This is a real life asteroid impact crater.
Made not up in space,
but rather, down here on Earth at the Texas A&M University,
where we fired a projectile much like this one
at 5.5 kilometers per second,
smashed it into the target, vaporizing itself,
and causing this rather massive hole.
Of course, this does remind us
that astronauts need some pretty insane protection
of their own in order to keep them safe.
And today we're going to find out how that was developed.
You're in for a pretty awesome episode.
(dramatic music)
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Before we go smashing our own very fast projectiles,
why are we doing this in the first place?
These are NASA astronauts Barry Wilmore and Terry Virts
during US EVA 30,
where they were packing up a glass fiber blanket
that had been installed
over pressurized mating adapter number two
on the International Space Station.
Previously used by the shuttle,
when that was canceled, the cover was put into place
until the port could be recommissioned
for used by the SpaceX Dragon Capsule.
Eventually one of these SpaceX missions
brought the cover back to earth
where today it's being housed inside Space Center Houston.
If you look really closely,
you can see that the surface is peppered in tiny holes
confirmed by chemical analysis
to be from sub-millimeter-sized micrometeorites
and fragments of space junk
traveling at between 4 and 12 kilometers per second.
Slightly larger pieces have temporarily disabled satellites
and cracked station windows.
This means that our engineers
are constantly trying to find better forms of shielding.
Let's leave the museum
and head to the front lines of this invisible battle.
To create the next generation of shields,
we need to really understand the underlying physics,
which is what brings me
to the Texas A&M Hypervelocity Impact Lab.
Located in the Center for Infrastructure Renewal
on the RELLIS campus.
Pride of place is, of course,
the 13.7 meter long Aeroballistic Range.
This thing can reliably shoot projectiles
at up to eight kilometers per second.
For reference, that's about 11 times faster
than a speeding bullet
and 2.5 times that of the Navy's experimental rail gun.
Entirely by coincidence,
exactly down range from the gun
is the home stadium of the Texas Longhorns.
Our projectile can make the trip
to Austin in about 15 seconds.
All guns operate with the same basic principle,
with an expanding cloud of high-pressure gas
pushing a projectile along for the ride.
However, this one has a few extra modifications
to make that projectile much faster.
The rate of a gases expansion depends
upon Newton's famous force equals mass times acceleration.
Where we get the most rapid expansion?
If we use a light gas under very high pressure.
To maximize the force term
we use a smokeless gun powder propellant
to produce lots and lots of nitrogen gas and carbon dioxide.
An igniter sets off a two gram primary charge
which simultaneously ignites a 300 gram secondary charge
to produce two and a half thousand bar of pressure.
These exhaust gases are pretty heavy,
which means that their expansion is slow
and maximum speed gets capped out
at about one kilometer per second.
The lightest gas is hydrogen.
It could expand and accelerate really quickly,
but only if we're able to get it
to a high enough starting pressure.
With this equipment we get that by using two stages.
First, the gun powder accelerates
a piston down the pump tube,
ahead of which is a lot of hydrogen gas
kept in place at the central breach by a steel disk.
The piston compresses our hydrogen
until its pressure is so high that the disc bursts.
Released from its high pressure confinement,
the super-light hydrogen accelerates itself,
and a conveniently placed projectile,
at 3,000,000 G's down the launch tube.
Meanwhile, the piston gets caught at the central breach
where it can be extremely annoying to remove.
Down range from the gun
we enter the first of two large tanks.
This one is typically filled with low-pressure nitrogen.
If we're using a sabot to hold a shot-type prop propellant
then the small amount of aerodynamic forces
will rip apart the container,
allowing the projector, and nothing else,
to continue forward.
The end tank is where we install our sample,
today, this 15 by 15 by 1 centimeter piece
of high-density polyethylene.
We'll keep the tank at a vacuum
to better represent the conditions of outer space.
If we add a bit of air back in
then this represents the upper atmosphere.
Add in too much,
then our projectile turns into a shooting star.
("Shooting Stars" by Bag Raiders)
We're going to see what's going on
by using a 1 million frame per second high-speed camera
illuminated by a panel of ultra-bright LEDs.
These noodly appendages are x-ray tubes
linked to a bank of capacitors.
They could allow us to see what's going on
inside the target during the collision
but for today, they're a little bit overkill.
Laser gates measure the time it takes
for projectile to move a known distance
and extrapolates this velocity forward,
starting the camera to record
just as the projectile is expected
to enter its field of view.
The team seals up the container,
pumps out the gas and we're ready for our test.
- [Jacob] Charging up.
Firing in 3,
2, 1.
(gun loudly booms)
- [James] We hit the target
at precisely 6.50098 kilometers per second,
releasing 63,394 Joules of energy
making this the most energetic collision
yet achieved in the facility.
- [Jacob] 2, 1.
(gun loudly booms)
(gentle ethereal music)
- [James] As the projectile impacts our target,
we see a bright flash as the outer layers vaporize
and resulting gases incandesce.
On the front face, ejecta are launched backward
while on the back debris are strewn forward.
Even our bank of ultra-bright LEDs
aren't enough to illuminate this cloud,
so dense the amount of material being spewed out.
Eventually it disperses enough to reveal its fine structure
but just as our view gets interesting,
the camera runs out of memory and the recording ends.
The A&M Hypervelocity Impact Lab
has today conducted more than 300 tests,
which they've used to develop
new forms of spacecraft shielding.
To show you how that's done,
I've set up a projector with some of my favorite shots.
You might expect that harder materials
are going to give us better impact protection.
But as this test with high-density polyethylene
versus ultra-high-molecular-weight polyethylene
is about to show, things are a bit more complicated.
Both have the same density but ultra-high-molecular-weight
is comprised of longer chains.
At slow speeds, below about one kilometer per second,
this gives ultra-high-molecular-weight
the better impact resistance
because these chains stick together more effectively
than the shorter ones of HDPE.
Repeating the experiment again,
but this time at five kilometers per second,
we get this.
Although the debris clouds are slightly different,
because these are literally different materials
with different failure mechanisms,
the general shape and mass loss is almost identical.
That's really important because it means
that when you have very-high-speed collisions,
it's not the material hardness
or strength that matters, but rather it's density.
If we'd used a denser material,
it would take physically more energy
to move the molecules away.
That is a big problem for spacecraft travel
because it means we need to send up really heavy shielding
which takes away mass
which would better be put to other uses.
One idea is to spread the original impact
over a very large area so that no one region
experiences a devastating amount of energy.
Developed in 1947 by Harvard astronomer, Fred Whipple,
this is the Whipple shield.
A bumper plate breaks up the projectile
into an expanding cloud of debris
which is spread over a large region of wall.
It works even better if you're able
to add in a second plate behind the first.
In this test, we'll get a bright blue flash
from the vaporized aluminium outer layer followed by a red
as we burn the second layer made of ceramic.
a good Whipple shield will half your mass requirements
but does so at the expense of a lot of volume.
In many ways this can be just as bad.
Fortunately, an ongoing research project shows some promise.
The idea for the new shield comes from this test
where our projectile hits an inclined piece of carbon fiber.
Rather going straight through as you might expect,
instead, the debris has come off
at right angles to the plate.
That's really important
because it means that if we keep adding in
more and more of these sheets,
then effectively we can turn the debris right back around.
Using a honeycomb structure,
we're able to get all of the required angles
while simultaneously achieving enough structural integrity
to replace the station's original pressurized hull,
overall, saving significantly on mass and volume.
When put into practice,
this means that our next generation of space stations
will be able to dedicate
more resources towards useful payload
while simultaneously being even safer for astronauts.
As we're able to test
an ever-increasing range of target materials,
projectiles, and impact velocities,
we're able to develop a much better understanding
of the underlying physics.
As we do, we're able to make this final frontier
just a little bit more safe,
or at least to better characterize
the dangers we are going to face
as we explore beyond our own Earth.
Until next time, this has been James Dingley
from the Atomic Frontier.
Keep looking up.
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