Countering constellations: Directed energy

Thanks to B. Hendrickx and Flyback for reviewing this article.

In the previous article of this series, we saw that using the large electrical power provided by a lighweight nuclear reactor the Russians could develop a jammer satellite able to severely degrade communications going through geostationary satellites. However, its utility against communications provided by constellations in low Earth orbit is more limited. To take them out of action, which is increasingly important for Russia since the USA is deploying a constellation providing communications, imagery and missile tracking, a more direct approach is needed.

So in this article, we will review how this electrical power can be used to bring to bear directed energy weapons. These “death ray” weapons were quite popular during the 1980s, when Reagan’s Strategic Defense Initiative planned to use them to destroy Soviet intercontinental ballistic missiles, and the decoys and warheads they carried. The plans did not pan out, but 40 years later the same technologies can be leveraged against easier targets. Let’s see what high power microwave, neutral particle beams, and lasers can do in this scenario.

High Power Microwave

High Power Microwave (HPM) is a form of radio-frequency weapon. It is becoming quite popular as a way to counter flying drones with negligible cost per shot, and being able to kill a whole drone swarm in a single shot due to a wide beam. US companies like Epirus have working weapons able to achieve this:

It is somewhat similar to jamming in that it uses radio waves, but contrary to jamming the effect can be permanent, and the frequency used does not have to match the one of the target’s receivers. So it is not a problem if the beam can not be kept on target forever, especially as many HPM weapons work in a pulsed mode. If the pulse has enough intensity at the target, then its electronics will become temporarily or permanently disabled.

There a two kinds of mechanisms that can damage the target. One is called front-door coupling, and consists in leveraging the fact that the target has RF hardware dedicated to collecting and amplifying radio waves, like antennas, amplifiers and analog to digital converters. These circuits are designed to work with a given level of incoming radio power. However, if they receive much more than that and do not have adequate protections, they will amplify it anyway and the resulting voltages will reach values that can cause permanent damage. An easy way to protect against that is to have an analog bandpass filter early in the chain, and that is quite common, so this kind of attack works best using the frequency that the target is supposed to receive. Further protection against in-band attacks is possible by using diodes to stop currents above a given threshold.

The other way, called back-door coupling, works even in the absence of a RF system on the target. Here, the radio waves penetrate the casing of the target and induce excessive power in all of its components. The accepted threshold to cause permanent damage with this is an electric field intensity of at least 15kV/m, meaning a power of 600 kW/m². The target can increase this threshold by being completely enclosed in a conductive shield (for instance a Faraday cage) that prevents radio waves from penetrating the casing. This is not too hard to do on a satellite, except maybe for outward-facing systems like optical focal planes.

Note the peak power is not directly related to the electrical power of the spacecraft. For pulsed HPM, the power is typically stored in fast-discharge capacitors, and the spacecraft can fill those over time. A low electrical power will only limit the repetition frequency of the pulses, but if one pulse is enough to disable the enemy there is no need to repeat them. In fact, the pulses are typically a hundred of nanoseconds, so a 1 GW pulse is only a radiated energy of 100 Joules. Even when taking into account the limited efficiency of the generators and amplifiers, there is not a need for a lot of power on the satellite, so HPM might be doable without a nuclear reactor or extra large solar panels.

Here is a table with the characteristics of 3 different models and their performance for a target 1000 km away:

	Peak Power	Frequency / Wavelength	Antenna diameter	Spot size diameter at 1000 km	Peak intensity at 1000 km	Max range for permanent back-door damage
Model 1	1 GW	10 GHz / 3 cm	10 m	7 km	50 W/m²	10 km
Model 2	1 GW	30 GHz / 1 cm	20 m	1 km	2 kW/m²	60 km
Model 3	100 kW	100 GHz / 3 mm	10 m	0.7 km	0.5 W/m²	1 km

The maximum range to reach the 600 kW/m² back-door coupling damage threshold is only 60 km for the best design. If the HPM is put in a random low earth orbit, that is a quite close miss distance so it will not be achieved often, meaning degrading a constellation without a proper orbital positioning of the attacker will take a lot of time. A good approach for that is to use the HPM satellite as a co-orbital ASAT. It could position itself in one of the orbital planes of the constellation. There, it can perform a series of orbital rendez-vous or fly-bys with each of the satellites of the plane and disable them, like this:

Neutral Particle Beam

Neutral Particle Beams (NPB) use atoms sped up by particle accelerators. Their main advantage is that they can achieve much tighter beams than optical or radio systems, because they are in practice not limited by diffraction. NPBs were all the rage during the Strategic Defense Initiative program. This led to the only launch of a NPB into space, the Beam Experiments Aboard a Rocket (BEAR) test of 1989.

BEAR was successful, the NBP fired as expected in space and since the launch was sub-orbital, it came back under a parachute and was tested again afterwards, and it still worked. Using 1980s technology, it fired a neutral hydrogen beam of 1 MeV energy and a 1 mrad divergence (ie a spot size of 1 km at 1000 km), using 100kW of power drawn from batteries.

Accelerators have come a long way since then. Notably, in 2020 the Pentagon wanted to resurrect the effort to have a NPB in orbit by 2023, but this was not funded. On the topic of NBP for space warfare, the best up-to-date resource is ToughSF’s detailed review. The main recent developments compared to BEAR is that it used a non-superconducting accelerator, and 70 kW of power was thus lost as heat due to Joule effects in the accelerating cavity. Modern accelerators cool the cavity with liquid helium to reach a superconductivity state and eliminate almost completely the electrical resistance. The main limit on efficiency then become the power converters, and those can reach up to 90% efficiency. Another aspect of BEAR was that obtaining a tight beam was not a priority: 1 milliradian is quite poor, ToughSF presents a design with current technology that is much better, reaching 40 nanoradians. And that comes in at less than 2 tonnes for 1 MW of power. Here’s an overview of its performance:

	Power	Aperture diameter	Spot size diameter at 1000 km	Intensity at 1000 km	Time to burn 1 cm deep at 1000 km
ToughSF NPB	1 MW	5 cm	8 cm	200 MW/m²	~1 s

	Power	Aperture diameter	Spot size diameter at 1000 km	Intensity at 1000 km	Time to burn 1 cm at 1000 km
scaled down NPB	100 kW	5 cm	8 cm	20 MW/m²	~10 s

Here’s what that means in practice:

Here we have the same target constellation as before, but the attacker has not matched the orbit of the plane. Still, when the planes cross, the attacker very often gets within 1000km, and thus could kill a satellite of the plane. Repeating this for all the planes, detailed simulations (saved for a future article) show the full constellation can be killed within less than a day.

It seems difficult to develop countermeasures against NPB, since they are so powerful. If somehow the targeted satellite can detect than it is under attack and react autonomously in a fraction of a second, it could put itself on a spin to spread the heat on the whole surface, but the maneuver would have to be very brutal to succeeded. The best defence is probably the offence, by shooting the NPB platform itself using a kinetic ASAT for instance, before it has significantly degraded the constellation. The US could do it with the SM-3 block II and GBI anti-ballistic missiles. Even it it creates debris, it can be worth it to save the constellation. Maintaining a co-orbital ASAT in the same plane as the NPB is also a possibility, but it is to close it will create a situation in which the one who shoots first has an advantage. That is destabilizing and increases the chances of a conflict. So better to keep it in a well-separated orbit in peacetime.

Contrary to HPM, NPB can not work from the ground, as the atmosphere stops the beam. The space versions appears to be a very interesting weapon against constellations.

Lasers

The last directed energy system we will consider is a laser powered by the electricity from the reactor. Modern lasers are based on optical fibers, and operate with around 30% efficiency. That means to have a 100 kW beam, we need 300 kW of electrical power, and radiators to dissipate 200 kW of heat. For a reactor-based spacecraft, there are already radiators. A Russian TEM spacecraft of 500 kW of electrical power already dissipates 2 MW as heat, so adding 200 kW will not modify the architecture too much. When using solar panels however, that means dedicated radiators have to be added.

The divergence of a laser is limited by diffraction, so to have a tight beam we need a high frequency (so a small wavelength) and a large optic. Regarding frequency, fiber lasers typically operate in the near infrared, with a popular choice being 1064 nm. On the optics, Russia operates the Razdan optical reconnaissance satellites, which reportedly have a 2 meter diameter mirror. They work in the visible bands, which is more challenging than in near infrared.

A single fiber outputs at most tens of kW of power, so multiple have to be combined. A first way to do this, called incoherent combination, it is to operate multiple fibers at frequencies around 1064nm, and pass their output through a kind of prism, where light coming at different frequencies and different angles goes out at the same angle for all beams. Then that beam is sent to the main optic.

The second way is to use coherent combination, that is feeding all the fibers from a common main source, so that their light is exactly identical. Then, each beam can be sent to its own smaller separate optic, and they will act as if they were the same beam out of a larger optic. That allows to reach higher power, since there is no single part having to withstand all the power like with the prism, and solves the issue of having to manufacture a single very large optic. However, maintaining and controlling the precise synchronisation between the fibers is challenging.

So we will asses the performance of two systems, each of 100kW beam power: a baseline one with a 2m single-piece mirror as optic, and a more advanced one with a 5m aperture, which is the largest than can be put in the fairing of a Russian rocket without having to fold it. The advanced version, if using solar power and an array of small optics with coherent combination, would be similar to the DE-Starlite concept:

	Beam power	Aperture diameter	Spot size diameter at 1000 km	Peak intensity at 1000 km	Time to burn 1 cm at 1000 km
baseline laser	100 kW	2 m	1.2 m	200 kW/m²	~1000s
large aperture laser	100 kW	5 m	0.5 m	1 MW/m²	~200s

Compared to the neutral particle beams, the spot size are larger, and consequently the time to burn though 1 cm of aluminium are much longer. Even the more advanced version, which requires 300 kW of input electrical power to the laser and and either a very large mirror or coherent combination, takes 10 times longer to burn through compared to the scaled down NPB, which only requires a bit more than 100 kW of power and much less heat management. Now these numbers are pretty conservative: they are for full vaporization of the target metal plate, whereas liquefaction takes 10 times less energy and might be enough to cause damage already, and furthermore there are parts that are more exposed and fragile than others, like solar panels. Also, lasers are more compact than a particle accelerator, so they could still be an interesting option for the Russians.

Lasers can work from the ground too. However, they are affected by weather, and even in clear sky atmospheric turbulence makes it much harder to achieve tight beams with large apertures. An adaptive optics system has to be added to compensate for turbulence. Alternatively, turbulence is lower at longer wavelength, so light further into the infrared can be used.

Counter-countermeasures against a laser-armed satellite are similar than against NPB, and since it takes more time to have an effect, a “barbecue mode” where the target satellites rotates on itself to spread the power on a larger surface might have a better chance of working.

Overall conclusion on reactor-powered satellite weapons

Russia has a credible path to developing a nuclear-powered satellite with hundreds of kilowatts of electrical power, with TEM as a main effort and the larger YaEU versions as an alternative. Even if it seems the complexity of such an endeavour is not worth it compared to just using large and efficient solar panels, the country might still do it for its propaganda value, or under the pressure of its nuclear development lobby.

The main military application that the Russian designers have considered publicly is jamming. A quick review of the jamming use cases shows that the most relevant one is to park the jammer in geostationary orbit and use it to render unuseable the communication terminals that use geostationary satellites. With the added power of the jammer compared to the standard GEO satellites, it could impact several GEO satellites.

A more advanced and definitive applications is to use directed energy against the satellites of low Earth orbit constellations. Compared to a kinetic approach, this creates no debris and has a lower cost per shot. Here, Neutral Particle Beams seem to be the most interesting option, with a long effective range allowing to neutralize a whole constellation in less than a day, with no maneuvering needed. Even advanced laser concepts do not achieve comparable performance, and they waste much more electrical power as waste heat. They would require a closer approach to be effective. High Power Microwave weapons even more so, forcing a quasi co-orbital operational concept.

In the next article, we will compare these low collateral damage methods to the brute force approach of just detonating a nuclear weapon at high altitude.