The DESTINY+ mission concept has recently undergone a major redesign following the transition from an Epsilon S launch vehicle to an H3 dual-launch configuration together with ESA’s RAMSES spacecraft. This new launch architecture provides significantly higher injected energy, enabling a direct Earth-escape injection, and eliminates the need for the originally planned complex sequences of Earth-bounded spiral orbit raising and Moon flyby. As a result, a comprehensive reassessment of the low-thrust interplanetary strategy has been performed, with a particular focus on enabling sequential flybys of (99942) Apophis and (3200) Phaethon via a low-thrust flyby-cycler orbit.
 
In the reconfigured mission concept, DESTINY+ is delivered by H3 directly onto a hyperbolic escape trajectory, replacing the many-revolution Earth-spiral trajectories of the previous Epsilon S scenario. The higher C3 level not only reduces early-mission operational complexity but also shortens time-to-NEO access and expands the number of reachable asteroid targets within the spacecraft lifetime. A primary driver of the updated trajectory architecture is the requirement of the dual launch with ESA’s RAMSES spacecraft, which in turn makes it feasible for DESTINY+ to intercept Apophis. The observation of Apophis before its April 2029 close Earth approach is essential for planetary defense. We analyze families of low-thrust solutions that achieve pre-2029 Apophis flybys with encounter geometries and relative velocities compatible with DESTINY+ system capabilities for high-resolution imaging.
 
Following the Apophis encounter, DESTINY+ transitions toward Phaethon, the original target of its science mission, via an Earth flyby. A key design challenge is rephasing from the Apophis-bound trajectory to a transfer capable of reaching the Phaethon flyby window while respecting mission constraints. We employ a low-thrust asteroid-cycler methodology using a machine learning-based surrogate model to construct feasible transfers, and further examine the influence of operational constraints including power availability and communication geometry.
 
The updated Apophis–Phaethon flyby-cycler architecture represents a valuable precursor for future planetary-defense reconnaissance missions. In addition to these two primary encounters, the increased launch energy and reduced early-mission complexity enable opportunities for three to four additional asteroid flybys during an extended mission phase, provided sufficient propellant and subsystem margins are retained. This multi-target capability positions DESTINY+ as a long-lived NEO survey platform, offering repeated characterization of small bodies across a broad range. Such an expanded sequence of encounters directly enhances planetary-defense readiness by supplying comparative measurements of shape, rotation, and surface morphology for objects of varying taxonomy and potential hazard level. Repeated validation of autonomous navigation and close-approach operations across diverse targets further strengthens the maturity of future flyby reconnaissance architectures.
 
This paper presents the reconfigured mission design, trajectory-optimization methodology, and associated performance assessments, providing an integrated evaluation of the feasibility, robustness, and operational implications of a low-thrust flyby-cycler connecting Apophis and Phaethon under the new H3 dual-launch scenario. The reconfigured mission analysis results indicate that the new architecture not only strengthens scientific return but also establishes DESTINY+ as a multi-encounter asset for the planetary-defense community, significantly increasing mission value through sustained small-body characterization across its operational lifetime.