Comet Halley will not return to the inner solar system until 2061, but a newly published mission design lays out how a spacecraft could finally do more than just glimpse it in passing. Researchers describe a trajectory that would let a probe catch up to the comet years in advance and stay alongside it for months, using only propulsion technology that already exists rather than the exotic engines that earlier rendezvous concepts assumed would be needed.
When Halley last swept past the sun in 1986, a fleet of spacecraft known informally as the Halley Armada, including the European Space Agency’s Giotto and the Soviet Vega probes, flew past it at high relative speed. Because those encounters were flybys rather than rendezvous, each mission had only a few hours inside the comet’s coma before racing past, leaving many basic questions about the nucleus unanswered. Comet 1P/Halley is not due back near the sun until 2061, but the authors of the new study argue that a rendezvous mission has to be planned decades ahead of time given how long the interplanetary transfer itself takes, and how long spacecraft development lead times typically run.
Why the 1986 flybys left so little time to look
The core problem is Halley’s orbit. The comet travels on a retrograde path tilted about 162 degrees relative to the plane most planets and spacecraft orbit in, which meant the Halley Armada probes approached it at enormous relative velocities. Matching that orbit closely enough to travel alongside the comet, rather than blow past it, requires an amount of energy that has kept a true rendezvous mission out of reach. Earlier rendezvous proposals tried to solve the problem with speculative hardware, such as high-power nuclear-electric propulsion systems or super-heavy launch vehicles, none of which has been fully developed or flight-proven.
A double gravity assist past Jupiter and Saturn
The new design, published by Roberto Flores, Elena Fantino and colleagues in a paper posted to arXiv, takes a different approach: using two planetary flybys instead of one. A spacecraft would launch with roughly 2,000 kilograms of instruments and propellant, firing a low-power Hall-effect thruster continuously along the way. A flyby of Jupiter would provide the first speed boost, sending the craft on toward Saturn. A second flyby at Saturn would then bend the spacecraft’s orbital plane to match Halley’s steep inclination, without burning through additional chemical propellant to force the turn.
Matching a steep, retrograde orbit without a prohibitive fuel bill
Combining the two gravity assists is what makes the trajectory affordable. A single flyby, the strategy previous designs relied on, cannot supply enough of a plane change on its own to line up with Halley’s orbit, which is part of why past rendezvous concepts stalled. By splitting the maneuver across Jupiter and Saturn, the researchers avoid the outsized propellant cost that a single-planet plan would demand, while still relying on power and propulsion hardware, a radioisotope thermoelectric generator paired with a Hall-effect thruster, that has already flown on other missions.
Simplifying the trajectory math to three design parameters
Part of what let the team explore so many possible paths is a mathematical shortcut. According to the paper’s abstract, the researchers derived closed-form expressions for the optimal geometry of each flyby and paired them with an explicit technique for describing the low-thrust arcs, cutting the number of variables that had to be tuned down to just three. That simplification made it practical to search a wide range of launch dates and flyby timings computationally, rather than relying on the kind of expensive, case-by-case trajectory work that has slowed earlier rendezvous studies. The team, led by Roberto Flores and Elena Fantino along with co-authors Alessandro Beolchi, Chiara Pozzi, Mauro Pontani, Ivano Bertini and Cesare Barbieri, used that search to identify two proof-of-concept trajectories they describe in detail in the paper.
Reaching the comet a year before its 2061 return
Under the proposed trajectory, the spacecraft would arrive at Halley in 2060, roughly a year ahead of the comet’s 2061 perihelion, while it is still safely outside the orbit of Mars. Arriving that early would let the probe watch as the nucleus begins to heat up and grow more active on its approach to the sun, with an instrument payload budgeted at 750 kilograms of the spacecraft’s mass. Rather than the few hours the Halley Armada managed, the mission is designed to stay in the comet’s vicinity for months, tracking its transformation as it nears the sun in far greater detail than any previous visit allowed.
A narrow decade-long window to build it
The design comes with a hard constraint on timing, as Universe Today reported: the best launch opportunities fall in August 2036 and September 2037, and missing that window would push the earliest realistic chance of intercepting Halley’s 2061 return out of reach. That leaves roughly a decade to move the concept from a published trajectory design to an approved, funded mission, a tight schedule for a spacecraft that would then spend more than 20 years in transit before it ever reaches the comet. The tradeoff, the researchers argue, is a mission built entirely around hardware that does not need to be invented first, using two gravity assists to do what earlier, more speculative designs could not.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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