Starfish Prime: The 1962 H-Bomb in Space That Lit Up Hawaii and Crippled Satellites

On the night of July 8, 1962, people all over Honolulu were looking west.  Radio stations had been broadcasting the countdown, and according to one veteran’s account, some hotels threw rooftop “rainbow bomb” parties so guests could watch.  About 900 miles away, over the open Pacific, the United States was about to explode a hydrogen bomb 250 miles above the Earth, about as high as the International Space Station flies today.

Nine seconds after 11 that night, Hawaii time, the sky lit up.  A white flash broke through the clouds, then the night turned green, then a deep, glowing red that hung over the ocean for minutes.  On Oahu, a few hundred streetlights went dark.  Burglar alarms went off.  A telephone company microwave link that connected Kauai to the other islands was damaged, cutting calls.  Nobody in Hawaii was hurt, but the blast had reached across 900 miles of empty ocean and tripped switches in their city.

The test was called Starfish Prime.  It was the largest nuclear explosion ever set off in space, and it surprised the scientists who planned it.  It knocked out instruments that were supposed to measure it, wrapped the planet in a man-made radiation belt, and over the following months helped kill several of the few satellites then in orbit, including the first commercial communications satellite.  Today, October 10, is the 63rd anniversary of the day the treaty that ended tests like it took effect.  Here is what happened, why it happened, and why military planners still study it.

Why explode a bomb in space?

By 1962 the United States and the Soviet Union had been in an informal pause on nuclear testing for about three years.  On August 30, 1961, the Soviet Union announced it was ending that moratorium, and within weeks both countries were testing again.  The U.S. answer was Operation Dominic, a long run of tests in the Pacific.  Within it was a smaller series of high-altitude shots called Operation Fishbowl, launched on Thor missiles from Johnston Island, a tiny, remote atoll southwest of Hawaii.

The military wanted answers about what a nuclear blast does at the edge of space.  In 1958 the United States had already set off six high-altitude explosions, including three small, secret shots over the South Atlantic called Operation Argus.  Argus tested an idea from the physicist Nicholas Christofilos, who argued that a nuclear blast in space would release a swarm of electrons that Earth’s magnetic field would trap, forming an artificial radiation belt.  In theory, a belt like that could scramble radar and radio or damage the electronics of incoming enemy warheads.  Earth’s natural radiation belts had been discovered only that year, by James Van Allen’s instruments on Explorer 1 and the satellites that followed.

Argus showed the effect was real but short-lived.  When The New York Times revealed the secret tests in March 1959, it called them the “greatest scientific experiment ever conducted.”  But the 1958 shots had been rushed and poorly measured.  A later government report said that the models built from them were too uncertain to predict what bigger bombs at other altitudes would do.  Planners had three main questions: how the electromagnetic pulse from a blast in space behaves, what causes the strange auroras such blasts produce, and how long radio blackouts last.  Fishbowl was meant to answer them.

Two misses, then a hit

The series got off to a rough start.  On June 2, 1962, the first Thor launch, named Bluegill, was lost by the radar tracking system.  With many ships and aircraft in the area, the range safety officer couldn’t be sure the missile was on a safe path, so he ordered it destroyed.  Investigators later found that it had been flying normally the whole time.

Starfish was next.  Just before midnight on June 19, the Thor flew normally for 59 seconds.  Then its engine cut out and the missile began breaking apart.  The safety officer destroyed it, along with its warhead, at an altitude of roughly 30,000 to 35,000 feet.  The warhead did not produce a nuclear explosion, but pieces of the missile rained down on Johnston Island and the surrounding sea, and some of the debris was contaminated with plutonium.  Navy divers and demolition swimmers spent the next two weeks recovering about 250 pieces.  Under the program’s naming rules, a failed shot got the same name plus the word “prime” on its next attempt.  So the second try was Starfish Prime.

It flew on July 9, 1962, at 09:00:09 Coordinated Universal Time, which was still the evening of July 8 in Hawaii.  The Thor climbed to about 1,100 kilometers, nearly 700 miles, and the W49 thermonuclear warhead went off on the way back down, at 400 kilometers, 13 minutes and 41 seconds after liftoff.  The yield was about 1.4 megatons, roughly 100 times the Hiroshima bomb, though only a tenth or so of the energy estimated for the Tunguska blast over Siberia in 1908. Twenty-seven instrument rockets were launched from Johnston Island to take measurements, and more flew from Kauai.

A sky full of rings and red light

In space there is almost no air, so there was no mushroom cloud and no shock wave to speak of.  Observers on Johnston Island, directly below, heard nothing they could attribute to the blast.  What they saw instead was a mottled red disk spreading across much of the sky overhead, with white and yellow streaks stretching north along the lines of Earth’s magnetic field.  Within about seven minutes most of it had faded.

From Kwajalein, about 1,600 miles to the west, the show lasted much longer.  One observer described a brilliant white flash through the clouds, then an expanding green ball, giant white fingers arcing toward the poles, and rings racing outward from the blast before freezing in place.  A dull red glow then spread across half the eastern sky, he wrote, interrupted by “tremendous white rainbows,” for at least 90 minutes.

The strangest effects showed up on the other side of the equator.  Charged particles from the explosion spiraled along Earth’s magnetic field lines, which arc high over the equator and come back down in the Southern Hemisphere.  Within a fraction of a second, auroras lit up the sky near Samoa, Fiji, and Tonga, the region planners called the southern conjugate area.  In New Zealand, scientists running a light-measuring instrument near Christchurch watched white auroral shafts turn into a red glow that covered most of the sky for about 12 minutes, even though the experts had expected Christchurch to be too far away to see much.  One early report noted that the Royal New Zealand Air Force found the light useful during anti-submarine exercises.  The Soviet Union, for its part, sent uninvited observation ships to watch from near Johnston Island and from the far end of the field lines near Samoa.

Why the lights went out in Honolulu

The biggest surprise was the electromagnetic pulse, or EMP. It was so much stronger than predicted that it drove many of the instruments set up to measure it off the scale.  Physicists had a theory of how high-altitude EMP should work, and Starfish Prime proved it wrong.  A Los Alamos physicist named Conrad Longmire worked out the correct explanation in 1963, measurements from the later Fishbowl shots backed it up, and his theory is still the one in use.

Here is how it works.  A nuclear explosion releases an intense burst of gamma rays.  The gamma rays heading downward pass through the thin upper air until, roughly 12 to 25 miles up, they start hitting air molecules in large numbers.  There they knock electrons loose and send them racing downward at nearly the speed of light.  Earth’s magnetic field bends those electrons sideways, and because trillions upon trillions of them are deflected at almost exactly the same moment, they radiate a single, enormous pulse of radio energy.  It reaches its peak in a few billionths of a second, far too fast for ordinary surge protectors.  Electrical engineers call this fast part of the pulse E1.

Later there is a much slower component, called E3, which lasts tens of seconds or longer.  The blast briefly distorts Earth’s magnetic field, and that change induces currents in long conductors such as power and telephone lines.  It works much like the solar storms that sent sparks flying from telegraph equipment during the Carrington Event of 1859, which is why engineers sometimes call strong geomagnetic storms “solar EMP.”

Honolulu got off lightly.  The pulse there was later estimated at about 5.6 kilovolts per meter, and only 1 to 3 percent of the streetlights went out.  Hawaii’s 1962 electrical system was simple and sturdy by today’s standards, and the damage was quickly repaired.  Later calculations suggested that the same bomb set off over the northern United States, where Earth’s magnetic field is stronger and angled differently, would have produced a pulse four to five times as strong.

The Soviet Union found out what that could mean.  On October 22, 1962, in the middle of the Cuban Missile Crisis, it exploded a 300-kiloton warhead about 180 miles above central Kazakhstan, a far smaller bomb than Starfish, but over populated land.  According to accounts that Russian scientists shared after the Cold War, the pulse blew every fuse along a 350-mile telephone line that had been wired up to measure it, started a fire at a power plant in Karaganda, and shut down about 620 miles of buried power cable.

A radiation belt made by hand

The explosion also released an estimated 10²⁹ electrons, a one followed by 29 zeros, into the space around Earth.  Some of them followed the magnetic field down into the atmosphere and made the auroras.  Many others were trapped, just as Christofilos had predicted, and bounced back and forth along the field lines between the hemispheres.  The electron levels in the inner Van Allen belt jumped by a factor of hundreds or more.  Some of those electrons were still up there five years later.

That was bad news for satellites.  In 1962 only a couple of dozen were operating, and their solar cells and electronics had not been designed for that much radiation.  Over the next several months the artificial belt helped disable roughly a third of them, including the U.S. satellites TRAAC, Transit 4B, and Injun I, the Soviet Kosmos 5, and Ariel 1, the United Kingdom’s first satellite.  Ariel 1 got a strange reprieve.  The radiation damaged its solar panels but also disabled the timer that was supposed to switch it off after a year, which ended up extending its working life.

The most famous victim was Telstar.  Built by Bell Labs for AT&T, it was launched from Cape Canaveral on July 10, 1962, the day after Starfish Prime, straight into the freshly charged belt.  Thirteen days later, it carried the first live television broadcast between the United States and Europe.  It worked well until November, when its command system began to misbehave, a problem later blamed on the radiation.  Engineers coaxed it back to life in December, but its transmitter failed for good on February 21, 1963.  The satellite is still in orbit, silent.

The damage changed the test plan.  A proposed shot called Urraca, a megaton-class blast planned for more than 600 miles up, was canceled to avoid harming more satellites.  Fishbowl went on with smaller bombs at lower altitudes, ending with a shot called Tightrope on November 3, 1962.  It was the last nuclear test the United States conducted in the atmosphere.

Closing the sky

Less than a year later, on August 5, 1963, the United States, the Soviet Union, and the United Kingdom signed the Partial Test Ban Treaty in Moscow.  It banned nuclear test explosions in the atmosphere, underwater, and in outer space, leaving only underground tests.  The treaty took effect on October 10, 1963, and more than 120 other countries later joined it.  Four years after that, the 1967 Outer Space Treaty barred countries from placing nuclear weapons in orbit at all.

Some useful science came out of the tests.  The Starfish warhead carried a radioactive tracer, cadmium-109, that let researchers follow how air from the poles and the tropics mixes over the seasons.  Measurements of the strange airglow over New Zealand helped confirm how oxygen atoms in the upper atmosphere give off light.  And the satellite failures taught engineers, the hard way, that spacecraft electronics need to be built to survive radiation.

Why Starfish Prime is back in the news

In 1962 there were only a couple of dozen satellites to break.  Today there are well over 10,000 working satellites in orbit, most of them in low Earth orbit, and much of daily life runs through them, from navigation and weather forecasts to banking and internet service.  A 2010 report for the U.S. Defense Threat Reduction Agency used the Starfish data to estimate what a nuclear blast in space would do to the satellites flying at the time, and its conclusions were not reassuring.  Unlike the crash-and-debris chain reaction described in our explainer on the Kessler syndrome, a radiation belt would not need to hit anything.  It would simply wear down unprotected satellites, one orbit at a time, for months.

In February 2024, U.S. officials said publicly that they believed Russia was developing a nuclear anti-satellite weapon, and they pointed to Cosmos 2553, a Russian satellite launched in 2022 into an unusual orbit about 1,200 miles up, as part of that effort.  Russia has denied the claims and says the satellite tests equipment against radiation.  In April 2024, Russia vetoed a United Nations Security Council resolution that would have reaffirmed the ban on placing nuclear weapons in orbit.  In 2025, analysts reported that Cosmos 2553 appeared to be tumbling and might no longer be working.

Engineers have responses, though none is complete.  Military and government satellites are often hardened against radiation, and researchers have studied whether powerful very-low-frequency radio waves could help drain trapped electrons out of the belts faster.  A U.S. Air Force Research Laboratory spacecraft called DSX, launched in 2019, studied how those radio waves move through the radiation belts.  But most commercial satellites are built to be cheap and numerous, not to ride out a nuclear storm.

Back in 1962, the people on Honolulu’s rooftops thought they were watching a fireworks show.  In a sense they were, but they were also watching the first real-world demonstration that one bomb, far beyond the edge of the atmosphere, could reach across an ocean to turn off streetlights and, months later, quietly shut down the newest technology in the sky.  That lesson is a big part of why nobody has repeated the experiment since.

Measure radiation yourself, or read how a nuclear war could unfold