If we want to one day explore the galaxy (let alone the rest of the universe), we have a speed problem. In late 2023, NASA’s Parker Solar Probe reached the fastest speed ever achieved by a man-made object, clocking in at 635,266 kilometers (394,736 miles) per hour.
While impressive, that’s only 0.059 percent of the speed of light. A visit to our nearest neighbor Proxima Centauri, 4.2 light-years away, would take about 7,700 years at these speeds, requiring generation ships (or robotic probes) to explore this star or another interesting star further away.
To reach these speeds, and on other missions to visit objects in the far reaches of the solar system, NASA has regularly used “gravity assistants.” When spacecraft approach a large body (planets or a star), momentum is transferred from the planet to the craft, slowing the object’s orbit slightly in exchange for a significant increase in velocity. Essentially, you’re stealing a bit of kinetic energy from the planet or star.
“Several robotic spacecraft have used the ‘gravity assist’ technique to reach their targets ‘high’ in the Sun’s gravity well. Voyager 2 was launched in August 1977 and flew past Jupiter for exploration and for a trajectory improvement to Saturn,” explains NASA out.
Voyager 1 launched the following month and did the same (it reached Jupiter before Voyager 2 did). Voyager 2 then received help from Saturn and again later from Uranus, and climbed all the way to Neptune and beyond. Galileo received one kick from Venus and two from Earth, while orbiting the Sun on its way to its destination, Jupiter received two boosts from Venus, one from Earth and another from Jupiter to gain enough momentum to hit Saturn reach.
It has been proposed that we could send ships to relativistic speeds by using gravitational support around a neutron star in a compact binary star system. However, such a mission would be quite dangerous, and in 2019 David Kipping, assistant professor of astronomy at Columbia University, suggested another way we could safely use this neat trick by firing protons around a black hole instead.
Black holes are a source of a lot of gravity and are formed by massive stars (or possibly by direct collapse) that have collapsed under their enormous mass and no longer even allow light to escape. But trying to fly a spacecraft around it is the behavior of someone who wants to become spaghetti.
But when light passes through gravitational sources, we know that it also gains energy. Because light travels at the speed limit of the universe—the speed at which all particles without mass must travel—it cannot gain or lose speed by falling into or out of a gravity well. Instead, when light falls into a gravity well, its frequency becomes higher and blue-shifted, while light coming from a gravity well becomes red-shifted. It is this that is exploited by the “Halo Drive”.
The basic idea is that you direct a beam of light around a pair of black holes that are orbiting each other prior to a merger, or a single black hole that is spinning rapidly, and use the higher energy blue-shifted light to accelerate your spacecraft.
“By using a moving black hole as a gravitational mirror, the kinetic energy of the black hole is transferred to the light beam in the form of a blue shift. Upon re-entry, the recycled photons not only accelerate the spacecraft, but also add energy to it,” Kipping writes. the newspaper. “Here it is shown that this extracted energy can later be used to reach a terminal velocity of about 133% of the black hole’s velocity.”
As the light travels around the black hole, it forms a halo, which gives the disk its name.
“The proposed system is for a spacecraft to send a collimated energy beam at a carefully selected angle to a black hole, so that the beam returns to the spacecraft – a so-called boomerang geodesic,” Kipping continued. ‘As the black hole moves towards the spacecraft, which can be easily achieved using a compact binary, this halo of particles will return with higher energy (and momentum). This energy is then transferred to the spacecraft, allowing acceleration. The halo drive then transfers kinetic energy from the moving black hole to the spacecraft through gravitational assistance.”
Using the drive, an interstellar civilization without fuel could jump between black hole binaries and use them to slow down as they approach. According to the article, the spacecraft’s mass is fairly unimportant as long as it is much lower than that of the black hole system, meaning it could propel Jupiter-sized ships to relativistic speeds.
Using a halo drive would have only minimally observable effects on binary black holes, because using it to slow down would effectively negate the effect of using it to speed up (thanks, Newton’s third law).
“However, finite time differences between departure and arrival would cause the binary star to spend time on a narrower semi-major axis than it would normally do, during which time it would experience faster gravitational radiation in a spiral,” Kipping added to. “Accordingly, a possible technosignature of the halo drive would be an increased rate of binary inspiration of black holes, compared to, for example, their neutron star counterparts.”
The study was published by the British Interplanetary Society and is available on arXiv.