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 one a clue to 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, warped, shifted, and stretched during its long journey through the expanding Universe.

What are talking apes to do in this situation but build another telescope adept at understanding a particular slice of all this noisy light? That’s what astronomers think we should do, to nobody’s surprise.

Due to the size of the Universe and its ongoing expansion, light from the Universe’s first galaxies is stretched into the infrared. This ancient light holds clues to the Universe’s origins and, by extension, our origins. It takes a powerful infrared telescope to sense and decipher this light. Earth’s atmosphere blocks infrared light which is why we keep building infrared space telescopes.

Infrared telescopes are also well-suited to 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 confounds optical telescopes, but infrared telescopes like SALTUS are designed to deal with it.

A team of astronomers from the USA and Europe has joined the chorus calling for a new infrared space telescope. It’s tentatively called SALTUS, the Single Aperture Large Telescope for Universe Studies. In a new paper, the astronomers outline the science case for SALTUS.

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

The paper is titled “Single Aperture Large Telescope for Universe Studies (SALTUS): Science Overview.” Gordon Chin from NASA’s Goddard Space Flight Center is the lead author. It’s in pre-print at arxiv.org.

If built, SALTUS will be different from the powerful JWST. The JWST has four instruments that cover an infrared frequency range from 600 to 28,500 nanometers, or 0.6 to 28.5 microns, which is from the near-infrared (NIR) to the mid-infrared (MIR). SALTUS would cover 34 to 660 ?m, which is in the far-infrared (FIR). SALTUS’ range is unavailable to any current observatory, space or ground-based.

Infrared telescopes need to be kept cool. They use sunshades and cryogenic coolers to keep temperatures down and IR light detectable. The longer the wave of infrared light, the cooler the sensor needs to be. Sunshades are passive and cool the primary mirror, but the instruments require active cryogenic cooling, and those systems have a limited lifetime that restricts mission length. In SALTUS’s case, the baseline mission length is five years.

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

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 Millimetre/submillimetre Array.

Its aperture is so large that it’ll be the only Far-IR observatory with arcsec-scale spatial resolution. One arcsecond is defined as the ability to show two posts standing 4.8mm apart from 1km away as separate posts. “This will permit an unmasking of the true nature of the cold Universe, which holds the answers to many of the questions concerning 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 during decades of use in ground-based telecommunications. A two-layer sunshield will keep the inflatable mirror cool.

SALTUS large aperture will provide high sensitivity and is aimed at a couple of foundational questions.

How does habitability develop while planets are forming? To address 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 different lattice modes of ice and some minerals. No existing telescope has this capability.

Habitability, as far as we understand it, revolves around water. Water begins its journey in the same molecular clouds where stars form. SALTUS will follow water’s journey from molecular cloud to protoplanetary disks to icy planetesimals and comets that deliver water to planets like Earth. A key part of SALTUS’s work will be deriving deuterium/hydrogen ratios.

How do galaxies form and evolve? SALTUS will measure how galaxies form and acquire more mass. It’ll measure heavy elements and interstellar dust from the Universe’s first galaxies to today. The telescope will also probe the co-evolution of galaxies and their supermassive black holes (SMBHs.)

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

There’s a causal link between star formation and active galactic nuclei (AGN) that influences galaxy growth and evolution. But the two phenomena take place on wildly different spatial scales, and the phase that links them together is obscured by dust. SALTUS’s high resolution and sensitive far-IR spectroscopy will give astronomers a clearer view of AGN and how they shape galaxies.

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

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

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

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