Landing a spacecraft on a comet or asteroid is a challenging task. These celestial objects not only move at astronomical speed; they also have relatively low gravitational pull, which makes it difficult to land on them. A spacecraft has to expend a lot of energy while trying to match the speed of a comet or asteroid so it can land on it. A recent proposal by Masahiro Ono of NASA’s Jet Propulsion Laboratory, Pasadena, California may have solved this conundrum by applying the principle of hitchhiking. Instead of racing to catch up with a comet, a spacecraft would bum a ride off the comet before it attempts to land on it.
Instead of using costly fuel to accelerate a spacecraft toward a comet or an asteroid, Ono’s proposed system would use a reusable tether system that would capture the comet. While approaching a comet, the spacecraft would release an extendable tether with a harpoon that would attach the craft to the object. The spacecraft then would reel out the tether much like a fisherman slackens and tightens his line when he catches a fish. As the craft begins to accelerate from the force of the moving comet, the spacecraft then can apply a brake and harvest the kinetic energy generated from this resistance.
Ono and his team have calculated what they call the “Space Hitchhike Equation,” a mathematical formula that describes the tether strength, mass of the spacecraft and other variables involved in the hitchhiking process. According to their calculations, the space-based rope would need to be anywhere from 62 to 620 miles long and must be made from a material capable of withstanding extreme tension and heat. Currently, Zylon and Kevlar are the only two materials capable of withstanding one of the slowest comet accelerations of 0.9 miles per second. A faster 6.2 mile-per-second propulsion would require advanced materials such as a carbon nanotube tether paired with a diamond harpoon.
Once the spacecraft is matching the speed of the comet or asteroid, the ship then can start to reel in its tether and inch closer to the asteroid until it is has made a gentle landing. It can stay attached and travel along with the comet as long as needed. When it is time to travel to another object, the craft can use its stored energy to release the tether and propel itself towards its next target.