Photon Rocket
We can talk about engines all day but in the end, fuel mass is a real troublemaker for a propulsion designer. Check out the most advanced chemical rockets today - up to 90% of the rocket is just fuel. In the future, as we explore farther reaches of space, even the more efficient, nuclear-powered spacecraft will suffer heavily from this problem. The reason is simple, accelerating all that fuel means more powerful engines are needed and more powerful engines ultimately eat more fuel - that's how you end up with truly massive spacecraft. Can we find a fuel that weighs nothing but can still accelerate our spacecraft? Sounds crazy, but yes. Actually, you are surrounded by it - it's called light.
Light can be harnessed to accelerate a spacecraft. In one of the previous editions, we talked about Light Sails. But sails require a nearby star or a laser as a source of light. In the space around the outer planets and beyond there is no energy source and so you want a self-reliant engine.
To design such an engine requires a bit of knowledge about light. Light is made up of individual packets of energy called photons. A photon is both a particle and a wave and although the photon weighs nothing (its mass is zero), it still has momentum. The higher the frequency of the photon, the higher is the momentum. As a result, if you eject a sufficient number of photons out of the back of a spacecraft at very high frequency you could generate thrust. This would be truly game-changing, an electrical engine that does not require propellant - surely sounds like work of science fiction!
The main problem is with the momentum equation for a photon. The momentum of an individual photon is incredibly small. Just hold a flashlight in your hand, not much kick, right? For there to be any noticeable effect, the photon must be emitted at a very high-frequency, in the far end of the light spectrum - this is called a Gamma Ray. Gamma rays are lethal to humans and today, can only be generated by stellar objects such as exploding stars.
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If we want to create our own gamma rays, the best device for generating focused light is a laser. If one can build a powerful gamma-ray laser, we may actually get some thrust out of it. The engineering challenge here is with the material inside the laser. A laser consists of a crystal - an applied voltage causes electrons inside this crystal to harmoniously bounce up and down to emit photons of the same frequency. The shorter the "bounce" (transition between one orbital state to another), the shorter is the wavelength of the photon produced, and the higher is the frequency. Gamma ray frequency would require incredibly tiny bounces and it is unclear at the moment as to what sort of material is needed. Yet another way to increase frequency is to use "frequency conversion crystals" currently being developed for the NIF (National Ignition Facility) Optical System.
The crystal inside the laser is heavy, and, in the past, studies have been conducted to eliminate the laser entirely from the concept. The idea was to utilize the photons from a red-hot glowing nuclear reactor to push the spacecraft along. Unfortunately, thermal heat is inherently low frequency, at the completely wrong end of the spectrum - called "infrared" light. Photons of this frequency are weak and carry along insignificant amount of momentum, so thermal energy could not be used for photon-based propulsion.
In the end, this technology has no major showstoppers. Speed of light is limited by physics, sure, but the frequency of light can be as high as our machine can produce. Keep in mind that although a photon rocket doesn't carry fuel, it cannot accelerate forever and will eventually run out of power (be it battery or nuclear). Still, the idea is awe inspiring...flying through space on gamma rays - just make sure there's nobody behind your spacecraft when you turn on the laser!