Close proximity operations at small bodies: orbiting, hovering, and hopping [chapter]

Daniel J. Scheeres, Michael J. S. Belton, Thomas H. Morgan, Nalin H. Samarasinha, Donald K. Yeomans
Mitigation of Hazardous Comets and Asteroids  
Central to any characterization or mitigation mission to a small solar system body, such as an asteroid or comet, is a phase of close proximity operations on or about about that body for some length of time. This is an extremely challenging environment in which to operate a spacecraft or surface vehicle. Reasons for this include the a priori uncertainty of the physical characteristics of a small body prior to rendezvous, the large range of values that its physical parameters can have, and the
more » ... rongly unstable and chaotic dynamics of vehicle motion in these force environments. To succesfully carry out close proximity operations about these bodies requires an understanding of the orbital dynamics close to them, a knowledge of the physical properties of the body and the spacecraft, and an appropriate level of technological sensing and control capability on-board the spacecraft. In this chapter we discuss the range of possible dynamical environments that can occur at small bodies, their implications for spacecraft control and design, and technological solutions and challenges to the problem of operating in close proximity to these small bodies. same orbits, due to solar radiation pressure effects, can be very unstable and lead to impact with the asteroid within a few orbits. Numerous other examples can also be found, indicating that small bodies, when taken together as a class, exhibit an extremely rich set of non-trivial dynamics. Spacecraft designs and mission operation concepts can be driven in very different directions as a function of the close proximity dynamical environment. As a case in point, the asteroid mission phase for the NEAR mission at Eros looks very different than the asteroid mission phase of the Muses-C mission to 1998 SF36. For NEAR, there was no choice but to use an orbital approach, due to the large mass of Eros. However, due to its shape, rotation state, and rotation pole orientation, the orbital mission had to be designed carefully to avoid destabilizing interactions with the asteroid. For Muses-C, due to the possible low mass of its target asteroid and the large mass to area ratio of the spacecraft, it could not be guaranteed that an orbital mission would even be possible. Thus, the entire mission consists of forcing the spacecraft to "hover" on the sun-side of the asteroid (discussed later in this chapter), with an associated cost in terms of fuel and ability to measure the asteroid's gravity field. A crucial point to make is that the placement of instruments on the spacecraft bus are fundamentally different for each mission. The NEAR mission plan required instruments boresights to be placed orthogonal to the solar array normals, while the Muses-C mission plan requires the boresights to be anti-parallel to the solar array normals, a fundamentally different spacecraft design dictated by the type of dynamics about the small body, not dictated by an abstract "design philosophy". Crucial small body parameters may not be known prior to rendezvous. In light of the previous point, it is clear that this can lead to increased design costs and impacts during the construction phase of the spacecraft. If relatively nothing is known about the target body, it will be necessary to design a spacecraft that covers a wider range of possible orbit and close proximity strategies -which will invariably lead to higher design and fabrication costs and to increased spacecraft mass. This places a strong driver on discovering as much as possible about the physical characteristics of potential target bodies prior to spacecraft and missin design.
doi:10.1017/cbo9780511525049.016 fatcat:qvhg73wozrho7ehifnw5364qmi