If one day we want to 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, reaching 635,266 kilometers (394,736 miles) per hour.
While that’s impressive, it’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, so generation ships (or robotic probes) would be necessary to explore this or any other star of interest at greater distances.
To achieve these speeds, and on other missions to visit objects in the far reaches of the Solar System, NASA regularly uses “gravity assist.” When a spacecraft approaches a large body (planets or stars), momentum is transferred from the planet to the craft, slowing the object’s orbit by a small amount in exchange for a significant increase in speed. Basically, you steal some kinetic energy from the planet or star.
“Several robotic spacecraft have used ‘gravity assist’ techniques to reach their targets ‘high’ in the Sun’s gravity well. Voyager 2 launched in August 1977 and flew by Jupiter to survey and boost its trajectory to Saturn,” NASA explains. .
“Voyager 1 launched the following month and did the same (reached Jupiter before Voyager 2). Voyager 2 then got an assist from Saturn and another later from Uranus and climbed up to Neptune and beyond. Galileo made one kick from Venus. and two from Earth, as Jupiter orbited Cassini en route to its destination, received two boosts from Venus, one from Earth and one from Jupiter, to gain enough momentum to reach Saturn.
It has been proposed that we could send ships to relativistic speeds with gravitational assistance around a neutron star in a compact binary system. However, such a mission would be quite dangerous, and in 2019 David Kipping, an assistant professor of astronomy at Columbia University, proposed another way we could use this neat trick safely, by firing protons around the black hole instead.
Black holes are the source of a lot of gravity to help, they are formed from massive stars (or perhaps outright collapse) that have collapsed under their massive mass and not even allowed light to escape. But trying to fly a spaceship around one is the behavior of someone who wants to become spaghetti.
But when light passes through gravity wells, we know that it also gains energy. Since light travels at the speed of space—the speed at which all matterless particles must travel—it cannot gain or lose speed by falling into or out of a gravity well. Instead, when light falls into the gravity well, its frequency increases and is blue-shifted, while light coming out of the gravity well is red-shifted. That’s what “Halo Drive” takes advantage of.
The basic idea is that you send a beam of light past a pair of black holes that are spinning around each other before merging, or a single black hole that is spinning rapidly, and use the higher-energy blue light to accelerate your spacecraft.
“Using the moving black hole as a gravitational mirror, the kinetic energy from the black hole is transferred to the beam of light as blueshift, and upon return, the recycled photons not only accelerate but also power the spacecraft,” Kipping writes. paper. “Here it is shown that this recovered energy can later be used to reach a terminal velocity of approximately 133% of the black hole velocity.”
As light passes around the black hole, it creates a halo, which gives the drive its name.
“The proposed system is for the spacecraft to send a collimated beam of energy towards the black hole at a carefully chosen angle so that the beam returns to the spacecraft – so-called boomerang geodesics,” Kipping continued. “If the black hole is moving toward the spacecraft, which could easily be done using a compact binary star, this halo of particles returns with higher energy (and momentum). This energy is then transferred to the spacecraft, allowing for acceleration. Overall, then the halo drive transfers kinetic energy from the moving black hole to the spacecraft using gravity.”
Using propulsion, an interstellar civilization could hop between black hole binaries without fuel and use them to slow them down as they approach. According to the paper, the mass of the spacecraft is quite insignificant as long as it is much less than the mass of the black hole system, meaning it could propel Jupiter-sized ships up to relativistic speeds.
Using halo propulsion would have only minimal detectable effects on binary black holes, as using them to slow them down would effectively cancel out the effect of using them to speed them up (thanks, Newton’s third law).
“However, the finite time differences between departure and arrival would cause the binary star to spend time on a tighter semi-major axis than would naturally be the case, and experience a faster spiral of gravitational radiation during this time,” Kipping added. “Accordingly, a possible technological signature of halo propulsion would be the increased spiral velocity of black hole binaries relative to their neutron star counterparts.”
The study was published by the British Interplanetary Society and is available on arXiv.