It's the night of the 9th July, 1962.

On a small  atoll in the Marshall islands the sky is clear   except for a thick band of clouds on the horizon.

Suddenly the eastern horizon lights up--but it's   not sunrise.

A white flash burns through the dense  clouds, and from that flash grows a vast sphere   of green light.

Nebulous white filaments leap  from the ball and arch across the sky,   before fading as a chain of circular shockwaves  erupted from the site of the flash.

It's over in   less than a minute.

But then a dull burning  red glow grows across the eastern sky,   cut with enormous white rainbows, and lasts  for an hour and a half.

If there was anyone   from the arctic present on that near-equatorial  island they may have recognized the tell-tail   patterns of aurora dancing in the sky.

But this was no natural aurora.

It was   the result of a thermonuclear detonation  in outer space--400km above another small   atoll 1400 km distant to the east.

This was the Starfish  Prime test, and its unexpected devastation   helped usher in an era of nuclear peace.

An era that may, or may not, persist.

Starfish Prime was the largest of the  five thermonuclear tests that the United   States conducted in space.

There was a big  bang, and there was radioactive fallout,   which is its own troublesome tale.

But today we’re  telling the story of the 10^29 electrons that were   produced in the explosion or released as gamma  rays ionized the upper atmosphere.

These particles   spiralled along the Earth’s magnetic field lines,  leading to the artificial auroras I described,   but also leading to the creation of a new radiation belt several orders  of magnitude stronger than the Earth’s natural   belts and they lingered for 5 years.

A third of all low Earth satellites were destroyed due to   repeated passage through this belt.

Admittedly  at that time a third of LEO satellites was six   satellites.

Today that would be thousands.

And right after the explosion, a cascade of   ionization caused an electromagnetic pulse—an  EMP—that disrupted electronics over much of the   Pacific ocean.

In Honolulu, 900km from the blast,  hundreds of streetlights just went dark.

Starfish Prime was a devastatingly clear  demonstration of the destructive power of   nuclear weapons in space.

Clear enough that in  1963 the world’s main nuclear powers ratified   the Partial Nuclear Test Ban Treaty banning  space and under-ocean nuclear testing.

This   was followed in 1967 with Outer Space Treaty, a  broad agreement for the peaceful use of space,   and clearly bans any nuclear or other weapons of  mass destruction being used or staged in space.

But treaties are only good as far  as we obey them.

In early February,   there were media reports that US intelligence  had evidence that Russia is developing a new   space-based military nuclear capability.

There  was frustratingly little detail—we don’t even   know what type of facility this might be.

But  various pundits have speculated that it could   be about developing the capacity to perform EMP  attacks—either an actual EMP-generating satellite   or the capability for detonating nuclear  explosions in orbit, similar to Starfish.

This episode isn’t about what Russia or any other  nation may or may not be doing.

But in light of   the media rumour mill, it seems like a good time  to bring science to one aspect of this issue.

What happens if we nuke space?

What would happen  if a Starfish Prime-like event happened today?

Let’s start with what’s probably the best known  effect of nuking space—the electromagnetic   pulse—the EMP, because that’s the main direct  effect on the ground.

EMPs are all over science   fiction—great for when you want to shut down a  spaceship without destroying it.

They’re also   a handy way to obliterate modern technology  to kickstart your post-apocalyptic drama.

But   EMPs aren’t science fiction.

Real militaries  are experimenting on real EMP generators,   and as Starfish Prime showed us,  space nukes can send powerful EMPs   to the surface.

So what exactly is  an EMP, and how dangerous are they?

Let’s start with the EM part of the EMP.

An EM  wave—an electromagnetic wave is—a self-propagating   ripple of electric and magnetic field caused when  charged particles accelerate.

They travel at the   speed of light—because light is an EM wave!

When  such a wave encounters another charged particle,   those rippling electric and magnetic  fields cause that particle to jiggle,   effectively transferring energy and  information between two locations.

The wavelength of the wave determines the scale  of the jiggling.

Microscopic wavelengths jiggle   things on the scale of atoms, for example  electrons jumping between atomic energy   levels on your computer screen create EM waves  several hundred nanometers in wavelength—just the   right wavelength needed jiggle electrons in your  retina—this is visible light.

A current bouncing   up and down a distant antenna creates an EM wave  from centimeters to meters in wavelength—radio   waves—and these cause electrons in your  cell phone’s antenna to jiggle in unison.

So what about an EM pulse?

These are very short,  very intense bursts of radio-wavelength EM waves.

They cause electrons to jiggle in wires, just  like any radio signal.

The difference here is   that the intensity is so strong that  it causes a lot of electrons to move,   potentially sending high currents through wires or  even circuitry.

Generating excess current in, say,   computer circuitry will at best cause the computer  to shut down.

But current also causes heat,   and heat melts wire.

A large EM pulse will  fry electronics.

Generating too much excess   current in a national power grid will melt  transformer coils and shut down the grid.

To make an artificial EMP you basically  need to apply an extremely high voltage   across a conducting coil.

The energy requirement  typically means you can only do this for a very   short time.

The simplest EMP device consists  of a bank of capacitors connected to a power   source and multiple loops of wire, with the circuit broken  by a spark gap.

Charge will build up in the   capacitors until there’s sufficient voltage  for the air between the charge gap to ionize   and close the circuit.

All those stored  electrons suddenly flow through the loop,   generating an enormous magnetic  field which kick-starts the EMP.

That seems like you need a pretty specific  set of conditions to generate an EMP.

And so it   seems surprising that a giant explosion  in the sky produces an EMP by basically   the same mechanism.

Where does the electrical  current come from?

Why is it moving in circles?

And how does the resulting electromagnetic  radiation pile up into such a sharp pulse?

Let’s start by remembering how nukes  work.

In general we’re talking about   modern thermonuclear bombs here.

These work  by compressing a bunch of hydrogen to such   high density that it mimics the processes  in the heart of a star—hydrogen fuses to   helium and releases an enormous  amount of energy in the process.

… A small fraction of this energy is released  in the form of a very rapid burst of gamma   rays—the highest energy electromagnetic radiation.

None of that radiation makes it to the ground,   thankfully.

To gamma rays and also X-rays, Earth’s  atmosphere is basically opaque—they can travel   very little distance before bumping into some  air molecule.

But although the actual gamma ray   photons don’t make it to the ground, some of their  energy does.

When a gamma ray hits a molecule,   it typically scatters off an electron.

The energy of gamma rays is so extreme   that the electron is knocked free of its  atom and sent hurtling downwards in roughly   the same direction as the gamma ray, which  itself continues on but with reduced energy.

Now that fast-moving electron finds itself in  the presence of a magnetic field—the Earth’s   geomagnetic field.

Magnetic fields  exert a force on moving charges,   and this charge is moving fast.

The electron  begins to spiral around the geomagnetic field   lines.

Remember how we made our artificial EMP?

By sending a huge current around a loop?

Well,   here’s a current moving in a loop.

The spiraling  electron creates a miniscule EMP that travels in   the same direction as the original gamma ray.

This  doesn’t sound too dangerous yet.

But remember that   the EMP travels at the speed of light—the same  speed as the gamma ray that created it.

It keeps   up with that photon, and so further scattering  events will add to the strength of this one   narrow pulse.

In fact, as the entire thin shell  of gamma rays passes through the atmosphere,   it gets converted one scattering event at  a time into the same increasingly powerful   electromagnetic pulse that always keeps  pace with the fading gamma ray shell.

By   the time the shell is gone, the pulse  is powerful enough to do its damage.

The potency and range of such an EMP depends on  the size of the explosion and the altitude.

In general, any explosion from orbit  will affect a continent-scale region,   ultimately limited by the curvature of the Earth.

The 400km-high Starfish prime explosion blew out transformers 1400km away in Hawaii.

It’s estimated that a similar thermonuclear  detonation above a large, populared landmass   would cause massive currents in the electrical  cables that now criss-cross the continent—burning   out hundreds of the insufficiently protected  transformers connecting this network.

This   would result in blackouts across that continent and could take years to repair.

The effect   would be similar to us getting a direct hit  from a major coronal mass ejection from the   Sun—a blast from a solar magnetic storm like the one that smacked us back in 1859, before   we had an electrical grid to worry about, or that  blacked out all Quebec in 1989.

Probably we should   put more of the relatively cheap safeguards  in place to avoid the multi-trillion dollar   cost of repair after an electromagnetic  incident—whether natural or otherwise.

So space nukes can make a mess on  the ground.

They’re also not very   good for space.

I told you that the  Starfish Prime test wiped out low   earth orbit satellites.

Let’s  see exactly how that happened.

The EMP we just talked about is generated  by electrons spiraling along the Earth’s   magnetic field lines.

Those electrons  don’t go away after generating the pulse.

As they get closer to the ground near the poles, the changing angle of the field lines means   the pitch of their loops relative to those  field lines change.

When the pitch is inverted,   the magnetic force on the electrons pushes them  in the opposite direction.

They’re reflected   before they reach the ground and start traveling back up the field.

This happens again when they reach   the opposite end of the field line on the opposite  hemisphere.

In this way, an electron can bounce   back and forth until getting absorbed by the  atmosphere or knocked free by a solar photon.

But until most of the electrons are so  liberated, they remain bound in this   band that traces the geomagnetic field.

At the  same time, east-west smearing of the electrons   causes the band to form a toroidal belt  around the whole earth.

A radiation belt.

Earth has two natural radiation belts—the Van Alen  belts—discovered in 1958, only a few years before   the Starfish Prime test.

These are also made of  charged particles bouncing along our geomagnetic   field lines—but here they’re mostly high-energy  electrons and protons from the solar wind,   with some cosmic rays for good measure.

They  extend from around a couple of thousand to   60000km above the Earth, with a gap between  them at around 2-4 Earth radii.

We do not put   satellites in the Van Allen belts because their  electronics would degrade pretty quickly.

We’re   lucky that geosynchronous orbits are in this  safe zone between the belts.

Low-earth orbit   satellites are also safe if they stay below the  inner belt—below around 2000km.

And these days,   that part of space is pretty densely  packed with something like 5,000 satellites.

So you can imagine the problem if a Starfish  Prime-like event happened today.

A space nuke   can target a specific if very large region on the ground with a EMP, but it’s indiscriminate in   its damage to satellites.

Low Earth Orbit would  gain a charged particle density similar to the   peak of the Van Allen Belts, perhaps wiping out a  good fraction of satellites there.

Hopefully this   is a strong enough deterrent against such  an attack by a nation dependent on those   satellites—which includes most nations  capable of launching such an attack.

There are ways to harden satellites  against increased charge particle flux   of a new Starfish Prime-like  event.

But that’s expensive.

The United States is moving more  in the direction of redundancy,   having just launched a prototype to test its  Proliferated Warfighter Space Architecture.

This program aims to blanket low-Earth orbit with  hundreds of small Starlink-like satellites.

The   philosophy is that it’s harder to wipe out  many small, cheap satellites—whether with   an artificial radiation belt or a targeted  EMP, and they’re also easier to replace.

The effects of the Starfish Prime test were  far more widespread and damaging than anyone   expected.

But the silver lining to this mushroom  cloud is that the world came together to first   ban the most irresponsible nuclear tests, and  then to commit to the peaceful use of space with   the Outer Space Treaty.

But good intentions don’t  last forever.

Given the global existential threat   of putting nukes in space, we should choose  carefully.

Do we want to move into an era of   militarized space?

Or do we reaffirm our commitment to our 60 year-long peaceful space time.