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The primary challenge of rocket propulsion is the burden of needing to accelerate the spacecraft’s own fuel, resulting in only a logarithmic gain in maximum speed as propellant is added to the spacecraft. Light sails offer an attractive alternative in which fuel is not carried by the spacecraft, with acceleration being provided by an external source of light. By artificially illuminating the spacecraft with beamed radiation, speeds are only limited by the area of the sail, heat resistance of its material, and power use of the accelerating apparatus.
A 2015 paper that I published here with my brilliant postdoc, James Guillochon, showed that leakage from a light sail propulsion apparatus in operation around a solar system analog would produce a detectable signal. For cargo transportation across planetary systems, we found the optimal beam frequency to be on the order of tens of gigahertz. Leakage from these beams yields radio transients for an observer viewing the source and destination exoplanets in conjunction on the sky. Because most travel within a planetary system would be conducted between the habitable worlds within that system, multiply transiting exoplanetary systems (like the seven planets in the TRAPPIST-1 system) offer the greatest chance of detection, especially when the exoplanets are in projected conjunction as viewed from Earth. We concluded that if interplanetary travel via beam-driven light sails is commonly employed in our galaxy, this activity could be revealed through a dedicated search for transiting exoplanetary systems.
Our new paper, available here, improves the previous calculation and extends it to optical and near-infrared wavelengths. We find that an Earth–Mars analog at a micrometer wavelength needs an aperture of only 20–100 meters rather than 1.5 kilometers in the radio band. In this case, the leaked signal falls in the wavelength bands of the Roman, Rubin and Euclid space telescopes. In particular, the Roman’s Galactic Bulge Time Domain Survey (GBTDS) would observe a pointed flash from anywhere in the Galaxy. If optical beam-driven light sails are commonly employed, this activity could be revealed by a matched search of Roman, Rubin and Euclid at no added cost. Photometry detects candidates, but the generic techno-signature of two or three consecutive flare detections at the same wavelength identifies uniquely a beamer.
Travel time between habitable worlds by chemical rockets is long, about a couple of years for an Earth–Mars analog, and the rocket equation requires an exponential penalty in fuel mass for every increment in speed. On the other hand, beam-driven light sails need no propellant, with the achievable speed set by the sail area, the thermal tolerance of its material, and the power of the driving array. The required beam powers are large, terawatts for ton-class payloads at an acceleration equal to the Earth’s surface gravity, 1 g=9.8 meters per second squared. The leakage of radiation past the sail is a techno-signature with a well-posed physical model. In our 2015 paper, we constructed that model for a microwave array conducting Earth–Mars transits, and found that the optimal frequency for a 1.5 km aperture lies near 68 GHz, producing transients with a radio spectral flux of of a few Jansky from a distance of 300 light years, lasting tens of seconds. The possible association of beams that drive a relativistic interstellar sail with some of the observed population of fast radio bursts, was proposed in a 2017 paper here that I co-authored with my postdoc then, Manasvi Lingam.
The same optimality argument that sets the beam frequency also sets the array diameter of the beamer, and at optical wavelengths the required aperture collapses from kilometers to tens of meters. Directed-energy concepts for gram-scale to ton-scale sails have been developed on Earth at optical wavelengths. A civilization minimizing capital cost may well land in the optical or near-infrared.
Three survey facilities now cover that window. The Nancy Grace Roman Space Telescope was launched on August 30, 2026 and is in a three-month commissioning period, with first images expected in early 2027. The Vera C. Rubin Observatory began the ten-year Legacy Survey of Space and Time (LSST) on June 29, 2026. The Euclid space observatory is midway through a 14,000 square degree survey. Between them these facilities cover the wavelength band of 0.3 to 2.3 micrometers and will accumulate of order a quadrillion (10^{15}) star-seconds of monitoring.
The beamer signal these new observatories could discover is not a nanosecond-to-second pulse and not a narrow spectral line, but a tens-of-seconds, spatially unresolved, single-band flash at the position of a star. Our new paper here presents the optical and near-infrared branch of the light-sail leakage transients.
The leaked brightness profile around the shadow cast by the light sail shows a periodic, so-called Kirchhoff pattern of peaks, as a function of the angle between the observer’s line-of-sight and the axis connecting the beamer array with the center of the sail mask. The profile shows a Poisson–Arago core on axis, equal in peak to the second Airy ring of the unocculted source. This core is a bright point of light that appears at the exact center of the shadow cast by a perfectly circular opaque object when illuminated by a coherent light source.
High-cadence archival surveys and vanishing-source searches have already sampled second-to-minute optical transients over large sky areas, but none has the bulge column of stars, the star-coincidence requirement, and the tens-of-seconds ramp sampling of Roman’s GBTDS. If optical beam-driven light sails are commonly employed in our galaxy, this activity could be revealed by combined search by the new Roman, Rubin and Euclid space telescopes of up to a billion stars, at no additional observing cost. Nearby multiply-transiting systems near projected conjunction would provide candidate targets for follow-up dedicated instruments.
The bottom line is that in the coming decade we might discover whether Elon Musk’s vision to transport cargos to Mars was realized at a grander scale using the light sail technology by more advanced technological civilizations in our cosmic neighborhood. We live in exciting times!
