Kerbal space program, how to transfer fuel

A drive for almost the speed of light

In 2016, physicist Stephen Hawking and billionaire Yuri Milner unveiled a plan for a trip to the stars. The Breakthrough Starshot project is a $ 100 million program to develop and demonstrate the technology needed to visit a nearby star system such as Proxima Centauri. Thousands of tiny spaceships the size of microchips will be accelerated to a speed close to the speed of light using extremely powerful ground lasers. Each “star ship” would be attached to a light sail the size of a badminton field. However, the extremely powerful lasers that would be required for such a mission are particularly difficult and expensive to develop. Which begs the question, what other ways are there to achieve these so-called relativistic speeds?

Technology like from Jupiter

Astronomer David Kipping of Columbia University in New York is working on an answer. His team has developed a new form of gravity sling, the same technology that NASA used, for example, to send the Galileo spacecraft to Jupiter. The idea is to accelerate a spacecraft by flying past a massive object like a planet.

Because in doing so, the spacecraft steals some speed from the movement of the planet. Gravitational slings work best around very massive bodies. In the 1960s, physicist Freeman Dyson calculated that a black hole could accelerate a spacecraft to relativistic speeds. However, the forces acting on the spacecraft as it approaches would likely destroy it.

Kipping has now found a clever alternative to this. His idea is to send photons around a black hole and then use the energy gained to accelerate a light sail. "Kinetic energy from the black hole is transferred to the light beam as a blue shift and when they return, the photons not only accelerate the spaceship, but also transfer additional energy to it," says Kipping. The process depends on the enormously strong gravitational field around a black hole. Since photons have a small but measurable rest mass, this field could force the light into a circular orbit.

No fuel needed

Kipping's work is now based on a slightly different orbit that controls a photon emitted by the spacecraft around the black hole and back to the spacecraft - a kind of boomerang orbit. During their journey, the boomerang photons gain kinetic energy from the movement of the black hole.

This energy can then accelerate a spacecraft fitted with a suitable light sail. Kipping calls this a "halo drive" (in German about halo drive). “The Halo drive uses gravitational assistance to transfer kinetic energy from the moving black hole to the spacecraft,” says Kipping, pointing out that the spacecraft would not use its own fuel in the process.

Because the halo drive takes advantage of the motion of a black hole, it would work best in what are known as binary systems, where a black hole orbits another object. The drive should also work for any mass that is significantly smaller than the black hole. According to Kipping, this could theoretically make spacecraft the size of a planet possible.

Jump through binary systems

So a sufficiently advanced civilization could move from one part of the galaxy to another at relativistic speeds by jumping from one black hole binary to another. "An advanced civilization could use the light sail concept for extremely efficient propulsion," says the researcher.

The same mechanism could also slow down the spacecraft. A correspondingly advanced civilization would therefore probably look for pairs of binary systems with black holes that act as accelerators and retarders. The Milky Way contains around ten billion such systems. However, Kipping points out that there are likely to be a limited number of flight paths connecting them, so these interstellar highways are likely to be valuable regions.

Of course, any technology to implement this concept is currently far beyond human capabilities. However, astronomers should be able to figure out where the best interstellar highways are, and then look up the technical signatures of the civilizations they may already be flying through.

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