Photon drives may offer a railway to the stars
Most scientists agree that slow speed travel to the stars is impractical. Any slow interstellar mission, defined as one that is at or below…
Most scientists agree that slow speed travel to the stars is impractical. Any slow interstellar mission, defined as one that is at or below 10% of the speed of light, 0.1c, would likely be overtaken by faster missions after technology becomes more advanced. The fastest human made object ever, the Parker Solar Probe, reached 0.064% the speed of light as it grazed the Solar atmosphere. At this velocity, only achieved by gravity assist, it would take about 7000 years to reach Alpha Centauri. Therefore, there is little reason to mount a mission that might take 1000’s of years to reach the nearest star if waiting 25 years or so might produce a mission that only takes 50 years or less.
Make no mistake, the problems of traveling to the stars are not simple. Besides needing to carry energy in a storage medium that is as efficient as possible, any propulsion system must not produce so much radiation as to kill the crew or destroy any probe equipment. It also can’t be so heavy that it would be impractical as an on-board propulsion system.
Dealing with impacting interstellar dust is difficult too, suggesting an interstellar probe may need to be constructed so that such impacts do not irreparably damage its systems. Many creatures from bacteria to tardigrades have evolved to withstand extreme radiation in space. An interstellar probe (most likely a colony of them for redundancy) would need a healing factor to rival Wolverine or Deadpool or maybe just a tardigrade. But that is a topic for another article.
Chemical, electric, and nuclear propulsion systems are all limited by Tsiolkovsky’s rocket equation, which is the same as the equation for continuously compounded interest and not in a good way. We must pay to accelerate each ounce of fuel so that we can burn it later, creating a compounding problem.
We cannot accelerate anything to much above 10 km/s by these conventional means. While gravity assist can boost this by several times, it is far below the 30,000 km/s or 0.1c we need to reach relativistic velocity. The venerable rocket equation applies to relativistic as well as non-relativistic rockets with some adjustment to the input parameters to account for any relativistic speed fuel exhaust. It mainly comes down to a parameter called specific impulse, the amount of boost you get from a given mass of fuel.
There are only two fuels available according to known physics that have a relativistic specific impulse: antimatter and photons. Both of these can, theoretically, provide thrust for the long periods of time necessary.
Most discussions of interstellar voyages involving macroscopic objects like people assume that thrust will be continuous throughout the voyage. Popular science fiction novels such as Project Hail Mary and We are Legion (We are Bob), stories about humanity’s first steps to the stars (with vastly different outcomes), center on sublight propulsion drives that provide thrust throughout an interstellar voyage.
There are several practical advantages to such a system. The first is that it provides gravity for any crew, supposing that the thrust is in the neighborhood of 1g (980 cm/sec/sec or more conveniently for relativistic calculations about 1 lightyear/year/year).
Secondly, the continuous thrust takes advantage of relativistic effects in that a traveler can reach almost any place in the universe via time dilation. For example, if I were to accelerate at 1 g for about 21 years and then decelerate for about the same amount of time, a total of 42 years, I would travel one billion light years. Of course, a billion years would have passed for everyone else.
Maintaining continuous acceleration for years is no simple feat however. Normally, rockets can only accelerate for seconds or minutes and expend enormous amounts of energy and mass doing it. We would need some serious tech upgrades to be able to do that for years.
Because of time dilation, it would take a probe constantly accelerating at 1 g for half the trip and decelerating the second half about 3.6 years in its own time frame to reach Alpha Centauri 4.4 lightyears away. For everyone else, 6 years will have passed.
Antimatter seems to be the obvious choice for power source. It is the only way to convert mass into 100% energy (via positron-electron annihilation). Any other type of fuel would add ballast when we need to avoid that as much as possible.
Yet, both of the novels mentioned above avoided antimatter. One of them used neutrinos but assumed neutrinos were Majorana particles such that they are their own antiparticles. Besides the apparent impossibility of storing neutrinos, we don’t know if they are Majorana particles.
The other novel used something like a sublight Alcubierre drive (without actually calling it that) with ordinary fusion reactors as power. This idea is physically a very unlikely combination since even under the best conditions a positive energy Alcubierre drive would require massive amounts of energy, far more than an ordinary reaction drive. Moreover the inhabitants of such a warp bubble would experience free fall the whole time making gravity a problem again.
Star Trek has made it a bit cliche in science fiction, but there are technical reasons as well to avoid antimatter propulsion. I will come back to that.
While on-board photon drives are possible, they require having a powerful source of black body radiation that can absorb an enormous amounts of light and re-radiate it. The only thing that might be able to do this is a small black hole which could absorb an absurd amount of energy and emit it as Hawking radiation, but we have no way of creating any black holes let alone one that could survive an interstellar voyage.
But there is a better and simpler way. Beam the energy to the probe only as needed.
Remove the propellant from the probe entirely and rely only on thin light sails (maybe made of graphene) to catch an externally generated laser or sunlight, and we escape Tsiolkovsky’s equation and vastly improve the chances of reaching a relativistic velocity. Photons from a ground based laser, generated in sufficient quantity, should be able to impart sufficient momentum to a relativistic probe.
The Breakthrough Starshot program, started in 2016 with $100 million in seed funding, has focused on this kind of photon drive as the only practical way to achieve an interstellar mission.
Starshot only plans to build one ground based laser and expects to push its probe to 0.2c in 10 minutes. Scaling up, star shot claims to be able to accelerate a probe up to 0.9c. At an estimated cost of about $10 billion, I think a photon drive like starshot would be a reasonable investment in humanity’s interstellar future, especially since, unlike an antimatter drive or even a rocket, the majority of it would be reusable for additional missions.
Consider that the Apollo program that sent astronauts to the moon cost $160 billion in 2020 dollars. and was also a “first” in sending humans to another world. Starshot may not include people but it would be a first dip into the interstellar ocean.
The goal is for a very small craft of about 1 gram of payload. Having the one laser also limits the possibilities for course corrections after the initial acceleration has occurred unless there is an on-board drive.
To deal with larger, more acceleration sensitive payloads, a series of lasers built at increasing distances from Earth through the Solar system could boost a probe as it gains velocity as well as provide an initial investment in rapid, interplanetary transport.
This a combination space elevator and photon-based railgun on an interplanetary scale. These lasers will be able to push cargoes back and forth from the outer to the inner planets. (Think of getting to Mars in a week with a constant 1g acceleration from a moon based laser and deceleration provided by a laser on Mars.) Fuel for these lasers (probably deuterium for fusion engines) can be produced and stored on various moons or Kuiper belt objects. Since Gigawatt or Terawatt lasers need a lot of cooling, outer space should be ideal for them.
Since rogue worlds exist in the interstellar medium as well, we could conceivably build these as far from Earth as necessary, even beyond the solar system.
Slowing down is a problem for a probe that is not visiting a star directly. (If it is, it can use the momentum of the blueshifted light of the destination star to brake.) To solve this problem, we can deploy light weight, steerable mirrors short hops of a few tens or hundreds of million kilometers at a time using more conventional rocketry (electric or nuclear), placing them on the surface of small moons or planetoids, and use those to convey the heavier material to build the next laser station since the mirror would be able to reflect the laser light and produce a braking effect without needing a star.
While the nearest stars are tens of trillions of kilometers away, we need not build our laser stations all the way there. We only need to build enough to get our probes to 99% or so of the speed of light without tearing them apart with too much acceleration. Having a series of boosting laser stations out to the Kuiper belt would create a Solar System size linear accelerator, a train to the stars. Ordinary reaction drives could provide course corrections enroute. Figuring out the return trip is a whole different problem, likely accomplished with seed factories — factories that seek out resources and construct themselves. That is a topic for another day.
The approach outlined above is vastly more practical than antimatter drives at current technology level, although in 50-100 years anything is possible. The possibilities of using light sails and ground based lasers seems infinitely more within reach but there are certainly advantages to be able to depend on your own rocket.
While we can produce tiny amounts of stable antimatter and may get better at it as time goes on, it is extremely difficult to store and produces high energy gamma rays when it interacts with matter. These gammas rays would, ideally, be used to propel the ship but would just as likely rip it apart, so they would have to be some how recycled, perhaps being absorbed or reflected by an atomic or electron gas medium.
If we assume a 1000 kg probe (Voyager 1 was about 700 kg) has perfect mass to energy conversion and perfect, light speed propellant, we would need about 41 metric tons of antimatter fuel to make the journey one way. This is a lower bound on the amount of fuel. Likely we would need many times this amount.
That said, let’s look at how antimatter propulsion works.
To calculate the antimatter fuel required, you need to figure out how long in the time frame of the probe it would be creating a 1g thrust. This is a hyperbolic function that relates rest frame time to distance traveled in the frame of Earth and Alpha Centauri. Once you know the time, you use a rocket equation to figure out what your starting mass would need to be given a speed of light exhaust and assuming constant fuel and constant acceleration to the half way point and then deceleration afterwards.
The total amount of antimatter created in particle accelerators is only about 20 nanograms, billions of times less than what we would need. One of the problems with creating anti-matter is that they tend to emerge from very high energy reactions at high speed and need to be slowed down for storage. This means there is a very high cost (low efficiency) to producing it.
A better way to produce antimatter might be using ultrapowerful gamma ray lasers. While light in vacuum normally doesn’t interact, at very high energies, it produces antiparticles out of vacuum.
It is impractical to use pure gamma radiation as a propulsion mechanism and would make more sense to accelerate particles, such as protons or pions, to near light speed using anti-matter reactions as the energy source. There are two ways to do this. One is directly by producing them from interaction between ultracold antimatter held in electromagnetic traps and matter, then marshaling them out the exhaust at nearly light speed. The other is to use the antimatter to create heat which would enable ordinary matter to be used as the propellant.
Storing positrons is not easy, however. Large amounts of antihydrogen, a stable anti-atom that can be used as an energy source, would be easier to control. Antihydrogen is formed of a single anti-proton and a positron.
When antihydrogen annihilates with hydrogen, you don’t get complete, 100% energy. You get a couple of gamma ray photons from the positron and electron and from the proton you get three pions, a neutral pion that is short lived and a couple of longer lived charged pions. The pion antimatter rocket essentially uses electric fields to shoot the high speed charged pions out the back. The gamma rays are all wasted energy.
All this waste means that you get an effective specific impulse of about 0.58c rather than 1c under our assumption of 1g acceleration. The wastage of the gamma rays, sadly, increases the fuel requirements by about 15 fold to 600 metric tons of antimatter since only about 40% of the fuel is being used as propellant.
Various gamma ray reflectors and absorbers can recycle the radiation. One of the most interesting proposals is a kind of gamma ray turbocharger in which the gamma rays are used to create more antimatter, enabling the resulting antimatter to be used as propellant. If all the gamma rays can be used, the specific impulse increases to about 0.96c and the requirement drops to about 45 metric tons of antimatter.
Antimatter production has improved in recent years. It turns out that scattering antiprotons off of an exotic particle called positronium, where an electron and positron are bound together in an excited state, can be used to produce a lot more antihydrogen than current methods.
Some have also suggested using only positronium as fuel and doing away with the antihydrogen. Positronium resembles hydrogen and may form molecules with positronium pairs. These can form a Bose-Einstein Condensate (a kind of quantum fluid) and be used to produce annihilation gamma rays. This could be used to generate heat to power the ship’s reactor.
Other suggestions, that may be further in humanity’s future, are using smallish black holes to produce antimatter. Black holes below a certain size are very efficient at converting protons into positrons near their event horizons and could be turned into antimatter factories.
Seeking out natural black holes and using them to harvest antimatter may be a practical use for our star train above since antimatter reacts to photons the same way as matter. We could also push the antimatter towards rockets that need refueling, potentially avoiding Tsiolkovsky’s there too.
Kadyrov, A. S., et al. “Antihydrogen formation via antiproton scattering with excited positronium.” Physical Review Letters 114.18 (2015): 183201.
Bambi, Cosimo, Alexander D. Dolgov, and Alexey A. Petrov. “Black holes as antimatter factories.” Journal of Cosmology and Astroparticle Physics 2009.09 (2009): 013.