- This episode of "Because
Science" is sponsored by
Star Wars: Jedi Fallen Order.
The fastest way for humanity
to expand into the stars
might not involve rockets at all
but giant gravitational machines.
Let's get technical.
(upbeat techno music)
How are we gonna get to other stars?
If humanity is to ever expand
beyond our own solar system
and become a truly interstellar
species, simply put,
we're gonna need more speed.
Our fastest rockets move
very quickly of course,
moving at many kilometers per second,
but for cosmic distances,
moving at 10 or 20 or 30 kilometers
per second just won't do.
For example imagine
traveling to the closest star
and the closest planet outside
of our own solar system.
It's Proxima Centauri B, 1.3 parsecs away,
which is 4.3 light years for
all you Han Solos out there.
Strap yourself to something
like a Saturn V rocket,
and there is no way we could
ever get to this system
within even dozens of human lifetimes.
It would take 79,000 years, and
this is simply unacceptable.
To explore the rest of the
universe, we must go faster.
Yeah, we must go, yeah, I
know, I'm going, I'm going!
All right, all right, because
of current technology,
even when we travel within
our own solar system,
we have to use all the
physics and engineering tricks
that we can to bring travel times down
and go as fast as possible,
and one of these tricks
that we use to get extra velocity for free
is called a gravity assist.
Very generally speaking,
a gravity assist is when
an object like a spacecraft
intentionally flies
very close to and around a
much, much larger body in space,
slingshotting around it, ending
up on a different trajectory
with extra speed, however,
if we look at this whole situation
from the planet or body's perspective,
we can see the spacecraft
slingshot around us,
but it has the exact same
speed it entered with
when it leaves.
These vectors are exactly the same.
And if we think about
what space-time looks like
during this maneuver, it
kinda makes sense, right?
If we imagine the planet or body creating
a depression in the fabric of space-time,
our spacecraft enters the
gravitational influence,
it picks up speed as it
gets closer and closer
to the deepest point of the gravity well,
and then as it leaves the gravity well,
it must lose all that
extra speed to return
to how much speed it had when it entered,
but we know that a gravity assist
can give spacecraft extra oomph,
so what are we missing here?
What we're forgetting is that during
a traditional gravity assist,
the planet or body that
we're using is moving, too,
and moving really, really fast.
If we change our perspective
during this maneuver
from being as though we're
standing on the surface
of this planet to being
an outside observer
watching everything happen,
we can see that the planet
in question has a lot
of velocity, too, velocity in orbit.
As you can see, the added
velocity of the planet
changes our math when we do
what's called vector addition.
The resultant velocity
when we enter the planet's
gravitational influence gets much bigger
when we leave the planet's
gravitational influence.
This is the gravitational
slingshot effect.
It's like jumping on
a giant merry-go-round
that's already spinning,
and then jumping off
with some extra velocity.
The reason we can get away
with this trajectory trick
is because of the
conservation of momentum.
Think back to jumping on a regular,
person-sized merry-go-round.
If you were to jump onto a
merry-go-round as it's spinning,
you would increase the mass of the system,
and so to keep everything conserved,
the rotational velocity of the
system would have to go down,
and you wouldn't get an extra velocity
after you jump off, but in
the case of a gravity assist,
what you're using as your
merry-go-round, so to speak,
is trillions and trillions
of times heavier than you,
if it's a planet,
so when you enter its
gravitational influence,
you do slow down the planet,
but insignificantly so,
and you add a significant-to-your-scale
amount of velocity
to your movement, and
this stealing of velocity
is why I call my ship the Velocity Thief.
You can't use that; it's trademarked.
Ha ha, thief away!
(rocket rumbling)
When a gravity assist is done
with precision and intent,
it looks like this.
Here are the Voyager I and
II probes using Jupiter
in separate missions to fling themselves
to other points in space.
No extra propellant required,
no crazy sci-fi engines needed.
Just math and science working together.
The promise of free
velocity is why in the 60s,
a prolific scientist came up with maybe
the ultimate gravitational assist,
one that could bring
interstellar travel times
to acceptable levels.
(rocket rumbling)
My head's not that big.
Slingshotting around
space has everything to do
with mass and velocity,
momentum, and so you may suspect
that moving yourself around
some celestial object
with more mass and more velocity
would make for a better slingshot.
Well, that's exactly
what legendary physicist
Freeman Dyson thought too,
yes, of Dyson Sphere fame,
and that's why in a 1963 paper,
he came up with a way to
slingshot yourself around
something with ridiculous
velocity using a body
much larger than Jupiter
and moving a lot faster,
white dwarf stars, but
this idea wouldn't involve
just any old stellar remnant
fighting off collapse
through electron-degeneracy
pressure alone.
No, this would involve a pair
of binary white dwarf stars
orbiting each other, either naturally,
or placed in an unnatural orbit by some
ridiculously-advanced civilization
and orbiting each other
really, really fast.
No, no I know, really
fast, I'm getting to it,
just a second, yeah yeah, geez, all right.
Yeah, I'll do the math, geez, stop it.
Gravity assists are so promising
as a transportation tactic
because of how the math works out,
to let's imagine that we
want to move our spacecraft
around our slingshot star in question
in a path that looks like this.
When the two gravitationally
meet and interact,
they're gonna interact with
each other in relation to
their relative velocity between them,
so if down is positive
in our picture here,
we can actually add these
two velocities together.
And on the other side of this maneuver,
our spacecraft will actually
add the star's velocity
to its own which sounds a little weird,
but you can think about it
like throwing a tennis ball
at an oncoming train and
having the tennis ball
bounce off at the train's velocity plus
the relative velocity between them.
A lot of textbooks use that example,
but don't throw things at trains.
With the right slingshot,
a spacecraft could add twice
a star's orbital velocity
to its own, and if that
star is going fast enough,
this might all get us
quickly across the cosmos.
The original paper, entitled
"Gravitational Machines,"
envisioned a gravity assist
that looks like this.
A spacecraft would enter this looping path
in a binary white dwarf star system,
and then fling itself out of the system
with twice this velocity,
and for orbiting stars like this,
all we need to know to
figure out what velocity
they're orbiting at is
their radius and their mass.
Dyson envisioned for this
scenario white dwarf stars
with one solar mass, the mass of our sun,
and radii of 20,000 kilometers each,
and you have this equation,
and you can look up Newton's
gravitational constant
and the mass of our sun, so pop quiz!
If a spacecraft could
leave a white dwarf system
with twice their orbital velocity,
how quickly could this slingshot
get our spacecraft going?
You can look up these values.
Here's your equation.
I want you to try the math
yourself; it's empowering.
I'll, I'll wait.
(Caribbean techno music)
The correct answer is C,
almost 1% the speed of light.
This is the true power
of a Dyson Slingshot,
a gravitational machine
that could get us to
the nearest star in under
500 years instead of 79,000,
but why stop here?
If these assists want even
more mass and more velocity,
why don't we use the densest
objects in the universe
and make them move even faster?
If you squeeze a white
dwarf star down even further
so that the only thing
keeping it from becoming
a black hole is neutron pressure,
then you get one of the
most extreme objects
in the universe, a neutron star.
When gravitational machines
were first conceived,
neutron stars were only theoretical,
but the math was done on them anyway.
You remember our equation, right?
Well let's give our neutron
star some realistic numbers.
Let's say they each have again,
one solar mass, the mass of our sun,
and each are just 20 kilometers in radius.
This star wouldn't even
over all of Los Angeles,
so I'm not gonna just
tell you what velocity
we could reach in relation to light speed
because pop quiz!
You have the variables,
you have the same equation.
I want you to try to get
the right answer for this.
It feels really good when you get it,
and I'll just wait here, hyah!
(Caribbean techno music)
The correct answer is A,
a little over a quarter
of the speed of light.
This is the true power of a theoretical
neutron star slingshot,
a gravitational machine
that can get you going so quickly,
you could reach the nearest
star, not in generations,
but in under 16 years, and
as you made this journey,
because of time dilation, you
would age seven months less
than everyone else in the universe.
This is interstellar travel made possible.
And it gets even more sci-fi from here.
Gravitational machines could
in theory be so effective
that we could imagine
an advanced space-faring
civilization rigging up a
network of them across a galaxy.
Think of a ship bounding across the galaxy
from one neutron star binary
to the next artificial system
traveling at a quarter
of the speed of light
between distant destinations
carrying goods and services
and people, making
interstellar life possible,
and if we couldn't do this,
we still might want to look for these
suspiciously-specific binaries
because if we could find them,
it might imply some advanced
extraterrestrial intelligence out there.
Of course, like any sci-fi-sounding idea,
there are a lot of problems here, too,
the first being we are talking about
solar-system-scale engineering here,
something we may be hundreds,
if not thousands of years away from,
or we just may never get there.
And we didn't even mention
how incredibly dangerous
it would be to manipulate
something like a neutron star,
something that would spread
you across its surface
into a thin film of protoplasm
just with its gravity alone,
and we're not talking about something
that is orbiting slowly.
In this binary star system,
the orbital period isn't like a year
like how long it takes the
earth to go around the sun.
It's like five milliseconds,
extremely quick,
and this kind of orbit would
of course rapidly decay
and explode and rip itself apart
if not constantly tended to.
Hey, hey, hey, hey, hey.
Trying to run an
interstellar operation here.
We would need a real
mastery over the cosmos
to make Dyson Slingshots a reality,
a humanity advanced enough to move
whole star systems around, and right now,
we kinda have trouble just moving
rockets and satellites around,
but if we did ever get to this
fantastically-advanced point,
gravitational machines
could be a realistic way
to extend our cosmic
horizon from what planet
should we explore next to which star?
Because science. (laughs)
(upbeat techno music)
Thanks again to Star
Wars: Jedi Fallen Order
for sponsoring today's episode.
You play Cal Kestis, a young Jedi padawan
who narrowly escaped the purge of order 66
following the events of Episode
III: Revenge of the Sith.
On a quest to rebuild the Jedi order,
you must pick up the pieces
of your shattered past
to complete your training,
develop powerful new force abilities,
and master the art of
the iconic lightsaber,
all while staying one
step ahead of the Empire
and its deadly Inquisitors.
Star Wars: Jedi Fallen
Order is available now
on Xbox One, PS4, and PC.
The reason you can get twice
a star's orbital velocity
with a gravitational machine
like we were talking about
is kind of a weird
gravitational interaction
that mimics a perfectly elastic collision.
This is the exact same thing
that happens when you drop,
say, a golf ball and
a basketball together,
and they both hit the ground,
and then the golf ball
goes shooting way off
because of the conservation of momentum,
because the basketball's so much heavier,
the velocity gained by the
golf ball is so much more,
and again, you could prove
this by throwing a tennis ball
at a moving, oncoming
vehicle, but don't do that!
(logo chiming)
