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

TL;DR

This paper compiles a comprehensive catalogue of all known simultaneous transits of Phobos and Deimos from Mars over a millennium, analyzing their patterns, predicting future events, and establishing geometric limits for such transits.

Contribution

It provides the first systematic catalogue of simultaneous Martian moon transits, including detailed predictions and geometric constraints, spanning from 1600 to 2600 CE.

Findings

8565 grazing double transits identified

49 partial-overlap double transits observed

17 full double transits documented

Abstract

I present the first systematic catalogue of simultaneous solar transits of both Phobos and Deimos as observed from the surface of Mars. Using the JPL mar099 ephemeris (Brozovic et al. 2025) and SPICE toolkit, I searched the millennium 1600-2600 CE for epochs at which both Martian moons project onto the solar disc at the same instant for at least one surface location. I identify 8565 grazing double transits, 49 partial-overlap double transits (both moons simultaneously on the solar disc, with at least one partially cut off by the limb), and 17 full double transits in which both moons lie wholly within the solar disc at the same moment. All events cluster tightly around the Martian equinoxes (Ls ~ 0 deg or 180 deg) and within +/- 9 deg of the equator, reflecting the near-equatorial orbital inclinations of both moons. I derive a hard theoretical latitude limit of +/- 13.1 deg beyond which simultaneous double transits are geometrically impossible. The next observable partial double transit, excluding less prominent grazing events, is predicted for 2034 April 17. The next full double transit, with both moons wholly inside the solar disc, with a gap between each silhouette and the solar limb, occurs on 2118 November 20. The geometries for both these events were confirmed with JPL Horizons. I provide uncertainty estimates based on the Brozovic et al. covariance model, with predicted position errors growing from ~1 km for near-term events to ~600 km at the catalogue boundaries, and note that the JAXA MMX mission (~2031) will dramatically reduce uncertainties for all post-2030 predictions.