Interstellar Travel

Classic Journey

Classic trip. Travel 16 light years to red dwarf Gliese 1002  in under six years. Ship accelerates at 1g (Earth's gravity) for half the distance, then turns and decelerates at 1g for the remainder. The orange bell curve shows time as measured by the ship's clock, teal curve shows time as measured on Earth, blue curve shows time in a hypothetical Newtonian universe. Relativistic effects are very pronounced with a maximum gamma factor over nine.  The green curve shows mass remaining based on the Tsiolkovsky  equation modified for constant acceleration models. Travellers have a comfortable journey that, with hibernation technology may feel like just a few months. Unfortunately, not a realistic mission, even for the far future. See below for more realistic scenarios. (Simulated using a Flox Calc script).

Introduction

The possibility of travelling beyond our solar system to a far distant planet has long been of major interest, but probably never more so than now. The last 30 years has seen the tantalizing discovery of hundreds of Earth like planets orbiting neighbouring stars. Many of these within a habitable zone where liquid water might exist. Soon, we will have telescopes able to look at their atmosphere and terrain. It now seems likely that at least a few will be much better colony candidates than any within the solar system. Our best local candidate is Mars, but with freezing temperatures,  thin atmosphere, low gravity and lack of standing water it is not an enticing prospect.  A Mars colonist must severely limit outside exposure even with a protective suit. Radiation levels both solar and cosmic are too high for long exposure. The low gravity will, over time, be deleterious. Our bodies require something close to Earth gravity for proper maintenance of bone density.  These considerations coupled with a lack of readily available water will probably mean any Mars colony will be small and impermanent, with colonists rotated back to Earth every few years. In contrast, an exoplanet could offer a permanent home. With a benign, temperate, protective atmosphere and Earth like gravity, colonists may be able to stay indefinitely. A non-breathable atmosphere is not a major constraint. Colonists can work and play outside with an oxygen pack if they do not need heavy protective suits.  But here's the thing, can we get there? That is what we will now explore.

First thing we must do is survive. As this is written, frankly, the prospects are not great. One explanation for the Fermi Paradox (lack of any response from alien intelligence) is that advanced civilisations self destruct before they are able to colonize other worlds. If that were the case, there is little prospect of bucking the trend. Still, we have to be optimistic and hope we defy the odds. What then? 

Hopefully, we will make steady progress, first with robotic missions then with sleeper vessels or, possibly, generational ships. Robotic missions can be expected within the next 500 years with human travel emerging at the end of this millennium. Shorter timelines are possible but less likely.  Some believe that interstellar travel is so difficult as to be next to impossible. All that can be said is that with current understanding there are no insurmountable obstacles. It was thought shielding might be a deal breaker over such a long distance. However recent work on the Breakthrough Starshot program as well as extensive testing of materials for Mars missions suggest otherwise.  There are issues at very close to light speed, but that is a problem for the far future. Attainable maximum speeds in the  near to medium term (1000 - 10,000 years) are likely much less than half light speed.  Passive, sacrificial shielding held forward of the ship should cope well, along with a thick coat of one of the advanced hydrogen rich polymers currently under development.  First though, we begin with a theoretical journey popular in fiction and widely discussed on the net.

The 1g Trip

If only we could. Why not simply arm a ship with sufficient fuel to allow acceleration of 1g for half the distance then turn around to decelerate at 1g for the remainder. With the comfort of Earth's gravity when awake, hibernation technology to extend sleep, we would arrive, refreshed and ready, in reasonable time to colonize our favourite exoplanet. The trip above  shows a six year journey to Gliese 1002, a red dwarf star with two potentially habitable Earth sized planets (b and c). 

Sadly for that comfortable trip, the deal breaker is fuel. Look to the left of the graph to see a green curve. That shows  the mass remaining of the ship over time as measured by the ship's clock. The readout on the right shows the percentage of mass remaining (Mr) down to four decimal places at the end of the trip. Our boat consumed itself in less than a year. This is a direct result of the Tsiolkovsky rocket equation. Basically, to move forward in space, matter must be exhausted out the back. The percentage of mass remaining depends on just three parameters, time as measured by the ship, the acceleration and the exhaust velocity.  It little matters how big the ship actually is. It is not simply a question of scaling things up. More fuel carried means more mass to accelerate. Plainly, some reasonable percentage of remaining mass will be required at journey's end.

Early Missions

So what might be achieved through the next millenium? First missions will be robotic and will occupy the next four to five hundred years. Some have already been proposed. Most notably the Breakthrough Starshot program. This uses light sail technology to propel gram sized craft to speeds up to 20% light speed. Problem is it does not scale well to above kilo masses. Solar pressure is not enough for high mass vessels. There may be some scope for huge space based lasers to provide enough push, but this technology is probably not for near future missions. 

It seems reasonable to expect drive technology to evolve well beyond the current generation of ion drives. They are good for  local missions, but with exhaust velocities in the 20 to 100 km/sec range it would take thousands of years to reach the nearest exoplanets. At those timelines, even robotic flyby missions are unlikely to proceed. However, it seems reasonable to suppose that advanced fusion or perhaps fission fragment drives will be developed with exhaust velocities in the 10,000 to 20,000 km/sec range. At those velocities interstellar travel is (hopefully) achievable within realistic time frames. 

Below is what may be a typical robotic flyby mission, this time to Ross 128 b. The ship accelerates at around 2% of Earth's gravity for a bit over 7 years, after which it coasts forever. Exhaust velocity is 10,000 km/sec. This leaves our benchmark of 1% of ship mass remaining to provide telemetry back to Earth. Even in flyby mode the mission takes a lengthy 78 years to reach its target. However, it should be noted that, from the start, valuable telemetry will be received about other useful aspects of a long interstellar voyage, particularly with respect to the nature of the local interstellar medium (ISM). Once started, it makes sense to launch new missions at say, one every 3 months, so a constant stream of data will be returned. Following a lengthy 80 year wait, detailed telemetry on all the nearby systems will start coming in over the following decades. Perhaps as far out as the Trappist System. Finally, after a longer wait, robotic missions will incorporate a deceleration phase to place some in orbit and even land on a favoured exoplanet. These missions are a precursor to the first human colonies and are discussed next. 

Early Robotic Mission

Possible robotic flyby mission to Ross 128 b. The ship accelerates at 2% of Earth's gravity for 7.42 years over a distance of 0.55 light years, then coasts forever. The vessel passes the planet after 77.78 years, returning telemetry to Earth. Earth receives the telemetry 89.6 years after the launch.

First Human Colonies

It is now the second half of the millennium. Robotic missions have returned detailed information on all of the Earth sized exoplanets with a reasonable chance of colonization. The best option is chosen for the first colony. Preparation includes landing food supplies, equipment and material for shelters. It is likely dozens of robotic missions will precede any human journey. The mission itself will probably require a global effort, with a massive budget and years spent planning. The size of the vessel will be an order of magnitude bigger than any robotic mission with a corresponding increase in the fuel load. Nonetheless, there is, currently, no obvious reason it could not be accomplished, given the will to do so. The reason a very large ship may be needed is because the journey will likely be 100+ years. A big leap of faith in drive technology could shorten the trip, but there is not enough evidence of such an advance given the near future timeline. Such a ship would be generational. That means significant resources to maintain and entertain the several generations living out their lives onboard. A sleeper vessel would, of course, enable an earlier and much less costly solution. Hibernation technology may evolve to allow that but the timeline for it is difficult to predict. 

One  possible scenario is shown below, assuming a drive that can support an exhaust velocity of 0.05 light speed (15,000 km/sec). The aim of the simulation is to complete the mission with at least 1% mass remaining of the ship. That means at the start, fuel will be 99% of the ship's mass. For example, a 10 tonne payload with a similar mass of an empty ship would require 2000 tonnes of fuel. This is an arbitrary figure. Much less than 1% mass remaining may not be feasible. It is up to the reader to decide on a sensible figure. Acceleration is roughly 10% of Earth's gravity and is maintained for just over a year, using 90% of the fuel. After that, the ship coasts at 11% light speed for the next 111 years. Finally, the ship uses the remaining fuel to decelerate again at 0.1g to arrive at Teegarden's star a year later. Time for the whole mission is roughly 113 years. 

Possible Near Future Mission

A plausible, if optimistic, near future mission to Teegarden's Star with two potentially habitable Earth sized candidates b and c. An exhaust velocity (Ve) of 5% light speed pushes the ship forward with an acceleration of roughly 10% of Earth's gravity for 1.12 years ship time, using 90% of its fuel. The ship then coasts over a distance of 12.5 light years taking 110.9 years, then turns and uses the remaining fuel to decelerate again at 0.1g for 1.12 years, arriving at Teegarden's Star after 113 years, 6 weeks.  

The Human Empire Begins 

A mission to Gl 625 b, a promising exoplanet some 20 light years distant. Relativistic effects are now apparent as seen by the separating graphs. The journey now is around 68 years ship time. As the vessel is a sleeper ship, the journey is over quickly for the hibernating passengers. They still lose a few years of life to senescence while asleep, but within acceptable limits. Colonization is now able to proceed at a faster pace and there is a dozen or more inhabited exoplanets with steadily growing populations.  

Looking across ten millenia should see the beginning of the human galactic empire. Colonies will spread into the closest systems, supported by hibernation technology which allows more rapid expansion than is possible in the near term. It is, of course, a guess that such technology is available. However, a serious look at the available literature would indicate that both hibernation and drive technology will be up to the task. Expansion will be limited given that a journey much beyond 100 years is unlikely due to natural limitations of hibernation. 

City sized colonies will offer a full range of opportunities for humans to enjoy. Robotic support will have long eliminated the need for any kind of drudge work, perhaps even many technological occupations can be standardized and automated. For example, hospitals will most likely be staffed by android doctors and nurses, with routine operations undertaken without human intervention. It will still be the case that advanced, non-routine work is driven and supervised by human researchers. The focus for the human population is no longer work but education and exploration. 

The most serious issue faced over this period is likely to be the problem of artificial intelligence. At some point, it will be possible to create sentient life far more capable than humans themselves. A huge ethical and possibly existential decision will then have to be made about how to proceed. The safest approach is to allow high levels of capability without the possibility of becoming self-aware. Whether or not sentient robots can be prevented, or, if allowed can then be controlled is an open question. Perhaps they will take over and eliminate human life. Perhaps humans and artificial life can coexist, after all, they may be morally and ethically more balanced than humans themselves. But then could humans cope with being a second-rate society, unable to compete at any level with their more advanced creations? The problem is greatly magnified by exoplanet colonization. It would only take one colony to allow artificial life to expand for it to ultimately take over.    

The latter half of the period  will also see the first stepping stone missions. These are missions which originate on an exoplanet, substantially extending the distance  of colonies from Earth. These far colonies will herald the next phase of expansion - the far future and beyond.

Far Future Scenarios

Visiting Kepler-1638 b for a 6 month holiday. The 5000 light year journey taking just 861 years is over in under a day (from the traveller's perspective) thanks to cryonic storage. An advanced antimatter drive provides a combined and directed pion and gamma ray beam giving an exhaust velocity (Ve) at 95% light speed. With a gamma factor of nearly 6 and a maximum velocity over 98% light speed this is well into relativistic territory. Your ship accelerates at about 20% g for 12 years, coasts for 836 years then decelerates again for 12 years, still with enough ship left to arrive with passengers and cargo. The mass remaining is now down to half a percent thanks to the low energy cost of cryonic storage. Naturally, your relatives at home will be long dead on your return, unless they are also willing to enter a freezer for the 10,000 years passing on Earth.

Move forward 10,000 years, still only a fraction of the time humans have existed on Earth and a mere blink in geologic time. Continuing with a utopian vision of the future and assuming survival there are many possibilities to consider. At some point cryonic storage may become possible and is a proverbial game changer for interstellar travel. Long voyages of thousands of years are feasible.  Even if drive technology lags, cryonics will still enable a rapid colonial expansion building to a human galactic empire. Such expansion may be stymied by alien interjection - as in a Star Trek scenario. However, some vast alien galactic presence is already looking increasingly unlikely. There may be other civilisations out there. But more than a very few would mean some far in advance of human technological society. So where are they? We can already see far enough for any such society to make itself known, given the likelyhood of their own colonial expansion. It is a big call, but it would seem advanced technological civilizations are rare, even on a galactic scale. In short, there may be little to prevent the inexorable spread of humanity across the Milky Way. 

It may be that even into the far future there is still no sign of alien intelligence. Will that compel us to search beyond the Milky Way? Many would claim those distances are too vast and we will never succeed in intergalactic travel. But even here there are really no overwhelming technological constraints. Next stop, the deep future - our journey on the good ship Brigadoon. 

Deep Future

An imaginary journey to the Andromeda Galaxy. The good ship Brigadoon is driven by a very advanced antimatter drive that accelerates slowly at 5% g to achieve 0.998 light speed after 92 years. Arriving 49,000 years later, the ship cruises alongside the galaxy searching for life. If complex life is found the crew, buried deep inside metres of frozen hydrogen, is revived. It is their decision whether to slow and explore or simply collect data and continue to the next galaxy. Naturally, if a technological society is detected they are expected to slow to meet and greet. The ship can refuel with antimatter from colliding white dwarf binaries or neutron stars to continue the journey.  

Welcome to the Deep Future. One hundred thousand years have passed, human colonies have expanded across the Milky Way. But still no sign of even complex life on other worlds. Simple unicellular life is not uncommon, but continual extinction events prevented further evolution. Turns out Earth just got incredibly lucky in avoiding the common fate of complex life on other worlds. So does the search continue or are we daunted by the vastness of intergalactic space? Eventually, work begins on a massive, intergalactic cruiser - The Brigadoon. 

The Brigadoon is 20 km long pencil shaped monster. It features a large sacrificial frontal shield plus a powerful laser to detect and deflect any critical object in its path. Operating at a speed that implies a gamma factor over fifty means any collision will hit with the energy equivalent of fifty times that of an antimatter explosion. However, such is the emptiness of the intergalactic medium (IGM) that the probability of a collision with anything destructive is small. An army of intelligent robots supervise guidance and periodically initiate repairs. However, even they are mostly asleep as Brigadoon cruises for thousands of years through the vast, empty IGM. The human crew lies frozen, buried in the metres of frozen hydrogen required to protect them from cosmic radiation. 

The mission is to search for intelligent life in other galaxies. The ship flies alongside a galaxy and, with its large array of sensors and telescopes, will likely detect any technologically advanced civilization. Pre-industrial societies or simply complex life are harder to find but are not a mission priority. If complex life is found, the human crew is revived and must decide whether to slow the ship and investigate or simply collect data and move on. Typically, they would only slow in response to detection of advanced technology. Otherwise, they collect data for transmission back to Earth colonies, then on to the next galaxy.  In any case, the crew is forbidden to make contact with other than an advanced society comparable to Earth. Star Trek's "Prime Directive" applies and for good reason. No point in finding something as rare and unique as life only to contaminate it with our own ideas and technology. 

If a slowdown is initiated, it will likely consist of some initial fuel expenditure to bring the gamma factor down to a level that will enable the ship to enter the denser ISM of a galaxy. Following that, the ship can use gravity assist and drag to further slow in the approach to a target planet. The whole process will take thousands of years, but the ship is designed for longevity. After the initial revival, the crew may only stay awake for for a month or two, so their next awakening will be on arrival. They then have the option to stay with their newly found society, or move on again. The ship can refuel with antimatter from a variety of sources, including neutron stars and from the collision of a white dwarf binaries.

Eventually, the crew is released from the mission. Either locating a friendly alien society to merge with, or starting a fresh colony on a planet in some far galaxy.  Finally, there is a chance they may find another Earth colony that established itself using a more advanced ship from their own future.  

Sombrero galaxy could be worth a look. It does take 600,000 years to get there, but why worry if you are frozen solid the whole trip. Maybe not a great prospect for a SETI search though. A massive black hole is a destructive beast.