Let's ride a Martian space elevator—from Pavonis Mons to areostationary orbit!

The left hand side looks down on the north pole, and the ellipse shows at each moment the free-fall trajectory the elevator car would follow if it detached from the cable

(More details in thread) 1/

The white part of the ellipse shows where that free-fall trajectory intersects Mars (i.e., if any path of the path is white we would crash into the planet) 2/

I've shown the cable extending from Pavonis Mons—a large equatorial volcano—since a space elevator has to be tethered at the equator. (Pavonis Mons is the location of the space elevator in Kim Stanley Robinson's Mars trilogy!) 3/

In a recent thread about rocket launches, I emphasised that the hard part about getting to orbit isn't getting to space, but getting up to orbital velocity, but with a space elevator we just go straight up at a sedate pace, so how does that work? 4/ https://twitter.com/AndrewM_Webb/status/1281554371713683456

As the car climbs the cable, it picks up horizontal velocity; the whole cable swings round once per (Martian) day, so if your the car's altitude plus the radius of the planet is R, then its horizontal velocity is such that would traverses distance 2π⋅R in a day 5/

The car gets its angular momentum from the cable, which gets it from the planet.

The car climbs to ~17000 km, where the cable is swinging around at the same speed as a circular orbit.

Since the cable is always above the same patch of ground, this is a synchronous orbit 6/

This is what riding a space elevator on Earth would look like.

A synchronous orbit on Earth is higher than on Mars, due to the higher gravity, so we need a longer cable 7/

You can ride the cable to higher than the synchronous point to reach escape velocity to travel to other planets.

To enter low orbit, detach lower down, wait until you're at the low point of the orbit, and then use engines to decelerate until the orbit is circular 8/

That might seem wasteful, but it's less wasteful than using rocket engines to get all the way to orbit. The main advantage of an elevator is that you don't need to carry any fuel! 9/

For more info on space elevators, see this excellent thread: https://twitter.com/solarrsystem/status/1279955806398697474 10/10