Portuguese 
Ra World Ship Concept Proposes an Interstellar Vessel Measuring 115 km in Length, 14.48 km in Diameter, and a Loaded Mass Exceeding 1 Trillion Tons, Capable of Departing with 1 Million Inhabitants, Sustaining a Population of Up to 10 Million, and Traversing Over Four Light-Years Using 1,024 Fusion Engines Over a Mission of Approximately 854 Years.
Imagine a ship so long that its ends would be separated by 115 kilometers, with a diameter of nearly 14.5 kilometers and a loaded mass estimated at around 1.0156 trillion tons. This is the scale of the Ra World Ship Mk2A(ii), a conceptual study published in the Journal of the British Interplanetary Society that attempts to answer how an entire human population could traverse interstellar distances over centuries.
This is not a ship under construction but an engineering exercise taken to extreme dimensions. The model provides space for 1 million people initially, with a demographic capacity projected to reach 10 million, five large habitable sectors, artificial gravity through rotation, and a battery of 1,024 inertial confinement fusion engines working in tandem to move a mass equivalent to hundreds of billions of tons.
Ra World Ship Would Measure 115 km in Length and Nearly 14.5 km in Diameter
The main structure of the Mk2A(ii) is a gigantic cylinder measuring 115 kilometers in length and a radius of 7.24 kilometers, equivalent to a diameter of approximately 14.48 kilometers.
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The design directly derives from world ship studies conducted by Alan Bond and Anthony Martin in the 1980s, but it expands the length of the original Mk2A configuration, which was 87.5 kilometers.
The comparison helps to contextualize the concept. The 115 kilometers do not represent a sequence of conventional modules like those currently used in space stations, but a single interstellar megastructure designed to function simultaneously as a vehicle, habitat, industrial infrastructure, and living environment for an entire society.
The name Ra was chosen by researcher Kelvin F. Long in reference to the Egyptian solar deity. This study updates the ancient architecture of world ships and attempts to replace the pulsed nuclear propulsion proposed in the 1980s with engines based on inertial confinement fusion.
Loaded Mass Would Exceed 1 Trillion Tons
The structural scale becomes even more extreme when the mass is considered. The study estimates the fully loaded ship’s mass at 1.0156 × 10¹² tons, or approximately 1.0156 trillion tons. Without fuel, the dry mass would be close to 1.989 × 10¹¹ tons, about 198.9 billion.
The structure alone corresponds to approximately 1.6788 × 10¹¹ tons. This calculation includes about 104 billion tons attributed to the cylindrical wall, nearly 49.75 billion tons of regolith, and approximately 14.03 billion tons related to the habitat’s atmosphere.
The article itself warns that several subsystem values are preliminary estimates and that some serve as illustrative parameters to allow for the model construction.
Nearly 848 billion tons would be allocated to the propulsion system
Most of the mass carried would not consist of people, cities, or machines, but rather the material necessary to move the ship.
The study’s budget allocates about 779.7 billion tons for additional propellant, along with approximately 36.96 billion tons for thermonuclear fuel and 30.9 billion tons for the tanks.
In total, thermonuclear fuel, propellant, and associated components add up to a calculated mass of approximately 847.56 billion tons.
This means that the fuel and the material expelled during acceleration and deceleration would dominate the total mass of the ship at the beginning of the journey.
This proportion underscores the main problem of a colossal interstellar ship: the greater the mass transported, the more material is needed to accelerate it, and this very fuel again increases the mass that needs to be accelerated.
The architecture aims to circumvent this issue with extremely high exhaust velocities and nuclear fusion.
During this phase, the expected acceleration would be just about 0.00142 m/s². This may seem extremely small compared to a conventional rocket, but if applied continuously over decades, it would allow the world ship to reach approximately 1,557 km/s.
At this cruising speed, the vessel would cover approximately 328 astronomical units per year, which is about 0.52% of the speed of light.
After the lengthy cruise phase, nearly 14.91 years of deceleration would still be required before arrival.
The base journey would take approximately 854 years
The updated profile of the Mk2A(ii) estimates a mission of approximately 853.92 years to traverse about 4.365 light-years. Of this total, just under 50 years would be consumed by the combined phases of acceleration and deceleration, while about 804 years would be spent at cruising speed.
Most of the people departing would not even come close to witnessing the destination. The article itself estimates that a mission lasting 854 years would correspond to approximately 17 generations, assuming generations based on 50-year lifespans; with 75 years, there would be around 11; and with 100 years, about nine.
The study emphasizes this very break from the conventional idea of a crew. A world ship would not just transport astronauts waiting to disembark.
For the overwhelming majority of inhabitants, the ship would be the only known world: they would be born, live, and die in space, leaving the arrival for descendants many generations later.
Population would start at 1 million and could grow to 10 million
The demographic architecture anticipates an initial population of approximately 1 million people, with space and planning designed to allow growth to as much as 10 million over the course of the mission. This transforms the Ra into something much closer to a mobile civilization than a conventional spacecraft.
The design divides the enormous structure into five separate habitats, meant for residential and work areas.
Large structural partitions between the sectors would also help reinforce the cylinder and serve as additional barriers against radiation between different regions of the ship.
The front section would primarily be dedicated to engineering systems, command and control. There would also be communication antennas, sensors, particle protection systems, industrial facilities, mining equipment, 3D printing parts factories, cargo vehicles, and structures for future colonization.
The entire cylinder would rotate to produce artificial gravity
Maintaining millions of people for generations would require solving a problem that arises even in much smaller space missions: the absence of gravity.
The solution considered for the Ra World Ship is to place the large cylindrical habitats in rotation, producing centrifugal acceleration inside.
A large engine system in the front region would initiate the spacecraft’s rotation. For those near the inner surface of the cylinder, the motion could create a sensation equivalent to a gravitational field, allowing people, water, soil, and structures to remain anchored to what would function as the “floor” of the habitat.
The concept would also require cooling systems, environmental control, recycling, pollution treatment, and even equipment to manage the internal conditions of the habitat. The spacecraft would need to operate as a practically independent artificial ecosystem for centuries.
Water, atmosphere, and regolith would be transported as parts of the spacecraft
It would not be enough to carry equipment and passengers. The mass budget reserves approximately 100 million tons of water, in addition to about 14 billion tons for the internal atmosphere and nearly 50 billion tons of regolith.
Part of the water and stored materials would serve a dual purpose. In addition to sustaining the biosphere, the design envisions that fuel, propellant, and water could help protect the habitats against cosmic radiation and energetic events from stars.
The challenge would be to maintain cycles of water, nutrients, gases, and materials for hundreds of years without relying on supplies from Earth. It is this autonomy that defines a world ship: it would need to operate as an independent world even before reaching another star system.
Heat from 1,024 engines would be one of the biggest challenges of the megastructure
Propulsion would not just produce thrust. The 1,024 reaction chambers would emit large amounts of radiation and residual heat, requiring the ship to carry enormous radiators in the rear section and possibly along different sectors of the cylinder.
The preliminary budget allocates about 100 million tons to the large radiators associated with the drive systems, in addition to other thermal control components. Preventing engine and laser heat from reaching the habitats would be critical for the survival of the population.
The team itself acknowledges serious limitations. Sustaining pulse fusion engines for decades, controlling continuous exposure to X-rays and neutrons, building thousands of giant components, and providing the necessary power to the lasers are among the obstacles that lead the authors to consider other forms of propulsion for a world ship of this scale.
Each engine cycle would require colossal amounts of energy
In the selected scenario, the fusion capsules would need approximately 165.7 gigajoules of drive energy per engine. Multiplied by the 1,024 engines, the study estimates approximately 170 terajoules of driver energy for each combined cycle.
Hypothetically considering a laser system with 25% efficiency, the work calculates that it would be necessary to provide something on the order of 680 terajoules per cycle to the drive systems. The author describes this demand as extraordinarily high and uses it to illustrate the practical difficulties of the architecture.
For this reason, the study does not conclude that inertial confinement fusion is necessarily the best solution.
The conclusion is more limited: while an architecture can be mathematically formulated, technological obstacles may favor different systems in the future, including external pulsed nuclear propulsion similar to that used in previous studies of world ships.
Ra World Ship remains a concept, but puts numbers to the idea of transporting a civilization between stars
Currently, there is no technology capable of constructing or launching a vessel that is 115 kilometers long and has a mass exceeding 1 trillion tons.
The Ra World Ship is a conceptual engineering study, developed to mathematically explore what dimensions, masses, populations, and propulsion systems would emerge when interstellar travel shifts from small crews to whole societies.
The Mk2A(ii) version takes this idea to a measurable extreme: 115 km of structure, a radius of 7.24 km, five large habitats, 1 million inhabitants at the start, potential growth to 10 million, 1,024 engines, a speed of approximately 1,557 km/s, and a loaded mass of approximately 1.0156 trillion tons.
Even in this scenario, crossing just over four light-years would still be a mission of approximately 854 years.
It is precisely this number that summarizes the scale of the interstellar problem: even a machine larger than many cities, continuously propelled by over a thousand fusion engines and traveling at over 1,500 kilometers per second, would still depend on dozens of human generations to complete the journey.
