Star Trek — as much as we love it — was guilty sometimes of a bit of hyperbole and more than its share of inconsistency. In some episodes, ion drives were advanced technology and in others they were obsolete. Make up your mind!
The ESA-JAXA BepiColombo probe is on its way to Mercury riding on four ion thrusters developed by a company called QinetiQ. But unlike the ion drive featured in the infamous “Spock’s Brain” episode, BepiColombo will take over seven years to get to Mercury. That’s because these ion drives are real.
The craft is actually two spacecraft in one with two different Mercury missions. The Mercury planetary orbiter will study the surface while the magnetosphere orbiter will study the little planet’s magnetic field. Check out a video about the mission, below. The second video shows [Neil Wallace] talking about how the ion propulsion — also known as solar electric engines — differ from traditional chemical thrusters.
According to [Wallace] the higher efficiency of the ion motors makes the mission feasible. A 6.5-meter tall vehicle that weighs about 9,000 pounds at launch would not be workable without the ion propulsion. The craft will also use gravity assists from Earth, Venus, and Mercury.
The ion engines are only about nine inches around and use xenon gas. Solar energy provides 4.5 kW to ionize and accelerate the xenon to over 100,000 miles per hour. Of course, xenon atoms don’t weigh much, so the thrust is only 145 mN. For point of reference, ANSI A117.1 (Standard on Accessible and Usable Buildings and Facilities) calls for interior doors to require no more than 22.2 N — that’s 22,200 mN — to open or close. That’s only five pounds of force, so the engines aren’t producing enough to open your bedroom door. However, in space, those tiny forces add up and over time they will act on the massive spacecraft and provide continuous braking against the sun’s pull until the craft orbits Mercury.
Chemical rockets, obviously, have a much greater thrust but use 10 to 20 times the fuel to get the same job done. It is also difficult to keep a highly energetic chemical rocket running for thousands of hours without failing. Additionally, that fuel adds to the weight of the craft. Ironically, NASA’s SERT ion engines, tested in the 1960s and 1970s used mercury as a propellant. Xenon, however, is much less prone to wear out the acceleration grids.
Sometimes in spaceflight, less is more. For example, an Electron offers an easier way to lift smaller satellites into orbit. If you want a more conventional rocket propulsion system, do like we did as kids and use water.