On July 9, 1962, the United States Air Force launched a 2,200-pound W-49 nuclear warhead into space. It exploded with a yield of 1.44 megatons 248 miles above the Pacific Ocean. Starfish Prime, the most powerful and highest space nuclear test ever conducted, produced a flash of light visible over much of the Pacific. For seven minutes after the explosion, an artificial red aurora danced in the skies over island groups as widely separated as Hawaii, Tonga, and Samoa (image at top of post). The blast's electromagnetic pulse damaged electrical systems on Oahu, 800 miles away from the explosion.

Van Allen radiation belts. Image: NASA. Simplified schematic of the Van Allen radiation belts. Image: NASA

According to some sources, Starfish Prime was intended to test whether nuclear explosions in low-Earth orbit (LEO) could augment and expand the Earth-girdling Van Allen radiation belts to create a barrier that would incapacitate Soviet intercontinental missiles launched against the United States. High-energy particles Starfish Prime pumped into the belts probably contributed to the failure of the Telstar 1 communications satellite just four months after its July 10, 1962 launch. No one knew how long the beefed-up radiation belts might persist. Some feared that the increased radiation might last until 1967-1968, when NASA hoped to carry out the first Apollo expedition to the moon. The Apollo spacecraft, launched from Cape Canaveral on Florida's east coast, would have to traverse the augmented Van Allen Belts, and no one could say what effect their radiation would have on Apollo crews.

D. B. James and H. J. Schulte, researchers with NASA's planning contractor, Bellcomm, analyzed the effects of Starfish Prime on NASA's moon plans in a memorandum they sent to NASA Headquarters on October 5, 1962. They based their analysis of LEO radiation during the first Apollo mission on a model of the post-Starfish Prime radiation belts developed by NASA Goddard scientist Wilmot Hess. His model placed the lower limit of the expanded inner Van Allen belt at an altitude of 600 miles.

Just two days after Starfish Prime, NASA had announced that it had selected the Lunar-Orbit Rendezvous (LOR) mode for accomplishing Apollo moon landings. LOR would see lunar mission functions split between two manned spacecraft - a command ship and a smaller moon lander - which would be launched together on a single Saturn V rocket.

LOR had won out over Earth-Orbit Rendezvous (EOR), which in James and Schulte's scenario would have seen a single manned moonship dock in LEO with a separately launched unmanned tanker holding its LEO departure propellants. Despite NASA's decision, James and Schulte examined the radiation environment for both LOR and EOR Apollo missions. This probably reflected lingering concern both inside and outside NASA that LOR might not work, in which case an EOR backup plan would be necessary.

James and Schulte assumed that, before an EOR Apollo spacecraft could set out for the moon, it would need to orbit the Earth at least six times in a 252-mile-high parking orbit inclined 28.5° to Earth's equator. During its first orbit after launch from Cape Canaveral, controllers on the ground would determine the manned EOR spacecraft's precise path. Rendezvous and docking with the tanker would need up to 2.5 orbits, then propellant transfer and final orbit determination/spacecraft checkout would require two more. After a final half orbit, the EOR Apollo spacecraft's orbital plane would become aligned for launch to the moon.

The Bellcomm researchers determined that, based on the Hess model, the EOR Apollo astronauts would receive a radiation dose of four rad in LEO before setting out for the moon. They would experience most radiation exposure during orbits five and six, when they would begin to pass through a magnetic anomaly that spanned the Atlantic from Brazil to South Africa. Within the South Atlantic Anomaly, as it is known today, the Van Allen belts dip to within 100 miles of Earth's surface. If the EOR Apollo astronauts could not depart LEO on schedule, then they would pass through the widest part of the South Atlantic Anomaly during orbits seven through 10 and receive up to six rads per orbit.

The Telstar 1 experimental communications satellite probably suffered damage from radiation the Starfish Prime explosion injected into the Van Allen Belts. Image: NASA. The Telstar 1 experimental communications satellite probably suffered damage from radiative particles the Starfish Prime explosion injected into the Van Allen Belts. Image: NASA

James and Schulte assumed that LOR Apollo astronauts would circle Earth once in 252-mile-high LEO while controllers precisely determined their orbit, then for half an orbit more so that their spacecraft's orbital plane would align for departure to the moon. They would stay far from the South Atlantic Anomaly during their one and a half orbits of the Earth, so their LEO radiation dose from the augmented Van Allen belts would amount to only 0.02 rad.

In both the LOR and EOR modes, the astronauts would receive a dose of 16 rad while crossing the augmented Van Allen belts en route to the moon. Thus, the minimum dose the EOR astronauts would receive would be 20 rad, while LOR astronauts would receive 16.02 rad. James and Schulte noted that future nuclear explosions in LEO could dramatically boost the dose moon-bound astronauts would receive during Van Allen belt passage. They noted that a bomb packed with Uranium-238 could increase the radiation in the belts "a hundredfold."

James and Schulte noted that the Van Allen belts are inclined relative to Earth's equator and do not cover its poles. If the belts became impassable, they wrote, NASA would have little choice but to launch Apollo astronauts through the Van Allen belt gaps over Earth's poles. Unfortunately, Cape Canaveral was poorly placed for polar launches because rockets launched due south or north would pass over populated areas (Cuba and Brazil to the south and the major cities of the eastern seaboard to the north). James and Schulte wrote that a country with polar launch capability might explode nuclear weapons in space to bar a nation without such capability from launching men to the moon. They did not mention the Soviets specifically, nor did they point out that the Soviet Union, with its extensive Arctic Ocean coastline, was well placed to carry out polar launches.

The Van Allen radiation belts returned to normal a few years after Starfish Prime. Nuclear explosions in space never menaced the Apollo astronauts, in large part because in August 1963 the U.S. and the Soviet Union negotiated the Treaty Banning Nuclear Weapon Tests in the Atmosphere, Outer Space, and Under Water.

References:

Memorandum, D. James and H. Schulte, Bellcomm, to W. Lee, NASA Headquarters, Radiation environment of EOR and LOR, Bellcomm, October 5, 1962.