The Feasibility of a Spacecraft Flyby with the Third Interstellar Object 3I/ATLAS from Earth or Mars

Abstract We investigate the feasibility of a spacecraft mission to conduct a flyby of 3I/ATLAS, the third macroscopic interstellar object discovered on 2025 July 1, as it traverses the solar system. There are both ready-to-launch spacecraft currently in storage on Earth, such as Janus, and spacecraft nearing the end of their missions at Mars. We calculate minimum Δ V single-impulse direct transfer trajectories to 3I/ATLAS both from Earth and from Mars. We consider launch dates spanning 2025 January through 2026 March to explore obtainable and hypothetical mission scenarios. Postdiscovery Earth departures require a challenging Δ V ≳ 24 km s −1 to fly by 3I/ATLAS. By contrast, Mars departures from 2025 July to 2025 September require Δ V ∼ 5 km s −1 to achieve an early October flyby—which is more feasible with existing propulsion capabilities. We further calculate the phase angle and flyby velocity for these trajectories, noting that the resulting flyby speeds would impose significant observational and engineering challenges that a mission would need to overcome. We discuss how existing spacecraft could be used to observe 3I/ATLAS and how spacecraft at other locations in the solar system could be repurposed to visit future interstellar objects on short notice.