Interstellar spacecraft have been built, but current technology limits their speed. Nuclear and solar sails are feasible options, while antimatter and electromagnetic launchers require more advanced technology. A nuclear pulse propulsion design could reach 10% of the speed of light, while antimatter and electromagnetic launchers could reach closer to the speed of light, but face technical challenges.
It is not extremely difficult in principle to build an interstellar spacecraft – we have already made five of them, namely Pioneer 10, Pioneer 11, Voyager 1, Voyager 2 and New Horizons. All of these space probes are moving at escape velocities out of the solar system and will one day reach other star systems.
The problem with these vehicles from a practical standpoint is that all of them will take millions of years to reach these star systems. While these probes won’t be exploring any other stars in the foreseeable future, some of them, notably Voyager 2, are already sending back data about the interface between our solar wind (the heliosphere) and the diffuse interstellar medium.
If you want to build an interstellar spacecraft that reaches its target star within a reasonable amount of time, say, 50 years, then that requires some form of propulsion significantly more powerful than chemical rockets, which are extremely inefficient. Possible sources include nuclear, in nuclear and pulse-propulsion reactor variants, solar sails, electromagnetic launchers, and antimatter propulsion systems. While antimatter propulsion and EM launchers require more sophisticated technology than we have now, nuclear and solar sail options are within reach of our current technology.
In the 1970s, the British Interplanetary Society conducted a detailed study of an interstellar probe design that would have been able to reach Bernard’s Star (6 light-years away) in just fifty years. This interstellar probe design used nuclear pulse propulsion, which meant it dropped atomic bombs behind it, allowing them to transfer some of their energy to thruster plates, which would accelerate the craft forward. Based on their calculations, the probe could reach speeds equal to 10% of the speed of light. This is around the limit for nuclear propulsion.
With antimatter or electromagnetic launchers, speeds closer to that of light could be reached. The technical challenges for antimatter include producing it in the quantities needed (today we can only produce picograms of antimatter, for millions of dollars) and containing it adequately. The challenges for electromagnetic launchers are providing the necessary energy (in the petawatt range) and length (hundreds of kilometers) to launch an interstellar probe at nearly the speed of light.
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