Preparing for the 2061 return of Halley’s comet. A rendezvous mission with an innovative imaging system

Abstract

The return of Comet 1P/Halley will promote a worldwide interest for ground and space observations of a celestial body of outstanding scientific and cultural interest. In addition to remote observations, space will open the possibility of in situ study, similarly to what was done during the passage of 1986. In this paper, we first discuss the scientific motivations for a rendezvous mission capable to overcome the limitations of the flyby missions that took place at that time. In the second part, we describe an example of a rendezvous trajectory that can be carried out with existing power and propulsion technologies, i.e., with radioisotope thermoelectric generators and a Hall effect thruster. Furthermore, the transfer is made possible by the gravitational assistance of a giant planet. The resulting mission concept, nicknamed HCREM (Halley Comet REndezvous Mission), selected from a number of cases treated in a previous paper of ours (Beolchi et al., 2024), will be capable to reach the comet beyond the distance of Saturn, when the sublimation of super-volatile species (e.g. CO and CO₂) will be ongoing, and well before the onset of the sublimation of water (expected to occur around 4 AU, namely at larger heliocentric distance than Mars). Following a direct transfer from Earth, a gravity assist with Jupiter inserts the spacecraft into the cometary orbital plane with retrograde motion. Electric propulsion modifies the trajectory so that the spacecraft reaches the target with zero relative velocity. After the rendezvous, the spacecraft will accompany the comet before, around and after perihelion, which will happen in July 2061, until the outbound crossing of the ecliptic and possibly even later. Given the large heliocentric distances reached by the spacecraft, our concept mission does not foresee the implementation of solar panels. In this way, some shortcomings deriving from the adoption of this technology onboard the Rosetta mission to comet 67P are avoided and operations can occur even inside the dense dust coma at short distance from the nucleus. In the third part of the paper, an innovative imaging system with a very large field of view of approximately 100°is proposed. This optical system allows the simultaneous capture of both details of the cometary surface and the surrounding space within a single image frame. For several degrees outside the borders of the nucleus, it allows following the trajectories of chunks and clouds ejected by pits or fractures, all phenomena crucial to the understanding of the cometary activity. In the conclusions, we stress that a concerted effort is needed in the current decade to plan and approve a rendezvous mission to 1P. Indeed, the scenario here described requires launching before 2040, less than 15 years from now. Later launches with existing rockets imply a severe loss of scientific knowledge, because the spacecraft will not be able to reach the comet before the onset of water sublimation.