NASA is investigating the possibility of building a hitchhiking spacecraft that could visit five to 10 objects in the asteroid belt or the more distant Kuiper Belt in the Solar System. The concept is being studied as part of the NASA Innovative Advanced Concepts (NIAC) Program.

Called the Comet Hitchhiker, the proposal would help a spacecraft visit multiple locations by reducing the amount of fuel needed to enter orbit or land on comets and asteroids. It is being developed by scientists at NASA’s Jet Propulsion Laboratory (JPL) in California.

For a spacecraft to land on a body, it must almost exactly match its velocity, which can prove rather costly in terms of propellant. But this proposal would use a harpoon and tether system to reel a spacecraft in to a target, saving additional energy in the process that could be used to visit another target.

"Hitchhiking a celestial body is not as simple as sticking out your thumb, because it flies at an astronomical speed and it won’t stop to pick you up," said Masahiro Ono, the principal investigator on the project at JPL, in a statement. "Instead of a thumb, our idea is to use a harpoon and a tether."

As the spacecraft approaches the object, it would fire out its harpoon and latch on. The act of firing this tether would push the spacecraft back (every action has an equal and opposite reaction, thanks Newton), so a brake would be used to slow the spacecraft down. Energy from this braking process would be harvested to use on the spacecraft.

Then, once the spacecraft matches the velocity of the comet, it would begin reeling in the tether, allowing it to land on the surface by descending gently. To leave the comet, the tether only needs to be retrieved back into the spacecraft; the force of doing so would push the spacecraft away (again, thanks Newton), allowing it to fly to another target with the use of propellant.

^{Philae's troubled landing illustrated how difficult it is to touch down on a comet. ESA.}

The main issue at the moment is finding a material that can withstand the rather large forces from the tension and heat of a spacecraft suddenly decelerating. Zylon and Kevlar have both been touted as possible solutions to this problem.

These would only be suitable for gentle hitchhiking maneuvers though, involving a velocity change of 1.5 kilometers (0.9 miles) per second. To save more fuel, a spacecraft could be brought in at a higher relative speed to the body, with 10 kilometers (6.2 miles) per second apparently possible, but this would require more advanced materials such as a carbon nanotube tether and a diamond harpoon.

Hitchhiking will also require a fairly larger tether – between 100 and 1,000 kilometers (62 and 620 miles) depending on what change in velocity is required – that can withstand impacts from small meteorites. The team plans to perform simulations and experiments on Earth to try out different ideas.

If the problems are solvable, however, this proposal could prove to be very attractive indeed. You only have to look at Philae's landing in November 2014 to see how hard it is to land on a small rock floating through space. A harpoon and tether approach could be an ideal system for future missions.