The Earth may not be our home forever.
Eventually, we may have to leave.
What if instead of finding a
potentially habitable exoplanet
light-years away,
we stayed in our Solar System 
and built a habitat so enormous
we could never overpopulate it?
This is WHAT IF,
and here's what would happen
if we built a ringworld in space.
Imagine you lived on a ring
with a radius of 150
million km (93 million mi)
encircling the Sun.
A gigantic artificial world
with its own gravity,
ecosystem and atmosphere.
Big enough for trillions
of humans to call home.
You'd live on an enormous landmass
on the inner side of the ring.
The outer shell would protect you
and all those trillions of people
from the hazards of outer space.
Problem is, assembling such a thing -
suspended out in the Solar System -
wouldn't be easy.
You couldn't just pull the Earth apart
and have an army of robots.
reassemble it into a ringworld.
Among the many problems you'd run into,
your first would be finding the material.
The International Space Station roaming
the Earth's lower orbit right now
weighs about 420 tons.
Something like a ringworld
would tip the scales at
no less than a million tons.
Where would we find all this material?
I know some places.
The Kuiper Belt beyond the orbit
of Neptune would do just fine.
This ring of icy bodies stretches out for
almost 3 billion km (1.9 billion mi).
Some astronomers think the Kuiper Belt
would have enough material
for this project, but...
it's hard to tell exactly how much we'd need
to construct a thing like this.
We might have to sacrifice all the planets,
moons and asteroids in the Solar System.
Our ringworld and the Sun
would be the only things left.
If we could manage to gather and
transport all the material available,
then construction would begin.
It would take a lot of physical labor,
an army of robots and maybe
hundreds of generations
to realize that our structure
was not stable enough.
Because the megastructure
would turn out so enormous,
it would break any
known molecular bonds.
We'd have to find a way to make use of
one of the fundamental forces of nature -
the strong nuclear force.
Of all forces, it's the grippiest.
It bonds material on the
scale of atomic nucleus
so that nothing can break it apart.
Or maybe we'd come up with a new
super-strong material altogether.
But until we figure that out,
every interstellar body passing
through our Solar System
would be a threat to our megastructure.
The next thing we'd have
to worry about is gravity.
That part is pretty easy -
we'd just have to spin the ringworld at
nearly 2,000,000 km/h (1,200,000 mi/h).
I know, that's really fast.
We'd have to build up the speed over time.
Luckily, maintaining it in the
frictionless environment of space 
wouldn't be too hard.
Such rotation would generate centrifugal force,
and that, in turn, would
create an artificial gravity
equal to the one we have here on Earth.
With gravity solved, we'd
bring in the atmosphere
and start populating the ringworld.
For the inhabitants of the megastructure,
it would always be daytime.
Unless we could create a day and night cycle
with extra panels inside the ring.
But for all the epicness of
the world we just created,
it wouldn't be stable.
A single asteroid strike could cause
the structure to drift closer to the Sun.
A hole punched through the ring
could let all our atmosphere out.
And a massive solar storm?
Don't even get me started on that one.
One failure inside the ring
could doom the entire structure
together with its inhabitants.
It's just too risky to build it around the Sun.  
We might have better luck
with a huge ring space
station in the Earth's orbit.
But that's a story for another WHAT IF.
