Astronomers propose a 14-meter infrared space telescope

The universe wants us to understand its origins. Every second of every day it sends us a multitude of signals, each indicative of a different aspect of the cosmos. But the universe is the original trickster, and its multitude of signals is an almost unrecognizable cacophony of light, distorted, shifted and stretched on its long journey through the expanding universe.

What are talking monkeys supposed to do in this situation other than build another telescope that is adept at understanding some portion of all this noisy light? That’s what astronomers think we should do, to no one’s surprise.

Due to the size of the universe and its continuous expansion, the light from the first galaxies in the universe is stretched into the infrared. This ancient light holds clues to the origins of the universe and, by extension, our origins. A powerful infrared telescope is needed to observe and decipher this light. Earth’s atmosphere blocks infrared light, which is why we continue to build infrared space telescopes.

Infrared telescopes are also great for observing planets as they form. Dense environments like protoplanetary disks are opaque to most light, but infrared light can reveal what’s going on in these planet-forming environments. The dust absorbs light, then emits it in the infrared and also scatters it. That confuses optical telescopes, but infrared telescopes like SALTUS are designed to deal with it.

A team of astronomers from the US and Europe have joined the chorus calling for a new infrared space telescope. It is currently called SALTUS, the Single Aperture Large Telescope for Universe Studies. In a new article, the astronomers outline the scientific case for SALTUS.

“The SALTUS Probe mission will provide a powerful far-infrared (far-IR) space observatory to investigate our cosmic origins and the possibility of life elsewhere,” the authors of the new paper write.

The article is titled “Single Aperture Large Telescope for Universe Studies (SALTUS): Science Overview.” Gordon Chin of NASA’s Goddard Space Flight Center is the lead author. It is preprinted on arxiv.org.

If built, SALTUS will be different from the powerful JWST. The JWST has four instruments covering an infrared frequency range from 600 to 28,500 nanometers, or 0.6 to 28.5 microns, from the near-infrared (NIR) to the mid-infrared (MIR). SALTUS would cover a range from 34 to 660 µm, which is in the far infrared (FIR). SALTUS’ range is not available to any current observatory, whether space-based or ground-based.

There are no precise definitions of which exact ranges make up NIR, MIR, and FIR, but this table is a useful representation.  Image credits: Wikipedia
There are no precise definitions of which exact ranges make up NIR, MIR, and FIR, but this table is a useful representation. Image credits: Wikipedia

Infrared telescopes must be kept cool. They use sunshades and cryogenic coolers to keep temperatures down and make IR light detectable. The longer the wave of infrared light, the cooler the sensor must be. Sunshades are passive and cool the primary mirror, but the instruments require active cryogenic cooling, and those systems have a limited lifespan that limits mission duration. In the case of SALTUS, the basic mission duration is five years.

During those five years, SALTUS will use its 14-meter primary mirror and its instruments to open a “powerful window into the universe through which we can explore our cosmic origins,” the paper’s authors said.

The two instruments are the SAFARI-Lite spectrometer (SALTUS Far-Infrared Lite) and HiRX (high resolution receiver). Using these instruments, SALTUS will complement the observing capabilities of the JWST and ALMA, the Atacama Large Millimeter/submillimeter Array.

The aperture is so large that it will be the only Far-IR observatory with arcsecond-scale spatial resolution. One arcsecond is defined as the ability to display two posts 4.8mm apart at a distance of 1km as separate posts. “This will enable an unmasking of the true nature of the cold universe, which holds the answers to many of the questions about our cosmic origins,” the authors write.

SALTUS has a unique design among space telescopes. It features an inflatable primary mirror, which is new to space telescopes but has been proven in ground-based telecommunications for decades. A two-layer sunshade keeps the inflatable mirror cool.

SALTUS’ large aperture ensures high sensitivity and focuses on a number of fundamental questions.

How does habitability evolve as planets form? To answer this question, SALTUS will trace carbon, oxygen and nitrogen in 1,000 different protoplanetary disks. It has the power to recognize numerous molecular and atomic species and various lattice shapes of ice and some minerals. No existing telescope has this capability.

SALTUS' far IR observing capabilities will make it possible to see some of the protoplanetary disks hidden at other wavelengths.  This will open a new window into planet formation and how habitability develops.  Image credits: Chin et al. 2025/Miotello et al. Protostars and planets 2023.
SALTUS’ far IR observing capabilities will make it possible to see some of the protoplanetary disks hidden at other wavelengths. This will open a new window into planet formation and how habitability develops. Image credits: Chin et al. 2025/Miotello et al. Protostars and planets 2023.

Habitability, as we understand it, is about water. Water begins its journey in the same molecular clouds where stars form. SALTUS will follow the journey of water, from molecular clouds to protoplanetary disks to icy planetesimals and comets that supply water to planets like Earth. An important part of SALTUS’ work will be the derivation of deuterium/hydrogen ratios.

This simple image shows how water gets to planets and can lead to habitability.  SALTUS will track the water's journey by observing hundreds of protoplanetary disks.  Image credits: Chin et al. 2024.
This simple image shows how water gets to planets and can lead to habitability. SALTUS will track the water’s journey by observing hundreds of protoplanetary disks. Image credits: Chin et al. 2024.

How do galaxies form and evolve? SALTUS will measure how galaxies form and gain mass. It measures heavy elements and interstellar dust from the first galaxies in the universe to today. The telescope will also investigate the co-evolution of galaxies and their supermassive black holes (SMBHs).

Tracking the rapid evolution of dust grains in galaxies in the first billion years of the universe is part of understanding the formation and evolution of galaxies. SALTUS can do this by observing PAHs, polycyclic aromatic hydrocarbons and their spectral lines. Some PAH spectral lines are very faint, but fully visible to SALTUS.

There is a causal relationship between star formation and active galactic nuclei (AGN) that influence the growth and evolution of galaxies. But the two phenomena occur on vastly different spatial scales, and the phase connecting them is obscured by dust. SALTUS’ high-resolution and sensitive far-IR spectroscopy will give astronomers a clearer view of AGN and how they form galaxies.

SALTUS would be placed in a Sun-Earth Halo L2 orbit. The maximum distance to Earth would be 1.8 million km (1.12 million miles). That orbit would give the telescope two continuous 20-degree viewing zones around the ecliptic poles, resulting in full coverage of the sky every six months.

The SALTUS concept was designed in response to the 2020 Decadal Survey and NASA’s Astrophysical Roadmap. It is a direct response to NASA’s request for 2023 Astrophysics Probe Explorer (APEX). The questions it will help answer come directly from those works.

“SALTUS has both the sensitivity and spatial resolution to address not only the open scientific questions of the year 2023, but, more importantly, the unknown questions that will be raised in the 2030s,” the authors write in their abstract. “SALTUS is forward-thinking and well-suited to meet the current and future needs of the astronomical community.”

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