70 April 2019 _ PopularMechanics.comTHE TOURIST’S GUIDE TO SPACEIN JANUARY, we ran an exclusive inter-
v iew w ith Elon Musk on PopularMechanics
.com, 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 inter-
view, 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 New tonian 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 goup 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 superfast, 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 superhot.
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 velocit y 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, [mean-
ing] you need an ideal velocity of about
Mach 30 to get to orbit so that you end up
with 25 and experience zero gravity, zip-
ping 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 Inter-
national 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 ver y 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 stand-
point. 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 doK NOW BEFORE
YOU GO
...W ITH
ELON MUSK
YOU CAN’T JUST FLY UP INTO SPACE WILLY-NILLY. A SMART SPACE TOURIST
UNDERSTANDS HOW TO GET THERE. THE CEO OF SPACEX EXPLAINS SOME OF THE
SCIENCE BEHIND HIS STAINLESS-STEEL STARSHIP ROCKET—WHICH MIGHT JUST
CARRY YOU TO MARS. INTERVIEWED BY RYAN D’AGOSTINO