There's an Earthlike planet orbiting our nearest neighbor, Proxima Centauri - but how might we actually get there?

The existence of a potentially habitable planet  orbiting our closest neighbor, the red dwarf Proxima Centauri, is both an extremely exciting event and sort of "more of the same." While Proxima b is just one exoplanet among thousands, it's the closest extrasolar planet we've found so far. We don't yet know if the planet is rocky or even has an atmosphere, but its mass and location imply the former. Its existence in the Goldilocks zone of its red dwarf, meanwhile, offers the tantalizing possibility of water.

There are significant questions about whether or not planets within the habitable zone of a red dwarf would actually be habitable. These planets orbit so close to their parent stars that they may become tidally locked, meaning one side of the planet always faces the star. It's possible that Proxima b has a 3:2 resonance orbit similar to Mercury, which would mean it rotates three times for every two revolutions around the star - but also, like on Mercury, it would mean that there was a major thermal differential between the day side and the night side. There are also questions about whether or not a planet in orbit around a flare star (a star prone to dramatic increases in brightness in a short time) can sustain life - and the cosmic environment surrounding young red dwarf stars is no picnic, either. But for the sake of our example, let's assume Proxima b is habitable and that we want to go there. Can we?

Probably not with conventional chemical rockets, is the short, unsatisfying answer. The trouble with chemical rockets is that they rely on huge amounts of propellant. The Saturn V may have taken us to the Moon, and NASA's SLS may one day reach Mars, but chemical rockets cannot reach other solar systems on human timescales. The problem is simple: The faster you want to go, the more propellant you need. The more propellant you need, the greater the mass of the rocket. The larger the rocket, the more propellant you need. Fuel and mass limits snowball like this. 90% of the weight of the space shuttle was its fuel, and it's been estimated that you'd need more chemical rocket fuel than there are atoms in the universe to complete a trip to Alpha or Proxima Centauri at any meaningful percentage of the speed of light.

Ion thruster technology, which uses a very low level of thrust produced over very long time periods, is theoretically capable of sending a mission payload to another star system without requiring a universe worth of chemical fuel -- but not within a sane period of time. The Space Shuttle would've taken roughly 165,000 years to reach Proxima Centauri; our current level of ion drive technology could perform the same feat in about half the time, or about 81,000 years.

Truly theoretical concepts for space flight, like the Alcubierre warp drive or the EMDrive NASA is studying that may or may not exist at all, aren't much more help here, because these technologies fundamentally rely on breakthroughs we aren't even close to making. Project Starshot, a recently announced initiative to accelerate tiny iPhone-sized satellites to a significant fraction of the speed of light, could possibly send a probe to Proxima Centauri within a human lifetime, but as my colleague Graham noted in his coverage, there are major technical hurdles to be overcome before we could send even a tiny probe to another star system.  We've got great ideas, but it's clear that any attempt to reach even our nearest neighboring star would require some actual technology development.

At present, it looks like the only candidate for human-scale exploration of another star system is nuclear propulsion. No nuclear rocket has ever flown -- while we've used nuclear power on a wide range of satellites and probes, the Partial Nuclear Test Ban Treaty of 1963 ended most serious research into using nuclear engines. The thermal nuclear rockets that were built and tested in the 1960s through to the present day would be insufficient to the task as well. Nuclear thermal rockets would be more powerful than their chemical counterparts if used correctly, but not enough to bridge the gap.

Now, it's true that concepts like Project Orion or Project Dadaelus have the theoretical capability to push a spacecraft quickly enough to accelerate it to some significant fraction of c. Project Orion(Opens in a new window) was a research project in the 1950s and 1960s that proposed building a space vehicle with a large pusher plate at the rear. Shaped nuclear charges would be ejected from the rear of the spacecraft, then detonated, with the resulting shockwave accelerating the ship to higher and higher velocities. In 1968 Freeman Dyson estimated that it could take between 130 and 133 years to reach Alpha Centauri using these designs; later research indicated that a fission rocket of this sort might reach between 9-11% the speed of light (top cruising speed is less if you intend to slow the ship down with the same kind of explosions as you approach your destination). In theory, such a fission rocket might reach Proxima Centauri within a human lifetime, if you expect a human lifetime to be about double what it is. This type of nuclear pulse drive, while functional in theory and buildable with current technology, is still strictly theoretical. Later projects, like Project Daedalus, proposed the use of fusion rockets, but no such rocket has yet been built.