This four-minute BBC podcast Mission to Mercury: BepiColombo spacecraft ready for launch highlights several challenges of the BepiColombo mission, including its orbital-mechanical aspects and is worth a listen
The BBC's James Menendez has been speaking to Elsa Montagnon, the operations manager for this European Space Agency mission - she's been working on the project for more than a decade.
You can hear her discuss the mission further in the Planetary Society's Planetary Radio podcast Return to Fiery Mercury With BepiColombo.
She is also the author of the cover article in the Planetary Report's Voyage to Mercury The BepiColombo Mission Prepares for Launch. This issue is also the first to be completely open-access and downloadable, and to have Emily Lakdawalla as its editor.
Montagnon's energetic descriptions of the mission have got me really interested in this orbit and reading further about it. I've run across the paper Interplanetary navigation along the low-thrust trajectory of BepiColombo (Acta Astronautica 59, Issues 1–5, July–September 2006, Pages 284-293)
For BepiColombo's five-year journey into the inner solar system, a combination of low-thrust arcs and six flybys (one at Moon and Earth, two at Venus and two at Mercury) will be used to reach Mercury with low relative velocity. At arrival a gravitational capture approach is foreseen, in which the Sun perturbation is exploited to get weakly captured around Mercury for a number of orbits. This trajectory imposes severe constraints from a navigational point of view. Very precise navigation is required due to the low flyby altitudes planned for Venus (300 km) and Mercury (200 km) and the level of accuracy needed for the final arrival through the vicinity of the Sun–Mercury L1 point. Besides that, the solar plasma effect severely degrades the quality of the radiometric measurements near superior solar conjunctions, which are more frequent for missions to the inner solar system. Moreover, perturbations, as the ones introduced by momentum wheel desaturation burns, entry into safe modes or solar radiation pressure, must also be taken into account. Delta-differential one-way range measurements are found to be required in periods of poor orbit determination prior to some gravity assists. Nevertheless, if a safe mode is triggered at a critical moment that produces a change in velocity in an unfavourable direction, the mission could be jeopardised. To avoid that risk, an increase in the flyby altitude and possibly a partial or total redesign of the trajectory to avoid flybys near solar conjunctions are considered.
BepiColombo will take several years to get to Mercury, and so there will be plenty of time for more questions about its orbit. Here I would just like to ask about certain aspects of this paper.
Question: In the context of this mission to Mercury so close to the Sun, what exactly does "weak capture" mean, and why might "delta-differential one-way range measurements" be necessary at times? If I understand correctly, these are not so commonly used in deep space missions, even during flyby maneuvers.