What’s on and in a star? What happens at an active galactic nucleus? Answering those questions is the goal of a proposed giant interferometer on the Moon. It’s called Artemis-enabled Stellar Imager (AeSI) and would deploy a series of 15-30 optical/ultraviolet-sensitive telescopes in a 1-km elliptical array across the lunar surface.
A U.S. team of scientists and engineers led by Dr. Kenneth Carpenter at the NASA Goddard Spaceflight Center, working in collaboration with Goddard’s Integrated Design Center, just finished a 9-month feasibility study for AeSI and published its findings. The study was funded through NASA’s Innovative Advanced Concepts (NIAC) program that allows scientists and engineers to examine visionary ideas for future programs.
AeSI is based on an earlier concept for a free-flying UV/optical space interferometer called Stellar Imager (SI). According to Carpenter, they watched the steady progress made on NASA’s Artemis campaign to establish habitats and supporting infrastructure on the lunar surface. The idea of a Moon-based facility started looking much more feasible and competitive with the free-flyer. “We thus proposed to the NASA Innovative Advanced Concepts (NIAC) program to develop a variant of the SI concept, named Artemis-enabled Stellar Imager (AeSI), that could potentially be built, deployed, operated, and serviced in collaboration with the Artemis campaign,” he said.
Artemis Opportunities and an Interferometer
NASA’s proposed return to the Moon via Artemis missions offers astronomers a chance to deploy an interferometer, and other telescopes. That would take advantaage of an environment supported by Artemis infrastructure and free of some of the constraints that Earth-based or space-based arrays could experience.
The NIAC-funded study focuses on a number of scientific goals. It states, “This mission would enable revolutionary science, including: imaging the surfaces of nearby (~4 pc) solar-type stars and more distant (>2 kpc) supergiants to study magnetically driven activity (plages, starspots, convection), imaging accretion disks around nascent stars, and resolving the regions around the central engines of Active Galactic Nuclei (AGN).”
Imaging the surfaces of stars gives clues to activities deep inside. If those stars are similar to the Sun (i.e. main-sequence stars), that would give deeper insight into what our nearest star is doing. AeSI observations will also help scientists understand the dynamo activity that drives the magnetic activity of the Sun and other stars, according to Carpenter. “Our proposed primary study of Sun-like stars uses a combination of high spatial resolution stellar imaging to observe the cyclical time evolution of surface manifestations of magnetic activity and high time and spatial resolution asteroseismology to probe the interior structure of the star, to obtain the information necessary to build truly predictive models of solar/stellar magnetic activity,” he said.
Delving into Stars from the Moon
Let’s take a look at a quick summary of the AeSI’s possible targets. It could study such main-sequence stars such as alpha Centauri A, Procyon A, Sirius A, and epsilon Eridani to gather details of their surface activities and the magnetic activity that drives them. That interferometric data could then be coupled to spatially-resolved asteroseismology studies to give more accurate insight into exactly what’s happening inside those stars. In addition, it could help scientists understand how stellar activity affects the existence and habitability of their planets.
Beyond understanding what happens to those stars (and implications for the Sun), interferometry studies would also have immediate applicability toward forecasting solar activity and its impact here on Earth. AeSI would provide high spatial- and temporal-resolution imaging capabilities, which would give us a look at stellar surfaces and how they vary through a magnetic cycle. Scientists would be able to “see” magnetically driven activity such as starspots (similar to sunspots), hot plages, and convection activity. Active regions on the Sun and other stars are very bright. On the Sun, they dominate wavelengths of light most important for predicting the impact of the Sun’s activity on its surround planets, including the Earth.
Simulations of AeSI observations of stars and the hearts of AGN. Courtesy NASA.
Studying More Complex and Distant Objects
The AeSI installation on the Moon would also provide highly detailed looks at accretion disks around other stars. These regions can be challenging to observe in high detail. That’s because they’re often tough to separate out from their star. Supernovae are another known target, particularly the ejecta from the catastrophic explosions that end the lives of supermassive stars. AeSI may help astronomers detect the expanding debris clouds during the earliest stages of a supernova outflow.
AeSI should also be able to image the complex events occurring in active galactic nuclei. In particular, AGN winds appear to exist around most of these objects. Their velocities and the amount of mass loss carry clues to the structure of the object at the heart of the galaxy. AeSI measurements of these regions may also contribute to more accurate distance measures to such objects (quasars) and help measure the cosmological constant. Such studies will need the capability of an expanded AeSI array, said Carpenter. “Because of the distance of even the brightest AGN’s, we would need large outer array diameters to be able to resolve the regions around the central engines, which are likely the only portions bright enough to be successfully detected by AeSI,” he explained. “We are exploring ways of increasing the UV sensitivity of AeSI by potentially using mirror coatings with higher UV reflectivity than currently possible, improved UV detectors, and perhaps larger mirror elements. These improvements would dramatically improve our ability to study a broader sample of AGNs and more portions of individual ones.”
Implementation of the AeSI
The basic mission design for AeSI depends on deployment by astronauts and/or robots during the upcoming Artemis missions. Each element in the array will be a one-meter telescope deployed on a small rover. The array will expand or contract as needed for specific observations. Data from the array will be collected by a central beam-combining “hub” and reconstructed to create images of its target stars or other objects.
Artist rendition of one of the primary mirror elements directing a beam to the central hub. (Credit: Britt Griswold)
The Moon presents a very good, stable environment for AeSI. It has no atmosphere to muddle the view for the telescopes, which means adaptive optics aren’t needed to correct for air movement. This also means the interferometer can operate at much shorter wavelengths than any Earth-based array. Two challenges to be considered (aside from the delivery of the telescopes and supporting hardware and the actual construction process) are dust and seismic motion during moonquakes. These can be dealt with, however.
Now We Wait for Artemis
If this mission concept is chosen for implementation by NASA, the biggest question will be: when and where will it be deployed? It all depends on the progress of the Artemis campaign and the capabilities it might supply for neighboring observatories. Currently, the first crewed mission won’t take place until spring 2026 (at the earliest). Followup flights will lay more infrastructure in place, and the cadence for those flights remains unknown. So, realistically speaking, the earliest AeSI could be implemented would be the late 2030s or early 2040s.
As to where the interferometer will be deployed, the team suggests several lunar south pole locations, preferably near where prior Artemis infrastructure is built, to enable easy access by Artemis astronauts or robots. However, the possibility of locating at more distant, lower latitudes is also of interest if Artemis could support that since it would enable observations of more of the sky. The next steps for the AeSI team are to do more R&D on the technology required for the interferometer and to continue to explore additional science investigations it can be adapted to support.
For More Information
NASA Innovative Advanced Concepts Phase I Final Report — A Lunar Long-Baseline UV/Optical Imaging Interferometer: Artemis-enabled Stellar Imager (AeSI)
AeSI – Artemis enabled Stellar Imager
A Lunar Long-Baseline Optical Imaging Interferometer: Artemis-enabled Stellar Imager (AeSI)