When CEO of SpaceX Elon Musk said during his TED interview (at 2:29) that
“… All modes of transport will become electric, with the ironic exception of rockets. There’s no way around Newton’s third law…”
it certainly turned a lot of heads. How a man who has literally revolutionized the world of electric cars could refute the possibility of creating an electric rocket? Google “Electric Rockets,” and you will find technology like beam-powered propulsion, Solar Electric Propulsion, etc. already existing. So how can Musk shrug off this possibility so casually?
To understand this, you first need to know that there is a difference between electric spacecraft and electrically powered spacecraft. And this subtle difference is what changes Musk’s “horrendous statement” to a more “yeah that makes sense.”
Elon Musk expressed the inability of a completely electrical rocket simply because everything in our world works on Newton’s Third Law (for every action, there is an equal and opposite reaction). This means if we want to move the spacecraft forward, it has to push back something in the opposite direction. While some boosters can be powered using electricity, it alone cannot be used to create the required thrust.
Musk’s electric cars do comply with Newton’s Third Law, as the electricity is used to move the crankshaft and the wheels across the ground, exerting backward push while the equal and opposite reaction thrusting the car forward. In space, there is no ground and friction to push against, so we can achieve Newton’s Third Law only by ejecting mass. A rocket moves forward by ejecting a mass at very high speeds, which moves the rocket forward by complying with the conservation of momentum principle (which Newton’s third law expresses).
So what Musk referenced is actually universally accepted. Although there is a certain technique called electric ion propulsion which uses electrical energy to accelerate and eject a very small amount of mass at an extremely high velocity moving the rocket spacecraft forward, clearly Musk doesn’t think it is effective enough, thus his insistence on the usage of propellant fuel.
If we generalize this to a relativistic model, hypothetically one can even use massless particles (like photons) to propel the aircraft since massless particles do have momentum in relativistic models. However, in real life, this would require so much energy that it would itself constitute a noticeable amount of mass expulsion via energy-mass equivalence.
Watch these astronauts prove all three of Newton’s law in space!