Designs of Multi-Spacecraft Swarms for the Deflection of Apophis by Solar Sublimation
Maddock, Christie¹; Vasile, M¹; McInnes, C²; Radice, G¹; Summerer, L^3
1University of Glasgow; ²Strathclyde University; ³ESA Advanced Concepts Team

This paper presents two conceptual designs of multi-spacecraft swarms used for deflecting Apophis. Each spacecraft is equipped with a large solar concentrator assembly which focuses and re-directs a small beam of solar radiation onto the surface of the asteroid. When the beams from each spacecraft are superimposed, the temperature on the surface is enough to sublimate the rock, creating a debris plume with enough to force to slowly alter the orbit of Apophis.

The first design employs a large parabolic reflector which feeds directly into a solar-pumped laser. A secondary mirror directs the beam to the determined position on Apophis. The orbits are designed to fly in formation with the asteroid around the Sun. These orbits are optimized for two opposing objectives: minimizing the spacecraft-asteroid range, up to a constraint limit in order to avoid the non-linearities of the gravity field of Apophis, and maximizing the diameter in the orbital plane of the spacecraft formation orbit in order to avoid the debris plume. The orbital maintenance is governed by a specially-adapted feedback control algorithm.

The second system option places the spacecraft at floating artificial equilibrium points around Apophis, balancing the gravitational effects of the Apophis and the Sun with that of the solar radiation pressure on the spacecraft. The mirror focuses the light directly onto the asteroid surface, controlling the beam by adjusting the focal point of the reflector. By altering the shape of the mirror surface, both the focal point and the vector of the SRP can be manipulated. A minimal amount of control, on the order of 1E-8 N, is required to keep the spacecraft at the artificial equilibrium points.

A key requirement for the successful implementation of the multi-mirror approach is that each spacecraft must know their position relative to both Apophis and the other spacecraft in the formation, and be able to find and maintain the direction of the beam onto a precise spot on the surface of the asteroid. The inertial attitude, position and velocity of each spacecraft are given by onboard measurement devices. An onboard camera is used to estimate the relative position of Apophis, in order to feedback into the attitude, dynamics and control algorithms. The navigation strategy uses the 2D camera image to estimate the centre of mass. By integrating the data from each spacecraft, the position of Apophis can be determined. This data is also used to determine the required pointing vector and range for the solar concentrator assembly. The point of intersection of the beams should be aligned with the velocity vector of Apophis and in contact with the surface.

The results of simulations of a hypothetical deflection mission to Apophis are presented for the dynamics, control, attitude and navigation, accounting for solar radiation pressure, the gravity field of the asteroid, and the deviation of the orbit of Apophis. A discussion on current and predicted levels of technology required for the mission is also presented, such as the deployment and design of the large reflectors.