When Does the Bus Leave for Mars?

Our solar system is a very very big place. Happily, we had the good sense to evolve on one of the inner planets (the planets closer to the sun than the asteroid belt), where the distances are merely enormous rather than gigantic, and don't even get me started on interstellar distances which are essentially unthinkably vast.

Just to give you a sense of scale, imagine that we could take one of Boeing's shiny new 737 Max passenger jets and fly it to the sun. It cruises at 850 kilometers per hour, but there's no air resistance blah blah blah, so let's pretend it "flies" at 1,000 kilometers per hour. How long would it take to fly from Earth to the Sun at that speed? Days? Weeks? Months? As long as a year? In fact, it would take just a tad over seventeen (17!) years in an aircraft that circles the planet in just over one day. When we look up at the sun, it just doesn't look that far away, but. Good thing we live in the inner part of the solar system; things get a lot more depressing on the outer side of the asteroid belt.

Mars is an excellent trainer-wheels planet for us. It has a surprisingly useful atmosphere. Gravity is low enough to be very pleasant while still high enough to (hopefully) alleviate most low-gravity health effects. No killer bugs that drill into our skulls and turn us into mindless zombies like here on Earth. No slavering alien space monsters (that we are aware of) to rape our cattle and eat our women. No volcanos, earthquakes, tornados, hurricanes. It's not too hot or too cold (relatively speaking), and it's right next door. Nice. Comfy.

Of course, "right next door" is also a relative thing. At its closest approach, Mars is just a tad less than sixty million kilometers, so our trusty 737 Max traveling at 1,000 kilometers per hour would take six-and-a-half years. Urk. Good thing we are not going by airplane! Lacking grown-up fission or fusion rockets, we're stuck with good old chemical rockets, which can do the job, but they require us to use a "least-energy" orbit to get us there. It is a surprisingly simple process; assuming you are already in Low Earth Orbit (LEO), you point your ship in the direction that Earth is traveling (hint: around the Sun) and accelerate until you have gained 3.8 kilometers per second (13,700 km/hour). And that's it, in about seven or eight months you will be in the same orbit as Mars. That path is called a Hohmann Transfer Orbit. There is one going both ways, there is one for every planet in the solar system, and until we develop grown-up rocket drives, that is pretty much the only way to get around in the solar system.

The green line is a Hohmann Transfer Orbit.

The tricky part is that "the same orbit as Mars" is not Mars. There is only a very narrow window during which you can leave Earth and transfer to Mars orbit, and upon your arrival Mars is actually there. Earth and Mars have different orbital periods, with Earth going around the sun fifteen times while Mars goes around eight times (they are in resonance). That means that once every 780 days you can launch from LEO and Mars will be there when you arrive in Mars Orbit. That number (780 days) is called the Earth-Mars Synodic Period, and it determines the launch schedule. There is a little wriggle room, you can burn a bit more fuel and get that transfer period down to perhaps five or six months, which also opens up the launch window to thirty to sixty days. But with the current state of chemical rocketry, that's it.

So the good news is we know how to get to Mars (we've made the journey without people dozens of times), and our chemical rockets will suffice if we have nothing better. The bad news is it will take 6 to 7 months, which has implications for food, water, air, heat, and radiation protection.

A fun diagram with approximate Delta-Vees for the Earth-Moon-Mars system.

I'm sure you noticed that the above diagram contains several red arrows. Red arrows are good, it means that we can burn off excess velocity by "aerobraking", which is essentially cheating death by diving into a planet's atmosphere just enough that we don't skip off and starve to death while drifting through deep space with no fuel, but not deeply enough that we all die screaming in a melting spaceship (It's important to aim carefully and don't mix up metric and imperial units like the NASA engineers did with Mars Climate Orbiter back in 1999.) But it saves tons of fuel! There are no red arrows when going to the Moon as it has no atmosphere to speak of; no atmosphere, no aerobraking. Surprisingly, it takes less fuel to travel from Earth to Mars Surface than to Moon Surface because we can save all that fuel by aerobraking.

So that's Orbital Realities 101. If you miss the bus, the next one doesn't leave for another 780 days, and it takes six-seven months to get there. In the next blog, we talk about what all that means to the people designing and building the bus, and to the people riding in the bus.

Thanks for reading along!