Astronomers propose a 14-meter infrared space telescope

The universe wants us to understand its origin. 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 on its long journey through the expanding Universe.

What are the monkeys to do in this situation but build another telescope capable of understanding a particular part of all this noisy light? That’s what astronomers think we should be doing, to no one’s surprise.

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

Infrared telescopes are also suitable 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. This confounds 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 tentatively called SALTUS, the Large Single Aperture Telescope for the Study of the Universe. In a new paper, astronomers outline the scientific case for SALTUS.

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

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

If built, SALTUS will be different from the mighty JWST. 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 near infrared (NIR) to mid-infrared (MIR). SALTUS would cover 34 to 660 m, which is in the far infrared (FIR). The range of SALTUS is not available to any current observatory, space or ground.

Infrared telescopes must be kept cool. They use sunscreens and cryogenic coolers to keep temperatures low and IR light detectable. The longer the infrared light wavelength, the cooler the sensor should be. The sun glasses are passive and cool the primary mirror, but the instruments require active cryogenic cooling and those systems have a limited lifetime that limits the length of the mission. In the case of SALTUS, the length of the base mission is five years.

During those five years, SALTUS will use its 14-meter primary mirror and its pair of instruments to open a “powerful window onto 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 Literally) and HiRX (High Resolution Receiver.) Using these instruments, SALTUS will complement the observing capabilities of JWST and ALMA, the Atacama Large Millimeter/submillimeter array.

Its aperture is so large that it will be the only Far-IR observatory with arc-scale spatial resolution. One arc second is defined as the ability to tell two poles standing 4.8 mm apart from 1 km apart as separate poles. “This will allow 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 inflated primary mirror, which is new to space telescopes but has been proven over decades of use in ground-based telecommunications. A double layer protection will keep the blown mirror cool.

The large SALTUS aperture will provide high sensitivity and targets several fundamental questions.

How does habitability develop during planet formation? To address this question, SALTUS will trace the carbon, oxygen and nitrogen in 1,000 different protoplanetary discs. It has the power to recognize numerous molecular and atomic types and different lattice modes of ice and some minerals. No existing telescope has this capability.

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

How do galaxies form and evolve? SALTUS will measure how galaxies form and gain more mass. It will measure heavy elements and interstellar dust from the Universe’s first galaxies to the present day. 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 entirely visible to SALTUS.

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

SALTUS will be placed in a Sun-Earth Halo L2 orbit. Its maximum distance from Earth would be 1.8 million km (1.12 million miles). This orbit would give the telescope two continuous 20 fields of view 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 2023 Astrophysical Probes (APEX) request. The questions it will help you answer come directly from those works.

“SALTUS has the sensitivity and spatial resolution to address not only the open science questions of 2023 but, more importantly, the unknown questions that will arise in the 2030s,” the authors write in their summary. “SALTUS is forward-looking and well suited to serve the current and future needs of the astronomical community.”