Above: Artist conception of NASA’s Project ORION, the smallest model of a craft that can take off from Earth, explore Saturn, and return in fourteen months with a manned crew. NASA
People have dreamed about interstellar travel for decades. Wouldn’t it be nice to escape political turmoil, climate change, overpopulation, and nuclear proliferation? Wouldn’t it be nice to move, with a few family members and friends, to a welcoming virgin world?
Hollywood makes it seem easy. We strap in aboard our starliner, and the craft gently lifts off. A button is pushed. The star field distorts. The ship slips into a wormhole, and we take a shortcut through space-time to arrive in a new planetary system and a blue-green oasis.
In reality, things aren’t that simple. We don’t know how to create a wormhole. Warping space requires immense energies — and forms of matter that might not exist. Space is so vast that if we were to depend on the conventional chemical propulsion systems that have taken us to the moon and other planets in our solar system, a voyage to the stars would last tens of thousands of years.
It’s so frustrating! Just as astronomers locate potentially habitable planets around nearby stars, we’re reminded of the difficulty of visiting these worlds.
Someday humans may be able to develop interstellar spacecraft powered by antimatter or fluctuations in the universal vacuum. (Antimatter combined with normal matter would be the most energetic possible rocket fuel. Tapping the universal vacuum implies a near-infinite energy source, if this proves possible.) But for now, let’s limit our view to proposed interstellar spacecraft powered by innovative propulsion techniques that do not violate the laws of physics as we now understand them.
The first realistic possibility, initially proposed in 1968 by physicist Freeman Dyson, was a craft powered by successive explosions of hydrogen bombs (separated from the crew compartment by colossal shock absorbers). The proposed spacecraft, dubbed Project Orion, was huge, with dimensions measuring in kilometers. Powered by the entire thermonuclear arsenals of the U.S. and the Soviet Union, Dyson calculated, it would have been capable of sending thousands of human colonists to the Alpha/Proxima Centauri system (at 4.3 light-years from Earth, the closest star system) within 1,000 years or so.
Starting in the 1970’s, scientists at the British Interplanetary Society and its offshoots, including the Institute for Interstellar Studies, proposed a series of interstellar probes and huge human-carrying space arks bearing classical names such as Daedalus and Icarus. These were essentially sanitized versions of Dyson’s Orion: Instead of hydrogen bombs, these conceptual craft were to have been propelled by small pellets of nuclear fusion fuel ignited by lasers or electron beams.
These proposed craft were supposed to have been able to reach Alpha/Proxima Centauri within centuries. But the fusion apparatus would be massive. It would be difficult to fund preliminary developmental in-space experiments because of the immense ship sizes required.
In the late 1970s, scientists started thinking about interstellar craft powered by not by chemical or nuclear processes but by “propellantless” systems. One of the most promising ideas is for craft propelled by the propulsive force of sunlight falling on a thin, highly reflective “sail.” Calculations indicate that such a system, pushed to its technological limits, could propel a robotic payload to Alpha/Proxima Centauri within only a few centuries. Human-carrying arks, whose speed would be boundby our bodies’ limited ability to withstand the high g forces of rapid acceleration, would take about 1,000 years to complete the same journey.
Unfortunately, the solar-photon sail has its limits. One is the inverse-square nature of solar luminosity: With every doubling of a ship’s distance from the sun, its acceleration would fall by a factor of four.
But there may be a way around this limitation.
During the next decade, a Silicon Valley billionaire named Yuri Milner intends to invest $100 million in the Breakthrough Initiative Project Starshot. His idea is to attach a tiny robotic payload to a super-thin, highly reflective sail measuring only about 100 square centimeters and hit it with a multi-gigawatt laser beam generated by a huge facility here on Earth. If scientists can find a way to keep it in the powerful beam for a few minutes without melting, it would fly off at a velocity sufficient to reach Alpha Centauri in a few decades.
This is all very challenging. We’ll need to figure out how to keep the tiny sail stable in the powerful beam without melting, maintaining the beam on target during the multi-minute acceleration run, and reconfiguring the tiny sail for both interstellar cruise and encounter with the Centauri system — but it is by no means impossible. And at least for now, it may be our best chance we — or, rather, robots — have for reaching the stars.
Greg Matloff is a professor in Physics at New York City College of Technology, CUNY, a consultant with NASA Marshall Space Flight Center, a Hayden Associate of the American Museum of Natural History, a Member of the International Academy of Astronauts, and a leading expert in possibilities for interstellar propulsion. He has authored and co-authored several books, including Starlight, Starbright: Are Stars Conscious (2016), Solar Sails: A Novel Approach to Interplanetary Travel (2008, 2015), Paradise Regained (2009), Living Off the Land in Space (2007), Deep Space Probes (2000, 2005), More Telescope Power (2002),Telescope Power (1993), Urban Astronomer (1991), and The Starflight Handbook (1989).
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