For millennia, skygazers have imagined what it might be like to wander among the stars. Long considered a quixotic pipe dream, interstellar travel is now taking shape as a quantifiable engineering challenge for space scientists around the world.
Last week, NASA's Innovative Advanced Concepts (NIAC) program awarded Phase I funding (roughly $125,000) to two proposals outlining propulsion systems that could enable future spacecraft to sail the extrasolar seas. NIAC is a pipeline for visionary, sci-fi-adjacent ideas for the future of spaceflight, so interstellar travel is right at home alongside other 2017 recipients (which include concepts for asteroid-dismantling softbots and synthetic organisms that create Martian farmland).
One of the proposals, entitled "Mach Effects for In Space Propulsion: Interstellar Mission," is backed by decades of experiments and research into the Mach effect, which predicts that an object undergoing acceleration will fluctuate in mass when there is a change in its internal energy.
James Woodward, a professor at California State University, Fullerton and the author of Making Starships and Stargates, pioneered the concept of a drive powered by the Mach effect, an endeavor that has interested scientists and casual space nerds worldwide (there's even a niche subreddit for it).
The idea is to use an electrostrictive transducer to convert the effect's predicted mass fluctuations into thrust. This would create a propellantless engine, one of the holy grails of propulsion.
"As soon as people figure out that the gizmos really are feasible, we will see a resource investment of significant proportions"
In place of traditional spacecraft fuel, this Mach Effect Gravity Assist (MEGA) drive would rely on the gravitational field of the universe to keep it in motion, according to physicist Heidi Fearn, one of the proposal's co-investigators (the other co-investigators are José Rodal, Marshall Eubanks, Paul March, Bruce Long, and Gary Hudson, based at the Space Studies Institute in Mojave, California).
"The real energy for the motion is coming from the gravitational field, not from the power supply that we've put in," Fearn told me over the phone. She borrowed an analogy from her colleague, Rodal, in which an object is placed at the top of a steep hill. All it needs is an initial push before gravity takes over and provides kinetic energy to keep it rolling. The same principle is at work in the MEGA drive, only instead of a hill, it's the entire universe, and the object is a badass starship.
"People get confused thinking we're getting out more energy than we're putting in but that's not really the case because we have the whole gravitational field of the universe to play with," Fearn said. "That's a huge ocean of energy out there that we're using."
A device like this could revolutionize spaceflight within our solar system, and open the gates to the stars beyond (it could also lead to stargates and warp drives, but that's a whole other story).
Yet the MEGA drive has taken a while to find mainstream traction, partly because it doesn't fit into the wild and speculative mould that people normally associate with revolutionary technology.
"It's conventional and breakthrough," Woodward, who is a consultant on the NIAC proposal, told me in a joint phone call with Fearn. "This case is an oddball because there's a new calculation using standard old physics that says if you do stuff in just the right way, you can produce this interesting effect."
"Where we are in this case is not the same place as where some people are with these sorts of [NIAC] studies," he continued. "We already have experimental evidence that shows that these effects can indeed be generated in the laboratory. As soon as people with resources figure out that this is actually real physics, that the gizmos really are feasible and it can be done, we will see a resource investment of significant proportions and it will get done a lot faster."
Resource-dependence is also a key factor for the other proposal with interstellar ambitions that NIAC just greenlit for Phase I development. Entitled "A Breakthrough Propulsion Architecture for Interstellar Precursor Missions," the concept involves a solar-powered laser array that stretches out over 10 kilometers (6.21 miles). The array is designed to beam power to spacecraft, propelling them to distances of up to 500 or 600 astronomical units (AU)—one AU being the distance between Earth and the Sun—in a mere 12 years.
This laser-based propulsion system would literally accelerate the pace of spaceflight. Passenger ships to Jupiter would take about one year, and robotic missions to Pluto could arrive at the far-flung dwarf planet in 3.6 years.
But as the phrase "interstellar precursor" suggests, the idea here is to create a potential facilitator of journeys between stars, rather than lay down a direct route.
"As good as this is, and as cool as this is, it is too wimpy to get to the nearest star, even the closest one," John Brophy, a propulsion expert at NASA's Jet Propulsion Laboratory and the principal investigator of the proposal, told me in a phone interview. "The nearest star is 268,000 AU. It is just really, really far."
Tailoring the mission for exploration applications within our solar system, and slightly beyond, is kind of like building a raft from the shallows. "It's a good stepping stone, because right now, nobody really knows how to do an interstellar mission."
On its face, Brophy's research seems similar to the Breakthrough Starshot concept mission, announced by Yuri Milner and Stephen Hawking in 2016, which aims to send a fleet of small spacecraft to the nearby Alpha Centauri star system. Both proposals build on research conducted by a fellow NIAC recipient, Philip Lubin, who detailed how high-powered lasers could be used to power a scalable variety of spacecraft.
But where Breakthrough Starshot is intended to quickly reach relativistic speeds, around 20 percent the speed of light, Brophy's NIAC concept would travel at a more comfortable clip conducive to crewed spaceflight and other exploratory missions in our solar neighborhood.
"While this spacecraft can go very fast, it doesn't accelerate very rapidly," Brophy told me. "It's actually a very gentle acceleration, but spread out over time, with no danger of hurting astronauts."
That's good news, because there's a sizeable population of space enthusiasts out there who are keen to explore the solar system, and the worlds beyond it, but would rather not die doing it. NIAC's latest round of selections could bring us closer to that next cosmic frontier.
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