Simultaneous Double Transits of Phobos and Deimos as Seen from the Martian Surface: A Millennium Catalogue

This paper presents the first systematic catalogue of simultaneous solar transits of Phobos and Deimos from the Martian surface between 1600 and 2600 CE, identifying 8,565 grazing, 49 partial, and 17 full double transit events that are geometrically constrained to the Martian equator and equinoxes, while providing precise predictions for future occurrences and updated uncertainty estimates pending the JAXA MMX mission.

The Gist

Imagine standing on the red sands of Mars, looking up at the Sun. Usually, you might see one of Mars' two tiny moons, Phobos or Deimos, drift across the face of the Sun like a dark speck. This is called a "transit."

But what if both moons crossed the Sun at the exact same time?

That is the question Samuel Cody answers in this new paper. He has created the first-ever "schedule" (or catalogue) for this rare cosmic event, looking 1,000 years into the past and future (from 1600 to 2600).

Here is the simple breakdown of what he found, using some everyday analogies.

1. The "Double-Decker" Bus Analogy

Think of the Sun as a giant, bright billboard.

• Phobos is like a large, fast-moving delivery truck. It zooms across the billboard in about 25 seconds.
• Deimos is like a tiny, slow-moving bicycle. It takes a couple of minutes to cross the same billboard.

Usually, the truck and the bike cross the billboard at different times. Sometimes the truck is there, and the bike is far away. Sometimes the bike is there, and the truck has already left.

Cody's paper asks: When do the truck and the bike cross the billboard at the exact same moment?

2. The Three Types of "Traffic Jams"

Cody found 8,631 times when the two moons got close to crossing together, but he sorted them into three categories based on how well they lined up:

• The "Near-Miss" (8,565 times): Imagine the truck and the bike are both heading toward the billboard, but they miss each other by a hair's breadth. To a person standing on Mars, one moon is clearly on the Sun, while the other is just barely missing it, hiding behind the Sun's bright edge. It looks like a single transit.
• The "Partial Overlap" (49 times): This is like the truck and the bike both getting on the billboard, but the bike is stuck halfway off the edge. Both are visible, but one is cut off. This is the next one we can see!
• The "Full Double Transit" (17 times): This is the "Holy Grail." Both the truck and the bike are fully on the billboard, floating in the middle of the Sun's face with room to spare. This is the most spectacular view possible.

3. The "Equator Rule"

You can't see this show from just anywhere on Mars.

• The Latitude Limit: Imagine Mars is a spinning top. The two moons orbit very close to the top's equator. If you stand too far north or south (above 13 degrees latitude), the moons will never line up with the Sun for you. It's like trying to watch a play on a stage from the back of the balcony; the actors are too far to the side for you to see them together.
• The Seasonal Rule: These events only happen around the Martian "spring" and "autumn" (equinoxes). Why? Because that's when the Sun is directly over the equator, making it easiest for the two moons to line up with it.

4. When Can We Watch?

Cody didn't just do the math; he gave us a watchlist for the future:

• The Next Event (April 17, 2034): This is a "Partial Overlap." If you were standing in a specific spot in the Elysium Planitia (a flat region on Mars), you would see Phobos crossing the Sun while Deimos just barely clips the edge.
  • Catch: No human or robot is currently there to see it.
• The "Postcard" Event (November 20, 2118): This is the big one. Both moons will be fully inside the Sun's disk, with a gap of only 12 seconds between them.
  • The View: You would see a large, potato-shaped shadow (Phobos) and a tiny dot (Deimos) floating side-by-side against the bright Sun.
  • The Location: This will happen near a famous volcanic plateau called Syrtis Major.

5. Why Is This Hard to Predict?

Predicting where exactly to stand on Mars is tricky because the moons are slowly changing their orbits due to gravity (tidal forces).

• The "Drift": Think of the moons as runners on a track. We know where they are now, but over 100 years, they might drift a few kilometers off their expected path.
• The Solution: The paper notes that a Japanese mission called MMX (launching around 2026) will visit Phobos, take samples, and measure its position with extreme precision. Once that data comes back, our predictions for the 2118 event will become as accurate as knowing exactly where a train will stop at a station.

Summary

This paper is essentially a 1,000-year movie schedule for a rare cosmic show on Mars.

• The Show: Two moons crossing the Sun at once.
• The Audience: You have to be standing near the Martian equator.
• The Timing: You have to wait for the Martian spring or autumn.
• The Highlight: The show happens very rarely (about once every 60 years), but when it does, it's a breathtaking sight of two shadows dancing on the face of the Sun.

If you are a human explorer planning a trip to Mars in the 22nd century, you might want to pack your camera for November 20, 2118.

Technical Summary

Here is a detailed technical summary of the paper "Simultaneous Double Transits of Phobos and Deimos as Seen from the Martian Surface: A Millennium Catalogue" by Samuel Cody.

1. Problem Statement

While single solar transits of Phobos and Deimos are well-documented phenomena observed by NASA missions (e.g., Viking, Spirit, Opportunity, Curiosity, Perseverance), the possibility of simultaneous double transits—where both moons cross the solar disc at the exact same instant from a single surface location—had not been systematically investigated.

The geometric constraints make this event highly improbable due to:

• Different Orbital Periods: Phobos ($\sim7.66$ h) and Deimos ($\sim30.3$ h) have vastly different orbital speeds.
• Opposite Shadow Directions: Phobos moves faster than Mars rotates (eastward shadow), while Deimos moves slower (westward shadow).
• Orbital Inclinations: The moons orbit at different inclinations relative to the Martian equator ($1.08^\circ$and$2.68^\circ$).
• Observer Parallax: Surface observers see the moons at different apparent positions compared to the Martian center due to the moons' varying distances from the surface.

The paper aims to determine if such an alignment is geometrically possible, how often it occurs, where it can be observed, and to provide a predictive catalogue for future robotic and human exploration.

2. Methodology

The author conducted a systematic search over a millennium (1600–2600 CE) using the following tools and algorithms:

• Ephemeris Data: The study utilized the JPL mar099.bsp ephemeris (Brozović et al. 2025), which incorporates over a century of astrometric data, including Mars Express, MRO, and Perseverance rover data. This supersedes the previous standard (mar097).
• Software: Computations were performed using the SPICE toolkit via the spiceypy Python binding.
• Search Algorithm (Three Phases):
  1. Conjunction Timing: Identified all Deimos superior conjunctions (angular separation from Sun $< 0.192^\circ$) and searched for Phobos conjunctions within $\pm30$ minutes. This yielded $\sim10^5$ candidate pairs.
  2. Shadow Geometry: Used ray-tracing to calculate the exact shadow-centre latitude/longitude on the Martian reference ellipsoid for each candidate.
  3. Overlap Classification: Classified events based on the latitude separation ($\Delta\phi$) of the shadow tracks relative to the track widths:
     • Edge-only (E): Near-miss events where tracks barely touch (8,565 events).
     • Partial Overlap (P): Both moons on the disc, but at least one is cut off by the solar limb (49 events).
     • Full Overlap (F): Both moons lie entirely within the solar disc simultaneously (17 events).
• Validation: Near-term predictions were cross-verified using the JPL Horizons online ephemeris service.

3. Key Contributions

• First Systematic Catalogue: The paper presents the first comprehensive list of simultaneous double transits for the period 1600–2600 CE.
• Theoretical Latitude Limit: The author derived a hard theoretical limit for observer latitude. Due to differential parallax, simultaneous double transits are geometrically impossible for observers located beyond $\pm13.1^\circ$ latitude.
• Uncertainty Modeling: The paper provides a rigorous error budget based on the covariance model of Brozović et al. (2025), quantifying how position uncertainties grow over time (from $\sim1$ km near-term to $\sim600$ km at the millennium boundaries).
• MMX Mission Impact: The study highlights that the upcoming JAXA MMX mission (launch 2026, sample return $\sim2031$) will drastically reduce tidal acceleration uncertainties, effectively resetting the error growth for all post-2030 predictions.

4. Key Results

• Event Counts (1600–2600 CE):
  • 8,565 Near-miss (Edge-only) events.
  • 49 Partial-overlap events.
  • 17 Full-overlap events (both moons fully inside the solar disc).
• Seasonal Clustering: All events cluster tightly around the Martian equinoxes ($L_s \approx 0^\circ$ or $180^\circ$). This is geometrically necessary because the sub-solar point must be near the equator to minimize the latitude gap between the two shadow tracks.
• Geographic Distribution: All observed events occur within $\pm9^\circ$ of the equator.
• Notable Future Events:
  • Next Partial Double Transit: April 17, 2034. An observer at $(-0.92^\circ, 160.9^\circ \text{E})$ will see Phobos fully on the disc while Deimos clips the limb. Uncertainty is low ($\sim2$ km).
  • Next Full Double Transit: November 20, 2118. This is the only full double transit in the next century. An observer at $(+0.90^\circ, 62.1^\circ \text{E})$ (Syrtis Major Planum) will see both moons fully framed within the solar disc with a conjunction gap of only 12 seconds.
• Observability: None of the predicted events coincide with current or planned surface mission locations. However, the 2034 event is within the traverse capability of current rovers (e.g., Curiosity, Perseverance), while the 2118 event would require specific mission planning.

5. Significance

• Scientific Value: These events serve as high-precision astrometric calibration points for lunar ephemerides and provide constraints on the tidal evolution of the Mars system.
• Exploration Planning: The catalogue provides critical data for future robotic and human missions. While the events are rare (approx. once every 60 years for a full double transit globally), knowing the precise location and time allows for targeted observation campaigns.
• Cultural/Visual Impact: The paper notes the visual spectacle of such an event, referencing its potential depiction in science fiction (e.g., Dune). The 2118 event is described as a "postcard" configuration, offering a unique view of two dark silhouettes against the solar photosphere.
• Methodological Benchmark: The work demonstrates the utility of modern ephemerides (mar099) and SPICE tools in predicting complex multi-body alignment phenomena over long timescales, establishing a framework for similar studies of other planetary systems.

In summary, this paper transforms a theoretical curiosity into a quantifiable, predictable phenomenon, providing a roadmap for observing one of the most rare and visually striking events in the Martian sky.