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 Sänger (1905-64) who is better remembered today for the so-called Silbervogel, a spaceplane bomber concept briefly studied by the Luftwaffe in WW2. Postwar Sänger 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. He revealed his ideas in 1957 and the idea was widely published.
Artists’ 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. Hardy’s New Challenge of the Stars (1977); I still remember the impact made on me by its painting by Hardy of an elegant “Photon Star-Ship” approaching a planet near the Trifid Nebula . The vehicle in Hardy’s artwork was a 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? Sänger’s 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 Einstein’s 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. A faster still photon rocket could span the 26 000 light years to the Galactic Core inside the crew’s 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!
Sänger’s 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 ‘particles’ of light are called photons, and Sänger reasoned that the highest performing rocket would push its way across the cosmos by spraying a stream of photons from its exhaust. Fantastic! Let’s go build one and see the Universe.
If only it were that simple. Sänger could only see one way to produce ‘pure’ 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. The dying electrons and positrons explode into gamma ray photons, which would be reflected rearward by a parabolic mirror to generate thrust. The rocket’s 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 ‘reflector’. To build a photon rocket that would not vaporise itself in flight we need a ‘perfect’ mirror, reflecting 100% of incident photons, no orthodox material known can do this (however hard science fiction fans will immediately recognise this as an ideal application for Larry Niven’s celebrated ‘stasis field’). Sänger suggested reflecting the gamma rays directly from a mirror of pure “electron gas”. 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. Apart from the difficulties it would cause to the ship itself, a photon rocket’s 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 rocket’s extraordinarily high power requirements and poor thrust. The photon rocket’s 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). 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.
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 Pournelle’s famous first-contact epic The Mote in God’s Eye (1974) used photon drives for sublight travel. However Niven and Pournelle’s 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. Niven’s solo work, The Warriors (1966) which is part of his popular Known Space sequence, features the Angel’s 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 Angel’s 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)