Nukes in space? Orbital detector could sniff out warheads

**Benjamin Thompson** (0:24)
Welcome back to the Nature Podcast. This week, Detecting Nuclear Weapons in Space and How the Forest of Knowledge is Under Threat.

**Nick Petruchow** (0:33)
I'm Nick Petruchow.

**Benjamin Thompson** (0:35)
And I'm Benjamin Thompson.
First up on the pod this week, we have a story about a potential way to detect whether satellites in orbit are secretly carrying nuclear weapons.
Now, a detonation in the vacuum of space wouldn't have the characteristic mushroom cloud and fireball we see here on Earth. What would happen is that there'd be an initial electromagnetic pulse capable of frying electronic circuits. Following this would be a massive buildup of high-energy electrons whizzing around the Earth. These could strike and damage things like solar panels, potentially rendering military, scientific and communication satellites useless, causing chaos back here on the ground. Back in 1962, the US conducted a test called Starfish Prime, which saw a nuclear weapon detonated about 400 kilometres above the Pacific Ocean.
Within months, and unexpectedly at the time, this event caused roughly one third of the satellites then in low Earth orbit to fail. This was by no means the only high-altitude test conducted by the US and by the Soviet Union, but the aftermath of this event ultimately led to countries signing the 1967 Outer Space Treaty, which prohibits weapons of mass destruction being put into orbit. Fast forward to today, and tensions are rising around the world, with claims and counterclaims about whether states are planning to sneak a nuke into space. After all, there are several orders of magnitude more satellites in low Earth orbit than there were back in the 60s. A detonation would be catastrophic. But there's a problem. If a nation had sent up a nuclear device and broken the Outer Space Treaty, how would you know? Well, that's the subject of a paper in Nature This Week, specifically a modelling study, suggesting that satellites carrying nuclear weapons in space would emit a telltale signal that could be detected using another satellite. The author of the paper is Areg Danagoulian, from the Massachusetts Institute of Technology in the US. I caught up with him on the phone about the work, and he explained why it's tough to detect a nuclear weapon-equipped satellite.

**Areg Danagoulian** (3:03)
In general, detecting nuclear weapons is not so trivial, because they have radioactive components, but those components are not very radioactive. If you move 50 meters away from them, the signature from a passive nuclear weapon is negligible. You are not really going to detect it. You have to come pretty close to it. So if you wanted to really do this, you would send some kind of an inspector satellite that would come up close to this device, and then it can employ something that is referred to as active interrogation methods. You shine something at the suspect, hoping to trigger a process that would reveal whether it has nuclear weapon on board or not. But that's a fairly hostile thing to do. The other side might think that you are trying to destroy their satellite.
So something that is politically much more feasible is, let's refer to them as passive things, meaning that you are not shining anything. Instead, you are trying to use existing processes that are taking place anyway, whether you are there or not, and try to identify processes that are specific to nuclear materials.

**Benjamin Thompson** (4:06)
You're looking at this passive method then, and in your paper you put forward a potential way then to detect a nuclear weapon equipped satellite. And it is an indirect method that begins with existing high energy protons trapped in Earth's magnetic field. Tell me about that.

**Areg Danagoulian** (4:22)
So, many of these protons have energies of approximately gigaelectron volt. We call it GV, gigaelectron volt. And those energies that are high enough, those protons, when they strike the uranium inside the nuclear weapon that the satellite is supposedly carrying, if it's carrying it, they basically rip this nuclei in pieces.
The binding energy of the nuclei, which in general is extremely strong, is nothing compared to the kinetic energy of the protons that are striking those nuclei. So, they shred this nuclei, okay, and they free up lots of neutrons. One proton for the right energy, depending on the circumstances, may generate 14 neutrons, 10 to 14 neutrons. And because there's a lot of protons to start with, in the end, you are having an enormous number of neutrons. We're talking about millions of neutrons generated per second. So, there's like a multiplying effect when these GLE protons interact with uranium.
And those neutrons, that's the telltale sign of the presence of uranium on the satellite.

**Benjamin Thompson** (5:22)
And so, this signature then of these neutrons is what you put forward in your paper as a potential way to pick up whether a satellite has a nuclear weapon on it. And the idea is this signature would be picked up by another satellite, a shoebox sized satellite. How would this detection work?