What the Study Found
- A pulsed electromagnet reached 1.58 tesla, roughly twice a standard permanent magnet’s field strength, in spacecraft reentry tests.
- The stronger field pushed a model’s glowing plasma shock layer about 16 percent farther from the surface than with no field at all.
- Unlike fixed permanent magnets, the new coil can be rewound to fit different spacecraft nose shapes and field patterns.
- So far the system has only been tested on 20 millimetre lab models across 19 shockwave runs, not on real reentry hardware.
THE model is barely bigger than a shot glass, 20 millimetres across, and it survives for well under a millisecond in the machine that tests it. A shockwave races down a steel tube at over seven kilometres per second and slams into the little capsule shape, and for a few dozen microseconds a coil buried inside it throws out a magnetic field strong enough to shove the resulting fireball away from the surface. Engineers at Tokyo Metropolitan University built the coil, the pulse of current that feeds it, and the timing circuit that syncs the two, and in doing so gave themselves a magnet that reaches roughly twice the field strength of the permanent magnets researchers had been stuck with. That upgrade is what let them push this kind of testing further than before.
Reentry has always been a fight against heat. A capsule dropping back into the atmosphere gets hit hard enough to raise the surrounding air to several thousand degrees, and for six decades the answer has been to wear the damage: heat shield tiles and ablative coatings that char, flake and get thrown away.
Magnetohydrodynamic aerobraking (MHD) offers a different bargain: use a magnetic field instead of a sacrificial shell. A spacecraft reentering the atmosphere sits inside a shock layer, a skin of superheated, weakly ionized plasma clinging to its surface. Apply a magnetic field there and the moving charged particles feel a Lorentz force, the same push that drives an electric motor, and it shoves the plasma layer further from the vehicle. Push the hottest air away and less heat reaches the hull. The same tug also adds drag, meaning the craft slows down faster on the way in, which is the second part of the appeal.
The physics has looked promising for years. Getting a lab to test it properly has not. Most experiments to date have relied on a permanent magnet stuffed inside a small model, and a lump of neodymium tops out around 0.8 tesla and comes in whatever shape it was cast in.
Tokyo Metropolitan University
A Magnet That Can Change Shape
Led by Associate Professor Kohei Shimamura, the team swapped the permanent magnet for an electromagnet built from a custom coil and fed by what is called a pulse forming network (PFN), a chain of capacitors and inductors that dumps a huge burst of current into the coil for about a millisecond. Because there is no solid magnet core to saturate, the field can climb far higher, and because the coil is just wound wire, its shape can be redesigned for whatever nose the model has. A trigger tied to the shockwave’s own pressure signal fires the pulse at exactly the right moment, so the field is already up and holding steady when the fireball arrives.
It took some redesigning to get two very different shapes working. One model was a simple blunt cylinder; the other copied the sharper, curved nose of a real reentry capsule. Each needed its own coil, wound to a different diameter and turn count so the field distribution matched the surface it was protecting, something a cast permanent magnet could never be talked into doing.
Watching the Glow Change Under the Field
Across 19 test runs split between the two models, the coils reached average peak fields of 1.24 tesla and 1.58 tesla at the point where the shockwave hits hardest, both comfortably ahead of what a standard magnet manages. With the field switched on, the glowing plasma layer clinging to each model measurably thickened, by about 16 percent, and a high speed camera running at 200,000 frames a second caught it happening in real time. The team also picked apart the light itself with a spectrometer and found a brighter band from ionized nitrogen when the field was live, a sign that the plasma nearer the surface had gotten hotter or the hot layer had simply grown, both consistent with the shock layer being pushed back rather than an artefact of the measurement. None of it happened by chance: switch the field off, rerun the identical shot, and the glow snapped back to its narrower, unmodified shape.
What the team has not yet measured is the payoff everyone actually cares about: how much heat gets kept out, and how much extra drag the craft feels. This experiment tracked the plasma glow and its spectrum as stand ins for that, because a sub millisecond shockwave test cannot easily wire up a full heat flux sensor, and the paper says so plainly.
The tests also stayed firmly on the bench. A 20 millimetre coil surviving a burst that lasts well under a millisecond is not the same problem as a coil surviving minutes of real atmospheric entry at full heat and full scale, and the team is upfront that this system is not built for that kind of duration yet.
Even so, a magnet that can be redesigned for any nose shape and pushed well past what neodymium allows is exactly the missing piece for testing MHD aerobraking properly, rather than just once, on whatever shape the lump of metal happened to fit. Reusable spacecraft are the obvious target: a heat shield that survives by pushing plasma away rather than by burning itself up could fly again without a repair crew standing by, which is also the pitch behind a separate superconducting-magnet reentry program at the University of Queensland. The same trick could matter for probes returning from Mars, or anything punching into an atmosphere thick enough to generate a shock layer worth deflecting, an idea a European Union-funded consortium is pursuing under a program called MEESST (Magnetohydrodynamic Enhanced Entry System for Space Transportation).
The next step, the team notes, is testing the system on real reentry hardware rather than scale models in a tube. Getting there means making the magnet survive longer, hotter, and at full size, and none of that is solved yet. But the bench now exists to try.
Reference
Muramatsu, T., Shimamura, K., Kakami, A., & Katsurayama, H. (2026). Quasi-Steady Magnetic Field Generated by Pulse Forming Network for Magnetohydrodynamic Aerobraking. Journal of Spacecraft and Rockets, 1–8. https://doi.org/10.2514/1.a36635
- Study type: Peer-reviewed laboratory experiment (pulsed-magnet expansion-tube shockwave test), Journal of Spacecraft and Rockets
- Sample size: 19 shockwave test runs (7 for the blunt-nose model, 12 for the capsule-shaped model)
- Test facility: MX-6.0 free-piston expansion tube, Tokyo Metropolitan University
- Field source: Pulse-forming-network-driven air-core electromagnet, compared against a permanent neodymium magnet baseline
- Duration: Sub-millisecond per shot; magnetic field held quasi-steady for 110 to 148 microseconds per run
- Funding / conflicts of interest: JSPS KAKENHI Grant Numbers 21KK0078 and 26KJ1885; no conflicts of interest declared
- Data availability: Not reported
- Main limitation: Heat flux and drag reduction, the ultimate payoff of the approach, were not directly measured; the paper infers them from shock-layer thickening and emission-spectrum changes, which it states are proxies, not direct measurements.
FAQ
Why does a spacecraft need protection from heat during reentry at all?
Reentry compresses and shocks the surrounding air so violently that it heats to several thousand degrees right at the vehicle’s surface. Without some way to keep that heat out, it would melt or vaporize the structure in seconds, which is why every reentry vehicle so far has relied on tiles, coatings or other sacrificial material to absorb and shed the heat instead.
Could this magnetic system replace heat shield tiles entirely?
Not yet, and possibly not on its own even eventually. The system tested here demonstrates that a pulsed electromagnet can push the hot plasma layer further from a model’s surface than a permanent magnet can, which is the mechanism a magnetic heat shield would rely on. But this experiment measured that pushing effect, not how much less heat actually reached the surface, so it is a step toward the idea rather than proof it can replace tiles outright.
Why did the researchers need a new kind of magnet instead of using the ones already available?
Permanent magnets are limited to whatever field strength and shape they were manufactured with, generally topping out around 0.8 tesla, and researchers previously had to accept that ceiling and that fixed geometry. Because the new system generates its field electrically instead, it can be pushed well past that limit and its coil can be rewound to fit a different model shape, letting the same lab test a much wider range of conditions.
Is this something that could work on other planets, not just Earth?
In principle, yes. The physics behind magnetohydrodynamic aerobraking depends on the shockwave forming a layer of charged, weakly ionized gas, a condition met whenever a vehicle reenters fast enough through an atmosphere, whether that atmosphere belongs to Earth, Mars or somewhere else. The team frames its own next step as testing on real reentry hardware, without specifying a particular destination.
Cite This Page