Propulsion in space is an extremely tricky affair, one that’s centred heavily on trade-offs. The engines we use to get into space are woefully inefficient due to the large amount of propellent that has to be taken along with them. The faster/further you want to go the more propellent you need which makes the rockets increasingly bigger, putting a soft upper limit on what makes for a feasible craft. On the flip side once you’re in space we have engines with efficiencies that are so good that they can achieve incredible speeds with fractions of a percent of the fuel that it takes to get them into orbit. It’s no wonder that these engines were chosen for the Dawn mission to Vesta and Ceres.
There’s also engines that straddle the boundaries of these two like the VASIMR which aren’t capable of getting payloads off the surface of the earth but are quite capable of performing the same tasks as chemical rockets in space with a fraction of the required reaction mass (fuel). The trade off here is that it requires a rather large power source for it to be effective, on the order of hundreds of kilowatts, which means that in order for it to fly you need an ultra dense power source, usually in the form of a nuclear reactor. They’ve also never been flown on an operational mission yet (they have been thoroughly tested and verified however) but we will likely see one aboard the International Space Station within the next 3 years or so.
Barring some technological breakthrough I was pretty sure these engines were going to be the ones powering most of our craft for the next couple decades or so as we’ve got most of the bases covered. However it turns out that there might be a way to improve on the high efficiency/low thrust idea by doing away with the reaction mass completely. Sounds impossible right? I mean what engine can run without any fuel to drive it? As it turns out there’s quite a lot of energy to be derived from the vacuum of space and NASA are investigating how to tap into it:
The lab will first implement a low-thrust torsion pendulum (<1 uN), and commission the facility with an existing Quantum Vacuum Plasma Thruster. To date, the QVPT line of research has produced data suggesting very high specific impulse coupled with high specific force. If the physics and engineering models can be explored and understood in the lab to allow scaling to power levels pertinent for human spaceflight, 400kW SEP human missions to Mars may become a possibility, and at power levels of 2MW, 1-year transit to Neptune may also be possible
Essentially the way a QVPT works is by harnessing the random fluctuating magnetic fields that are present throughout the vacuum of space and using them to propel the craft. This works by polarizing a block magnetoelectric material leading to a force in one direction on the block whilst the field, or more accurately the bosons they’re made up of, are pushed in the other. Technically QVPTs are drives that uses photons as its reaction mass but it doesn’t have to bring them along which is a pretty big distinction between them and ion thrusters.
Much like VASIMR and ion thrusters QVPTs main limitation is the size of the power source that they can bring with them. However unlike their predecessors QVPTs have a far greater upper limit on how long they can run (referred to as specific impulse). These means for long distance missions like those to Mars and beyond there’s great potential to cut much of the transit time off by utilizing a QVPT. To put it into perspective the fastest craft ever launched, New Horizons, will take approximately 9 years to reach Pluto at its current speed. A QVPT powered craft is theoretically capable of getting there in just on a year, almost an order of magnitude faster. Of course this will rely on the effect being experimentally verified but since NASA has dedicated an entire team, dubbed EagleWorks, to verifying the idea I’d say that there’s at least some credence to it.
It’s quite exciting as new ideas like this don’t come along very often and it’s not common for NASA to simply dedicate significant resources to them in order to see if they pan out. This is what they’re good at though and it makes me incredibly happy to see NASA engaging in some good old fashioned envelope pushing. It might be a while before this bears fruit but the potential for unlocking our solar system is just too good to pass up.