It’s an old idea, nearly 300 years old, at least as old as Gulliver’s Travels from 1726 because Laputa was supposed to be lifted and moved in this way.

Use the Earth’s magnetic field to lift objects.

My first objection to this has been that in a uniform magnetic field, there’s plenty of torque on a magnet but no net force because forces on the north and south poles of a bar magnet cancel each other out. But that objection can be overcome by noting that on a large enough scale the Earth’s magnetic field is not uniform. The length scale required would be much less than that needed for a space elevator.

My second objection is that it would be an unstable equilibrium, any deviation from perfect alignment with the Earth’s magnetic field creates a torque that makes the deviation worse. But that objection can be overcome using computer controlled stability, as is done in for example fighter aircraft and quadcopters.

My third objection is that it wouldn’t work near the equator where the Earth’s magnetic field is near horizontal. But in fact it could work there when the magnet is held horizontal, the upward force would be created by the reduction in the strength of the Earth’s magnetic field with height.

So to my question.

Given present (or future) magnet technology, how much mass could the Earth’s magnetic field lift?

How would you harness this magnetic field?

Shades of Zarkov.

Bubblecar said:

How would you harness this magnetic field?

Put a magnet in it.

There will be a net upwards force because the top of the magnet is further from the centre of the Earth than the bottom of the magnet. eg. assume the magnet is about 1 km long.

To give another example, take two bar magnets (one is the Earth) and place then with their south poles nearly touching – there is a repulsion force, and that’s what I’m looking for here. How big a repulsion force can magnets generate?

The force of a dipole magnetic field (like Earth’s) drops off as the cube of the distance from the centre of the Earth. But let’s ignore that. Then the force on any one pole of our magnet is:

https://en.wikipedia.org/wiki/Force_between_magnets#Force_between_two_magnetic_poles

The force follows an inverse square law like that of gravity:
F = c*q ₁ *q ₂ / r ²
To use this I’d have to approximate the Earth as a bar magnet with two poles.

Say my magnet sits over the south pole on Earth. Then the south pole at the bottom has smaller r so is pushed upwards more than the north pole of my magnet is pulled downwards. And so (in the absence of gravity) the magnet would accelerate upwards.

So, let’s start by choosing a place south of Australia and north of Antarctica where the Earth’s field strength is 0.66 Gauss.
Earth’s radius there is about 6,362 km.
So the top of my magnet would be 6,363 km.

Earth’s field strength at the top of my magnet, because the Earth is a dipole, is 0.66*(6,362/6,363)^3 = 0.65969 Gauss.

Therefore the downwards force on my magnet’s north pole is less than the upwards force on my magnet’s south pole, the difference being proportional to 0.00031 Gauss.

The magnetic field from an MRI machine can be 700,000 Gauss. So if my magnet is that strong …

Could you have two human created magnets with opposite poles facing each other and use that repulsion force to lift objects into orbit.

Cymek said:

Could you have two human created magnets with opposite poles facing each other and use that repulsion force to lift objects into orbit.

For small enough values of gravity, yes.

(A variation of this is the “rail gun” where high speeds are produced by magnetic repulsion, but here I’m interested in gentler acceleration using the planet’s magnetic field).

I’d be interested in how small gravity would have to be for it to work.

On a slight tangent, it hadn’t occurred to me until yesterday that a big hazard of asteroid mining would be when the spacecraft gets magnetically stuck to the asteroid.

mollwollfumble said:

Cymek said:

Could you have two human created magnets with opposite poles facing each other and use that repulsion force to lift objects into orbit.

For small enough values of gravity, yes.

(A variation of this is the “rail gun” where high speeds are produced by magnetic repulsion, but here I’m interested in gentler acceleration using the planet’s magnetic field).

I’d be interested in how small gravity would have to be for it to work.

On a slight tangent, it hadn’t occurred to me until yesterday that a big hazard of asteroid mining would be when the spacecraft gets magnetically stuck to the asteroid.

Small gravity as in the planets gravity.
Mars has lower gravity than Earth but not much of magnetic field to work with.

Cymek said:

mollwollfumble said:

Cymek said:

Could you have two human created magnets with opposite poles facing each other and use that repulsion force to lift objects into orbit.

For small enough values of gravity, yes.

(A variation of this is the “rail gun” where high speeds are produced by magnetic repulsion, but here I’m interested in gentler acceleration using the planet’s magnetic field).

I’d be interested in how small gravity would have to be for it to work.

On a slight tangent, it hadn’t occurred to me until yesterday that a big hazard of asteroid mining would be when the spacecraft gets magnetically stuck to the asteroid.

Small gravity as in the planets gravity.
Mars has lower gravity than Earth but not much of magnetic field to work with.

Yes. Best for Earth is rockets, best for Mars is space elevator, best for Moon is railgun. Best for Venus is ?

Magnetic levitation may be best for launch from Ganymede. It could also be that the magnetic fields of Jupiter and Saturn are strong enough to launch spacecraft from their moons .

Magnetic levitation may also be the best for launch from mining asteroid 16 Psyche, which almost certainly has a strong magnetic field.