A Starship to Visit a Neighbor
A slightly modified version of an article that appeared
Our celestial neighborhood looks more homey all the time. As if slowly zooming in with a cosmic camera, previously unresolvable points of light are expanding into star systems and becoming real places. Space artists are already having great fun imagining what our new neighbors look like up really close.
The Exosolar Atlas so far lists over 100 planets orbiting other stars. Because of the limitations of the current planet detection techniques, only large gas giants have been found and most of these orbit very close to their suns. Recently, however, Jupiter sized planets at Jupiter sized orbits have been discovered. It could well be that small rocky type worlds inhabit the inner orbits of such systems just as in our Solar System.
When an earth-like planet is discovered, we will, of course, continue to observe and study it closely with ever more powerful telescopes, interferometers, and other sensors. SETI instruments will scrutinize it for signs of civilization. But what about actually going there to visit or, at least, sending a probe to check it out up close?
Not impossible but it won't be easy. I will review here the enormous challenges of starflight and some of the proposals for surmounting them.
Bigger than Big
The distances involved in stellar travel truly stagger the mind. One analogy (via Paul Woodmansee) gives perhaps the best visualization of the scales involved. If the distance from the earth to the Moon is scaled to the thickness of a sheet of paper, then the distance to Proxima Centauri, the closest star, would scale to a stack of paper 17 miles (22.8km) high!
Proxima Centauri, furthermore, lies a mere 4.3 light years from us. The currently known exosolar planetary systems lie from 15 to 322 light years away. Closer ones may yet be found and it's possible some systems have earth-like planets and no gas giants so they won't be detected until NASA's TPF observatory is launched. Nevertheless, even if Earth II appears at, say, 5 light years from us, sending a spacecraft there that will pose perhaps the greatest technological challenge humanity has ever faced.
According to special relativity no object of non-zero mass can accelerate to the speed of light. Instead it requires ever increasing amounts of energy to achieve ever smaller increments in velocity to get closer and closer to “c” but never reaching or exceeding it. Most starship proposals aim for about 10% of the speed of light (30,000km/sec) as their top speed. This means that our trip to Earth II at 5 light years (ly) will take at least 50 years one way, not counting the time for acceleration and also for deceleration if we want the spacecraft to do more than a fly-by. (Note that at this small fraction of c the bizarre time and space distortions of moving near the speed of light, such as the famous twin paradox, are not very significant.) The amount of energy needed to accelerate a good sized ship up to 10% of c is comparable to the total energy output on Earth in a year.
Humanity, in fact, has already launched four starships: the Pioneer and Voyager spacecrafts. These ships used conventional rockets plus kicks from encounters with our own gas giants to achieve the fastest speeds human-made objects ever achieved and will escape from our solar gravitational well. However, the 17km/sec speed of Voyager 1, for example, is still only 0.006% of c. If our probe to Earth II (at 5 ly) left at that speed, it would take over 83,000 years to get there!
Over the past few decades many starship designs have been proposed and many variations on these designs soon followed. I will only briefly review a few basic approaches here. Note that no design is yet an unambiguous winner. The goal of starship designers so far has been simply to provide general concepts that don't break any known physical laws and stay at least close to the hairy edge of practical feasibility.
Furthermore, all the designs so far have various key technologies, such as nuclear fusion power systems, that are not available today and require substantial development. Most also involve components of very, very large scale. No cheap off-the-shelf parts for starships!
I'll concentrate on the different propulsion schemes but I should note a number of other important problems the starship designer must solve. For example, the ship's internal energy source must provide minimum power levels for several decades. All of the systems, from electronics to large mechanical components must work reliably over this long period. For unmanned spacecraft this will require sophisticated robotics that can autonomously carry out complex maintenance and repairs. A deceleration system at the destination should be a requirement since the huge investment in a starship doesn't seem worthwhile for just a brief fly-by. Other challenges include interstellar navigation, communication, and protection from cosmic radiation, dust and micro-meteorites.
Conventional chemical rockets are completely out of the running for starships. The relatively small amount of energy available from chemical reactions means that only a infinitesimal payload could be accelerated to a fraction of c. Even nuclear fission falls short of the required efficiency.
With nuclear fusion reactions, however, we can start to get serious about a starship design. When lightweight atoms such as helium-3 and deuterium are forced into close enough proximity, they fuse together, releasing significant quantities of energy. Fusion reactors have been under development for many decades but, while showing great progress, have yet to produce more power out than put in.
The more cruder approach of a fission triggered fusion bomb, however, has shown exceptional energy output! The famous Orion project attempted to take advantage of the power of nuclear explosions for propulsion. The audacious concept was developed by several top physicists in the late 1950's and early 1960's. The basic version used small fission bombs fired in succession behind a ship with a large pusher plate that absorbed a significant fraction of the bomb blast to provide pulses that could launch the enormous vehicle into space and provide for fast interplanetary flight. Freeman Dyson, a member of the Orion team, went on to show that if fusion bombs were used instead, speeds up to 10,000km/s (3.3% of c) could be achieved, allowing a ship to reach Proxima Centauri in 150 years. Unfortunately, such a ship would be of the scale of a good sized asteroid: 240 million tons, a pusher plate of 150km diameter, and 25 million bombs.
The Daedalus project took a different approach to fusion propulsion. Daedalus was a detailed engineering study, led by the British Interplanetary Society, of a starship that could reach Barnard's Star (5.9 ly from Earth) within 50 years. The engine used small pellets of deuterium and helium-3 that were compressed by electron beams until they fused and exploded. The 450 ton payload could be accelerated by a two stage rocket to 12% of c with the use of 50,000 tons of pellets that were fired 250 times a second.
As Scotty and fans of Star Trek know, anti-matter is the ideal rocket fuel. In fact, at particle accelerator labs around the world, sizable quantities of anti-electrons and anti-protons are produced routinely. However, the amounts needed for spaceflight propulsion, are billions of times larger than what could be produced yearly with current techniques. So instead of pure anti-matter engines, there has been considerable work on using attainable quantities of anti-matter as catalysts for conventional fusion reactions. This could allow for smaller fusion powered starships. However, the challenges of storing and handling even small amounts of neutral antimatter have yet to be solved.
The interstellar medium is a hard vacuum but not a complete one. There are about a billion hydrogen atoms per cubic meter. Robert Bussard proposed in 1960 that a ship could scoop up these atoms and light them in a fusion reactor and fire the exhaust out the back for propulsion. There was tremendous excitement about this proposal as it eliminated the need to carry fuel (except what was needed for an initial acceleration phase).
The scoop would consist of a large magnetic field extending out to hundreds of kilometers. The hydrogen atoms are mostly neutral so they must be ionized (the electron stripped from the nucleus) by a laser so that the charged nuclei will follow the field lines into the ship's mouth.
However, over the years the practicalities of the approach have shown the initial design to be untenable. The cross-section for proton-proton fusion is extremely small. A much higher rate reaction comes from deuterium-deuterium (hydrogen in which the nucleus includes both a proton and a neutron) but these are even more rare so the scoop has to be even bigger.
Several variations on the Bussard Ramjet have been offered to overcome its shortcomings. For example, a combined electrostatic and magnet scoop would significantly reduce the size and field strength required for the electromagnet. A catalytic approach with either 12C or 20Ne could greatly enhance the rates of p-p fusion. Another approach is to carry fuel for a fusion reactor and use it to heat the scooped up hydrogen, which enhances the thrust. This allows much less fusion fuel to be carried to achieve a given velocity.
Separating the energy supply from the vehicle has become a popular way to greatly increase the capacity and efficiency of the starflight systems. In these schemes the vehicle is pushed along typically by a laser beam but other proposals use microwaves, particle beams, or massive “bullets” of some sort.. The source of the beam would be based in space and must be of very high power and run reliably for several decades.
There has been recent excitement about new carbon fiber mesh materials that would be ideal for sails. The material is extremely light yet very strong and can withstand high temperatures. Experiments have already shown that lasers can push the material.
Even a solar sail could obtain a strong initial acceleration by first diving in close to the sun to obtain the highest intensity light possible for a short time. An unmanned sail craft could obtain several hundred g's of acceleration this way. A manned craft could only withstand a few g's, however, but the boost would make it easier for a secondary propulsion system to reach cruise speed.
A new propulsion concept called Mini-Magnetospheric Plasma Propulsion (M2P2) comes from Robert Winglee of the University of Washington. M2P2 uses a magnetic bubble of plasma particles in a field produced by a solenoid to interact with the charged particles in the solar wind and absorb some of their momentum. This magsail could allow a vehicle to ride the solar wind up to 100km/s. Such a system is intended for travel within the solar system but it could also provide an initial first stage boost for a ship with a secondary fusion or other propulsion system. Note that a magsail would also help in decelerating a ship as it approaches the destination star.
The SailBeam concept of Jordin Kare of Lawrence Livermore Labs involves a magsail as well but in this case it is driven by a stream of micro-sails that are in turned pushed by a solar system based multi-Gigawatt laser. The micro-sails are launched once per second and quickly reach 10% c. When the micro-sails reach the main magsail craft they are vaporized by an on board laser and the resulting plasma pushes on the magnetic fields of the magsail. The constant stream of pulses drives the main ship up to 10% c as well.
For human star flight one approach is to accept lower speeds and instead plan how to survive a thousand, or even several thousand, years in transit. A popular plot in science fiction involves the generation ship in which a large habitat of perhaps several thousand people set out for a star on long voyage on which only their descendants will live to see the new world. Recently, a study was released that showed that a crew as small as a couple of hundred was sufficient to maintain healthy genetic diversity on a 1000 year trip.
It might seem like a lonely voyage but many isolated communities, on small islands for example, have survived and thrived for long periods. It's really just a matter of scale. We are, after all, traveling at this very moment through the interstellar cosmos on Spaceship Earth.
If large Gerard O'Neill style space habitats, which rotate to provide artificial gravity, are developed for communities in space then they could be adapted for interstellar travel. Perhaps a flotilla for such habitats would travel together, each habitat offering a different culture and environment to provide variety as people moved among the ships.
Proposals for keeping a single crew alive for a long voyage usually involve hibernation schemes of some sort. Considering the current growth of knowledge in biological sciences, it seems rather odd, though, to assume tremendous advances in the technologies needed for starships but still to hold that human life spans will remain unchanged. In fact, it is quite likely that humans will achieve indefinite lifespans within the next century.
The psychological implications of living for hundreds or thousands of years are obviously unknown but they are fun to speculate about. For example, after experiencing a long full life and several careers, boredom may set in and the chance to take a ship to another star could seem quite attractive to some people. Perhaps, on such a long voyage the occupants, instead of going into a deep sleep hibernation, would choose the option of going into a “Matrix” type simulation in which they can experience alternate lives in completely synthesized worlds.
Wormholes, Warp Drive and All That
So why not just wait for Star Trek type science and technology to appear and not waste a lot of money and effort on the crude approaches discussed so far? After all, people setting off on a thousand year voyage would feel rather silly if 10 years into their trip a warp drive cruiser catches up with them on a two day jaunt from Earth.
In fact, prestigious physics journals now publish articles on sci-fi sounding topics like wormholes and space warp drives. The concepts come out of Einstein's general relativity theory and satisfy the requirements of special relativity (no exceeding the speed of light) at the small scale local level. However, to open a wormhole shortcut from one point in space time to another, or to create a warped space bubble in which your starship can safely reside, requires theoretical materials and technologies that seem as far fetched as the wormholes and space warps themselves.
For example, a key ingredient for implementing [most of] these concepts is a substantial sample of negative mass. This a bizarre material that, for example, repels positive matter but is attracted to it. So a ball of negative matter would chase a ball of positive matter that it is pushing away. How to find or create negative matter is not yet known (anti-matter is not believed to be negative matter.)
Also, despite satisfying relativistic requirements, the proposals bring up various time travel and causality conundrums that must be dealt with. These paradoxes, in fact, imply to many physicists that some basic laws of physics will prevent practical implementation of wormhole type concepts just as the laws of thermodynamics prevent perpetual motion machines from running off the surrounding heat energy.
Full development of these concepts requires a theory of quantum gravity that combines general relativity and quantum mechanics. Substantial progress has been made in developing quantum gravity but great strides must yet be made. It should be noted, though, that James Maxwell wrote down his famous equations of electrodynamics in the 18th century yet we are still attempting to grasp their full implications and continue to expand on ways to implement them such as, for example, in containing a hot plasma to produce fusion power. Similarly, even after a quantum gravity theory is developed, it could be centuries, if ever, before it leads to practical applications of any kind, much less star travel.
If an earth-like planet were discovered within a few light years of us, and especially if it showed signs of holding life, there would be tremendous interest and support for contacting it directly. But all of the proposed starship designs require components of enormous size and complexity and they demand long term commitments to implement and operate. It's clear that only after the development of a space infrastructure consisting of sizable communities and industries could such vehicles be realized.
Thus the best way to begin developing starships is to begin developing that infrastructure. Many of the starship designs are just large scale versions of spacecraft that would be ideal for transport within the Solar System. Fusion drives, for example, could reduce transit times between the planets from years to months. The starships will just involve scaling up these systems. Similarly, generation ships would naturally evolve from space habitats that eventually will populate the Solar System in large numbers.
At a time when it is difficult to obtain even modest resources to develop reusable vehicles to reach low earth orbit, proposals to build gigantic starships seem wildly optimistic to say the least. But the story of life is one of starting small and then steadily and incrementally growing and developing. To travel to the stars we begin by building a road to our own solar system.
© 2003 - Clark S. Lindsey