Imagine it's July 20, 1969 and no one is paying much attention as Neil Armstrong sets foot on the Moon, because all eyes are on the first manned mission to reach Saturn. That may sound absurd, but while NASA was figuring out how to use rockets to reach the Moon, a super secret US government project was developing a gigantic reusable spaceship powered by atom bomb explosions that was designed to carry a crew of 20 to the outer Solar System by 1970 as a first step to the stars. New Atlas looks at the story behind the original Orion Project.

The most powerful rocket in NASA's inventory is the Space Launch System. When completed, it will weigh 2,875 ton fully fueled, will measure 322 ft (98 m) tall and be able to lift 130 tons into low Earth orbit (LEO). It will be the largest booster ever built and is the key to NASA's ambitions to send crews of up to seven astronauts to cislunar orbit and smaller crews to Mars.

Now picture a spacecraft 200 ft (60 m) tall, weighing around 4,000 ton, and capable of lofting 1,600 ton into LEO along with a crew of eight living in quarters more like that of a small submarine than a cramped capsule. Rather than exotic, lightweight aerospace alloys, the ship's hull is made of welded steel a quarter inch thick using standard submarine-building techniques. In addition, the whole spacecraft is completely reusable and its orbital mission is mere muscle flexing before much bigger things.

If Orion had been completed, Apollo 11 might have been a footnote in history (Credit: NASa)

This monstrous craft was designed as a multi-mission platform. The idea was to go to Mars in 1965, then to Saturn in 1970 with a stopover on Mars along the way. And how would it do this? By dropping atomic bombs out the stern and riding the blast waves.

The Orion Project may sound like one of a hundred back-of-the-envelope speculative proposals that the aerospace field is always throwing out when engineers play around with "what ifs," but this was something very different. Orion was under very serious consideration by a US government looking to create the world's first true spaceship that, if completed, would have made Apollo look like a toy.

So serious was this commitment that from 1957 to 1965, Washington spent US$10.5 million (US$85 million in today's money) on the idea, with many of the same minds behind the Manhattan Project turning their expertise from creating weapons of war to developing a means of space travel. The project was so advanced that much of the work is still classified half a century later.

Beyond the Moon

The reasoning behind Orion was that, even though chemical rockets were being developed around the world as the primary motive force of space travel, the technology was already straining at its limits when the first German V2 traveled into space in 1944. Today's chemical rockets may be more sophisticated and have allowed us to send unmanned probes out of the Solar System, but as far as manned missions are concerned, they're not at all practical beyond lunar orbit and reaching Mars with a small, precarious expedition is the absolute outer limit.

The German V2 rocket was already working at the limits of chemical rocket technology (Credit: UK goverment)

It's simply a question of energy or, to put it another way, how much change of velocity chemical rockets can produce per unit of fuel. Engineers describe this as a rocket's specific impulse, which is measured in seconds. The more seconds, the more efficient the rocket is. Though the calculations are fairly complicated, it's a way of comparing very dissimilar systems, like chemical rockets, that produce a very high thrust for a very short time, and ion drives that produce very small amounts of thrust for a very long time.

While chemical rocket launches are spectacular, they are also very inefficient. The maximum exhaust velocity is only about 6,000 mph (10,000 km/h), which means that the best specific impulse is only 430 seconds. By comparison, the ion thruster that will be used on the Jupiter Icy Moons Orbiter mission has a specific impulse of 9,620 seconds. Unfortunately, the latter's thrust is so low that it can only be used to push unmanned probes at very low accelerations through deep space.

In practical terms, chemical rockets require about 16 tons of fuel to place one ton of payload into orbit. To reach the Moon, that works out to 1,000 tons of fuel for every ton of payload. That's the reason manned spacecraft are so cramped, carry limited crews, and are made out of extremely lightweight aerospace alloys. It's also the reason the Apollo missions started off with a ship the size of a skyscraper and came back with a scorched capsule the size of a garden shed.

Scientists and engineers were aware of these limitations even in the 1920s when the likes of Robert Goddard and Wernher Von Braun were flying primitive liquid-fuel rockets built on a shoestring budget in scrub-covered fields. What was needed was something vastly more powerful than burning two chemicals and riding on the flame.

Atomic rockets

When Einstein developed his famous E=mc² formula, suggesting that splitting the atom could release incredible amounts of energy, it looked like a godsend to early rocketeers, and science fiction writers like E E Smith were quick to adopt atomic power as a way to hurl their heroes into space. Through the 1920s and '30s, the idea stayed in the pages of science fiction until the first atomic test at Alamogordo, New Mexico on July 11, 1945.

Almost immediately, proposals for practical atomic rocket engines started to appear and reached a surprisingly advanced state very quickly. In fact, it was so fast that some of the early memoranda on the subject from 1947 remain classified, though the principle was already being discussed in magazines and even Robert Heinlein novels for young adults.

The principle behind an atomic rocket, more formally known as a nuclear thermal rocket, is very simple. Instead of the arrangement in a chemical rocket where the fuel and the propellant are the same thing, in the atomic rocket the two are separate. The fuel is a graphite-core nuclear reactor, which sits where the combustion chamber is usually found. This reactor is wrapped in beryllium and pierced from end to end with tubes that allow liquid hydrogen to pass through it. The hydrogen acts as the propellant and as it passes through the reactor, it heats up to 3,000 K (2,727° C, 4,940° F), causing it to expand and exert thrust as it shoots out the nozzle.

It's a much more efficient system than chemical rockets, providing specific impulses of up to 840 seconds compared to chemical's 430 seconds. The US government was so impressed by the potential of nuclear rockets that it implemented a pair of projects called Kiwi and Rover, which culminated in the NERVA engine project that was once considered for a manned Mars mission, but ended up being canceled in 1972.

One reason for the cancellation was that, though nuclear engines appeared promising, they suffered from one of the same problems as the chemical rockets. In order to be practical, they had to operate at the extreme limit of their design envelope. To heat the propellant, the reactor had to run at the peak of criticality where the heat was just below the point of melting the engine. This meant that the liquid hydrogen wasn't just acting as propellant, it was the vital coolant keeping the engine from destroying itself.

NERVA flow diagram (Credit: AEC)

That might be acceptable in a static test, but in flight that involved living with the prospect of the engine suddenly melting, catching fire, or disintegrating. Worse, unlike power reactors where the reactor core is isolated by a heat exchange loop that feeds the reactor heat to the steam system running the turbines, the atomic engine core is in direct contact with the outside world. This raised the possibility of a reactor failure in the atmosphere raining chunks of nuclear fuel down on the Earth.

Definitely not a desirable thing.

So, chemical rockets weren't powerful enough, and nuclear engines, though they were twice as efficient, ran far too hot and had other drawbacks. Something better was needed.

What about something with 14 times the specific impulse of chemical rockets at 6,000 seconds? What about riding into space on an atomic bomb?

The birth of Orion

Like the nuclear engine, Orion was conceived literally the day after the first atomic bomb exploded. It was then that Polish-American mathematician, Manhattan Project team member, and one of the fathers of the H-bomb, Stanislaw Ulam, asked himself what would happen if instead of using missiles to deliver bombs, one used bombs to deliver missiles?

The principle behind Orion is very simple and goes back to 1880 when German inventor Hermann Ganswindt suggested building a spacecraft that would be propelled by tossing dynamite into a giant steel bell. It's also how an internal combustion engine works. It's a way of harnessing forces and temperatures that would destroy the engine by making them in a pulse rather than a continuous blast.

In other words, while propelling a spaceship by detonating a nuclear bomb underneath it may seem like lighting a cigar by sticking your head in a blast furnace, it's actually a way of making the nuclear rocket idea practical. Think of it as like the nuclear rocket engine, only instead of fighting to keep the reactor from melting, it uses a stream of hundreds of expendable reactors. In other contexts, these "reactors" might be called "bombs," but it depends on how you look at it.

For an Orion spacecraft, a nuclear bomb would be dropped out the stern of the ship and when it reaches a preprogrammed distance, the bomb detonates. The bomb is wrapped in a material, like polystyrene, that creates a plasma shock wave and pushes the ship forward. As the shock wave disperses, another bomb is dropped, and the process continues.

Before the Orion project proper was started, a number of versions of this principle, some still classified, were looked at in the 1950s, but none as a serious proposal. Then on October 4, 1957, the USSR shocked the world by launching the world's first artificial satellite and the public realized that where a satellite could go, a nuclear weapon could follow and potentially strike any spot on Earth.

In the aftermath of the Sputnik launch there was demand for immediate action, and the United States space program was shifted into high gear as the Eisenhower administration struggled to catch up with the Soviets. This led to the foundation of NASA, the grudging acceptance of using military rockets for the civilian program, and the eventual race to the Moon.

It also opened the way for a number of projects that normally wouldn't get a look in. One of these was Orion. Sponsored by DARPA's predecessor, ARAPA, the Orion project was started by leading nuclear weapons designer Theodore Taylor in 1957, about a month after Sputnik, at the California-based company, General Atomic.

To the drawing board

General Atomic (now called General Atomics) was founded in 1955 as part of the US Atoms for Peace program. Aimed at finding peaceful applications for nuclear power, this project is famous for inventing the idiot-proof research reactor TRIGA. From 1957 to 1965, General Atomic would employ up to 50 people on the Orion project and spend US$10.4 million.

To provide more credibility in Washington, Physicist Freeman Dyson was brought in from Princeton as a consultant, and Taylor's team included many of the scientists behind the Manhattan project. In fact, the atmosphere was very like that of Manhattan, with a mixture of secrecy and enthusiasm as the scientists grasped the opportunity to once again enjoy the fun of playing around with nuclear devices without the guilt of doing weapons research.

Taylor encouraged an open, bureaucracy-free style by adopting a management model of Germany's pre-war Verein fur Raumschiffahrt (VfR) – the famous amateur rocket society that included such space pioneers and Willy Ley, Hermann Oberth, Eugene Sänger, and Wernher Von Braun. This meant that there was no division of labor, with scientists working on engineering problems, engineers on science, and both on odd tasks like model building.

General Atomic was famous for its TRIG research reactor (Credit: General Atomic)

In a BBC interview, Dyson described the team as "a bit mad" as enthusiasm gripped scientists who had a deep familiarity with nuclear weapons and bomb dynamics now had the security clearance and freedom to pursue their ideas on a project of fantastic potential. In addition, Orion soon went beyond a mere feasibility study based on speculation to one that could see a fully-functional atomic ship ready to launch by 1964.

By late 1958, the team was looking at a long list of questions as they sorted out the mathematics of the vehicle – much of it done by Dyson. How big would it be ? How big a bomb would be needed? How many? How many explosions? At what rate? What payload could it carry? What would be the specific impulse? How much radiation shielding would the crew need? Could the ship survive the shocks? What about blast temperatures? Acceleration? How would the shock wave propagate?

Testing the theory

In addition to the theoretical work, the Orion team also dealt with field tests of both the principle and the technologies needed to harness it. It would be years before they could test the bombs that would propel Orion, but plans were already being drawn up for ways to carry out atomic tests both above and below ground.

Meanwhile, in 1959, the Orion team gained permission from the US Navy to use its Point Loma facility in San Diego to carry out initial tests using conventional explosives. One key member of the team was a member of the Czech resistance during World War II, and when the Navy said that no cranes would be available to tear down the steel structures on the test site, he made short work of it using plastic explosives.

Nicknamed "Putt Putt," the tests involved hanging half-pound balls of plastic explosives under various test models and detonating them. The first model was made out of stainless steel mixing bowls from a supermarket, but they soon became more sophisticated, until in November 1959 the tests culminated in the flight of "Hot Rod."

This was a model 3.3 feet in diameter that was designed to drop a series of six RDX explosive charges beneath it on length of cord. As each charge reached the end of its tether, it detonated and another dropped. It flew for 23 seconds and reached an altitude of 56 m (184 ft) in controlled flight before being recovered by parachute.

A problem of scale

What the Orion team soon discovered from its tests and calculations was that the basic problem of building the spaceship was exactly the opposite to that of a chemical rocket. The sort of craft that lift off from Cape Canaveral are dominated by one overriding concern: to keep weight down as much as possible. The skin of the boosters has to be thin and the electronics must be miniaturized – every strut, every bolt, and every screw must be shaved of every microgram of excess mass.

Orion was very different. The calculations demonstrated that if the spacecraft was anything like the size of, for example, the rocket used to send the first Mercury capsule into space, the atomic blast would obliterate it on lift off. To survive the yield of a 0.03-kiloton bomb, the ship would have to weigh at least 800 tons and wouldn't be able to place more than a very small payload into orbit. Worse, making such tiny bombs was extremely difficult.

The answer was to think not just outside the box, but as if the box was in another country. The mathematics indicated that the Orion principle scales up beautifully – the bigger the ship, the more efficient it is. In practical terms, that meant an Orion ship had to weigh thousands or even tens of thousands of tons to work.

In 1959, a base model for Orion emerged. It would be as tall as a 20-story building, 40 m (131 ft) in diameter, weigh 4,000 ton and able to put 1,600 ton into orbit before returning to Earth in one piece. As soon as the team realized this, they embraced the incredible size of the ship and ran with it. Dyson said the Orion would be "built like a submarine, not an airplane," using steel and standard shipbuilding techniques. In fact, they expected the ship to be built by submarine manufacturer Electric Boat rather than an aircraft firm.

This leap in scale had a number of knock-on effects. For one thing, the sheer mass of the ship acted as shielding to protect the crew from the bombs' radiation bursts. Also, the crew module for the large crew of astronauts could be roomy enough for bunks, ward rooms and showers (plural). The designers even included a commercial barber's chair among the fixtures and fittings in case any of the crew needed a trim.

Indeed, the problem wasn't to find a small enough payload for an Orion to fly, but one large enough to justify its use, such as transporting solar power plants, setting up nuclear material production on the Moon, or carrying orbital observatories – with 10 entire families aboard.

Powered by bombs

Kicking A-bombs out of a ship and riding the blast waves may seem a trifle risky, but there was very sound reasoning behind it. Pre-Orion concepts envisioned a bomb-propulsion system has having a giant bell or steel ball to contain the explosion, but this was soon rejected as producing too many problems. This was a system that generated a million times the energy of a chemical engine with a hundred times the exhaust velocity and temperatures ranging from 10 million degrees in the fireball to 120,000 degrees where the shock wave struck the ship. Those sort of numbers dictated a more novel approach.

Though the Orion team did regard the project as a way to use nuclear weapons for peaceful purposes, the bombs used by the spaceship were not off-the-shelf military hardware. These were small, clean bombs that used as little uranium or plutonium fuel as possible with a yield of about 0.15 kt, though this varied depending on the size of the craft and when the bombs were needed during a mission. Many details of their design are still classified.

Each bomb was designed to be about 6 in (15 cm) wide and weighed about 300 lb (140 kg), and thousands of these would be carried in jettisonable drum magazines stored in stacks in the middle of the ship. When accelerating, these bombs would be fed by conveyors to a Gatling gun-like mechanism to be fired through a hole in the stern at a rate of up to four per second before detonating about 20 to 30 m (65 to 100 ft) away.

The size and shape of the bombs was dictated by the need to move them rapidly from storage to launch, and so they could be swapped out for different yield bombs at an instant's notice. To do this, and to illustrate how little the team cared about weight, the plan was to use a scaled-up version of the mechanism from a Coca-Cola bottle-vending machine.

Each bomb was in itself special. Not only was the design small and used as little fissile material as possible, but it was a shaped charge similar to that of anti-tank missile warheads. In these warheads, the explosive charge is formed into a hollow cone lined with copper. When detonated, the cone concentrates the explosive force and focuses it in a straight line. The copper instantly melts and becomes a supersonic blob pushed by the blast wave straight through the tank's steel plates, slicing through it like a hot knife through very soft butter.

The Orion bombs worked in a similar way, only the blast was a nuclear one that was coaxed into a cigar shape instead of a sphere and the copper was replaced with tungsten, polystyrene, or even ice. This was the actual propellant that would transfer the blast energy to the ship as thrust.

For the standard Orion, it would take 800 bombs unleashed over a six-minute period to push it into a 300 mi (483 km) orbit. Each blast would add 20 mph (32 km/h) to the velocity and several thousand bombs would be needed for an interplanetary voyage. Oddly, once the ship left the atmosphere, there wouldn't be anything to see because most of the explosion flash would be in the ultraviolet.

The mother of all shock absorbers

Of course, the shock wave wouldn't be delivered to the unprotected fantail of Orion. Instead, there would be a massive pusher plate made out of steel or some other heavy metal and weighing in at 500 to 1,000 ton. This would be coated with ablative plastic like a space capsule's heat shield to protect it against wear, and shaped to absorb the maximum energy from the shock wave while allowing the cooling plasma to quickly disperse before the next blast.

One bit of serendipity came from the open air atomic tests being conducted by the Americans. In one test shot, oily fingerprints left on a steel plate protected it from ablation, so an oil spray mechanism was incorporated into Orion's design to oil the plate between each blast and almost eliminate wear.

This is sort of propulsion is all fine except for one tiny detail. The acceleration from an atomic bomb is in the neighborhood of 10,000 g. Since the usual safe limit for a human being is about 5 g, this means that Orion would very quickly reduce its passengers and crew to a collection of grease spots on the deck unless something was done.

The team's solution was to equip Orion with the mother of all shock absorbers. In the design, directly behind the pusher plate was a huge accordion bag filled with an inert gas. Behind this was a ring of giant pistons that work like the steam catapult on an aircraft carrier, only in reverse, and it uses a system of cylinders, pulleys, springs, and magnetic clutches to absorb and spread out the impact of each shock wave.

By tuning the frequency of the shock absorber, the acceleration could be cut to an acceptable 4 g. However, the entire pusher plate and absorber system was a bit more sophisticated because it had to handle the problem of misfires. If a bomb failed to detonate, the absorber might shoot out and not contract, causing it to overextend and jam or rip the air bag, so the mechanism had to compensate with a two-stage detuned spring and piston shock absorber.

Worse, the bomb might detonate, but not reach critical mass. That would mean a conventional explosion, but no nuclear one – showering the pusher plate with shrapnel that could pit it. This, too, had to accounted for in the design.

The crew module

As we've said, the crew module on the Orion was nothing like what has flown even today. The multi-deck module was not only spacious and well appointed with multiple gangways, control rooms, and crew quarters, but it's was also protected against cosmic radiation by thick hydrogenous shielding and there was even a bunker in case of solar flares and the like.

Even more impressive, the whole ship rotated when not accelerating, so the crew could enjoy artificial gravity. There were landing craft, which were only penciled in, but resembled the Space Shuttle, a pair of space taxis, and the entire module had its own rocket system, so it could act as a lifeboat.

But not everything was high tech on Orion. Attitude control still used chemical thruster rockets and navigation was by good, old-fashioned chart and sextant, and trajectories plotted on graph paper. This was around 1960, remember.

Destination Saturn

If Orion had become reality, what would it have done? The Orion team had very clear and immediate ideas about that – they fully expected test flights by 1964 and to fly on a mission to Mars by 1965 and reach Saturn by 1970. And unlike NASA's Moon landings, these weren't envisioned as quick walks on the surface, but beachheads for colonization.

Of course, no missions were officially authorized and the plans were more along the lines of options based on how large a ship was available at the time, but the Orion team were confident that the missions would happen.

Though calculations were made for a Moon landing, the team seemed to regard the goal of Apollo as not worthy of Orion's maiden voyage, so they opted for a Mars flight in 1965, which would consist of two or three Orions flying in convoy. The journey to the Red Planet would take 258 days. This could be faster, but that would mean carrying more bombs.

Where today a Mars mission aims to set down a rover and, one day, a small group of perhaps three astronauts, the Orion plan was to put down 20 to 50 explorers in a huge bullet-shaped lander that would act as the nucleus for an outpost. The lander's propulsion module and the other ships would remain in orbit to protect Mars from radioactive contamination. After 454 days, the convoy would return to Earth, leaving behind a team with lander to await the next expedition. Total mission time: 32 months.

However, even the 1965 Mars mission was just a benchmark. Dyson saw the real challenge in sending an expedition to the Saturnian moon of Enceladus in 1970. Styled on the voyage of HMS Beagle, this would be a grand multi-year mission starting with a brief stop off at the Moon to plant the American flag, then on to Mars for an extended stay of up to four years to explore and take on water as propellant for the bombs, then on to Saturn.

And it didn't end there. With the ability to hurl thousand ton payloads on interplanetary trajectories, Orion was seen as a way of opening up the entire Solar System and paving the way toward interstellar flight. Soon, an advanced Orion of 10,000 ton was on the drawing boards.

In addition, Orion was seen as an all purpose technology that could be adapted for a merchant ship, research vessel, reconnaissance platform, orbital communications center, or battleship. Closer to home, there were plans for lunar shuttles complete with separate crew and passenger quarters, bunk beds, and a passenger lounge, as well as a 20-passenger lunar lander.

The military option

This ambitious program of peaceful exploration was all well and good, but Orion was still a fledgling project operating on a begrudged budget that in seven years would total what NASA spent per day on the Apollo project. Between the incredible nature of the project and its lukewarm reception by the purse holders, Taylor and Dyson spent a lot of time trying to drum up interest and funds.

Since ARPA was part of the Department of Defense, Orion was technically a military project and to keep the money coming in, military applications had to be considered. The team went to the US Air Force and showed that the ability to put thousands of tons into orbit had some formidable implications – especially if it turned out that the Soviets were working along the same lines.

One of the most obvious of these was that Orion could have been the ultimate nuclear deterrent. Being able to spit out bombs like watermelon seeds might already seem formidable, but the payload of Orion meant that it could park a bomb in orbit big enough to wipe out a third of the United States. In a polar orbit, it was a weapon that could, theoretically be delivered to any point desired. More than that, the 1,600-ton bomb carrier could carry a crew, armor plating, and enough defensive weapons to easily see off any anti-satellite attack the Soviets might mount.

It was an idea that caught the imagination of many in the Air Force. At the peak of interest in the early 1960s, the supporters saw the US fielding a space force on constant patrol consisting of 20 ships sitting in orbit for 15 to 20 years with rotating Blue and Gold crews of 20 to 30 living in a shirt-sleeve environment with artificial gravity, ample room, recreation gear, and no weight restriction.

This was especially attractive because as well as being a deterrent force, it was an ideal platform for reconnaissance, early warning, electronic warfare, and anti-ICBM systems using depleted uranium rods to take out silos in enemy territory.

Most importantly, it had the potential to cool down the Cold War considerably. Such a fleet would not be vulnerable to the split-seconds the Earth-based systems had to make the decision between launching a nuclear strike immediately or risk destruction. Equally, the deterrent would be based far from civilian areas. Combat would be more like during the Age of Sail, where strategic conflicts took place far from inhabited regions and when decisions could be taken in hours and days instead of minutes and seconds.

Troubled waters

Just as Orion's design and mission was coming into focus the project struck trouble. There was only so much that could be done with computers and conventional explosives and the project needed to quickly move on to detonating nuclear bombs, but the US government was extremely reluctant to grant permission.

Worse, ARPA was growing unhappy with the project and Orion was handed over to the more sympathetic Air Force, but this resulted in an end to all field model testing and the end of Taylor's freewheeling management style. On top of all that, Kennedy Administration Defense Secretary Robert McNamara made it clear that he did not regard Orion as a military asset and refused any increase in funding beyond that of a feasibility study.

Taylor and General Atomic now started courting NASA as a possible sponsor. Von Braun had already shown enthusiasm after seeing film of the flight tests and the Orion teams drew up plans for a stripped-down version of Orion that could be launched into orbit in sections using three Saturn V boosters, which was tiny by Orion standards and not very efficient.

Orion lunar ferry (Credit: NASA)

NASA looked over the new plans, but was unenthusiastic about being attached to a project based on atomic bombs. The Orion team wasn't very enthusiastic about the mini Orion either, and spent more time working on the noise levels in the crew module.

The fallout

Orion was also held up by questions about the dangers of nuclear fallout from its use. The original idea was to use a bomb to raise Orion off its launch pad, but this was discarded due to huge cloud of radioactive dust that would be thrown into the atmosphere. It was therefore decided to rest Orion on a giant ring of conventional explosives to throw it into the air. Once aloft, the atomic bombs would take over. Detonating in midair, there would much less fallout.

The problem of fallout was one that exercised Freeman Dyson in particular, and he undertook the task of calculating the danger an Orion lift off or landing posed. He started by adopting the no safe threshold hypothesis that was then current. Because of how radiation affects living tissue and genetic material especially, it was assumed that the effects of radiation exposure are cumulative and that there is no safe minimum level of exposure.

The Orion bombs were designed to use as little fissile material as possible and exactly how much fallout one would create is intimately related to the bomb's layout, so the specifics are still highly classified. Dyson estimated that if a 6,000 ton Orion used conventional nuclear weapons it would produce as much fallout during launch as a 10 megaton nuclear weapon airburst. That would produce somewhere between 0.1 to 1 deaths from cancer worldwide. A Mars mission launch might cause 10 deaths.

Bear in mind that this isn't a direct causal relationship, but a statistical probability based on the no-threshold hypothesis. Dyson felt that while the probability of one death was acceptable, 10 was out of bounds. At the same time, Taylor believed that special bomb designs could reduce the fallout hazard by a factor of 10 or even eliminate it if a pure fusion explosion could be produced.

This is assuming that the bombs went off as intended. The prospect of one misfiring and falling to Earth or burning up in the atmosphere was even less pleasant. Another problem was if the bombs contained any metals, like tungsten, they would not only become a radioactive gas, but would ionize in space and be pulled down along the lines of the Earth's magnetic field and back into the atmosphere.

Obviously this made Orion less attractive; particularly when the Kennedy administration was trying to negotiate a nuclear test ban treaty with the Soviets. These negotiations and the fallout issue made is almost impossible to get anyone to lobby on the project's behalf.

Too hot to handle

Orion was a political football. It started out under ARPA and bounced around the bureaucracy for a while. It was rejected by NASA due to the nuclear weapons element and the high degree of secrecy around it. The Air Force, which took control in 1960, was sympathetic, but the Defense Department didn't like the exploration part, were unconvinced as to its military applications, and loathed the whole space fleet idea. In addition, the bombs made the team look like crackpots to the politicians and an embarrassment during the test ban talks.

In 1963, the Partial Test Ban Treaty was signed between the Britain, the US, and the USSR, banning atomic tests above ground, in the air, or in outer space. The Orion team fought hard for an exemption for small underground tests and a 50 kt space test. The American negotiators did try to include an exception for space propulsion in the treaty, but the Soviets rejected this out of fear of a military loophole.

The end of Orion

The Orion project was now in a hopeless position. Without atomic tests, there was no way to develop the spacecraft any further. Committed to the Space Race, the government threw its backing behind Apollo and the Saturn V, divorcing Orion from the mainstream space program. Meanwhile, the Air Force was stuck with a non-military project that, by law, they couldn't back.

As Orion became more scientifically feasible it became less palatable, and by late 1963 the only interest came from the Strategic Air Command, which was intrigued by the idea of a deep space fleet. Ironically, one of the biggest nails in Orion's coffin was an eight-foot model the team built to show off a complete spacecraft. Long vanished, it was an impressive display of what the finished product would look like in cutaway detail, but to Washington, this was less convincing than a vague proposal had been.

Arguing that Orion was not only a civilian project, but one that was diverting funds from conventional defense projects, the Air Force handed Orion over to NASA in 1964, whether it wanted it or not. NASA already had its own nuclear rocket project and saw Orion as a DoD relic. The space agency was also suffering budget cuts now that Apollo was built and ready to go – so no thanks.

In 1965, in one final attempt to keep Orion alive, General Atomic went to the US Atomic Energy Commission for permission to conduct underground tests. The AEC was receptive, but NASA said that this was pointless because it had no plans for manned planetary missions.

Orion had no backers, no mission, and no money. On June 30, 1965, the Air Force officially shut the project down and seven years of work was shelved.

The public was barely aware it even existed.

Obituary

In 1968, Dyson wrote the obituary for Orion in Science, where he stated, "[T]his is the first time in modern history that a major expansion of human technology has been suppressed for political reasons."

Here Dyson made public for the first time many of the details of Orion, its significance, and lost promise. Of course, he was hampered by being restricted by security regulations, but he did make a detailed case. However, he didn't leave it there.

That same year, Dyson in Physics Today published his paper, "Interstellar Transport," where he outlined his plans for a next-generation Orion capable of reaching the star Alpha Centauri. This variation on the super Orion abandoned using fission bombs in favor of a system using deuterium to create a pure fusion reaction.

An updated NASA Orion design (Credit: NASA)

Dyson had two versions of this new Orion in mind. The more practical of the two would be a mammoth craft 100 m (330 ft) in diameter and weighing in at around 400,000 ton. Large enough to carry an entire colony to Alpha Centauri, it would accelerate at 1 g for 10 days until it reached 3.3 percent of the speed of light, then coast for 133 years. As a neat bit of forward planning, Dyson suggested building the pusher plate out of transuranic elements, so it could be broken up by the arriving colonists and used for nuclear fuel.

How well the colonists would be doing after five generations in a spinning starship was not looked into very deeply.

In the 1970s, the British Interplanetary Society took Dyson's idea and refined it for Project Daedalus. A team of scientists under Alan Bond calculated that by using a fusion drive similar to Dyson's, it would possible to build a two-stage 54,000 ton unmanned probe that could reach a maximum velocity of 12 percent of speed of light. This would allow it to reach Barnard's Star, 5.6 light years away, in about 46 years.

In 1989, the US Naval Academy and NASA unveiled Project Longshot, which was intended to launch from Space Station Freedom, the planned predecessor to the International Space Station. This would have used a 300 kW nuclear reactor powering a laser-based fusion drive to accelerate the unmanned 400-odd ton probe to 4.5 percent of the speed of light on a 100-year mission to Alpha Centauri.

Even today, interstellar spacecraft based on Orion are being studied. Some, like the Mini-Mag Orion, use superconducting coils to compress fissile material. Others rely on anti-matter. What they all have in common from Dyson's colony ship on is that, unlike the original Orion, they depend on technologies that have yet to be developed. However, it does show that the idea is still very much alive.

Is Orion really dead?

The chances of setting up colonies on Mars using A-bomb ships would be politically impossible today, even if it all the bugs were sorted out. The intervening decades have also unveiled new problems, including the effects of a bomb's electromagnetic pulses on unshielded satellites and the danger of heavy ions from the bombs getting trapped in the Van Allen belt.

Then there's the problem of the bombs themselves. A whole new infrastructure would be needed to construct the bombs and to mass produce them in enormous numbers. Then there's the fact that these small bombs using minimal fuel are exactly the design that terrorists and rogue states have been looking for all these years.

And that's not to mention the shaped-charge technology developed for the bombs. The ability to aim the blast of a small nuclear weapon like the jet of a flame thrower would make such arms practical as a tactical field weapon, which is not a pleasant thought.

But there is one role that Orion could still fulfill, and it may one day justify building such spacecraft. One of the unlikely, but frightening threats that the Earth faces is that of a rogue asteroid on a collision course with our planet. Johndale Solem, theoretical physicist and the Los Alamos National Laboratory and others have advocated Orion as one answer to this danger.

Because of the size of Orion and its ability to build up huge amounts of kinetic energy, the impact of a single unmanned Orion could deflect a 14 million ton asteroid a week before impact, and do it fast enough that a second could be launched if it failed. In the case of larger asteroids, a small fleet could be kept at the ready.

Whether Orion will one day be resurrected as an anti-asteroid weapon is a question for the future, but the basic premise of the Orion project is as relevant today as it was in 1945. The inescapable fact is that we are still faced with is that we are operating at the limits of what chemical rockets can do.

To date, many of the efforts of scientists and engineers have essentially been workarounds for this problem. Slingshot orbits, miniaturization, lightweight alloys and composites, solar panels, robotics, autonomous systems – these are all ways of carrying out missions with as little mass as possible and cutting people out of the equation almost entirely after leaving the launch pad.

None of these address the fact that one day we will have to be able to move the sort of payloads that container ships do on the Earth's seas and to carry hundreds of people at a time from one planet to another – otherwise we will only ever be tourists in space. According to the inflexible laws of physics, that means that the next century or two must see the development of ships that can handle the kinds of energy that Orion used. If not a revamped version of Orion, then something that works on the same scale.

In the here and now, Orion still has relevance. Many of the things developed by Orion are still active programs. Much of it is still classified down to the titles of some reports. Even the declassified ones are buried so deep that when researchers ask NASA and other agencies that are supposedly the official curators of these reports for a look, they are often met with a denial of having them and a request to be sent of copy if the researcher finds one.

It's been sixty years since the Orion project began in the wake of Sputnik. Today, it's seen as a bizarre dream – as fantastic as Cyrano de Bergerac flying to the Moon in a chair with sky rockets strapped to it. But this fantasy was very much a thing of reality that died more from politics than engineering. It is also one that may well play a key role in the future of the human race as it expands its horizons to the stars.

If you are interested in learning more about the project, check out Project Orion: The True Story of the Atomic Spaceship by George Dyson (the son of Freeman Dyson).

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