Flyby Missions to Interstellar Comets Are 'Feasible and Affordable,' SWRI Study Finds

On Wednesday, Sept. 3, the Southwest Research Institute put out a statement with a plain claim: sending a spacecraft past an interstellar object is something we can do now, with technology already in hand. The institute's own study, it said, shows that such a flyby is both feasible and affordable. The statement added that the nearest case in point, comet 3I/ATLAS, could have been reached.

Matthew Freeman, the study's project manager and SWRI's director of space instrumentation, said the trajectory of 3I/ATLAS falls within the interceptable range of the mission his team designed. The comet is the third confirmed interstellar object to cross our solar system, after 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. The "I" in each name stands for interstellar, and the number marks the order of discovery.

There is a limit, though. Putting a probe into orbit around one of these visitors for closer study is not currently possible. Interstellar objects travel on hyperbolic trajectories, meaning they pass through the neighborhood and leave rather than circling the sun, and they move too fast relative to native solar system bodies for a spacecraft to match them. A flyby offers only a brief look at close range.

To test the idea, SWRI built software that generated a representative but artificial population of interstellar objects. It then calculated how much energy a spacecraft would need to leave Earth and approach each one. The institute's statement said numerous interstellar objects pass inside Earth's orbit every year, and as many as 10,000 come within the orbit of Neptune, which lies roughly 30 times farther from the sun than Earth does. Earth sits about 93 million miles, or 150 million kilometers, out.

The results, according to the statement, showed that in many cases a flyby would need less launch and in-flight velocity change than many other solar system missions. The team then estimated costs and payloads, with the idea that a mission concept could later be proposed to NASA. The statement gave no details on likely costs.

Several caveats come with the work. The mission is a proposal only, and nothing implies that NASA or any other entity will fund it. The study is internal to SWRI and apparently has not been submitted to a journal, so it has not been through peer review.

Alan Stern, an SWRI associate vice president who led the project, said flybys would give unprecedented insight into the composition, structure and properties of these objects, and would significantly expand understanding of how solid bodies form around other stars. Stern is best known as principal investigator of NASA's New Horizons mission, which made the first flyby of Pluto in 2015 and went on to study other small bodies in the Kuiper Belt. In 2023 he also flew to suborbital space with Virgin Galactic on a research mission.

A future spacecraft could examine an object's physical properties to learn how it formed and evolved, or its composition to learn where it came from. It could also study the coma, the loose envelope of material sublimating from a comet as it nears the sun's heat and pressure. Mark Tapley, an orbital mechanics specialist at SWRI, said the arrival of 3I/ATLAS strengthens the case, and that the study showed nothing is needed beyond the technology and launch performance of missions NASA has already flown.

The timing may matter. Interstellar visitors have been hard to spot, but SWRI says newer all-sky surveys, such as the National Science Foundation's Vera C. Rubin Observatory, may make discoveries more common within a decade. SWRI argues that laying groundwork now would let scientists reach comets from other star systems with relative ease, without leaving our own neighborhood.