Particles that damage satellites can be flushed out of orbit
Summary
- All it takes is very long radio waves
Electronic systems do not respond well to highly charged particles. Accelerated to near the speed of light by Earth’s magnetic field, such particles pack enough kinetic energy to cause damage. History offers ample evidence of the consequences: in 1859 a massive solar storm known as the Carrington event started fires at telegraph stations. A century later, a big American nuclear test called Starfish Prime released enough energetic particles to fry a third of all satellites then in orbit.
“Hot" or “killer" electrons, as such harmful particles are also known, tend to be dragged by Earth’s magnetic field into one of two doughnut-shaped belts of radiation that circle the planet. For decades scientists have wondered if these belts could be emptied of their electrons on command, a process they call radiation-belt remediation (RBR). In recent years, work on RBR has accelerated, with encouraging progress. Much of the R&D is undertaken, in some cases discreetly, by America’s departments of defence and energy.
Such work has acquired a new urgency. In early 2024 allegations emerged of Russian plans to park a nuclear weapon illegally in a crowded band of space. A detonation there could destroy a greater proportion of satellites than Starfish Prime did, as most of today’s space kit is commercial and lacks military “hardening" against a nuclear detonation’s particle storm. Dennis Papadopoulos, a professor emeritus at the University of Maryland, sees this as a catalyst for a new (and secretive) RBR initiative at the Naval Research Laboratory (NRL) for which he is to serve as adviser.
Such efforts are not as fanciful as they sound, says Allison Jaynes, a space physicist at the University of Iowa. Lightning bolts, for example, do a form of RBR all the time. This is because the radio waves they generate exert pressure on hot electrons, causing them to descend into the atmosphere, where they collide with air molecules and rapidly lose energy in a process known as precipitation. Dr Jaynes describes this effect as “a complete wipeout" of nearby radiation belts.
The wavelengths needed to precipitate energetic particles range from roughly 10km to 100km. Generating such great wavelengths, and therefore low frequencies, is hard, though not impossible. A few navies emit such “very low frequency" (VLF) radio waves for communication with submerged submarines, with the help of antennae hundreds of metres high. As it stands, though, says Craig Rodger, a physicist at the University of Otago in New Zealand, if America’s navy switched its handful of transmitters from “talk-to-the-subs mode" to the best frequencies for RBR, they wouldn’t be able to precipitate hot electrons fast enough to save satellites.
The problem is the ionosphere, which begins about 80km above Earth’s surface. Here, incoming solar radiation strips electrons from atoms and molecules, creating a layer of electrons that interfere with radio waves. At night, the ionosphere weakens them by a factor of about 100, says Dr Rodger. By day, the attenuation is an order of magnitude greater.
Aiming higher
Some are searching for workarounds with experimental antennae. Using $750,000 of American air-force money, Morris Cohen and his team at Georgia Tech in Atlanta built a specialised 275m-long antenna, designed to hop between different frequencies, and laid it horizontally in a field in Oklahoma in the summer of 2023. The flexibility such frequency-hopping confers means particles with a range of energy levels can be targeted. What’s more, Dr Cohen notes, as particles in different regions of space are susceptible to different frequencies, such a tool should make it easier to clear priority orbits. Though no breakthroughs have been achieved so far, Dr Cohen hopes a similar—but costlier—upright antenna may offer improvements.
Others hope to bypass the ionosphere altogether. In 2019 America’s Air Force Research Laboratory (AFRL) launched a unique satellite called DSX to a height of between 6,000 and 12,000km above Earth. In a feat of engineering, it deployed an 80-metre, 10,000-volt transmitting antenna (with the longest span of any uncrewed spacecraft). For nearly two years, the spacecraft generated VLF radio waves that knocked energetic particles down into Earth’s atmosphere. Michael Starks, AFRL’s head for RBR, says the mission showed that an orbiting craft could protect space assets from nuclear attack.
Wilder ideas are afoot. The Department of Energy’s Los Alamos National Laboratory aims to use a space-based electron beam to, in essence, manufacture lightning. The NRL, for its part, seeks to launch a rocket that would release 1.5kg of barium, a metal, into the ionosphere, where sunlight strips off electrons. Earth’s magnetic field would cause the resulting barium ions to spiral, generating an electric current that could, in turn, produce electron-precipitating radio waves.
Such approaches come with risks. By-products created by the precipitation of the highest-energy particles nibble away at stratospheric ozone, which shields Earth from ultraviolet solar radiation. Just how damaging a big RBR operation would be remains unknown, says the University of Iowa’s Dr Jaynes. The hope, says Dr Starks of the AFRL, is that if America can demonstrate an effective countermeasure to a nuclear attack on satellites, no adversary would give it a shot. Solar storms, however, will not be so easily deterred.
