Under NIAC Phase I funding, JPL Robotics is performing a feasibility study of a new concept called Comet Hitchhiker. The comet hitchhiker concept is literally to hitch rides on comets to tour around the Solar System. This concept would be implemented by a tethered spacecraft that accelerates or decelerates itself without fuel by harvesting kinetic energy from a target body. First, the spacecraft harpoons a target as it makes a close flyby in order to attach a tether to the target. Then, as the target moves away, it reels out the tether while applying a regenerative brake to give itself moderate (less than 5g) acceleration, as well as to harvest energy.
We estimate that a comet hitchhiker spacecraft could obtain up to about 10 km per s of deltaV by using a carbon nanotube (CNT) tether. This level of deltaV would enable a spacecraft to land on or orbit around long-period comets and Kuiper belt objects (KBOs), which have not been visited by any spacecraft. With existing technologies, only a fly-by is realistic for these targets. Furthermore, a comet hitchhiker could obtain about 5 km per s of additional deltaV by utilizing just 25 percent of the harvested energy for reeling in the tether or driving electric propulsion engines.
The Comet Hitchhiker concept would enable ambitious exploration missions that are otherwise impossible or impractical at best. For example, orbital insertion around KBOs, including dwarf planets such as Pluto and Haumea, typically requires about 10 km per s deltaV, assuming a reasonable (less than 15 years) flight time from Earth to the target. To achieve this deltaV with a chemical rocket with 250 sec ISP, about 98 percent of the mass of the spacecraft must be occupied by propellant. Solar electric propulsion cannot be used at this distance from the Sun. Aerocapture cannot be used without atmosphere. The only realistic pathway to detailed exploration of KBOs seems to be the comet hitchhiker. We strongly believe that the comet hitchhiker concept will advance the frontier of space exploration to the most exotic worlds in the Solar System.
