Magnetic Repulsion

One way to accelerate a spacecraft is to use an internal magnet to repel an external magnetic field. Such a method would not require a single gram of propellant - acceleration without the use of reaction mass - the “holy grail” of propulsion. The spacecraft would need to be flying through a strong magnetic field and be capable of powering an internal magnet. This magnet will lose energy over time as it gets converted to vehicle kinetic energy. In essence, onboard electrical power will be converted directly to propulsion via the coil.

Unfortunately, this method is highly impractical. Let’s consider planet Earth for instance. The magnetic field generated by current flows in the molten core is sufficiently strong to protect it from solar radiation, but it is nowhere strong enough to lift a spacecraft - barely strong enough to point a needle North. Furthermore, in order to maintain good acceleration over a long period of time the spacecraft must fly along straight magnetic field lines. For this reason, the best place to launch is at the poles of the planet. But the field becomes weak and curves away as the spacecraft gains altitude so maintaining consistent acceleration is virtually impossible. Even if we do overcome this problem, there is still the issue of attitude control which could only be accomplished through the use of ordinary rocket thrusters.

Let’s look beyond Earth. There are celestial objects with much stronger fields such as our sun and neutron stars. Magnetars (a type of neutron star), for instance, generate field lines that are strong enough to propel large spacecraft with built in superconducting coils. But here too there are at least two fundamental problems - the spacecraft must be far away from the star to avoid getting sucked in but not too far where the magnetic field is too weak. Ultimately, there is no real freedom of flight, the spacecraft goes where the field takes it - not a very useful method of transportation.

However, there is one more concept that we can explore - magnetic sails. Instead of repelling against a field, we repel charged particles. A magnetic sail works in a similar manner as how the Earth repels solar radiation. It can also repel charged particles that permeate nebulas allowing fast and controlled flight through these regions of the universe. Magnetic sails too have limitations. The spacecraft would be stranded if it runs out of charged particles. For this reason, the most practical application is interstellar spacecraft deceleration. Imagine for a moment that we have “kicked“ our spacecraft out of the solar system using a powerful laser, but now it cannot decelerate to the target star. As the craft nears the star, it may deploy the magnetic sail which would then work much like a parachute to decelerate the vehicle.

For the time being, as we work to leave Earth and colonize Moon and Mars such technology has no merit. But in the far future, it will certainly provide many benefits for exploration of deeper space.

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