Extended-body motion in black hole spacetimes: What is possible?

TL;DR

This paper investigates the universal constraints on extended-body motion in black hole spacetimes, revealing how internal structure influences motion and potential control mechanisms for spacecraft without propulsion.

Contribution

It identifies specific quadrupole components affecting forces and torques, and demonstrates how internal control can manipulate orbit stability in Schwarzschild spacetime.

Findings

Two gravitational torque components must vanish for universal motion.

Four quadrupole components are irrelevant to motion.

A spacecraft can stabilize or escape orbits by controlling its internal structure.

Abstract

Free-fall is only approximately universal in general relativity. Different extended bodies can fall in different ways, depending on their internal dynamics. Nevertheless, certain aspects of their motion are universal. This paper examines the universal constraints on extended-body motion in vacuum type D spacetimes. Working in the quadrupole approximation, we show that in addition to the (previously-known) constraints imposed by Killing vectors, two components of the gravitational torque must vanish. Furthermore, of the ten components of a body's quadrupole moment, four are found to be irrelevant, two can affect only the force, and the remaining four can affect both forces and torques. As an application, we consider the capabilities of a hypothetical spacecraft which controls its motion by controlling its internal structure. In the Schwarzschild spacetime, such a spacecraft can control its mass, and in doing so, it can stabilize unstable orbits, escape from bound orbits, and more -- all without a rocket.