Spiderman Spacecraft: Tethered Asteroid flybys in the Asteroid Belt
Swinging between asteroids by attaching a string (or tether) to them, just like Spiderman does when he's flying between the skyscrapers of New York. That is the basic idea of tethered asteroid flybys. By holding on to these relatively small celestial bodies, a spacecraft can change direction "for free", i.e. without spending any fuel.
The use of the gravity of celestial bodies for gravity assist maneuvers is quite common in astrodynamics. The spacecraft gravitationally interacts with the celestial body in a carefully designed way so that it is deflected in the desired direction. This technique works well for large bodies such as planets and allows large fuel savings in mission design. However, the gravitational attraction of small bodies, such as asteroids, is typically too small to perform such maneuvers.
In this work we analyze a different type of fly-by orbit using a variable length tether. During a fly-by of the spacecraft at an asteroid, a tether is attached to the asteroid which is then reeled out maintaining a tension within the limits of the tether. We have developed a dynamical model describing such a tethered flyby. Using this model, it is possible to perform fly-by maneuvers that conceptually are very similar to traditional gravity assist maneuvers. Unlike other proposed space tether missions, our proposed flybys are already feasible with currently available tether technology.
One possible application of tethered fly-by maneuvers is for multiple rendezvous orbits. A spacecraft in the main asteroid belt has to visit a long sequence of asteroids via tethered fly-bys. The optimization is then performed with the goal of maximizing the number of asteroids visited with minimal (ideally zero) fuel consumption.
The figure on the right shows such a tour of 7 asteroids in the main asteroid belt in which requires only 51 m/s delta-v from chemical propulsion, while the tethered flybys provide the bulk of the required delta-v at 373 m/s.
 Wittig, A. and Izzo, D., Spiderman Spacecraft: Tethered Asteroid Hopping in the Main Belt, 6th International Conference on Astrodynamics Tools and Techniques (ICATT), 14-17 March 2016. (link)