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Beyond Sgr A* and M87*: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry

This paper proposes that lunar-based extremely long baseline interferometry (VLBI) could achieve sub-microarcsecond resolution at 230 GHz, enabling the detection of black hole shadows and photon rings for a significantly larger sample of supermassive black holes beyond the current capabilities of the Event Horizon Telescope.

Original authors: Shan-Shan Zhao, Ru-Sen Lu, Lei Liu, Zhiqiang Shen, Yosuke Mizuno

Published 2026-06-19

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CC BY 4.0

Original authors: Shan-Shan Zhao, Ru-Sen Lu, Lei Liu, Zhiqiang Shen, Yosuke Mizuno

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Idea: Building a "Super-Eye" on the Moon

Imagine you are trying to read the fine print on a coin from a mile away. The Event Horizon Telescope (EHT) is like a giant pair of glasses made by connecting radio telescopes all over Earth. It has successfully taken pictures of the "shadows" (the dark silhouettes) of two massive black holes: M87* and Sagittarius A* (Sgr A*) at the center of our galaxy.

However, Earth is too small. Even with all its telescopes connected, our "glasses" can only see those two specific black holes clearly. To see more, we need a bigger lens.

This paper proposes a bold new idea: Put a telescope on the Moon.

By connecting a telescope on the Moon with the telescopes on Earth, we create a "baseline" (the distance between the two eyes) that is 30 times wider than the Earth itself. This would act like a super-magnifying glass, allowing us to see the shadows of 31 different black holes across the universe, not just the two we can see now.

How It Works: The "Moon-Earth" Team

The authors simulated what would happen if we built a radio telescope on the Moon and pointed it at these 31 candidates.

1. The Location Matters (The "Viewing Deck")
Just like being on a balcony vs. a basement, where you put the telescope on the Moon changes what you can see.

The team tested five spots, including the Apollo 11 landing site and the South Pole.
They found that the best spot is the lunar antipode (the point on the Moon farthest from Earth). From there, the telescope can see almost all the target black holes for a good amount of time. Other spots would block the view of many targets because the Moon's surface gets in the way.

2. The Size Matters (The "Dish Size")
The telescope needs to be big enough to catch the faint signals. The team tested four sizes: 5 meters, 10 meters, 20 meters, and 40 meters (about the size of a small house to a large apartment building).

The Goal: To see the "shadow" of a black hole, you need to catch a specific signal pattern called a "null" (a dip in the signal). Think of it like listening to a song; if you can hear the silence between the notes, you know the song is there.
The Result: They found 6 specific black holes that are "shadow-detectable."
- M104 (The Sombrero Galaxy): This is the easiest target. Even a small 5-meter telescope on the Moon could see its shadow.
- NGC 524 & PGC 049940: These need a 10-meter telescope.
- NGC 5077: Needs a 20-meter telescope.
- NGC 5252: This one is tricky and faint; it needs a massive 40-meter telescope.
- NGC 1052: Despite being small, it's very bright, so a 5-meter telescope works.

3. The "Photon Ring" Bonus
Black holes don't just cast a shadow; they have a glowing ring of light around them. Some of this light has orbited the black hole multiple times before escaping, creating "echoes" or inner rings (called photon rings).

The paper suggests that if we fill in the gaps between the Moon and Earth with a few extra space satellites, we could see the second inner ring (the $n=2$ photon ring) of our own galaxy's black hole (Sgr A*) and M87*.
This would be like seeing the ripples inside a ripple, testing Einstein's theory of gravity with extreme precision.

Why This Is a Big Deal

Expanding the Family: Currently, we only have pictures of two black holes. This setup could give us a "family album" of 6 new black hole shadows, helping us understand if they all behave the same way.
Testing Gravity: Seeing these shadows and rings allows scientists to test Einstein's General Relativity in the strongest gravity environments in the universe.
Feasibility: The paper argues this isn't just science fiction. With upcoming lunar missions (like China's Chang'e program and the International Lunar Research Station), building a telescope on the Moon is becoming a realistic engineering goal.

The Catch

The paper admits that this is a "first step." It assumes ideal conditions. In reality, the Moon's surface is rough, and the signals are weak. Also, because the Moon orbits Earth, the view changes constantly, which makes the "picture" a bit blurry in some directions. To get a perfect, round picture of all these black holes, we might eventually need more than just one telescope on the Moon—perhaps a whole fleet of satellites.

In short: By moving a telescope to the Moon, we can turn our current "standard definition" view of black holes into "4K Ultra HD," revealing the shadows of dozens of cosmic giants that were previously too small to see.

Technical Summary: Beyond Sgr A and M87: Sub-Microarcsecond Black Hole Shadow Detection via Lunar-based Extremely Long Baseline Interferometry**

Problem Statement
The Event Horizon Telescope (EHT), a global 1.3 mm (230 GHz) very long baseline interferometry (VLBI) array, has successfully imaged the black hole shadows of M87* and Sgr A*. However, its angular resolution is fundamentally limited by Earth's diameter ( $D_\oplus$ ), restricting high-fidelity shadow imaging to only these two sources. While next-generation ground arrays (ngEHT) operating at 345 GHz may improve resolution to $\sim15$ $\mu$ as, atmospheric opacity prevents further frequency increases. Achieving sub-microarcsecond resolution ( $\sim0.7$ $\mu$ as) requires baselines extending far beyond Earth, such as the Earth-Moon distance ( $\sim30.1 D_\oplus$ ). Although space VLBI concepts exist (e.g., BHEX, Millimetron), the specific feasibility of using a lunar-based telescope to detect black hole shadows for a broader sample of supermassive black holes (SMBHs) has not been quantitatively assessed.

Methodology
The authors perform a quantitative assessment of shadow detectability for 31 SMBH candidates with predicted large angular sizes. The study utilizes the following methodological framework:

Target Selection: 31 candidates were selected based on predicted shadow diameters ( $\theta$ ) larger than the theoretical Moon-Earth beam size ( $\theta_{beam} \approx 0.7$ $\mu$ as at 230 GHz). Sources were drawn from the ETHER database and recent flux density detections at 230 GHz.
Lunar Site and Telescope Assumptions: Five potential lunar sites were evaluated (including the lunar antipode, Apollo 11, Chang'E-5/6 sites, and the South Pole). The lunar antipode (S1) was selected as the optimal location because it provides an observable window for all 31 candidates, whereas polar sites suffer from hemispheric visibility constraints. The study assumes a lunar-based telescope operating at 230 GHz with variable antenna diameters (5 m, 10 m, 20 m, 40 m), a system temperature ( $T_{sys}$ ) of 100 K, and 50% aperture efficiency.
Baseline Coverage Simulation: Using the OmniUV toolkit, the authors simulated $(u, v)$ coverage for a Moon-Earth array (combining the 2025 EHT array with the lunar telescope at S1) over one lunar sidereal month. Constraints included a $15^\circ$ minimum elevation angle and a $5^\circ$ separation from solar/lunar limbs.
Detectability Criteria: Detection was analyzed in the visibility domain using a geometric ring model. Two primary criteria were applied:
1. Resolvability: The first null in the visibility amplitude profile (a key signature of a ring structure) must lie within the range of projected Moon-Earth baselines.
2. Sensitivity: The secondary peak of the visibility amplitude (post-null) must exceed 10% of its value and remain above the baseline sensitivity threshold (defined as $7\sigma$ of thermal noise).
Advanced Modeling: For the most promising candidate (M104), the authors verified the geometric ring results using General Relativistic Magnetohydrodynamic (GRMHD) simulations to generate synthetic visibility data. Additionally, a separate analysis assessed the detectability of higher-order photon rings ( $n=1, n=2$ ) under the assumption that space telescopes could fill $(u, v)$ coverage gaps.

Key Results

Shadow Detection Candidates: Under the strict criteria of resolvability and sensitivity, six sources were identified as detectable for Moon-Earth VLBI: M104, NGC 524, PGC 049940, NGC 5077, NGC 5252, and NGC 1052.
- M104 is the most favorable target, with a large shadow size ( $\sim10$ $\mu$ as) and high flux density, making it detectable even with a 5 m lunar telescope.
- NGC 524, PGC 049940, and NGC 5077 require 20 m, 10 m, and 5 m antennas, respectively.
- NGC 5252 and NGC 1052 have smaller shadows ( $\sim1$ $\mu$ as); NGC 1052 is detectable with a 5 m telescope due to high flux, while NGC 5252 requires a 40 m telescope.
- Notably, M87* and Sgr A* do not meet the resolvability criterion for their primary shadow (first null) with Moon-Earth baselines alone, as their first nulls occur at baselines shorter than the minimum projected Moon-Earth baseline.
Photon Ring Detection: Assuming gaps in $(u, v)$ $(u, v)$ coverage are filled by space telescopes:
- The $n=2$ photon ring region is detectable for Sgr A and M87** using a 10 m lunar telescope.
- The $n=1$ photon ring region is detectable for 12 candidates using lunar telescopes up to 40 m. Five of these (including M104) require only a 5 m telescope.
Site Sensitivity: The study highlights that the lunar antipode (S1) is critical for maximizing the number of observable targets, as other sites (like the South Pole) render northern sources unobservable.

Significance and Claims
The paper claims to provide the first quantitative scientific and technical motivation for deploying lunar-based telescopes specifically for black hole shadow studies. The results demonstrate that:

Extension of Sample: Moon-Earth VLBI can extend shadow detection beyond the current two targets (M87* and Sgr A*) to a diverse sample of SMBHs, addressing the observational gap for black holes with masses between $10^6$ and $10^9 M_\odot$ .
Technological Pathway: The study validates the feasibility of using lunar infrastructure (motivated by missions like Chang'E-7/LOVEX and the International Lunar Research Station) to achieve sub-microarcsecond resolution.
Stepwise Progression: The work outlines a progression in VLBI capability: from ground-based arrays (horizon scale) to Earth-orbital baselines (photon ring $n=1$ ) and finally to Moon-Earth baselines (photon ring $n=2$ and diverse SMBH shadows).

The authors maintain a modest tone, noting that their results rely on idealized ring models and that real-world factors (source asymmetry, variability, and incomplete $(u, v)$ coverage) pose challenges. They explicitly state that detailed observing strategies, technical implementation (e.g., data downlink, dish synthesis), and optimization of space telescope configurations for denser coverage are left for future work.

The Big Idea: Building a "Super-Eye" on the Moon

How It Works: The "Moon-Earth" Team

Why This Is a Big Deal

The Catch

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