World Ships 2

In an earlier post we detailed the publication of a paper that discussed the design of a world ship, a large vessel with a carrying capacity of 1 million people (growing to 10 million) over a trip time of order one millennia to the nearest stars. This was based on an earlier 1984 study for a Mk2A world ship, and we renamed it Ra after the Egyptian God of the Sun.

The first paper mostly dealt with demographics issues but also some discussion on the creation of artificial gravity through spinning of the habitat. This first paper was:

K. F. Long, Population Demographics and Other Issues for the Massive Ra World Ship Model - Part 1, Journal of the British Interplanetary Society, Volume 76, November 2023.

We are pleased to report that a second paper has also now been accepted for publication and this paper deals specifically with the propulsion issues for how to propel such a massive vessel. This is using inertial confinement fusion engines, via laser drivers, as opposed to the original 1984 study which used external nuclear pulse propulsion such as in the historical Project Orion.

The new paper is:

K. F. Long, Inertial Confinement Fusion Propulsion for the Massive Ra World ship Model - Part 2, Journal of the British Interplanetary Society, Volume 77, April 2024.

After some research on the different design configurations for the engine, a layout of using 1024 engines was adopted as a compromise between minimising the number of engines (which increases the system mass) and minimising the ICF capsule mass; since too high a capsule mass then requires much higher laser drivers to implode and ignite them.

For the world ship Ra the large spacecraft would accelerate for around 35 years, coast for around 853 years and then decelerate for a further 15 years, reaching the target destination after a total trip time of 854 years. Its maximum cruise velocity would be of order 1,557 km/s or 0.52% of the speed of light. Throughout the voyage many generations of humans would be born, live out there life and die on the ship.

With its huge 10^12 tons mass the parallel thrust engines would deliver a thrust of 978 GN, using an exhaust velocity of 1,880 km/s and a mass flow rate of 5.2*10^5 kg/s. The individual ICF capsules would have a mass of 230 grams but would be augmented by 4.853 kg of expellant propellant which under thermalisation helps to boost the mass flow rate and thereby thrust. Each capsule would require 166 GJ of driver energy and correspond to an energy gain of 240. For all of the 1024 engines combined the total driver energy requirement would be around 170 TJ.

This paper, along with the earlier one, may be one of the most up to date comprehensive reviews of a world ship architecture but within the nuclear pulse propulsion paradigm. Also discussed in the most recent paper were the sourcing of the world ship regolith, atmosphere and water.

Who is to say if such vessels are to become a part of our plausible future, or will other options be adopted such as much smaller and lower mass vessels but containing Artificial Intelligence and embryo seed banks for growing at the destination. Yet it is interesting to see these architectures sketched out and to visualise what may be possible and what it would take.

Fusion Energy Exhaust

In studies of advanced fusion spacecraft the engine exhaust is often depicted as a white jet of collimated gas emitted from the rear of the vehicle. A case in point is the Daedalus study, a theoretical research project into an interstellar flyby probe conducted by the British Interplanetary Society in the 1970s. In almost all of the depictions it is shown with a very bright white emitting jet from the engine. An example of this is shown in the beautiful image below by Nick Stevens. The powerful engine associated with this vehicle would have a thrust of around 7.5 MN and an associated jet power of 40 TW, releasing energy at a rate of 4.42*10^13 J every second.

Illustration of Daedalus by Nick Stevens

Yet, realistically what would the colour of the exhaust plume be and would you even be able to see it. This is worth some examination. The table below shows several calculations for key fusion reactions that might be expected to be utilised within an advanced fusion engine. The last one is the proton-proton reaction which is very difficult to ignite due to its incredibly small cross section, except at the centre of the Sun where it is achieved thanks to the massive gravitational pull.

Also shown in the table is the expected frequency range of the total energy emission. This is then translated to a wavelength for the radiation. It is shown that the wavelength range is given as 20.6 - 123.5 pm, where 1 pico = 10^-12. To write this another way this is a range of 2.0610^-11 m (0.02 nm) to 1.23510^-10 m (0.1235 nm). Then looking at these wavelength range on any electromagnetic spectrum it is shown that they correspond to x-rays.

These x-rays are generated from several physics processes within a fusion engine. This may be from Bremsstrahlung radiation, Synchrotron radiation (relativistic) or Cyclotron radiation (non-relativistic) and it really depends on the type of fusion engine being used, i.e. magnetic bottle trapping versus laser induced inertial confinement fusion.

For the wavelengths calculated above, these x-rays are outside of the visible part of the electromagnetic spectrum. This means that a lot of it would not be visible to the naked eye, although it could be detected in x-rays detectors. That said, its possible that some of the emission would approach ~100 nm range which begins to tend towards the extreme ultraviolet. Even though there is less of this light being emitted, compared to those in the x-rays, it will be incredibly bright from our perspective, and would tend towards a light purple colour - perhaps violet.

So this then might be a recommendation for any artist depicting an advanced fusion propulsion engine, to perhaps paint the jet exhaust lightly with a tint of purple/violet haze within it. This might not look as spectacular as say the images like the one shown above, but it would be more realistic. That said, any humans looking at this light would have to be a a very safe distance and also behind an x-ray shielding screen, since the dose of radiation from the jet emission would be too dangerous to observe directly.

This also means that launching any such vehicles would have to be away from Earth orbit with its orbiting satellites and human crewed space stations or we can expect much damage to occur. The vehicle would have to be ‘tugged’ out first to a safe distance before switching on the engine. Although astronomers from Earth would be able to observe it through their detectors speeding off towards infinity and that would make for some good images.

SunVoyager Revisited

A second paper has now been published on the concept of the SunVoyager spacecraft. This is a robotic probe mission to 500 - 1000 AU propelled by inertial confinement fusion engines. Its main purpose is to deliver a large astronomical observatory to the solar gravitational lens focal point so as to image exoplanets around nearby stars in the opposite direction of the Sun.

In the redesign the vehicle would use 5.76 mg ICF pellets fuelled with a mixture of deuterium and helium-3, but this would be augmented with 10 mg of expellant propellant for the purpose of increasing the mass flow rate and therefore the thrust.

The spacecraft would travel at a speed of 720 km/s reaching its destination in a trip time of 6-10 years. Although such a mission has much potential, it is not without its problems. In particular, the laser drivers required for the mission would have to be highly efficient, and for this model they were calculated assuming 24% efficiency which is beyond the current state of art.

Yet such a mission may be possible in future decades and this is especially since in the last two years the National Ignition Facility in San Francisco has achieved ignition and energy gain within a laboratory experiment. This is an exciting time to be alive and once we have figured out the physics all that remains is the engineering and economics to build a better machine.

The paper is titled “Development of SunVoyager Interstellar Precursor Probe Driven by Inertial Confinement Fusion Propulsion”, published in the Journal of Spacecraft & Rockets, May 2024.

https://arc.aiaa.org/doi/10.2514/1.A36045

Interpreting the Aliens

by Kelvin F Long

Quite a few years ago now I worked on a project with the U.S military. This was a dozen officers mostly of rank Captain and Major who had the task of developing a strategic contingency plan for a potential ET invasion. To be clear, this was a scenario training exercise and did not reflect an actual fear of a real threat to humanity. It was a lot of fun to see how these outstanding officers grappled with the various issues and assessed the limited data they were given in the scenario for events unfolding in the distant cosmos that pointed towards artificial megastructure engineering that was headed our way.

Since they were members of the military, they were very much threat oriented and this indeed was a part of their brief. To identify the threat and develop a mitigation strategy for the purpose of defence or even elimination of the threat. But this bothered me a little, since there was insufficient information to lead to a firm conclusion that a threat was imminent and I wanted to encourage the officers to think more out of the box in terms of questioning the chain of command instructions. In the military this is not really permissible of course and a chain of command is important for minimisation of errors, but given the gravity of the subject matter I wanted to encourage them to see a bigger picture and to develop an ability to challenge authority when it is appropriate.

Around the same time, a film had just came out titled ‘Arrival’ which was based on the short text ‘Story of Your Life’ written by Ted Chiang. In this story objects arrive on Earth and it is the task of the scientists to establish a line of communications with them - a species referred to as the heptapods. It was an excellent film and I have seen it several times since.

But one of the main issues that the story centres around is a misinterpretation of the intended message. The scientists believe that the heptapods are communicating “offer weapon”, which is then also interpreted by China to mean ‘use weapon’, when in fact the species is communicating the word ‘offer tool’ or ‘offer technology’. They simply want to establish a trading relationship with our species.

Knowing about this misinterpretation in the film, I decided to have a bit of fun with the excellent officers. I told them that in this hypothetical scenario a message had been received from one of the distant ET craft and that it was symbolic in nature, constructed of a group of Hieroglyphic like symbols.

I asked them in groups of three to see if they could work out what the message said. I gave them a set of 7 picture cards and firstly they had to put them into the right order. If you want to have a go at this yourself, then don’t scroll down yet since the answer is at the bottom of this post.

The officers split into their respective groups and they were given half an hour or so to come to a consensus about the message and also to put the cards into the right order. The chosen symbols were actually borrowed from ancient history on Earth and I used a combination of Egyptian Hieroglyphics and proto-Cuneiform since I had familiarity with both languages. I wanted to see how they would interpret the images.

A historical example in the history of Earth where a miscommunication occurred leading to tragic consequences was the Voyage of Captain James Cook who in the 1700s had landed on the coast of New Zealands North Island. Pulling alongside the East side of the Turangunui river near present day Gisborne he encountered the Maori inhabitants. But a leader from this tribe was immediately shot and killed whilst they were undertaking a ceremonial challenge to greet the new arrivals. Cook and his crew had misinterpreted this as a hostile gesture of war.

The military officers correctly put the cards into the right order as should be expected from what was highly intelligent individuals. But next came the issue of translating the message. There was a variation in the answers and this is to be expected since the challenge was somewhat ambiguous but they really enjoyed the task having spent the prior few days immersed in physics and engineering lectures so this was a welcome distraction. It was so long ago now that I don’t recall all of the group answers but for sure there was a variation.

The truth is that there was no right or wrong answer, since I had made up the message, but I wanted to leave in their minds a clear impression that (1) instructions for the annihilation of another should always be questioned when the consequences are so large (2) that messages can be misinterpreted and it was important to develop a critical thinking capacity and come to your own conclusion.

This is especially important when we live in a world with an incompetent leadership class across the political spectrum who neither serve the interests of the people who elect them, or make sensible decisions that lead to the improvement of society or the global community. Indeed, I would argue it’s even worse than that, in that we, the people of planet Earth, are deliberately deceived by every government of every country in the world on a regular basis. This points towards a profound problem in the human culture and the models we have adopted for how we elect our leadership class.

Given this, if ET was to come and visit us, we should be very careful about who we listen to and what information is communicated to us and therefore what are the motivations? Indeed, even on Earth one has to wonder in a geopolitical context what one nation is saying that is misinterpreted by another nation and how this can lead to dramatic consequences for everyone. Actions matter, but words matter too. Yet leaders across the world of all political persuasion are not careful with their words, but in fact they practice the opposite. We appear to live in a time where nation state diplomacy for the settling of disputes and civil discourse as a means for solving arguments is all but at an end. I worry about that. I worry about how we misinterpret each other.

When I was a child we learned the phrase “sticks and stones can break my bones but words can never hurt me”. But in the real world of nation state rivalry words can lead to chemical, biological or thermonuclear Armageddon. Words matter a lot!

So what did you come up with? My proposed answer is shown below. Remember this was just a bit of fun and I hope you enjoyed the challenge too.

Message read from Right to Left. The actual message intended was a request to meet in our system (star) at the first light (sun rise) and walk (foot) in the place where the mountains are (mountain), then to greet each other and in exchange a gift of precious metals (jewels) and tools (arrow) would be given to the humans. In return the aliens requested permission to mine (plough) hydrogen fuel (water) from the Sun (star) upon which their starship (boomerang) would then move off on a new trajectory away from our solar system (star).

Interstellar Probe Project

by Kelvin F Long

n 2018 I attended an awesome meeting at the Explorers Club in New York. The meeting was to discuss the design of a potential interstellar probe to travel to 1,000 AU, to derive its possible design definition and also the compelling science goals to be achieved. The project was led by scientists at the Johns Hopkins University Applied Physics Laboratory. The main requirement was to develop a probe that could travel at 50 AU per year and therefore reach the distance within around 20 years. Other than the initial meeting in New York, I attended meetings in various other locations around the United States including in New Orleans and California. In those meetings it was insightful to learn how professional spacecraft designers go about designing an interstellar probe and it was a privilege to be involved with the original concept definition as it began to materialise, with one of the earlier goals to generate a 30 W power capability within a 30 kg payload mass. Such a probe would explore the solar heliosphere, Kuiper belt objects and circum-solar dust disk and even a flyby of a Trans-Neptunium object. The interstellar probe would pave the way, scientifically, technically and programmatically, for longer interstellar journeys that would require future propulsion systems. It would go beyond the Grand Tour of the 1970s Voyager probes. It was recognised that a high solar system escape speed would be required, and probably at least twice the asymptotic speeds of Voyager 1 at its ~34 km/s. Two approaches were considered to include a passive one with a C3 launch and unpowered gravity assist, and an active one with a powered gravity assist and the use of in-space propulsion. This might include solar sails, solar electric, radioisotope electric, nuclear electric or nuclear thermal propulsion.

Although I was only involved with the project for the first year or so, I was highly impressed by the professionalism of the scientists involved who led the project. People like Pontus Brandt, Ralph McNutt Jr, Michael Paul and Jim Kinnison. Inspirational leaders reaching for the far frontier in space. The project was initiated in support of the Decadal Surveys of the United States and focussed on the time frame for a possible launch of 2023 – 2032. The final report is listed below and I was honoured to have my name listed as one of the collaborators for such an inspirational study. Below shows a photo from the meeting of the initial team in New York City in October 2018 held at the Explorer's Club.

INTERSTELLAR PROBE, HUMANITY’S JOURNEY TO INTERSTELLAR SPACE, NASA Solar And Space Physics Mission Concept Study for the Solar and Space Physics 2023 - 2032 Decadal Survey, led by Ralph L McNutt Jr, Michael V Paul, Pontus C Brandt, Jim D Kinnison et al., 13 December 2021.

https://interstellarprobe.jhuapl.edu/Interstellar-Probe-MCR.pdf

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Inaugural Meeting of Interstellar Probe Working Group

First Contact with ETI

by Kelvin F Long

Recently I published one of my more speculative papers to date. I derived a very simple equation to estimate the time of first contact with Extraterrestrial Intelligence (ETI). This calculation neglected the possibility that they would come here and only considered (as a thought experiment) the possibility of us interacting with them either in deep space (in the space between the stars) or in their planetary system of ETI origin by one of our probes. The paper emphasised two key developments which will enable first contact being; advanced propulsion capability and the discovery of many exoplanets through astronomical observations. This calculation depended upon two factors (1) the growth rate to mature our technological propulsion capability to the appropriate velocity, where as a case study I examined up to 0.1c (2) the distance to the astronomical target where it is assumed an exoplanet has been discovered with the appropriate conditions for life, for which we may choose to send such a probe. I concluded that for interstellar targets out to 10 LY first contact may occur any time in the next ~25 - 174 years. For targets out to 100 LY first contact may occur any time in the next ~114 - 1,044 years. For targets out to ~200 LY first contact may occur any time in the next ~214-2,000 years. The range of values was a function of the growth rate parameter index. Any maturation of technology into the relativistic velocity regime will only serve to highlight these conclusions.

On the basis of the above I recommended that astronomers should give particular attention to any exoplanets within ~200 LY distance. Since this neglected the possibility of them coming here, and given the age of our sun relative to other stars, the probability that they would have been here already or at least had knowledge of us was argued to be high. This also implied that the scenario of an ETI presence in our own solar system was also high. Although, these conclusions do depend on the assumption of a galaxy that is filled with independent biogenesis which emerges purely as a function of chemistry and the rise of intelligence in the first place and so is the major uncertainty on any conclusions. This also makes certain assumptions about the nature of intelligent life as organic/chemical in origin when in fact it could be defined more broadly but this is beyond our current knowledge (i.e. Schrodinger's definition of life being a resistance from decay to thermodynamic equilibrium).

The paper was:

K. F. Long, The Temporal Contact Equation: An Estimate for the Time of First Contact with ETI, JBIS, 76(11), 279 - 282, November 2023.

Vorlon Cruiser Spacecraft from Babylon 5

Project Starshot

by Kelvin F Long

Last year I published a paper on dust erosion for the Project Starshot probe. This is in relation to my work on the Breakthrough Initiatives Project Starshot, where I sat on the advisory committee since it was first formed in 2016. It was a lot of fun and I got to work with people like Martin Rees, Louis Friedman, Phil Lubin, Geoff Landis, Greg Matloff, Avi Loeb, Jim Benford, Greg Benford, Pete Klupar, Mason Peck, Kevin Parkin, Pete Worden, to name a few...and even got to meet Frank Drake a couple of times before his passing. It was an honour to have worked on the project and I feel that the theoretical results from the study have advanced our knowledge about how to do beamed propulsion missions of this type. Project Starshot still continues of course and I hope will pioneer new and interesting results in the future. In my paper I estimate that the erosion rates over the ~20 year journey would be of order 10^-11 - 10^-8 g/s and including a design margin would suggest a shielding size of order ~3 - 7 mm thick for the Gram scale probe. Interesting to wake up this morning to the news that the New Horizons probe may have discovered an additional dust layer to the outer solar system. The paper was:

K. F. Long, Calculations of Particle Bombardment Due to Dust and Charged Particles in the ISM on the Project Starshot Gram-Scale Interstellar Probe, JBIS, 76, 94-111, 2023.

Below is a photo of the original Project Starshot advisory committee meeting at the house of our sponsor Yuri Milner back in 2016. I'm at the back next to Greg Benford. Below is also a photo from the meeting and it illustrates what it is like to work on such projects with many amazing people all dedicated to the vision of advancing spaceflight on behalf of humanity.

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Typical Project Starshot Meeting discussing the technical program

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Inaugural Meeting of Advisory Committee at the home of the sponsor Yuri Milner

World Ships

by Kelvin F Long

In some recent studies I have been looking at World Ships. These are massive vessels 10^11 - 10^12 tons travelling at 1,500 km/s (0.5%c) to reach the nearest stars in 1,000 years or less. They carry 1 million people at the start of the journey and allow for growth of that population. The design concepts were to be propelled by 1024 engines based on inertial confinement fusion systems and this requires a staggering 170 TJ driver energy assuming an optimistic 25% wall plug efficiency of any lasers. The individual pellets in the design are 230 grams each of TN fuel along with 2.43 kg/shot of expellant propellant for thrust augmentation, all detonated at 100 Hz pulse frequency. The first paper was published and examines population demographics, power supply requirements for the habitats and spin gravity to contain the atmosphere and regolith. The second paper is currently under submission and examines the propulsion system. These studies were based on earlier papers from Bond/Martin in 1984 which utilised external nuclear pulse propulsion and seeks to advance concepts to the next stage. Its not easy to push such a large vessel and requires a thrust of 978 GN.

K. F. Long, Population Demographics & Other Issues for the Massive Ra World Ship Model - Part 1, JBIS, 76(11), 262 - 272, 2024.

K. F. Long, Inertial Confinement Fusion Propulsion for the Massive Ra World Ship Model - Part 2, JBIS, Submitted, February 2024.

World Ship Frontal View

World Ship Rear View

World Ship under Construction in Earth Orbit

STV Hypothesis for the Fermi Paradox

by Kelvin F Long

Recently I published a paper giving an explanation for the Fermi Paradox which I call the Spatial-Temporal-Variance (STV) hypothesis. It relates to the temporal and spatial scales of emerging intelligent civilizations in the galaxy. Calculations were conducted for populations of von Neumann probes expanding out through the galaxy as a dispersal problem. I used geometrical series as a representation of the growth of a population and diffusion theory as a representation of the decay of a population. This results in two possible extremes including Drake-Sagan chauvinism for a crowded galaxy and Hart-Viewing chauvinism for a quiet galaxy, and then everything else in between. I argued that our galaxy is likely to be populated but sparsely.

For more information see the paper:

K. F. Long, "Galactic Crossing Times for Robotic Probes Driven by Inertial Confinement Fusion Propulsion", JBIS, 75, 118-126, 2022.

Discovery III

by Kelvin F Long

I have recently been working on a mission concept for the outer planets to Jupiter and Saturn. This is based on an earlier NASA study called Discovery II which was driven by a spherical magnetic torus fusion engine (which itself is based on the earlier Discovery I from 2001 A Space Odyssey driven by a gas core nuclear fission Cavradyne engine). The difference is that the Discovery III is also an ICF driven engine. As with the NASA mission it is designed to carry a 172 tons artificial gravity payload and 6 - 12 astronauts, completing the mission to Jupiter at ~5 AU in around 118 days and the mission to Saturn at ~10 AU in around 212 days.

The Discovery III engine would be 26 MJ driver energy delivered to a target of 2.88 milli-gram fuel with an assumed laser efficiency of 7.69% with a peak intensity of 4.7e17 W/cm2. Preliminary calculations suggest that the ICF engine may only require half as much thermonuclear fuel mass when compared to the magnetic fusion engine (Discovery III versus Discovery II) but this research is ongoing. A paper will be submitted on these calculations to a journal shortly.

NASA Discovery II

ICF Driven Discovery III