An icy rock is flying through the Solar System on an open trajectory — coming in from interstellar space, swinging around the Sun, and heading back into the dark. Such interstellar comets already carry everything needed for flight: ice that can be vaporized and used as fuel, and rotation that naturally changes the body's orientation. All that's left is to steer a little.
The researchers asked: how many engines does it take to turn such a comet into a controllable ship? The answer is surprisingly modest — four. One main engine and three auxiliary ones arranged in a circle every 120 degrees. As the comet rotates, the auxiliary engines fire and cut off at the right moments, producing thrust in the desired direction. The main engine fires less often, but it sets the overall course, using rotation as a natural steering mechanism.
This approach doesn't require a complex attitude control system. The trick is to synchronize the engine firings with the body's rotation. The authors formally described which trajectories are achievable under limited turn curvature and showed that even with a slight tilt of the rotation axis, the result holds. This opens the door to inexpensive missions lasting decades, and to the possibility of protecting Earth from dangerous space objects.
An interstellar comet can carry so much ice that vaporizing it could fuel the engines for years — the fuel is already on board, you just need to melt it.
The plot of building a base on a comet and riding it to another star appears in science fiction, for example in Arthur C. Clarke's novel "Rendezvous with Rama".