Hitchhiking on Comets: The latest trend in space travel

September 24, 2015 | Sarah Tse

Illustration of harpoon system for hitchhiking on comets or asteroids
Photo credit: NASA

Our exploration of the outer solar system has been costly and slow in part because of fuel limitations. New Horizons, having passed Pluto this summer, is now destined to hurtle further and further into space until it runs out of power and loses contact with Earth. But what if it could hitch a ride with a moving object, like a comet or an asteroid?

Passing comets aren’t generous enough to pull over for a stranded spacecraft but a proposal developed at NASA’s Jet Propulsion Lab may get around this problem. Using a new technique, spacecraft will hitchhike from comet to asteroid without having to ask for permission.

The Comet Hitchhiker concept uses a harpoon and tether to reduce the need for propellant in entering the orbit of a small space body and landing on the surface. According to the model, a spacecraft can cast a harpooned tether as it flies by its target comet or asteroid. The backlash from firing the harpoon would push the spacecraft back, so it compensates by applying the brakes. As the target drags the spacecraft along, it loosens the tether in the same way a fisherman slightly lets out his line once he has a big catch, to reduce the tension and make sure the line doesn’t snap. This combination of braking while letting out the tether lets the spacecraft harvest kinetic energy for future use.

Once it’s moving at the same velocity as the comet or asteroid, the spacecraft can slowly reel in the tether to approach the surface and land. When the time comes to progress to the next target, the hitchhiker can quickly retrieve the tether, using that energy reserve to slingshot away. This method could allow a spacecraft to visit up to 10 different comets or asteroids on a single mission.

SEE ALSO: Soar Over Pluto In This Bird's Eye Tour

The harpoon process is much simpler and more efficient than using propellant (fuel). Using only fuel, a spacecraft would have to catch up to a comet’s or asteroid’s average speed of 24 km per second (15 miles per second), and then brake to enter its orbit. Since these small celestial bodies also have low gravitational pulls, the spacecraft would require even more power to land on the surface.

Up until now, such missions have required a lot of expensive fuel. The hitchhiking method could allow a spacecraft to visit up to 10 different comets or asteroids in a single mission, without requiring any propellant.

Hitchhiking also has the potential to speed up long distance voyages. The team calculated that this method would have shaved off 4 years from the 9.5 year journey taken by New Horizons.

Of course this model will require a bit of testing before it can be implemented. The team has analyzed simulations to figure out how strong the tether and harpoon have to be to survive the impact and tension involved in such extreme game of tag. They have accordingly designed the Space Hitchhike Equation to calculate these measures according to the mass of the spacecraft and the velocity that will be required for each specific maneuver.

In particular, the rapid decrease in velocity as the hitchhiker slows down to enter the orbit of its target will cause a lot of heat and tension. So far, the materials Zylon and Kevlar show some promise in being able to withstand the pressures of intense deceleration. But as the model aims for further and faster targets, the hitchhiking spacecraft will require more durable materials, like a carbon nanotube tether and diamond harpoon. The tether will also have to ride out the jerks and jolts that accompany high-speed celestial maneuvers, especially considering how much junk and debris is flying through space.

Before this innovation can be practiced, researchers will continue to create more simulations, and then test the technique using miniature models that replicate conditions in space. It’s a promising approach to space travel that will make it much easier to sneak up on comets and asteroids, and vastly expand our ability to study these celestial bodies. However, we don’t recommend that human hitchhikers try it out on unsuspecting truck drivers.

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