Friday, December 9, 2022

Phobos as a space elevator for Mars

Building a Martian space elevator would be complicated by the Martian moon Phobos, which is in a low orbit at ~6,028 km above the Martian surface and intersects the Equator regularly, thus getting in the way of a traditional geostationary space elevator. But there is an idea instead to build a space elevator from Phobos itself.

Phobos is tidally locked to Mars (as the Moon is to Earth), where the same side of the moon stays facing the planet. A space elevator could extend down from Phobos to Mars 6,000 km, about 28 kilometers from the surface, and just out of the atmosphere of Mars. A similar space elevator cable could extend out 6,000 km the opposite direction that would counterbalance Phobos. In total the space elevator would extend out over 12,000 km which would be below Areostationary orbit of Mars (17,032 km).

A rocket launch would still be needed to get the rocket and cargo to the beginning of the space elevator 28 km above the surface. The surface of Mars is rotating at 0.25 km/s at the equator and the bottom of the space elevator would be rotating around Mars at 0.77 km/s, so only 0.52 km/s of Delta-v would be needed to get to the space elevator. Phobos orbits at 2.15 km/s and the outer most part of the space elevator would rotate around Mars at 3.52 km/s.

Infographic: Phobos as a space elevator for Mars

In our speculative Mars Colonization Timeline we have estimated such a Phobos-based space elevator could be built as soon as in 2080s–2090s. In adition to the space elevator there could be built a shuttle port at the summit of Pavonis Mons – the Martian volcano practically on the equator – for shuttles heading to and coming from Phobos space elevator. The summit of Pavonis Mons is standing 14 km above Mars' mean surface level (way above the denser part of the atmosphere), halving the needed trip to the shuttle platform at space elavator's lower tip 28 km above the surface.


  1. Some of this would depend on the structure of Phobos--it seems to be a bit like an asteroid--a rubble pile--with some indications of large voids in the interior so not sure it is the best for building a space elevator and counterbalance on. Maybe after probes visits Phobos we can understand its geology better and discover it is strong enough for such structures.

    Also, as Robert M says on Facebook, the speed of the planet-end terminus would be moving quite fast (Phobos orbits Mars 3x a day). Not sure how useful that is for this space elevator design as the rockets launching would have to catch a very quick moving target. And you still need to launch a rocket/shuttle from the surface to get to the terminus, so I am not too sure what the great advantage of building this kind of space elevator would have--a lot of costs and engineering and you still would need to launch rockets from the surface anyway. Marspedia states that a tether from the planet-end terminus could lower cargo to the surface, but, again, Phobos is not geostationary but moving quite fast so those on the ground would have to catch a fast moving cargo delivery unless it just gets dropped using parachutes or retro rockets to softly land the cargo (and you would have to wait until Phobos flies over the target drop zone to lower costs and trouble of traveling to the cargo to pick it up on the surface). Getting back up the elevator would be extremely difficult (again, fast moving terminus end).

    1. We know that Phobos has a density of 1.88 t/m3 (Earth has 5.51, the Moon 3.33). So, much less than Earth and the Moon, but almost 2x of water. If it's a rubble pile, than a quite dense one.

  2. I do like the idea of space elevators, though. I think they would work best on the Moon and Mars rather than on Earth (or at least are presently far more feasible on the Moon and Mars). Maybe we just need to get rid of Phobos :-). Mine it for materials to build the space elevator and get rid of it in the process (OK, just joking...well, half-joking).