Could humanity ever travel to the stars? Today this is an unattainable dream but world-wide researchers are studying the possibilities of starships. One concept for an interstellar craft is the photon rocket, an idea once popular but less prominent today. What happened to this appealing idea for voyaging into deepest space?
The photon rocket concept was the brainchild of Eugen S†änger (1905-64) who is better remembered today for the so-called Silbervogel, a spaceplane bomber concept briefly studied by the Luftwaffe in WW2. Postwar S†änger continued to explore innovative aeronautical and astronautical ideas and was the first to propose the use of electron-positron annihilation for propulsion as a photon rocket. This was pure speculation, a thought-experiment assuming technology could be taken to the limits. It was never meant as near-future possibility.ˆà He revealed his ideas in 1957 and the idea was widely published.
ArtistsòÀÙ impressions of photon rocket starships were common in the sort of coffee table books promoting the wonders of space and astronomy I lapped up in the 1970s. One of these books was Patrick Moore and David A. HardyòÀÙs New Challenge of the Stars (1977); I still remember the impact made on me by its painting by Hardy of an elegant òÀÜPhoton Star-ShipòÀÝ approaching a planet near the Trifid Nebula . The vehicle in Hardy’s artwork wasˆàa huge vehicle, perhaps kilometres in length. Visible components of the starship included the crew quarters at the tip of the craft’s spine, two plate structures intended as a radiation shield, two propellant tanks and a huge parabolic reflector. Little tubes spaced around the reflector were more conventional rocket devices for use maneuvering near habitable worlds. This would be a huge vessel which could never land on a planet, rather it would carry smaller spaceplanes to ferry down the crew.
Why was this speculation so exciting? S†ängeròÀÙs fantastic concept seemed to suggested it was possible to build a rocket vehicle capable of reaching near-light speeds. Imagine that! A craft like this could conquer the unimaginably huge gulfs of space between the stars, perhaps carrying intrepid crew to explore the mysterious worlds around other stars. That is the least of the possibilities, a speeding photon rocket and its crew would be subject to the strange (to slow-moving folk) magic of EinsteinòÀÙs Special Relativity. A journey to say, 51 Pegasi, 50 light years distant and with its own family of planets might take the ship a little over five decades to complete to outside observers, but less than a decade to the crew.ˆà A faster still photon rocket could span the 26 000 light years to the Galactic Core inside the crewòÀÙs life time while millenia passed outside the hull. Even intergalactic journeys would be possible, if we could build photon rockets it would really be time for the stars!
S†ängeròÀÙs starting point was the familiar (to space geeks) rocket equation, a simply derived formula which predicts the maximum speed a rocket vehicle can attain. Any rocket, from the simplest leaking rubber balloon to the mightiest launch vehicle, pushes matter, reaction mass, out of an exhaust, thrusting the rocket forward. The rocket equation shows that maximum speed the rocket attains in its flight is directly proportional to the speed the reaction mass leaves the exhaust, the exhaust velocity. Note that it does not matter what the reaction mass is made of, just that its speed, and therefore momentum, be as high as possible. What would the highest possible speed be? Why, 300 000 km/s, the speed of light. Individual òÀØparticlesòÀÙ of light are called photons, and S†änger reasoned that the highest performing rocket would push its way across the cosmos by spraying a stream of photons from its exhaust. Fantastic! LetòÀÙs go build one and see the Universe.
If only it were that simple. S†änger could only see one way to produce òÀØpureòÀÙ photons and that was by bringing electrons and positrons (which are anti-electrons) together. These violently annihilate on contact, so just like any of the USS Enterprises a photon drive starship is powered by a matter-antimatter reaction.ˆà The dying electrons and positrons explode into gamma ray photons, which would be reflected rearward by a parabolic mirror to generate thrust. The rocketòÀÙs exhaust is essentially a gamma ray laser (or “graser”) beam!
Physics buffs will straight away see issues with this. There are no vast reserves of positrons ready to exploit anywhere near Earth. A photon rocket’s propellant would have to be manufactured somehow, a process which would demand unimaginable quantities of energy. Gamma rays are not easily reflected by normal matter, but instead absorbed, rapidly heating the òÀØreflectoròÀÙ.ˆà To build a photon rocket that would not vaporise itself in flight we need a òÀØperfectòÀÙ mirror, reflecting 100% of incident photons, no orthodox material known can do thisˆà (however hard science fiction fans will immediately recognise this as an ideal application for Larry NivenòÀÙs celebrated òÀØstasis fieldòÀÙ). ˆàS†änger suggested reflecting the gamma rays directly from a mirror of pure “electron gasòÀÝ. Creating this would be an astonishing technological achievement in itself as electrons repel each other, forcing them together into a reflecting surface would require us to be able to exert and maintain pressures like we see at the centre of an exploding supernova. ˆàApart from the difficulties it would cause to the ship itself, a photon rocketòÀÙs exhaust would be hideously destructive to any planets it was directed at, with disastrous effects similar to a nearby gamma ray burst.
Worst still are the photon rocketòÀÙs extraordinarily high power requirements and poor thrust. The photon rocketòÀÙs unparalleled exhaust velocity comes at a price, it is straight-forward to show that its every single newton of thrust requires 300 MWatts of power (a calculation based on fundamental physics, there is no way technological advances could improve this).ˆà This is ludicrously inefficient; a small modern turbofan engine to power a modest jet aircraft might have a thrust of 27 000 newtons, while a respectable power station might be rated at 650 MWatts. A photon rocket would have to be huge, perhaps moon-sized, to accommodate its reserves of propellant. Its acceleration would be sluggish, perhaps needing decades to reach near-light speeds.
The Large Magellanic Cloud is one of the closest galaxies to our own. A photon rocket spacecraft could reach there in 160 000 years. Thanks to relativity, the crew could still see the star-forming region NGC 2035 there in their lifetimes. (Image credit:ESO)
Photon rockets show up occasionally in science fiction. The earliest reference I know of is in the 1959 novel The Land of Crimson Clouds by Boris and Arkady Strugatsky, I have yet to read this work so I cannot comment on its realism. A few years later Stanislaw Lem used a photon rocket to propel a human starship in his remarkably foresighted novel of conflict with alien nanotechnology The Invincible (1964). A decade later the human starships in Larry Niven and Jerry PournelleòÀÙs famous first-contact epic The Mote in GodòÀÙs Eye (1974) used photon drives for sublight travel. However Niven and PournelleòÀÙs fictional creations were powered by nuclear fusion and capable of accelerating multi-thousand tonne craft at multiple gees of acceleration, so have little in common with “real” photon rockets.ˆà NivenòÀÙs solo work, The Warriors (1966) which is part of his popular Known Space sequence, features the AngelòÀÙs Pencil, a slightly more credible photon drive starship capable of attaining 80% of lightspeed. The story famously demonstrates just how dangerous a photon rocket might be to other ships in its vicinity. However Niven seems to have had second or even third thoughts on photon drives, in later works, the AngelòÀÙs Pencil is said to have been propelled by a light-pressure drive (boosted by a laser system) or even as a Bussard ramjet. Into Infinity (1975) was a one-off TV movie from Gerry Anderson, featuring BRIAN BLESSED! (an actor famed for his loud declarations of dialogue) as the patriarch of a space-faring family enduring an incident-packed voyage on a photon-drive starship, the oddly-named Altares. This was probably a terrible movie but it made a great impression on me back in the day: the Altares was represented by a stunning model and the movie introduced me to the mysteries of special relativity and the Doppler effect. I would not be sitting here in the Planetarium today without the childhood inspiration from stories like that.
When you closely examine the feasiblity of photon rockets the concept falls apart. They require materials and techniques that may never exist in the real Universe and need outrageous resources and time to attain their amazing performance. Sadly, photon rockets appear to be forever an intriguing fantasy.
(If you are interested in the physics of photon rockets, see the paper at this link.)
(Article by Colin Johnston, Science Communicator, you can learn more about exotic space propulsion concepts at the wonderful Atomic Rockets site)
Excellent article!! It is nice to see some attention given to the intriguing old idea of photon rocket propulsion… S†änger’s photon rocket was the very first serious proposal for reaching near-C speeds, and he was one of the first to point out his colleagues that time dilation would permit space crews to reach very faraway destinations in their lifetimes, even though centuries to millennia would pass back home.
The photon rocket was one of the first serious relativistic propulsion concepts I came across, and it introduced me to the rocket equation and (along with the solar photon sail!) the concept that light has momentum.
One thing troubles me- many sources describe S†änger’s photon rocket as attempting to direct the gamma rays produced by positron-electron annihilation into an exhaust stream, but this description of the photon rocket concept appears inaccurate. In his paper “The Photon Rocket and the Weapon Beam” (New Scientist, Sept. 1959), S†änger writes that the three main ways of producing radiation are i. directly, by radioactive materials that emit gamma rays, ii. electrically, like a radio transmitter, and iii. thermally, by hot matter, like the filament of a light bulb. Heat he considers the most promising considering the intensities required.
The photon rocket’s core is described as a “lamp” made of a heavy plasma at 150,000¸Ú C, radiating intense radiation consisting mostly of soft X-ray, UV, and visible wavelengths. The electron gas reflector is supposed to only absorb a millionth of the energy falling on it at these wavelengths- in theory. The lamp is to be kept “hot” by nuclear energy. He considers both a plasma-shockwave mirror and a cold electron-gas reflector to direct the photon jet, as no material reflector could withstand the intensity of the photon exhaust jet.
Later researchers seem to have considered only option i., direct conversion of mass into gamma ray photons, for the most part. Recently, Friedwardt Winterberg proposed a gamma-ray laser photon drive that, it is claimed, would somehow persuade a “matter-antimatter ambiplasma” to completely convert itself into a stream of gamma ray photons all neatly directed aft!!
Anyway, thanks for sharing this succinct and accurate description of one of the most intriguing early “starship engine” concepts!! Both the wonder and the difficulties of the concept are well communicated in this post, as well as the impact it had on popular imagination and art. I might have to read some of the SF sources mentioned now, ha ha…
Thanks for your kind words and the link to the Photon Rocket and Weapon Beam piece, i wasn’t aware of it.
I wouldn’t waste any time on the science fiction works if you want to learn about the photon rocket concept as in all of them it is just used as a handwaving way way to make a spaceship go fast without any technical detail. The books are fine as entertainment.
I’m hoping to do a similar article covering the other great relativistic spacecraft concept, the Bussard ramjet, watch this space.
I agree, most SF stories tend to use technical sounding terms without really bothering to find out the real details behind them… in this way they really are not so very different from Star Trek and its dilithium crystals.
There are many books and papers on photon rockets and relativistic flight out in the technical literature. It won’t be too hard for me to find some good resources that discuss all the relevant physics of the concept.
I look forward to reading your post on the Bussard Ramjet! Few propulsion concepts have captured popular imagination as much as that one. Except for warp drive, perhaps.
Do you think you might do another article after that, this time on Generation Ships? If the above do not work (and they may never be feasible), the question becomes surviving a journey that can take centuries using our remaining feasible propulsion options…
Some people prefer to sidestep this problem by suggesting we freeze the travelers into a suspended state, as first suggested by Robert Goddard in his musing on The Ultimate Migration!!
I will cover the generation ship concept some time, until then you might be interested in some articles about starships in the old printed version of Astronotes, you can download them as PDFs. See the Ultimate Project, Bussard ramjet and Daedalus (links).