In January, we ran an exclusive interview with Elon Musk in which he explained, for the first time, his full thinking—and the complex engineering questions—behind his decision to construct SpaceX’s Starship rocket and booster with stainless steel. The previous design for the rocket (which was then known as the BFR) had called for carbon fiber, but Musk recalculated and went with steel due to its durability, cost-effectiveness, and ductility.
Here, in a continuation of that interview, Musk goes deep on what it takes to actually travel beyond orbit and into space. Also, it sounds like Mars will have a nice park.
Ryan D’Agostino: What don’t most future potential space tourists understand about the practicality of traveling to space?
Elon Musk: Some basic concepts about orbit and gravity, which are counterintuitive because they’re not what we experience. Like, a lot of people think if you go up high enough, gravity stops. This is not true. Earth’s gravitational reach is infinite. You could be at the other side of the universe— given enough time, if you have no relative velocity compared to the Earth, you will come right back to Earth.
The simple Newtonian formula for gravity is GMM over R squared—you know, gravitational constant times the two masses divided by the distance from the centroids. So if you were to go up 100 miles, you’re not changing the distance between you and the center of the Earth very much. The pull of gravity would seem the same to you. The reason there’s this thing called zero gravity, or microgravity—it’s really due to zooming around the Earth very fast. A rocket doesn’t go straight up. It does this arc. It only goes vertical briefly and then turns over and accelerates horizontal to the surface of the Earth.
The reason it’s going horizontal is because it’s trying to increase its radial acceleration. Its outward acceleration. If you swing a ball on a rope—like tetherball— you can get it to be basically horizontal by swinging it around very fast. That outward radial acceleration is what’s keeping it there. So what you’re actually trying to do is have the outward radial acceleration be equal to the inward acceleration of gravity. So you have net zero acceleration, and that feels like zero gravity. But actually, you’re like a tetherball.
Another way to think of it is like a marble in a funnel. Gravity looks like a funnel in space-time. And if you spin a marble around anywhere on that funnel, it’s gonna roll right into the middle. Even if the funnel’s really big. That’s what gravity looks like. It’s gonna orbit the center of the funnel, very slowly at first, and then it accelerates, in terms of revolutions per second, as it gets closer and closer, and then in the middle it’s going super fast, okay? That’s gravity.
So you’ve got to spin the marble up. That’s what you’ve got to do with a rocket.
RD: So then what are the implications for reentry—or entry, on Mars?
EM: Entry is hard. Sure. It’s super hot. These things all kind of add up when you think about it. Like, why is entry so fast? It’s because you’re coming in from a very high velocity. You’ve got to slow it down from a crazy-high velocity.
Minimum velocity needed to stay in low-Earth orbit is about 25 times the speed of sound once you get there. You generally scrub about 5 Mach getting there, [meaning] you need an ideal velocity of about Mach 30 to get to orbit so that you end up with 25 and experience zero gravity, zipping around the Earth. In low-Earth orbit, you’ll be zooming around the Earth every 90 minutes.
Something that really surprises a lot of people is when you tell them that the International Space Station, which looks like quite a large ungainly structure, is going around the Earth at 17,000 miles an hour. It completes an orbit every 90 minutes.
It’s very fast.
RD: Let’s talk about Mars. You must have spent some time thinking about those first minutes, hours, and days.
EM: On Mars?
RD: On Mars.
EM: Ahh, not really. I mean—not at the granularity of minutes.
RD: Is that because the focus right now is so much on getting there?
EM: Yeah yeah, you need to get there. That’s a big deal. I think Starship will also be good for creating a base on the moon. We’ll probably have a base on the moon before going to Mars.
RD: Could you simulate a Mars Base Alpha on the moon?
EM: It would be quite a bit different because the gravity on the moon is much less, and the moon has an atmosphere. But once you get there, it’s quite manageable. That’s not the hard—there’s a lot of work to do once you get there, but it’s not like, Oh my god, we’re on Mars!
There will be that, from an awe standpoint. But if you got there and you’re alive, the hard part is accomplished. That’s the hard part.
RD: But the planning that will have gone into knowing what you’re going to do when you get there—for food, for water, for fuel.
EM: Once you get there, that stuff is relatively straightforward.
RD: Food. What’s the plan?
EM: I mean, the easy way to do the food would be just to do hydroponics. You essentially have solar power—unfoiled solar panels on the ground, feed that to underground hydroponics, either underground or shielded by wires, dirt. So then you can be sure that you don’t have to worry about excessive ultraviolet radiation or a solar storm or something like that. Really pretty straightforward. You could just use Earth hydroponics. Earth hydroponics will work fine.
For having an outdoorsy, fun atmosphere, you’d probably want to have some faceted glass dome, with a park, so you can walk around without a suit. Eventually if you terraform the planet, then you can walk around without a suit. But for say, the next 100-plus years, you’ll have to have a giant pressurized glass dome.
RD: You seem unimpressed by the people who say you can’t terraform Mars.
EM: Of course you can terraform Mars. Why would they think you can’t? You totally can.
RD: And making fuel once you’re on Mars, you don’t see as a problem?
EM: Well, it’s a tricky engineering problem, especially in terms of the energy required—you need a lot of solar panels or you need a nuclear power plant. Then, if you have a methane-oxygen system—which is what we’re talking about, since the atmosphere of Mars is primarily CO2, and there’s a massive amount of water ice, so you’ve got CO2 and H2O, from which you can make CH4 and O2—it’s a simple Lego. You’ve got three types of blocks. That’s it. C, H, and O.
Technical challenges: power generation, and a good way of getting ice. There’s ice in the soil, but it’s dirty. And it’s not necessarily all in like ice form. Sometimes it’s like permafrost or something.If you can land somewhere near a glacier, where there’s only a small amount of dirt with the ice, that would be quite helpful. So you’ve got to mine some ice, basically. You’ve got an ice-mining engineering challenge, and you’ve got an energy-generating engineering challenge. Those are your problems. On Mars.
The logical thing to do is basically outfit one of the ships as a propellant plant itself, and just land it on the planet as a working propellant plant. And then you just need little miner droids to go dig up ice and bring it back and unfurl the solar panels.
RD: What are you hoping to learn from the InSight lander? Do you have people here who monitor what NASA’s finding? [Long pause.]
EM: Certainly we pay attention to any new discoveries with respect to Mars. Yeah. But I mean Mars is fairly well understood. You know, it would be helpful to have finer details about, where are the high concentrations of water ice? The more you can find high concentrations of water ice, the less work your miner droids need to do and the less work you need to do to basically heat the soil and evaporate the water and get rid of whatever isn’t water. So having high-purity ice? Very helpful. Less energy to heat it.
Still, one of the toughest things that’s really hard to explain to people is orbit versus space. Getting to space is easy. Getting into orbit is hard. It’s 100 times harder to get to orbit than to get to what you’d call, in quotes, “space.” Which is, say, the Karman line at 100 kilometers, which is an arbitrary point at which the atmosphere is fairly thin. This is typically called space. It’s arbitrary. Obviously it would be quite a coincidence if space started at 100 kilometers. I think it was randomly [named that] so X-15pilots could get astronaut wings back in the ’50s or something. You can’t orbit a satellite at 100 kilometers because the atmosphere is still too thick.
RD: Is that one of the things you most want to communicate to people? Orbit versus space.
EM: Yeah. Giant, giant, giant difference. And if you want to go to orbit and come back from orbit, now this is way harder than going to 100 kilometers and just falling back. Not that it’s easy to get to 100 kilometers and fall, but if you go to 100 kilometers and just fall back down, you don’t even scorch the paint. But if you are coming in from orbit, unless you have a heat shield, you will get vaporized. Okay? There’s meteors coming in all the time, but they generally get pretty much vaporized or pulverized into tiny pieces before they touch the ground, which is good. You don’t want to be the meteor.
You need something that allows you to reach the ground intact.
Originally posted on Popular Mechanics