Reaching For The Stars – Practical Interstellar Travel

Artistic representation of the Project Daedalus. Credit: David A. Hardy/www.astroart.org

Will we ever be able to send spacecraft to the stars in a reasonable amount of time? The big problem—and it’s huge—is distance. The nearest star system to the Sun is far, but close.

Alpha Centauri, lies about 4.3 light-years (40 trillion kilometers) away, or roughly 10,000 times the distance of Pluto. At its present speed of about 17 kilometers per second (60,000 km/hr), Voyager 1, which is the fastest of the five spacecraft currently on exit trajectories from the Solar System, would take more than 75,000 years to reach Alpha Centauri (assuming it was heading in the right direction, which it isn’t).

The highest speed ever attained by any spacecraft from Earth, relative to the Sun, was 240,000 km/hr in the case of the Helios 2 Sun probe in 1976. But even at this rate the nearest star would be 19,000 years away, or the equivalent of more than 600 generations.

Clearly, a mission that takes many thousands of years to reach its goal is not acceptable. Not only would no one be interested after all that time, but such a slow spacecraft would inevitably be overtaken by much faster vehicles launched at later dates with more effective methods of propulsion, rendering it obsolete.

Having said that, small steps toward interstellar exploration have already been taken and more ambitious interstellar precursor missions have been proposed. These early flights will provide valuable scientific data about conditions at the edge of the Solar System and in the space between the stars that lies beyond.

The twin Voyagers, 1 and 2, launched in 1977, together with Pioneers 10 and 11, and New Horizons (at present on course for a rendezvous with Pluto in 2015), are all on trajectories that will ultimately take them into the interstellar void.

Both Pioneers have fallen silent, their power reserves drained past the point at which they can transmit a detectable signal. Both Voyagers, on the other hand, are still returning information as part of what in 1989 became known as the Voyager Interstellar Mission.

Having crossed the terminal shock, where the solar wind slams into the interstellar medium, the two Voyagers have entered the heliosheath, heading in different directions.

The latest data from Voyager 1, which take 17 hours to travel to their home planet, suggest that the spacecraft has entered a region dubbed the “magnetic highway,” in which the magnetic field between the stars is making itself increasingly felt. In a matter of months, or at most a couple of years, mission scientists believe the probe will break through into interstellar space.

Proposals have also been put forward for faster spacecraft that would be launched specifically to explore the near interstellar medium. The first detailed analysis of an interstellar precursor mission was made at the Jet Propulsion Laboratory in 1977. It focused on a probe that could reach a distance of 370 astronomical units (55 billion kilometers) from the Sun in 20 years after launch, and 1,030 AU (155 billion kilometers) in 50 years after launch, using a nuclear-electric propulsion system.

In 1990, the Interstellar Precursor Mission was revised as one of three NASA “frontier probes” concepts to explore the heliosphere and local interstellar space. Of these three, the “fields and particles” Interstellar Probe, powered by a solar sail, was endorsed for further study. However, neither this nor any subsequent proposed interstellar probe has so far gone beyond the drawing board.

Artist impression of the Project Orion spacecraft traveling through the solar system. Image Credit: NASA

The idea of journeying to the stars is nothing new. As early as 1929, the British scientist J. D. Bernal wrote about the possibility of generation ships—enormous spacecraft, like miniature worlds, which would take hundreds or thousands of years to reach their goal and aboard which many generations of travellers would live out their lives. In both science and science fiction, concepts such as suspended animation have also been used to allow people to cross the light-years to other stars, even at relatively low speeds.

But, almost certainly, practical interstellar travel will demand that vehicles reach far higher speeds than any that have been achieved in spaceflight to date. This in turn will mean that new forms of propulsion have to be developed that go beyond the capabilities of chemical rockets or even ion engines.

One of the first practical designs for a robotic interstellar probe was that of the British Interplanetary Society in the mid-1970s. Known as Project Daedalus, it called for a ship to make the voyage to Barnard’s Star, a red dwarf 5.9 light-years away, in a travel time of 50 years, powered by a nuclear-pulse rocket that could propel the craft to about 12 percent of the speed of light (36,000 kilometers per second). This type of engine, which would use a series of nuclear fusion explosions—effectively, hydrogen bombs—had already been studied by Freeman Dyson and his colleagues as part of Project Orion in the 1960s.

The distance to the nearest star, in perspective. Notice the logarithmic scale. Image Credit: NASA

Daedalus would be constructed in Earth orbit and have an initial mass of 54,000 tons, including 50,000 tons of fuel and 500 tons of scientific payload. The first stage would be fired for two years, taking the spacecraft to 7.1 percent of light speed, before being shut down and jettisoned. Then the second stage would fire for 1.8 years before being shut down to begin the 46-year cruise to Barnard’s Star. Since the design made no provision for deceleration upon arrival, Daedalus would carry 18 autonomous probes, equipped with artificial intelligence, to investigate the star and its environs.

In 1988, researchers from NASA and the US Naval Academy came up with a design for an interstellar probe to Alpha Centauri. Project Longshot, as it was called, would have been assembled at the International Space Station, and propelled on a 100-year flight to the nearest star system by a pulsed fusion engine of the type proposed in the Daedalus study. The 6.4-ton spacecraft would have carried a 300-kilowatt fusion reactor to power instruments and engine startup and used a 250-kilowatt laser to transmit data to Earth.

The trajectories of the twin Voyagers and Pioneers shown together with the more direct route to the edge of the solar system, which would be followed by a purpose-built interstellar probe. Image Credit: NASA

Of course, no one expected these early proposals to be put into action straight away. However, they showed that practical interstellar travel is possible, at least in terms of the propulsion systems. Other problems remain for any spacecraft, crewed or robotic, that must travel for many years at a significant fraction of the speed of light. These include surviving collisions with even small particles, such as dust grains, at speeds of tens of thousands of kilometers per second, and the durability of sensitive components, especially electronic ones, over such long time periods. Source: AmericaSpace

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