature.com/2tUYAN7). 
However, restraining development times 
and costs on future large missions is still a 
critical goal.
Some helpful steps are being 
investigated within the LUVOIR study. 
Easily repairable, upgradable, and scalable 
observatory concepts can better respond 
to uncertain technological and budgetary 
landscapes. Technology gaps should 
be filled much earlier in the mission 
development process. Important products 
of the LUVOIR study will be a thorough 
assessment of technological maturity 
and a technology development plan that 
includes estimated schedule and cost, all 
of which will be independently checked. 
Finally, an observatory like LUVOIR 
can and should draw new resources to 
space astrophysics, through expanded 
international partnerships and non-
traditional partnerships with other scientific 
and technical disciplines. But the greatest 
potential source of support comes from 
the public and their representatives in 
government. Ambitious and revolutionary 
science goals — like those of LUVOIR — are 
the way to win and sustain that support. ❐
Aki Roberge1* and Leonidas A. Moustakas2
1NASA Goddard Space Flight Center, Greenbelt, MD, 
USA. 2Jet Propulsion Laboratory, California Institute 
of Technology, Pasadena, CA, USA.  
*e-mail: aki.roberge@nasa.gov
Published online: 1 August 2018 
https://doi.org/10.1038/s41550-018-0543-8
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Acknowledgements
L.A.M.’s work was carried out at the Jet Propulsion 
Laboratory, California Institute of Technology, under 
a contract with the National Aeronautics and Space 
Administration. This article contains pre-decisional 
information and is for planning and discussion only.
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