Destination: Proxima Centauri, Part II

Building Spaceships: Life Among The Stars

By Cam Wipper

This is the second part of a multi-part series, ‘Destination: Proxima Centauri’.
This series will explore the various issues surrounding interstellar travel and what would be found once we reach Proxima Centauri, the closest star to the Sun. ——————————————————————————————————————— “You should look at this.”
And through his pain, Kirk does… and is amazed…
The shuttle approaches a massive DOCKING STATION where a dozen STARFLEET VESSELS ARE DOCKED.
But the ship we’re approaching is miraculous: “U.S.S. ENTERPRISE
NCC-1701” IS REVEALED ON THE HULL OF THE FLAGSHIP.
THE QUEEN OF THE FLEET IS FULLY CONSTRUCTED AND READY FOR HER MAIDEN VOYAGE.

Even in science fiction stories like Star Trek, where interstellar travel is commonplace, the vessels which ply the cosmic seas still inspire awe. The ability these ships provide—to journey across the galaxy, to live among the stars—often seems at first-glance to be sheer folly. Perhaps the notion of interstellar travel, while mathematically feasible (as we discussed in the first part of this series), is technically impossible. The machines we would need to travel among the stars seem to be the stuff of dreams. History, however, has taught us an important lesson about dream machines: they don’t always remain dreams: they can become reality.

Consider the humble telephone: in the year 1800, the idea that it would be possible to transmit ones voice around the planet to talk to someone thousands of miles away would have seemed ridiculous. By 1900, it was possible to do so. By 2000, not was it commonplace, but it could be done with a wireless machine that could fit into a pocket, communicating invisibly with other such devices.

Consider the airplane: in less than 70 years, the idea of powered flight went from a dream to so well developed that men traveled 250,000 miles to the Moon and back.

Consider medical imaging: for thousands of years, the only way to see inside the body of another creature was to physically make an opening. Now, it is possible to create an image of the inside of a body just by shining light through it.

These are just three examples. Quite literally every modern technology we possess today would have been the stuff of dreams for an earlier age. We live today in a dreamworld, unimaginable to those who came before us. Today’s dreams are tomorrow’s machines. Spaceships are today’s dreams. They are tomorrow’s machines, and tomorrow always comes sooner than it seems. With that in mind, how would we build a spaceship? To build one, we need a few key things: chiefly engines and a way to support life.

Engines, or more broadly propulsion systems, are quite simply the single largest challenge to interstellar travel. Without a propulsion system capable to reaching speeds between 10% and 20% of the speed of light, it wouldn’t be possible to reach other stars—like Proxima Centauri—within a single human lifetime. A number of technologies have been proposed that would allow for these sorts of speeds, including nuclear pulse, ion engines and solar sails. As of this writing, one of the most exciting is also one of the most controversial: the radio frequency resonant cavity thruster—or more commonly, the EmDrive.

RF resonant cavity thruster built by NASA Eagleworks lab for experiments in 2013-2014
RF resonant cavity thruster built by NASA Eagleworks lab for experiments in 2013-2014

This technology is the latest in a series of proposed ‘reactionless drives’ which seem to violate Newton’s Third Law of Motion; the famous, “every action has a equal and opposite reaction”, and by extension, the conservation of momentum. It does so by appearing to produce thrust and induce motion despite having no propellant—the equivalent of pushing on the steering wheel of your car and expecting it to move. Such violations to one the most fundamental laws of nature have made the drive extremely contentious, with some dismissing it as pure fiction—especially since no proposed mechanism to explain this violation has yet been proven. Despite the apparent impossibility of this machine, a number of experiments have each, independently, shown that this drive appears to produce thrust. Additionally, a peer-reviewed article is rumored to be coming out in the coming months. Finally, it was reported last month that the EmDrive is heading to space. A small version of the drive is being built to fit into a CubeSat—a type of miniature satellite used for space research. The CubeSat will be approximately the size of a shoebox with the drive taking up around a quarter of that space. According to a Popular Mechanics article, a satellite of this size at an altitude of ~150km, normally remains in orbit for about six weeks. The EmDrive CubeSat is slated to remain in orbit for six months or longer—kept elevated by the EmDrive. The longer the CubeSat stays in orbit, the more obvious it becomes that it must be producing thrust, despite the apparent physical impossibility of this feat. If it wasn’t producing thrust, it would plummet to Earth due to atmospheric drag relatively quickly. While it remains unknown if the EmDrive will ultimately be a viable propulsion system—and if it does work, what this means for our understanding of physics—the prospect that the results so far may be independently verified and shown to work in space is extremely exciting. The applications for such a dream machine are nearly endless.

Our other chief concern with interstellar travel is life support. While supremely adapted to the various environments on Earth, the human body is very fragile in the alien environment of space. This has been clearly demonstrated thanks to the long duration crews who live on the International Space Station. These astronauts, who live on the station for between six months and one year, have returned to Earth with various issues: including bone and muscle deterioration due to weightlessness, vision problems due to fluid redistribution, and having been exposed to much higher levels of radiation while beyond the Earth’s atmosphere. Weightlessness, while an iconic facet of space travel, is one of the greatest issues due to this deterioration experienced by the human body. One way to get around this issue is to create artificial gravity.

Artificial gravity, a staple of science fiction, would rid space travel of the ills of weightlessness. It is also—unlike the EmDrive—very feasible. As it was mentioned in the first part of this series, gravity is simply acceleration. To create artificial gravity, one simply has to accelerate. That feeling of being pinned back in your seat while driving a fast car or riding a roller coaster is the same force as gravity. In the previous article, we used the rate of acceleration due to gravity—9.8 m/s2—as a convenient example when discussing the acceleration needed to reach interstellar travel speeds for this reason: acceleration at this speed would feel the same as living on Earth does. While this would be a very convenient way to create artificial gravity, we also discussed the immense energy requirements needed to sustain such acceleration over a long period of time. We do, however, have another option: centrifugal force.

This force (in physics it is technically known as a ‘pseudo-force’, but that is beyond the scope of this article) is well known. Anyone who has ever spun an object on a string knows of the tendency for the object at the end of the string to move as far from the axis of rotation as possible. A spacecraft which rotated in such a way would create artificial gravity along the outer wall of the ship parallel to the axis of rotation. All objects contained in the spacecraft, including the crew members, would be forced as far from the axis of rotation as possible by the centrifugal force. If spun at the right rotational speed (a speed that would vary depending on the radius of the spacecraft), the centrifugal force would be the same as the force of gravity felt on Earth.

This is often the method used to create artificial gravity in science fiction, but there are some technical drawbacks. One of these is that the smaller a spacecraft is, the faster it would have to rotate to create artificial gravity comparable to Earth. This would, one, make for an incredibly dizzying view from any windows in the spaceship as stars and planets rapidly whirled by and, two, mean that the “gravity” felt would be stronger near the feet of any standing astronaut than at their head—a situation totally impossible on Earth, that would make movement extremely difficult.

To get around these issues, a spacecraft would have to be built larger. The larger the radius of the spinning ship, the slower it would need to rotate to create Earth-like artificial gravity. Additionally, the larger the ship, more gradual changes in the strength of the force would exist, the further one got from the axis of rotation, meaning that normal movement would be possible. Of course, the larger the ship, the more expensive it is to build and maintain and (depending on the outcome of the EmDrive tests) the more fuel it may need.

Artificial gravity would solve a number of issues, but not all. Radiation for example, doesn’t care if one is spinning or not. Currently our only way to protect against radiation is for astronauts to shelter behind thick metal plating. Dense heavy metals like lead are good radiation shields as they can block all but the most energetic of particles. What makes it good for this purpose, it’s mass, also makes it poor. Heavy objects like lead plating are difficult and expensive to get into space. In fact, because of this, astronauts aboard the International Space Station only have lightweight aluminum to protect them. This is known as passive shielding.

photo credit:NASA
photo credit:NASA

The future is active shielding. This is more like the shields seen in science fiction where an energy field is created to deflect, absorb or otherwise block incoming radiation. It may be possible to develop such technology using magnets and high voltages to create artficial magnetospheres around spacecraft. These artificial magnetospheres would then deflect incoming radiation before reaching the occupants. Such technology has been rudimentary thus far. The size and cost—in terms of energy and money—of such systems are prohibitive. This may be changing though. NASA is currently testing a technology called ‘HTS Coils’ that show much greater deflection of charged particles over much more manageable physical scales. HTS Coils, or high temperature superconductivity coils use superconducting magnets and are, according to NASA, lightweight and allow for simpler cooling systems due to their higher operating temperatures than previous systems. These coils would surround the spacecraft creating artificial magnetospheres and protect the crew from harmful radiation.

Spaceship design is one of the most challenging aspects of interstellar travel. Based on what we discussed above, we would need a large spinning ship, surrounded by superconducting magnets, propelled by an engine that many believe to be impossible. So, it may seem that we are at the end of the line, but remember: all of these technologies appear plausible (if, in the case of the EmDrive, inexplicable). They are today’s dreams, but possibly, tomorrow’s machines. Proxima Centauri is still a possible destination for the human race. What we would find there is coming up in the next installment of ‘Destination: Proxima Centauri’ here on the CFHT Hoku blog!

For some additional reading:
Popular Mechanics article on the EmDrive:
http://www.popularmechanics.com/science/energy/a22678/em-drive-cannae-cubesat-reactionless/
Popular Mechanics article on Artificial Gravity
http://www.popularmechanics.com/space/rockets/a8965/why-dont-we-have-artificial-
gravity-15425569/
NASA article on Active Shielding
http://www.nasa.gov/directorates/spacetech/niac/2012_phaseII_fellows_westover.html

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