Trajectory Mission Analysis for Plasma Physics Observations during the 2029 Apophis Flyby
Thesis project at KTH Royal Institute of Technology.
Author: Lucas Barbero Sánchez in collaboration with Vinterstellar.
Abstract
On April 13th, 2029, asteroid (99942) Apophis will pass within 31600 km of Earth’s surface, offering a unique opportunity for plasma physics observations as it moves through the magnetosphere and interacts with the outer radiation belt, ring current, and edges of the plasmasphere. This thesis explores a low-thrust trajectory strategy for a small 12U CubeSat platform named Onuris to intercept Apophis during its close flyby. The asteroid’s retrograde motion with respect to Earth, its high relative velocity, and the narrow interception window make this a challenging mission. To address these challenges, a trajectory framework is developed based on periodic orbits and their associated Invariant Manifold (IVM) in the Circular Restricted Three Body Problem (CR3BP). A Phase-Space Informed (PSI) design strategy is introduced, with transfer arcs originating from periodic solutions around the Sun–Earth Lagrange points. A Lunar Gravity Assist (LGA) maneuver is also considered to patch realistic launch opportunities with computed PSI strategy profiles. The resulting trajectories demonstrate flexibility in timing and feasibility under tight mission constraints, contributing not only to the design of a potential flyby mission to Apophis, but also to broader applications in planetary defense, such as rapid-response asteroid interception.
Problem Definition and Scientific Interest
Apophis’ close flyby offers a unique opportunity for space plasma physics, as the asteroid will pass within 31 600 km of Earth’s surface in a retrograde orbit, moving through the magnetosphere and encountering the outer radiation belt, ring current, and outer edges of the plasmasphere.
As it moves through these regions, particle interactions may release ions and neutrals through mechanisms such as surface charging, revealing surface composition and influencing the dynamics of levitating dust. Moreover, Apophis’ flyby presents itself as a natural experiment capable of providing a better understanding of the solar system’s evolution. This is due to the presence of magnetic field and dust interactions similar to those believed to have occurred during the primordial solar nebula.
To investigate these effects, a 12U1 spacecraft named Onuris, equipped with plasma and neutral gas analyzers, could provide valuable measurements during the encounter. This is particularly interesting, since none of the current missions to Apophis carry, to this day, instruments specifically dedicated to plasma physics.
Consequently, the idea arises of designing a low-cost low-thrust propulsion mission capable of intercepting Apophis. However, this idea presents significant challenges, particularly due to Apophis’ incoming direction (retrograde motion with respect to Earth) and its high relative velocity of 7.42 km/s at closest approach, which results in a very narrow time window for interception. Therefore, the desired trajectory profiles will need to be flexible in terms of time-window availability.
Additionally, with Apophis approaching in less than four years, this work serves as an exercise in rapid space mission development. Furthermore, the techniques discussed in this thesis could very well be applied to other potential asteroid or Near-Earth Object (NEO) threats, with the mission goals shifting toward planetary defense.
Solution Summary & Future Work
In order to meet the challenges presented the project developed a Phase-Space Informed (PSI) strategy, combining periodic orbits around Lagrange points, invariant manifolds, and segmented optimization to enable flexible and feasible transfers under tight mission constraints.
The complete transfer from a northern L1 Halo orbit to Apophis interception lasted approximately 215 days. At closest approach, Onuris successfully intercepted Apophis with a relative velocity of 4.89 km/s, lower than Apophis’s 7.42 km/s, but sufficient to conduct plasma physics observations and fulfill the mission’s scientific objectives.
The results demonstrate that the developed PSI strategy can produce realistic and flexible trajectories for asteroid interception, particularly by exploiting the natural dynamics of the multiple available IVM paths to fall along from. A complete mission profile was presented, from a hypothetical translunar orbit patched via a Lunar Gravity Assist (LGA), all the way to Apophis interception. This was modeled in low fidelity and only for the Lyapunov case, but the same approach could be extended to the Halo trajectory in future work.
The trajectory framework developed in this thesis could be directly extended to other asteroids or near-Earth objects. Depending on the characteristics of their incoming trajectories, either the unstable L1 or L2 Invariant Manifold (IVM) would be more suitable. From a planetary defense perspective, this naturally leads to the idea of placing two dedicated platforms in orbits around L1 and L2, similar to ESA’s Comet Interceptor mission [16], which will await its target near L2. In the event of an incoming object, an interceptor spacecraft could be deployed along one of the precomputed manifold-guided trajectories, enabling a fast and cost-effective response.
