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The best options for flying to faraway stars

Experts say it’ll take at least 200 years to build up the technology and the energy to travel to other stars, but some researchers are already trying to find ways to shorten that time frame. Commentary by Tau Zero Foundation’s Marc Millis.
Image: Warp Drive
A warp drive works by creating folds in the underlying fabric of spacetime, as shown in this artistic concept created by Alexandre Szames. The concept is based on theories proposed by physicist Richard Obousy.Alexandre Szames / Antigravite
/ Source: Special to

Just last month, scientists announced the discovery of the first possibly habitable planet, orbiting a star 20 light-years from Earth. That's relatively close in astronomical terms, but beyond today's reach.

Estimates based on three key factors — finances, technologies and energy sources — all come to the same conclusion: The first missions to others stars will not be possible for another two centuries.

While that's a sobering answer, it's not the last word on the topic. Volunteers at the Tau Zero Foundation, the nonprofit organization I founded, are working to improve humanity's prospects in the decades ahead.

Interstellar flight is quite possible in principle, whether it's launching a probe to Alpha Centauri or sending a colony ship out of the solar system. The catch is that it takes a lot of work to pull it off. Global commitments beyond historical precedents would be needed. Sending spacecraft to other stars is as much a sociological challenge as a technical challenge.

The technical progress being made toward interstellar flights was one of the topics discussed two weeks ago at the 61st International Astronautical Congress in Prague. Almost 3,000 professionals from the aerospace community gathered in Prague, the Czech capital, to bring each other up to day on the latest developments in space exploration.

Most of the discussions were about current space missions and the rise of commercial launch services, but there were also sessions on interstellar precursor missions, advanced space propulsion and the search for extraterrestrial intelligence. A few of the researchers in attendance are already making progress on meeting the challenges of interstellar flight. Here's a quick scan of that progress:

Solar sails:
Simple in principle, solar sails are large sheets of ultra-thin materials that are pushed by sunlight. Successful flight tests occurred this summer with Japan's Ikaros spacecraft. For interstellar missions, however, the sails would have to be at least thousands of times larger, and it would still take thousands of years to reach other stars. Using beamed power could reduce the requirements for sail size and travel time, but building those beaming systems would require global commitments.

Rockets work by blasting propellant rearward to push a spacecraft forward. If you want to go farther and faster, or send a bigger payload, you'll need more propellant ... and then you'll need even more propellant to propel that extra propellant.  It adds up exponentially to the point where interstellar rockets are astronomically difficult (and in some configurations, flatly impossible).

An international team of volunteers is pushing interstellar rocketry to its edge.  Finishing the first year of a five year study, this "Project Icarus" is a sequel to the 1970s-era "Project Daedalus," designed at creating a fusion-based interstellar probe. This study aims to deliver realistic estimates of what such technology could accomplish, along with estimates of what other milestones would be needed to make it happen — such as making the business case for extracting helium-3 from the atmosphere of Uranus, and building the communication network for deep-space exploration.  By reaching beyond the near-term horizons, such work sets the stage for a new wave of advancements to follow.

Precursor missions:
With actual interstellar flight still so daunting, numerous ideas are being discussed for smaller, learning missions.  These include missions through the heliopause, where our solar system meets with the galactic background, and missions far enough out to study the gravitational lensing of our own sun. Even now, NASA's Voyager 1 and 2 spacecraft are on their way through the heliopause. Among the mission concepts being considered for solar gravitational lensing are FOCAL and TAU.

Beyond technology by advancing physics:
Rockets and space sails operate within the laws of physics that we already understand, but their ultimate performance falls far short of timely missions.  This brings us to the next ambition — exploring the realm of unfinished physics in search for spaceflight breakthroughs.

A book about these prospects, "Frontiers of Propulsion Science," was published last year by the American Institute for Aeronautics and Astronautics. It's not light reading. Weighing in at 730 pages, this technical volume is aimed at professional researchers and graduate students. But the basic concepts for breakthrough propulsion are easily understood by anyone who's watched "Star Trek."

If it were possible to move a spacecraft without propellant, the energy requirements drop exponentially. This is where notions of manipulating gravity or inertia come into play.  Such advances would also ultimately allow zero-gravity hotel rooms on Earth as well as crew cabins with artificial gravity for long-duration space missions, plus any number of other revolutionary advancements.

This might sound like science fiction, but the subject has matured to where several rigorous investigations have commenced, but mostly at the level of asking the right questions, with a few embarking on experiments. The key issues involve unsolved questions about the origin of inertial frames, the interactions of matter and energy when moving through those frames, and the coupling of gravity with the other fundamental forces.

Beyond the speed of light:
Before 1988, traversable wormholes were considered merely make-believe. And before 1994, warp drives were seen as impossible science-fiction plot devices. These challenges have now matured into normal scientific discourse and even appear as homework problems in general-relativity textbooks.

Rather than attempting to break the light speed limit through spacetime, these theoretical approaches manipulate spacetime itself to create shortcuts (wormholes) or to move 'bubbles' of spacetime (warp drives). The rate at which spacetime can move is inferred from the faster-than-light expansion that physicists say occurred just after the big bang.

The contentious issues are the implications of time travel, the magnitude of energy required, and the assertion that the energy must be "negative." Although negative energy states are observed in nature, there are unresolved debates regarding how much energy these states can hold, and for how long. Another recent conclusion is that wormholes appear to be theoretically more energy-efficient than warp drives.

To clarify a common misunderstanding, the category of space drives is distinct from the notions of faster-than-light space warping. As an analogy, consider moving an automobile across a landscape. The space-warping theories would move whole sections of the landscape to carry the automobile toward the desired destination. This requires substantial energy expenditures, but also opens the way for creating faster-than-light pathways.

The space drive perspectives, on the other hand, consider how the automobile might move under its own power relative to that landscape, in a way analogous to tires pushing against the ground. The energy requirements are minimal, but travel is limited to less than light speed.

Regardless of whether such abilities are achievable, pursuing interstellar flight adds another perspective for advancing technology and solving the lingering mysteries of physics. In addition to the unsolved connections between gravity and the other fundamental forces, cosmological observations have presented us with the mysteries of dark matter and dark energy, plus anomalies surrounding the trajectories of deep space probes.

Quantum physics — which is profoundly useful on extremely small scales — has not yet been successfully extended to cosmological scales. There is plenty of physics yet to be discovered.

Keeping up:
If you are interested in such activities, stay tuned to the "Centauri Dreams" news blog. This blog is part of the Tau Zero Foundation — a volunteer network of more than four dozen researchers, educators and journalists who collaborate on the methods, lessons and inspirations of interstellar flight. We hope the work of our nonprofit foundation will pave the way to eventually reach other habitable worlds, and you can contribute as well.

Marc Millis is the founder and president of the Tau Zero Foundation, and the former head of NASA's Breakthrough Propulsion Physics program.