Gravitational slingshots and Interstellar, the Movie

By Philip Hall

The major motion picture Interstellar has had one of the most eminent scientists of our generation, Kip Thorne, as advisor. He classifies the science of Interstellar in three ways, known, barely feasible, but based on known science, and speculative. This makes Interstellar that unusual creature, a science fiction story which is science based. Most so-called science fiction is more properly science fantasy introducing elements which we are rather sure are impossible in reality.

Much of the fantasy genre is based on unexplained magical techniques. However we have to allow the creative imagination of the writer to flow. It was eminent science fiction writer Arthur C Clarke who said “any sufficiently advanced technology is indistinguishable from magic”. However the boundary between science fiction and science fantasy may be drawn in a rather blurred way between those which don’t contradict known laws and those which ignore them.

One of the features employed in the film is gravitational slingshots. This is a phenomenon actually employed in space missions. Space is very, very big. Even the solar system is huge and interstellar distances are vast. The distances are so large that they are often measured in relation to the time taken by light to traverse them. Thus the nearest star, Proxima Centauri, is about 4.22 light years away ie. at the massive speed of 186,000 miles per second it would take light 4.22 years to cover the distance. It is convenient to label the speed of light c, and nothing can travel faster. Mars is about 15 light minutes from Earth.

The distances even within the solar system are so large that rockets simply cannot carry enough fuel to travel fast enough to cover the distance in a reasonable time. The solution is to use gravitational assistance. As planets orbit around the sun they have massive kinetic energy. The spacecraft is directed behind the planet at an angle so it is pulled by the planets gravity speeding it up but sending it off an angle to the original direction. Effectively a minute part of the planet’s kinetic energy is passed to the spacecraft.

This technique was employed for the first time with the Voyager missions to explore the outer planets. The mathematics was developed in time to take advantage of this complex manoeuvre and the planets were aligned in the right configuration. Voyager ( there were two ) was not intended to slow but to fly by the outer planets. Voyager is about to pass out of the solar system into deep space. Thorne illustrates the gravitational slingshot by the later Cassini-Huygens mission to Saturn. Cassini flew by Earth once, Venus twice and Jupiter once on its way to Saturn and then slowed by a slingshot around Saturn’s moon Io. The illustration below is taken from the book Thorne has written about Interstellar.

Cassini Interplanetary Trajectory – Image courtesy of NASA –

These craft were far too slow for interstellar travel. Even at c/3 the journey takes 12.5 years. To reach this colossal speed Thorne postulates a slingshot around a black hole. A black hole has such massive gravity that even light cannot escape hence it is black. By comparison the gravity of a planet is puny. A slingshot around such a gigantic gravity could provide the spacecraft with the energy to achieve interstellar speed. On arrival it would need another black hole slingshot to slow down.

This method suggested by Thorne and used in the film, while speculative, is based on physical reality. This, and much other advanced science, is detailed by Thorne in his book “The Science of Interstellar”.


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