Interstellar Travel: The Space Age and Nuclear Rockets - Universe Today

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Given time and the right kind of commitment, could the dream of interstellar travel become a reality? Credit: Pablo Carlos Budassi

The Universe is unfathomably huge, with over 2 trillion galaxies in an "observable" volume measuring 93 billion light-years in diameter. Even our galaxy, the Milky Way, is incredibly huge, measuring an estimated 100,000 light-years across and containing between 100 and 400 billion stars. As we explored in a previous article, it would take centuries to millennia to travel to even the nearest star in our galaxy.

As with launching spacecraft to orbit, which is subject to the tyranny of the Rocket Equation, achieving interstellar travel is subject to Einstein's Theory of Relativity. Just as it takes a significant mass of propellant to place even 1 kg (2.2 lbs) into space, reaching even a fraction of the speed of light would require tremendous amounts of energy, the discovery of exotic physics, or both. However, since the dawn of the Space Age, several concepts have been proposed that could achieve the dream of interstellar flight.

As time and technology have progressed, proposals for interstellar spacecraft have evolved from huge, ambitious projects to miniaturized and automated vehicles. The propulsion methods, paradoxically, have also become more exotic and more practical. As the Space Age wound down, the source of the proposals began to transition away from space agencies towards non-profit organizations and private research institutes.

Alas, the dream of reaching another star is fraught with monumental, nay herculean, challenges. As with most advanced concepts (such as space elevators, O'Neill Cylinders, interplanetary settlements, etc.), it all comes down to two questions:

"How much are you willing to spend, and how long are you willing to wait?"

Having addressed that, what are the options for an interstellar mission? First, let's take a look at possible destinations.

Worlds Next Door

As noted, the field of exoplanet studies has exploded in the past two decades. To date, scientists have confirmed the existence of 6,333 exoplanets in 4,747 star systems. While Earth-like (terrestrial) planets account for a small minority, only 222 confirmed to date, a number of these are relatively close (in astronomical terms) to the Solar System. Within 50 light-years of Earth, there are 31 known terrestrial planets, 30 of which orbit low-mass, M-type red dwarf stars.

The closest of these is Proxima b, a rocky planet comparable in size and mass to Earth that orbits within its sun's habitable zone (HZ). Discovered in 2016, scientists estimate that it has an average surface temperature of -39 °C (-38 °F), compared to 15 °C (59 °F) for Earth. Given that this planet is tidally locked with its star, where one side is constantly facing its sun (which is common with M-type stars), scientists are divided on the question of its habitability.

Nevertheless, unless some potentially habitable planets exist within the Alpha Centauri system, Proxima b is the nearest confirmed terrestrial planet beyond the Solar System. It shall therefore serve as our reference point, at a distance of 4.25 light-years from Earth. With that covered, let's examine the propulsion systems proposed over the years, in chronological order.

Nuclear Propulsion (1950–1973)

Our first stop is the Cold War, a time characterized by two superpowers locked in a constant game of one-upmanship and a nuclear arms race. It was also a time of intense scientific discovery and experimentation. Having benefited from breakthroughs in rocket science, including the wartime development of ballistic missiles, the U.S. and Soviet Union (USSR) were also determined to achieve spaceflight before the other. From 1958 to 1972, this competition would come to be known as the "Space Race."

Even before the first astronauts and cosmonauts would reach space, scientists, engineers, and mission planners were contemplating how to send humans to orbit, the Moon, and beyond. Naturally, the idea of sending humans to the nearest stars was also considered, leading to the emergence of several projects. By the 1950s, having achieved many breakthroughs in the development of nuclear reactors that relied on the slow decay of fissile material (uranium) to generate heat and electricity.

This led the U.S. and Soviet Union to initiate programs to create reactors that would power nuclear rockets. By 1958, President Dwight D. Eisenhower signed the National Aeronautics and Space Act into law, which established NASA as the country's space program. From that point onward, NASA would take over existing programs and mount its own development efforts. By the end of the Space Race, both the U.S. and the Soviets created and tested multiple nuclear reactors designed to power nuclear engines.

This established the foundation for nuclear...