Prepare for warp speed

Science fiction has shown us spaceships travelling at enormous speeds, some of them had faster-then-light capabilities (and some have done the Kessel run in 12 parsecs). Which metods of transportation that are being developed or thought about in the near/far future you think are the most promising?
Paul Carr (Space Systems engineer at NASA, podcaster, blogger, investigator)

I'm not optimistic about faster than light travel at any time in the future, although I would love to be proved wrong. Not only do we not have the technology to travel faster than the speed of light, we don't know what technology we need, or even if it's possible.

For the near future, something we could make happen would be nuclear space propulsion - first fission reactors, and then fusion reactors. My dream reactor would be a Helium 3 fusion reactor. Helium 3 is stable, and the Helium 3 fusion reaction produces Helium 4 (also stable), a proton (or two) (that can be used to generate electric power), and energy, but no neutrons. Neutrons are a problem that make most fusion reactors unusable for space applications. Such a reaction is far more mass efficient than chemical rockets, and with some work, could open up the entire solar system to us.

Fraser Cain (publisher at Universetoday.com, co-host of Astronomy Cast)

In the near term, I'm mostly excited about the potential for light sails, like the Breakthrough Starshot. If this technology can be developed, we could see spacecraft traveling out to Pluto within a few weeks or even days. Once we've mastered this tech, we can start sending spacecraft out to other stars.

Ciro Villa (technologist, application developer, STEM communicator)

Ever since human have been able to use their imagination they have been dreaming of traveling far away in space to explore and discover new worlds. Unfortunately, as much as our brains can dream it, we are limited by our physical and technological capabilities to only be able to travel very nearby.

So far in the history of space travel, chemical rockets have been the main mean of propulsion and other new propulsion technologies are only at their infancy. Many studies are underway and much literature has been created to envision the design of new ways to propel human made spaceships further in space and in shortest amount of times. In the shortest term, more efficient forms of propulsion are being developed such as electric variants like Ion, Plasma and Hall-effect thrusters some of which are already operational on some space crafts (https://en.wikipedia.org/wiki/List_of_spacecraft_with_electric_propulsion). Also, Solar sails which are still somewhat experimental in nature with their size challenges and limitations, are being investigated as another promising mean to accelerate spaceships beyond the confines of our Solar System.

More futuristic forms of propulsion are unfortunately still only on paper at this time and it will take willpower, new discoveries, money, time or most likely all the above to be further developed. The hope is that with the accelerating pace of technological advancements, some of these new, exotic propulsion technologies will materialize at some point in our future make human exploration of deep space a reality.

Andrew Rader (SpaceX engineer, MIT PhD, author)

For faster than light travel, it's always possible that there will be some breakthrough that we can't anticipate. Apart from that, I think we're going to end up taking a long time to get to other stars, possibly in some kind of suspension or by just sending robots or human embryos. In terms of advanced propulsion in general, anti-matter offers the best mass to energy ratio we know of, but that's a long way off (hundreds of years?). Fusion rockets might be possible before the end of the century. These would be great for travel in the solar system, but probably not to another star.

Robert Novella (co-founder and vice-president of New England Skeptical Society, co-host of Skeptics’ Guide to the Universe)

Chemical rockets have served humanity very well for many decades. They have launched satellites into orbit and blasted our probes and landers into the nooks and crannies of our solar system. They have lifted humans to low earth orbit and our moon. All of this has given us a priceless cornucopia of images and data and mind-boggling discoveries.

These types of rockets however are not nearly as adept at ferrying our fragile bodies much beyond the moon. To keep us healthy and happy requires vast ships that are prohibitively slow and expensive for trips to the closest practical planet, Mars.

Luckily, conventional rockets are only a tiny subset of all rocket types, yet I've been disappointed for literally decades that we have made so little progress on other types of rocket technology for transporting humans.

I'm still holding out hope for the widespread realization that rockets using nuclear fuel are the only real option we have in the near future for getting humans well past our moon. The energy density of nuclear is orders of magnitude that of chemical energy. Nuclear thermal rockets using fission for example could weigh half as much as similarly powerful chemical rockets. Directly comparing chemical vs nuclear rockets is complex but many have concluded that such nuclear rockets would be at least as twice as efficient as chemical rockets. This would allow trips to mars requiring half the time, or less, which is especially important considering the more time spent in space, the more time you're exposed to life-threatening solar radiation and cosmic rays. Fission rockets would also allow for some serious maneuvering during a flight which is too expensive for modern chemical engines. You're just not much of a spaceship in my book if you can't maneuver easily.

A little beyond these fission rockets (which we can build now), we will create fusion rockets which should quickly predominate since they are even more efficient and produce less radioactive waste. Remember, a significant limitation to any ship's maximum velocity is the amount of fuel required to reach that velocity. You could actually reach 10% of the speed of light with chemical engines but you'd need a gas tank the size of our sun to do that. Doable? Yes, theoretically. Practical? Ummm, no. Fission would require far less fuel to reach that speed and fusion even less. So what would require the least amount of fuel? Read on...

Long-term scenarios for Space Travel will certainly offer humanity many fascinating hi-tech options but some type of antimatter engines will probably be required if you want to move something space ship sized as close as possible to the speed of light. Sure, there may be some bizarre quirk of physics that allows for superluminal travel but...probably not, so don't get your hopes up.

We know for certain right now that as you approach appreciable fractions of the speed of light, your mass starts increasing alarmingly fast (kinetic energy). To continue accelerating, your ballooning mass requires an exponentially increasing amount of energy. Eventually, to reach the speed of light itself you'll need infinite energy to move your infinite mass. Unless you have infinite energy in your back pocket, you'll never hit that speed.

To get as close as possible however, you'll need an efficient method of energy conversion and that's exactly what matter/antimatter annihilation provides. The energy released from such interactions is truly huge even if the masses involved are tiny (that is, after all, a key take-away from E=mc^2). The primary problem though is that we can't practically convert all the byproducts of matter/antimatter collisions into the kinetic energy of our spaceship. The bottom line then is that we will probably not be able to ever get arbitrarily close to the speed of light. The estimates seem to be all over the place but somewhere between 40 and 70 percent of the speed of light could be attainable eventually.

I'm totally ok with a spaceship going 753 million km per hour.

Antonio Paris (Astronaut Candidate, Astronomy Professor, Planetary Scientist, Space Science Author)

For generations, science fiction has attempted to shape our future. From cameras on a watch as depicted in Dick Tracy; to warp speed, a common mode of travel used extensively in the Star Trek franchise. However, traveling faster than the speed of light or at warp speed, from a practical purpose, is not possible according to the laws of physics. The energy required to achieve the speed the speed of light, for example, would be infinite – sort of a an impossibility.

Today, and for the foreseeable future, spacecraft are limited to local orbits and interplanetary missions. There are numerous factors that shape spacecraft design and capabilities, but predominantly they are due to budget constraints, its intended function, and policy requirements. Extraordinary specific power and the ratio of jet-power to total spacecraft mass are required to reach interstellar targets within sub-century time frames. Some heat transfer is unavoidable and a tremendous heating load must be effectively handled. Thus, for interstellar rocket concepts of all technologies, a key engineering setback is controlling the heat transfer from the exhaust stream back into the spacecraft.

Based on research in the late 1950s to the early 1960s, it is technically possible to build spacecraft with nuclear pulse propulsion engines (i.e. driven by a series of nuclear explosions). This propulsion system contains the prospect of very high specific impulse and high specific power. This type of spacecraft, in my opinion, is our best hope for achieving interstellar travel.

In 1968, Project Orion team members proposed an interstellar spacecraft using nuclear pulse propulsion, which used pure deuterium fusion detonations with a very high fuel burn-up fraction. They calculated an exhaust velocity of 15,000 km/s and a 100,000-ton spacecraft able to achieve 20,000 km/s allowing a flight-time to Alpha Centauri of roughly 130 years. Later studies suggested that the top cruise velocity that can theoretically be achieved by a Teller-Ulam thermonuclear unit powered Orion spacecraft, supposing no fuel is saved for slowing back down, is about 8% to 10% of the speed of light. An atomic Orion can reach perhaps 3%-5% of the speed of light. A nuclear pulse drive spacecraft powered by Fusion-antimatter catalyzed nuclear pulse propulsion units would be comparably in the 10% range and pure matter-antimatter annihilation rockets would be theoretically capable of achieving a velocity between 50% to 80% of the speed of light.

In closing, although there have been numerous proposals and design concepts, spacecraft propulsion for interstellar flight is not an easy endeavor or economical. At current pace, we are at least hundreds or perhaps thousands of years before capable of interstellar travel to even the closest stars. Nevertheless, there are no doubts we will become an interstellar species in the foreseeable future.

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