This thread about the potential future of a crewed Mars mission got me thinking and rather than cluttering up that thread, I ask my questions here. I want to be clear though. I know this is a fantastically expensive and technically daunting proposal here. The long term cost is probably unsustainable and the political will to do this is non-existent. None of that is relevant here. All I’m asking about is the orbital mechanics and the scheduling of crew rotations.
I know that due to the orbital paths of the planets, we periodically have a good launch window for a relatively fast and efficient trip from Earth to Mars. My memory of this is vague and probably wrong on many level but ISTR that roughly every 18 months, we get a small time frame where the travel time to Mars is as short as it can be - roughly nine months to get there. True?
If so, how soon is the next available return to Earth window? Is it on a regular schedule or is it more chaotic?
Basically what I’m envisioning is that we send a crewed mission up and they begin construction of what is to become a permanent Mars base and a potential start to a self sustaining colony. At the next launch window from Earth, we send up a second crew. Nine months later, after the second crew arrives on Mars, the first crew takes the next available return window to Earth. Keep going like this with every available window to Mars, send another team from Earth with each team bringing supplies to expand the facility’s functionality and the previous team taking the next window home. What is this launch and travel schedule going to look like? Is it predictable as a clock or is it more complex than that?
What I recall with both planets in the best position, it would take 6 months for a Terra to Mars flight. It would take a Terran year for the planets to come to the best positions for a Mars to Terra flight, and that flight would take 6 months. Unmanned supply flights could launch from less advantageous positions and take longer, but they won’t need life support or rations for a crew/passengers.
Uncrewed Mars support are outside of this thread since there is probably no need to account for their return trip. This discussion is about the scheduling of crew rotations for a continuously occupied Mars base.
It actually takes about 780 earth days for Earth and Mars to come to the same position relative to each other, so optimal launch windows would be a bit over two years apart.
ETA: It takes one year for Earth to return to the same point in space. However, during that year Mars has moved more than halfway through its orbit so is on the other side of the sun. Earth has to “catch up” to Mars before they are in the same relative position.
EETA: As to the time between when a vessel arrives at Mars on the optimal path and when the window for an optimal return trip opens - that is beyond my pay grade.
EEETA: And speaking as a local, we don’t want you kind up here anyway.
Your’re thinking of a Hohman transfer orbit. The launch window is every 26 month for those and the travel time is nine months.
However, those are not the fastest way to get between two planets. They’re the most efficient, that is get the most payload there for a given amount of fuel. But if you need to get there faster, there are shorter transfer orbits. With a larger amount of fuel, you could get there in only a couple months or even less. For fragile payloads (living beings, for example), these are probably the way to go, since they minimize radiation exposure and time in zero g.
The OP assumes that there will be regular return trips from Mars. Is that really possible? Given Mars’ gravity, is it really realistic to expect regular returns to Earth? Since the Mars colony will be starting from scratch, how long will it take to have a Cape Kennedy / Johnson Space Centre equivalent for launching the return trips?
All the early missions will take a landing craft with them that is also capable of attaining orbit from Mars’ surface. Once in orbit this launch module will rendezvous with an orbiting craft, which will then return to Earth. Mars missions have an interesting profile, and lots of things could go wrong; but there won’t be a need for a Mars JSC equivalent for many decades to come.
Totally unrelated with the topic at hand, but I noticed your signature, and I in fact remember being very impressed when my ex’s father, then about 85 years old, planted 100 apple trees (doing the work himself). That’s an anecdote I often repeat.
For the record, he’s now 93, and can seat in the shade.
Exactly - given unlimited power, say a much-hyped fictional fusion drive or something, there is no limit to how fast you can get to Mars other than how much acceleration the human body can endure. For ship that have to carry enough fuel, and enough fuel to get back too, the Hohmann transfer uses the least amount. You use just enough fuel to raise to an elliptical orbit that has as its low point earth’s orbit (perihelion) and as it’s high point (aphelion) the orbit of Mars. Time the launch from earth orbit to reach Mars orbit when Mars is right there. Then burn enough to match Mars’ solar orbit and be captured into an orbit around Mars. Ditto for return - slow your orbit so it’s elliptical and when it reaches it’s lowest point, it is at Earth at the same time earth reaches that point in its orbit.
Look at the moon landings… but more advanced. We won’t send a giant ship to Mars that is intended to land and take off and return to earth. The main ship will sit in orbit around Mars until departure. There will be landers similar to the lunar lander just to shuttle stuff up and down, to meet arriving ships. The first few trips (or several? All?) may include larger one-way landers to bring bigger components o a Mars base to the surface to assemble a base, with no intention of launching. Then the humans going back will use a much smaller craft to return to the ship in orbit. (I.e. leaving pieces like the lunar lander’s base, only bigger)
The key point will be finding/synthesizing rocket fuel to minimize the amount of cargo that needs to come from earth. This is why finding water is so important. Solar power can use electrolysis to separate that into H2 and O2 for use in rockets, and then store it in tanks until the next launch. Someday they may refuel the orbital ships as they arrive, freeing up capacity for more cargo coming from earth.
The idea with SpaceX’ “Starship” is that it won’t need to launch from a specially-built launch pad. I don’t know how realistic that possibility is, even stainless steel rockets are fairly thin-skinned things, with lots of explodey-stuff on the inside, and blasting the ground underneath it with crap-loads of thrust upon lift-off is bound to kick around dirt and rocks and things. Could one of those rocks bounce up the wrong way and puncture the hull? Maybe. It might even be an issue on landing. For example, they built fairly specialized landing pads for Falcon 9’s to RTLS. They’re not just landing in a random field.
I guess I haven’t “read” it either. I sort of inferred it from some of the early concept art they’ve shared, like this or this. The “Lunar Surface Missions” slide from here shows a BFR landing on the Moon, where we currently have a distinct lack of landing pads.
Admittedly, some of the later Mars base buildup and “Moon Base Alpha” slides show BFR’s on landing pads. I suspect SpaceX hasn’t really worked through the mechanics of “once we land on Mars, what GSE do we need to launch again” in great detail.
Well, it’s easy to say that. But Apollo is a misleading comparison. Luna is a small, lightweight body, with no atmosphere, and so it’s easy to launch from it. You can get two men, and all of their life support equipment, into orbit using a vehicle about the size of a car. But while Mars is lighter than the Earth, and has less atmosphere, it’s still a lot harder to launch from Mars than from Luna: You’d need a return vehicle much bigger than the Lunar Ascent Stage. And now consider, how do you get that return vehicle there? To get that vehicle the size of a car from Earth to a destination beyond required the mightiest rocket ever built. What, then, would it take to launch Mars’ significantly larger return vehicle? That glib answer of “just build the rocket here, and take off in the same vehicle you landed in” elides over a lot of very big engineering challenges, big enough that it might even be easier to build the return rocket on site from local materials.
The novel The Martian used an idea of read of before and like very much; the ascent vehicles are sent before hand with supplies. It’s nice to know that your ride home is waiting there for you.
What did Apollo need to get to orbit with only astronauts, no LMs or moon cars?
But Space-x wants to land many colonists, ans that makes two or three dozens landers very expensive. I guess Musk wants to land the Big Funny Rocket on Mars.
