Sciency Words: Mars Direct

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


In November of 1989, NASA published the findings of a 90-day study on the future of the American space program. That report came to be known as the 90-Day Report and established a goal of putting humans on the surface of Mars within thirty years. The methods to achieve this goal were complicated. Very complicated. Stupidly complicated, or so thought aerospace engineer Robert Zubrin.

So in 1991, Zubrin and colleagues published a paper outlining an alternative plan which they called “Mars Direct.” Zubrin further elaborated on the Mars Direct plan in his book The Case for Mars.

Mars Direct means exactly what it says: astronauts would go directly to Mars. This is in contrast to the elaborate and expensive space infrastructure ideas proposed in the 90-Day Report, which involved enormous space stations and moon bases and orbital fuel depots and fleets of giant starships, all of which would have to be built before even one person could set foot on the Red Planet.

I won’t go through all the details of how Mars Direct is supposed to work (there’s a good reason Zubrin had to write a whole book about this); I’ll just cover the basics.

Launches would take place every twenty-six months, coinciding with the regular planetary alignments of Earth and Mars. Specifically, Zubrin advocates for launches during Earth/Mars conjunctions, when Earth and Mars are on opposite sides of the Sun. That may seem counterintuitive, but because of the math and the delta-v and the orbital mechanics and… you know what, let’s just say it’s because you end up using less fuel.

Once we get this plan started, the launch schedule would go as follows:

  • First Conjunction: A single, unmanned spacecraft heads to Mars. This will be used as the first Earth Return Vehicle (ERV-1) and it will spend the next twenty-six months making fuel for itself.
  • Second Conjunction: A pair of spacecraft head to Mars. One is another Earth Return Vehicle (ERV-2) and the other will carry a habitat module (HAB-1) and four astronauts (Expedition-1).
  • Third Conjunction: Expedition-1 returns to Earth aboard ERV-1, leaving HAB-1 and ERV-2 behind. Meanwhile HAB-2 and ERV-3 launch from Earth, along with the crew for Expedition-2.
  • Fourth Conjunction: Expedition-2 returns to Earth aboard ERV-2. HAB-1 and HAB-2, now connected together, are left behind. So is ERV-3. Meanwhile Expedition-3, HAB-3, and ERV-4 launch from Earth.

The cycle keeps going after that. With each expedition to Mars, the habitat complex grows a little bigger, laying the groundwork for full-scale colonization later on, and because of the way Earth Return Vehicles are staggered, each crew on Mars always has access to two ERVs, which seems like a wise precaution.

One of the key selling points for Mars Direct is that it’s cost-effective, at least in relative terms; it certainly costs a whole lot less than what was proposed in the 90-Day Report. Also, Mars Direct would only use currently available technology, so we could start doing this right now.

But for some reason, at least as far as I can tell, no government agency or private organization (aside from Zubrin’s own advocacy group, the Mars Society) has committed to Mars Direct. Oh yes, lots of people talk about it. Sometimes people borrow bits and pieces of the plan, but no one—not NASA, not Buzz Aldrin, not even Elon Musk—seems willing to adopt it in its entirety. And I’m not sure why.

9 thoughts on “Sciency Words: Mars Direct

  1. From what I understand, most of the industry recognized Mars Direct as an important conceptual milestone, breaking everyone out of “Battlestar Galactica” type thinking, but also just about everyone realized it had flaws, including Zubrin himself, who later modified it. The modified version became the basis of the first version of Nasa’s Design Reference mission design.

    Liked by 1 person

    1. Some of this is familiar to me and some is not. I was considering including Mars Semi-Direct in this post, but it was already getting to be kind of long.

      I guess what I was trying to say here is that a lot of the proposals I’ve been reading about sound more like “Battlestar Galactica”-style missions. I’m thinking specifically of the Aldrin Cycler and Elon Musk’s giant colony ships. And even NASA seems to be stuck on the idea that we need more space stations and/or a moon base before a Mars mission can get underway.

      Liked by 1 person

      1. Hmmm. Maybe things tend to drift back to the Galactica model due to risk. One issue with a Mars Direct style plan is if an Apollo 13 type failure happens on the way to Mars, in a single craft by itself, the crew is toast. But if there’s a redundant ship or two nearby, the situation seems far less dire.

        Of course, redundancy (and lower risk) is expensive.

        Liked by 1 person

      2. It could also just be my perception of things. Maybe I’m reading the wrong articles and missing something important. Or maybe there’s a lot of talk about these bigger types of missions while simpler mission designs are quietly moving toward launch-readiness. I don’t know.

        Liked by 1 person

  2. this is what I know about Mars Direct:

    The programme called Mars Direct entails first sending an unmanned module, which once it arrived on the planet would start producing the fuel for the return journey through a procedure of methanation conceived by the same Zubrin (by causing hydrogen brought from Earth to react with CO2 obtainable on Mars in large quantities) and afterwards sending a spacecraft with human crew. This research team, waiting for the proper alignment of the planets in order to return to Earth, would have 1 year and a half to conduct scientific research.

    With this system it could be possible to send humans to Mars systematically, thereby establishing a permanent outpost.

    next step is the terraformation of the red planet….

    Liked by 1 person

      1. Musk has given me a lot of hope. So has Jeff Bezos, even though Mars seems to be less of a priority for him. But Musk’s plans for Mars are very different from Mars Direct. He’s much more interested in huge colony ships than the kind of small, incremental process Zubrin wrote about.

        Liked by 1 person

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