# Is a midspace space station between Earth and Mars practical?

A recent answer included the following:

For example if you were to start a colony on mars, in this order for supplies and other necessities to be delivered there would need to be a network of space stations or ports midspace so many smaller transport or supply vessels can travel between them and supply the various stations with necessary supply’s coming from earth.

The Earth and Mars are moving in different orbits around the Sun. To me it seems that having a halfway station has significant issues. The first that comes to mind is the fuel to keep the station in a halfway orbit. The second would be the fuel required for a ship to stop at the station and start again.

But, maybe I am wrong?

Is a midspace space station between Earth and Mars practical?

• If we're talking about a Mars Cycler, then yes it's practical. (FSVo practical) – JCRM Dec 5 '19 at 15:34
• Why would (significant) fuel be needed to "keep the station in a halfway orbit"? Are there no possible stable orbits intermediate between Earth and Mars? – Kenny Evitt Dec 5 '19 at 22:21
• @KennyEvitt Intermediate? Sure. Aldrin cyclers sound like a pretty good way to handle transit between Earth and Mars (and back) periodically. But something that always sticks on a path between Earth and Mars? Nah. Keep in mind that at different distances from the Sun, things orbit at a different time. The distance between Earth and Mars varies like crazy - sometimes we're almost next doors (on the same side with respect to the Sun), sometimes we're at the opposite ends of the inner solar system. The very idea that there is a halfway "point" is flawed. – Luaan Dec 6 '19 at 7:41
• Would the moons count as halfway stations? Seeing how hard it is to actually escape earths gravity, in terms of effort, getting to the Moon is already halfway to Mars. – Pieter B Dec 6 '19 at 17:10
• Halfway in distance would be silly, halfway in delta-V could work: it would just mean a station in e.g. Earth orbit, Mars orbit, or on one of the moon. – jpa Dec 7 '19 at 6:42

You're right: this has issues. You can insert a station into a circular orbit halfway between Earth and Mars, but because this has an orbital period also in between those of Earth and Mars, your station won't be in a usable position most of the time. So you'd have to fill the orbit with several station to always have one reasonably close.

The fuel is also an issue, as you say. A spacecraft on its way to Mars would have to spend energy braking to dock at the station.

And there's the logistics. There's no point in sending supplies to a station "halfway" because of the fuel issue, you might as well launch them to Mars in one go.

• This is a good answer but I think the OP thinks that the station would be expending fuel to maintain its orbit, this answer would be even better if it made clear that wasnt the case (unless they want to do something really crazy like make the station orbit at an unnatural period to always be between earth and Mars, but I'm not sure you can really call that an "orbit") – Richard Tingle Dec 5 '19 at 23:15
• WRT fuel & supplies, "might as well" is wrong. You're much better off launching them directly to Mars. If you want some sort of emergency "pit stop", the most (indeed, I think the only) viable solution would be to launch several ships simultaneously, to follow the same orbit. (And if you connect them with cables, you can spin the assemblage for artifical gravity...) – jamesqf Dec 6 '19 at 3:14
• Is there no trajectory which would meet a midway station with minimal delta-v? Can the space ship be captured somehow? – Peter - Reinstate Monica Dec 7 '19 at 21:43

It's hard to guess what is in the mind of another contributor, but...

Using a cycler model could involve a network of space ports, and the cycler would take the role of a "midway" space station (it taking care for the cargo for the portion of the journey between Earth and Mars)

In Earth orbit, there would be a staging post or spaceport.

Launch vehicles would stock this up over time with consumables. The dock would provide thermal control and ballistic and radiation protection, but the lightweight launch vehicles would not, instead relying on the short duration of the flight.

When the cycler approached, a transfer spacecraft would move the cargo (possibly in the equivalent of a shipping container) to the cycler, again relying on the short duration of the flight to reduce the mass of protection needed.

Once docked with the cycler, it would take over the thermal control, and radiation/ballistic protection.

At the appropriate time, the cargo would again be moved,this time from the cycler to a Mars orbit staging post - again with minimal mass for payload protection.

From the Martian spaceport, EDL craft would take goods to the planet surface.

The advantages of this type of arrangement is the delta-v needed at the four short duration stages (launch, transfer, capture and landing) is applied to as little support infrastructure as possible.

Of course, this involves sending up more than three times the infrastructure that a single trip from earth to Mars would take, and sending twice the infrastructure needed into a Mars transfer orbit, but with enough usage it becomes economic.

Is a midspace space station between Earth and Mars practical?

Here's a naive view:

The problems with notions like a "halfway" station is that the locus of points halfway between Earth and Mars is probably a thick disk with an radius about equal to the average of the average of Earths and Mars orbital radius. Mars also has the second most eccentric orbit of the planets. The lateral cross section of that disk might have an area of over 110,000,000,000,000,000 square kilometers. I think you'd have to construct hundreds of thousands of "midspace" stations if you wanted at least one to be within a million km of the current midpoint between Earth and Mars.

A single "midpoint" station in an orbit between Earth's orbit and Mars' orbit would, I suspect, mostly be in completely the wrong direction (e.g. from Earth in the opposite direction than Mars) and mostly further from Earth than Mars is. Of course, space vehicles in the solar system don't travel in straight lines, but even so, you are going to need a huge cloud of space stations, most of which will never be used unless there are vast numbers of spacecraft making the journey.

I don't doubt a clever person could plan and construct a network of fewer stations that might be useful if the spacecraft making the voyage are constrained to specific infrequent start and end dates over the following half century.

The second would be the fuel required for a ship to stop at the station and start again.

Yes, I don't see any benefit to a "midpoint" station.

Using an interplanetary journey calculator I got this result

Mercury to Earth:          3.2 months   with a delta-v of 12852
Mercury to Venus to Earth: 8.5 months†  with a delta-v of 18570


So using Venus as a "midpoint" station between Mercury and Earth doesn't seem useful if you just want to get to your final destination efficiently. I suspect the same sort of arithmetic applies to an Earth-Midpoint-Mars journey.

† Probably plus waiting time in orbit around Venus until Venus and Earth positions in their orbits are optimal.

Posing this question (and most answers) indicate severe lack of understanding of basic orbital physics. Here's a shortlist of facts and probably reasonable ideas:

• Orbital period is in direct relation to the radius of the orbit (we speak of circular orbits here).

This means that anything orbiting the Sun between Earth and Mars is at some point at this "mid" point between them, and mostly completely somewhere else. Any such station will have orbital period shorter than that of Mars and longer than that of Earth.

• Acceleration is expensive, coasting is free. If you are on your way to mars, it makes absolutely no sense to make effort to stop in between and then insert to Mars-bound orbit again. Practically speaking we are always using Hoffman transfer orbits. I assume you don't know what those are, and that's why you are asking this question anyway. All probable propulsion methods are quite performance constrained as is when it comes to Mars missions.

What might make sense is a station on elliptical orbit between Earth and Mars. Incidentally this is a Hoffman transfer orbit too. Such a station would be constantly cycling between Earth and Mars orbits, and one could just board it at one end and exit at another. This would make it possible to have comfortable and spacious living conditions for most of the trip without investing to big crew compartments in every ship. Here's the problem though: I did some quick math and such a station would complete orbit once every 1.417 years. This is not a nice ratio of the orbital periods of either Earth of Mars, and the planets would have no responsibility to be anywhere nearby when the station reaches the desired point. You would be forced to take some extra measures to rendezvous with the destination in time and space.

• Chewing people out for 'lack of understanding of orbital physics' (which, btw, I saw no problem with in the question and answers), and then going on to misspell 'Hohmann' as Hoffman is not a good look. – Ingolifs Dec 9 '19 at 8:21
• complaining about a lack of understanding of orbital mechanics, then displaying a lack of understanding of cycler orbits – JCRM Dec 9 '19 at 9:04