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borgendurp t1_izkkbvq wrote

Reply to comment by Icee777 in Phobos' orbit prevents a traditional geostationary space elevator on Mars, but it is possible instead to build a downward space elevator from Phobos itself by Icee777

Yeah okay.. but why? The atmosphere only goes up to 10.8 km. After that, why do you need 6000km of elevator for?

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manicdee33 t1_izlwy08 wrote

Escape velocity from Mars is about 5km/s. The article explains that the bottom end of the elevator would be travelling at ~770m/s while the outer end of the elevator would be travelling at ~3.25km/s. This means a two-stage escape from Mars (one stage to get to the elevator, second stage to get from the elevator to escape velocity) would only need to provide ~0.8km/s to rendezvous with the elevator, and another ~1.75km/s to escape Mars, saving ~2.4km/s in delta-v overall. This results in significantly lower propellant requirements for cargo moving between Mars and Earth (and thus smaller spacecraft to carry the same payload).

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borgendurp t1_izn1x0a wrote

I'm not too familiar with what you're talking about but from wiki;

In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non-propelled object to escape from the gravitational influence of a primary body

So I'm not entirely sure how that applies?

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manicdee33 t1_iznadnx wrote

In celestial mechanics, escape velocity is the speed an object needs to reach in order for the force of gravity to never reduce the outward velocity to 0. As you travel further away from the primary body the force of gravity gets smaller and smaller, so the deceleration gets smaller and smaller, and the limit at the distance approaches infinity is for the deceleration from the force of gravity to reach 0. If the starting speed of the object was such that by the time it reaches that infinite distance it still has some radial velocity, it has escaped.

Also in celestial mechanics, a "propelled object" is one that can thrust forever (basically a torch ship). A non-propelled object includes an object which has accelerated by burning a rocket engine and has stopped consuming propellant (it's no longer propelled). If that rocket can reach escape velocity, it can coast out of the influence of that primary body. This is how rockets can push space craft from one planet to another: they reach escape velocity to escape the gravity well of one planet, and carefully aim to be caught in the gravity well of the destination.

Hope this helps.

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