Optical interferometry has been a long-proven science method that involves using several separate telescopes to act as one big telescope, thus achieving more accurate data as opposed to each telescope working individually. However, the Earth’s chaotic atmosphere often makes achieving ground-based science difficult, but what if we could do it on the Moon? This is what a recent study presented at the SPIE Astronomical Telescopes + Instrumentation 2024 hopes to address as a team of researchers propose MoonLITE (Lunar InTerferometry Explorer) as part of the NASA Astrophysics Pioneers program. This also comes after this same team of researchers recently proposed the Big Fringe Telescope (BFT), which is a 2.2-kilometer interferometer telescope to be built on the Earth with the goal of observing bright stars.

Here, Universe Today discusses MoonLITE with Dr. Gerard van Belle, who is an astronomer at the Lowell Observatory in Flagstaff, Arizona, regarding the motivation behind proposing MoonLITE, the science they hope to achieve, lunar surface location preference, the cost of MoonLITE, and next steps to make MoonLITE a reality. Therefore, what is the motivation behind proposing MoonLITE?

“The real barrier to doing super sensitive high resolution optical interferometry is the Earth’s atmosphere,” Dr. van Belle tells Universe Today. “It’s a boiling, turbulent medium that means the exposure time of your telescope is ultimately limited to less than a millisecond or so. Telescopes that expose longer than that can achieve greater sensitivity, but at the expense of angular resolution – things smear out. MoonLITE, with two-inch (50mm) apertures, would be more than a thousand times more sensitive than terrestrial apertures is 8-meter collecting apertures, because it can stare for many minutes at a time. In comparison to millisecond exposure times on the Earth, the amount of light grabbed by these tiny dime-store sized telescopes exceeds giant industrial facility telescopes within the first second of having the shutter open.”

Much like with the recently proposed BFT, MoonLITE has a number of scientific objectives it hopes to accomplish, as the study notes three science cases, including studying the radii of low-mass stars (M-dwarfs) and brown dwarfs, young stellar objects (YSOs), and active galactic nuclei (AGN). For the M-dwarfs and brown dwarfs, the team aspires to obtain long-sought data regarding their sizes and temperatures since observing them from ground-based telescopes has proven difficult.

For YSOs, the researchers hope to gain greater understanding of the formation and evolution of habitable exoplanets within the protoplanetary disks of pre-main sequence stars, with MoonLITE being capable of ascertaining the inner regions of these stars and the star sizes, as well. For AGNs, the researchers aspire to learn more about supermassive black holes, and specifically the jets that emanate from them, with MoonLITE being able to observe these objects in optical wavelengths for the first time. But what else can we learn from these three science cases?

“So, we actually have more science cases than that – so many, in fact, that we realized the new capabilities of MoonLITE were beyond our collective imagination for covering all the bases,” Dr. van Belle tells Universe Today. “So, we built into the program a 20% slice of the overall observing time to put up for competitive selection by the community – you know, crowdsource for the really creative ideas. The three we wrote up were just what we felt highlighted what one could do with greater sensitivity from the Earth’s surface. For example, the stars that are the smallest – 10% the size of our sun – are also the faintest. And measuring the sizes of those is out of reach of terrestrial interferometers. Same for YSOs and AGNs – there’s a few that can be done from Earth, but for more general samples – ones that represent the more typical objects, not the super-bright oddballs – you need lots of sensitivity.”

One of the unique aspects of MoonLITE is it could be brought to the lunar surface via NASA’s Commercial Lunar Payload Services (CLPS), which is a collaboration with the private sector to deliver scientific and technical payloads to the Moon to test technologies that can help with both human missions as part of the Artemis Program, and scientific missions to further our understanding of the universe, like MoonLITE. Examples of companies participating in upcoming CLPS missions include Intuitive Machines, Astrobiotic, Firefly Aerospace, and Draper, all of which are delivering payloads to various locations on the lunar surface. But is there a specific location where MoonLITE would work best?

“We designed MoonLITE to be entirely site agnostic,” Dr. van Belle tells Universe Today. “For a small experiment like this, it’s going to catch a ride on board a NASA CLPS lander as a minor guest – and putting a minimal number of requirements on your ride improves one’s chances of getting a ride assignment. So polar or equatorial latitudes both work, as well as nearside versus farside.”

As noted, this same team of researchers recently proposed the Big Fringe Telescope, which is slated to be a 2.2-kilometer interferometer telescope comprised of 16 smaller telescopes that are 0.5-meters in diameter. Along with conducting cutting-edge science, including observing binary star systems and making exoplanet transit “movies”, one of the most notable features of the BFT is its extremely low cost compared to current optical interferometers around the world, coming with an approximate price tag of $28,496,000.

In contrast, the cost of the European Southern Observatory’s Very Large Telescope Interferometer (VLTI), which is comprised of four 8.2-meter telescopes and four movable 1.8-meter telescopes, has been estimated in the hundreds of millions of dollars. Therefore, what is the potential cost for MoonLITE compared to other Earth-based interferometers?

“MoonLITE was designed to work within the cost box for the NASA Pioneers call for proposals,” Dr. van Belle tells Universe Today. “This CfP [Call for Projects] stipulates a couple of things: a $20M cost cap, including a 25% uncommitted reserve, so the actual budgeted level of activities and hardware was $15M. The CfP does let you request some things – first off, a CLPS ride, though you have to then fit within the CLPS mass cap of 50kg. The notional CLPS lander in the CfP was to provide some other things as well – power, communications, mobility with a rover. So, there’s actually quite a bit of in-kind support wrapped up in that CLPS ride.”

Submitting a proposal to NASA is a very in-depth process involving several steps, also known as phases, resulting in a very small acceptance rate, often with several rejections and improvements before being accepted. These proposals range from CubeSats to full-blown, multi-billion-dollar space missions, with most taking years to become real-world missions even after selection, if at all. For example, of the four proposals selected for further development in January 2021 Astrophysics Pioneers Program, (Aspera, Pandora, StarBurst, and PUEO), only two of them have definitive launch dates (StarBurst in 2027 and PUEO in 2025). Therefore, if MoonLITE is to be selected for advancement, it could be years, or even decades, before it officially lands on the lunar surface to conduct science. Unfortunately, while Dr. van Bells says the 2024 Pioneers proposal term was canceled due to federal budget issues, what are the next steps to make MoonLITE a reality?

“We submitted for the 2023 NASA Pioneers call and got turned down,” Dr. van Belle tells Universe Today. “But we got a good review and have been encouraged to improve things, address perceived issues, and resubmit. We’re trying to reduce risk by doing some lab and ground-based tests. This is another nice element of MoonLITE – we can just build a representative system on the ground and test it straight up here. We don’t get the exquisite sensitivity like we would on the moon, but otherwise it’ll work just the same – we just need to look at bright things here from Earth. So, we’re keen to address some of these issues from the review panel and resubmit for 2025.” 

As NASA prepares to send humans back to the Moon with the Artemis Program for the first time since 1972, the level of science that can be achieved on the lunar surface is unprecedented. This is specifically evident given the lack of a Moon’s atmosphere, allowing for more accurate data to be obtained and potentially providing scientists with a greater understanding of our universe, and our place in it. With MoonLITE, scientists hope to gain insight into low-mass stars and brown dwarfs, young stellar objects, and active galactic nuclei from potentially anywhere on the lunar surface, allowing for greater diversity is site selection and what celestial objects can be observed.

Dr. van Belle concludes by telling Universe Today, “MoonLITE is super exciting, not just because it’s a really high-impact experiment in a remarkably affordable package – but because it will show the whole approach works and can be taken much, much further. As an example, high precision astrometry from a lunar interferometer could characterize the masses of terrestrial-scale extrasolar planets. Mass measures are needed in advance of the Habitable Worlds Observatory of the 2040’s, to understand the spectral HWO will get, and disentangle those spectra for signs of life.”

How will MoonLITE contribute to optical interferometry on the lunar surface in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!