I’ve discussed the difficulties of interstellar exploration before. To get a spacecraft to another star within a human lifetime requires accelerating it to an appreciable percentage of c (the speed of light), say 10-20%. In general that requires titanic amounts of energy. (Forget about the common sci-fi scenarios of going into warp drive or jumping through or into hyperspace. Those are fantasy plot devices with either no science or highly speculative science behind them.)
The mass ratio of fuel-propellant to the rest of the craft, using the most plausible short term option, nuclear pulse propulsion, is something like 10,000 to 1 to reach 10% of c, that is, for every kilogram of spacecraft you want to reach the destination, you’ll need 10,000 kilograms of fuel. Although multiple stages would help, when we consider everything that would be required to send humans, things start to look pretty bleak. It’s a little bit more hopeful with uncrewed probes.
One solution being considered is Breakthrough Starshot. Use tiny probes with light sails attached, which are accelerated by ground based lasers to 20% of c. The biggest issues with this plan include the cost and logistics of the ground based lasers, the challenges in successfully miniaturizing the craft, and the fact that there’s no way to slow the probes at the destination, so they’d have to collect what data they could during the few hours they had when flying through the destination system. And their small size limits their transmitting power, meaning sending back the resulting data would require decades.
Another old solution proposed in 1960 by Robert Bussard, is to collect fuel from the interstellar medium. The Bussard Ramjet (BR) has a tremendous electromagnetic scoop in front of it, which brings in the diffuse hydrogen floating ahead of the craft, compresses it so that it undergoes nuclear fusion, and expels it as propellant. The idea is that the faster the craft is moving, the more fuel available to it, and the faster it can accelerate. The biggest issues with the BR is that the interstellar medium has been found since Bussard’s proposal to be far thinner than he believed, and the drag of the scoop limits its overall effectiveness.
Alex Tolley has a post up at Centauri Dreams discussing a new proposal: the Q-Drive, as put forward by Jeff Greason, chairman of the Tau Zero Foundation. Like the BR, the Q-Drive uses the interstellar medium, but in a different manner. Unlike the BR, this craft uses an inert stored propellant: water. (The water is stored as a giant cone of ice in front of the craft, acting as a shield against interstellar particles.) The water is ionized and accelerated out the back of the craft, propelling it forward.
What comes from the interstellar medium is the power to accelerate the water. This involves two large magnets that create a couple of magsails (sails made of magnetic fields), but instead of using them as sails, they function sort of like a wind turbine, in that they collect energy by slowing down (relative to the craft) the passing ionic interstellar matter, transferring the difference in kinetic energy to the drive. The faster the craft is moving, the more energy collected, the faster it can accelerate the propellant, and the higher the thrust.
Notably, the craft has to be initially accelerated to some small percentage of c by other means, such as the nuclear pulse propulsion I mentioned above. But once that is achieved, that first stage can be jettisoned, allowing the Q-Drive to supposedly then reach speeds around 20% of c. It can decelerate by pointing the drive in the direction of travel. (It’s less clear to me how the final stages of deceleration near the destination would work.)
If you’re interested in the details, I recommend reading Tolley’s post. Tolley is clear that this is something that seems possible in principle, but the practicalities may be another matter. In particular, there is a question of whether the energy conversion can be efficient enough to make the Q-Drive more effective than just using something like the nuclear pulse rocket for the whole trip.
However, if it does work, or it can be developed into something that works, it may make interstellar exploration far more practical than it currently looks.
If you’re interested in the hard core technical details, check out Jeff Greason’s paper, or his talk on the subject.
Greason’s primary message in the talk is to emphasize the key idea, that drag energy from the surrounding medium (the interstellar medium in interstellar flight, the solar wind in solar system flight) represents an untapped energy source.
It’s been a while since I saw a new idea in this space. Interstellar travel is a very hard engineering problem. Hard enough that some scientists think it’s effectively impossible. It’s nice to see someone make a possible dent in that problem.