How Long Would it Take to Travel to the Nearest Star?

by Ian O'Neill on July 8, 2008

Project Orion, using nuclear explosions as a propellant (NASA)
We’ve all asked this question at some point: How long would it take to travel to the stars? And could I do it in my lifetime? There are many answers to this possibility, some very simple, others in the realms of science fiction. To make this easier to answer, we’ll address how long it would take to travel to the nearest star to the solar system, Proxima Centauri. Unfortunately, any route you take to the stars will be slow, even if you are powered by the most powerful nuclear propulsion technology…

In April, I examined how long it takes to travel to the Moon. We took the fast-track with New Horizons Pluto mission, powering past Earth’s only natural satellite in a mere eight hours and 35 minutes. We also had the leisurely ion drive-propelled SMART-1 mission that trundled its way to the Moon for 13 months. So, from the speedy rocket-propelled spacecraft to the economical ion drive, we have a few options open to us when flying around local space (plus we could use Jupiter or Saturn for a hefty gravitational slingshot). But say if we build a dedicated mission to somewhere a little more extreme?

Hertzsprung-Russell Diagram (NAU)

The nearest star to Earth is our Sun. It is a fairly “average” star in the Hertzsprung – Russell diagram’s “Main Sequence.” Our Sun is surprisingly stable, providing Earth with just the right sunlight for life to evolve on our planet. We know there are planets orbiting other stars near to the Solar System, but could they support life as efficiently as our Sun? In the future, should mankind wish to leave the Solar System, we’ll have a huge choice of stars we could travel to, and many could have the right conditions for life to thrive. But where would we go and how long would it take for us to get there?

First choice would probably be Proxima Centauri, the closest star to the Solar System. Part of a triple star system called Alpha Centauri; Proxima is 4.22 light years from Earth. Alpha Centauri is actually the brightest star of the three in the system, and so the system is named after this star. Alpha Centauri is part of a closely orbiting binary about 4.37 light years from Earth, but Proxima Centauri (the dimmest of the three) is an isolated red dwarf star 0.15 light years from the binary. Red dwarf stars generate far less energy than our Sun, so we’d have to find a planet in a closer orbit to this red dwarf to sustain life as we know it.

Red Dwarf star and planet. Artists impression (NASA)

Interstellar travel probably conjures up some outlandish theories about the technology we could use to get there. Star Trek‘s warp drive will have to wait and stay in the “sci-fi” category for now, it is more likely any deep space trip will take generations rather than a few days. So, starting with one of the slowest forms of space travel, how long will it take to get to Proxima Centauri? Remember, this is all conjecture as there is currently no benchmark for interstellar trips…

Slowest: Ion drive propulsion, 81,000 years
Ion drive propulsion was considered to be science fiction only a few decades ago. In recent years however, the technology to support ion propulsion has moved from theory and into practice in a big way. The ESA SMART-1 mission for example successfully completed its mission to the Moon after taking a 13 month spiral path from the Earth. SMART-1 used solar powered ion thrusters, where electrical energy was harvested from its solar panels and used to power its Hall-effect thrusters. Only 82 kg of xenon propellant was used to propel SMART-1 to the Moon. 1 kg of xenon propellant provided a delta-v of 45 m/s. This is a highly efficient form of propulsion, but it is by no means fast.

The SMART-1 mission, slow, but economical (ESA)

One of the first missions to use ion drive technology was the 1998 Deep Space 1 mission to Comet Borrelly. DS1 also used a xenon-powered ion drive, consuming 81.5 kg of propellant. Over 20 months of thrusting, DS1 was designed to reach a cometary flyby velocity of 56,000 km/hr (35,000 miles/hr).

Ion thrusters are therefore more economical than rocket technology as the thrust per unit mass of propellant (a.k.a. specific impulse) is far higher, but it takes a long time for ion thrusters to accelerate spacecraft to any great velocity. As the maximum velocity of ion thruster-powered spacecraft depends on the amount of fuel it can carry and the amount of electricity it can generate, although slow, if ion thrusters were to be used for a non-time critical mission to Proxima Centauri, the ion thrusters would need a huge source of energy production (i.e. nuclear power) and a large quantity of propellant (although not as large as less-economical forms of space travel, such as rockets). As interstellar ion engines do not exist yet, I will quickly calculate how long it would take for an interplanetary ion engine spacecraft, like Deep Space 1 to travel to our nearest stellar neighbour.

Ion engine test (NASA)

Assuming all the 81.5 kg of xenon propellant translates into a maximum velocity of 56,000 km/hr (assuming there is no other forms of propulsion, such as a gravitational slingshot, and this velocity remains constant for the duration of the journey), Deep Space 1 would take over 81,000 years to travel the 4.3 light years (or 1.3 parsecs) from Earth to Proxima Centauri. To put that time-scale into perspective, that would be over 2,700 human generations. So I think we can categorically say, interplanetary ion engine mission speeds are far too tiny to be considered for manned interstellar missions. But, should ion thrusters be made bigger and more powerful (i.e. ion exhaust velocity would need to be higher), with enough propellant for the spacecraft’s entire 4.3 light year trip, the 81,000 years would be greatly reduced.

Fastest: Gravitational assists, 19,000 years
The Helios solar mission (Deep Space Network)
The 1976 Helios 2 mission was launched to study the interplanetary medium from 0.3AU to 1AU to the Sun. At the time, Helios 1 (launched in 1974) and Helios 2 held the record for closest approach to the Sun. However, to this day, Helios 2 holds the record for fastest ever spacecraft to travel in space. Helios 2 was launched by a conventional NASA Titan/Centaur launch vehicle (the craft itself was built in Germany) and placed in a highly elliptical orbit. Due to the large eccentricity (e=0.54) of the 190 day solar orbit, at perihelion Helios 2 was able to reach a maximum velocity of over 240,000 km/hr (150,000 miles/hr). This orbital speed was attained by the gravitational pull of the Sun alone.

Gravitational assists are a very useful spaceflight technique, especially when using the Earth or massive planets for a much needed boost in velocity. The Voyager 1 probe for example used Saturn and Jupiter for gravitational slingshots to attain its current 60,000 km/hr (38,000 miles/hr) interstellar velocity. Technically, the Helios 2 perihelion velocity was not a gravitational slingshot, it was a maximum orbital velocity, but it still holds the record for being the fastest manmade object regardless.

So, if Voyager 1 was travelling in the direction of the red dwarf Proxima Centauri, how long would it take to get there? At a constant velocity of 60,000 km/hr, it would take 76,000 years (or over 2,500 generations) to travel that distance. And what if we could attain the record-breaking speed of Helios 2’s close approach of the Sun? Travelling at a constant speed of 240,000 km/hr, Helios 2 would take 19,000 years (or over 600 generations) to travel 4.3 light years.

Again, these speeds are prohibitively slow for any quick forms of transportation to the stars. Other technologies are required (wormholes, warp drives and teleportation will remain in the “sci-fi” drawer for now)…

Fastest (theoretical): Nuclear Pulse Propulsion, 85 years
Project Orion, using nuclear explosions as a propellant (NASA)
Nuclear pulse propulsion is a theoretically possible form of fast space travel. Very early on in the development of the development of the atomic bomb, nuclear pulse propulsion was proposed in 1947 and Project Orion was born in 1958 to investigate interplanetary space travel. In a nutshell, Project Orion hoped to harness the power of pulsed nuclear explosions to provide a huge thrust with very high specific impulse. It is a major advantage to extract maximum energy from a spacecraft’s fuel to minimize cost and maximize range, therefore a high specific impulse creates faster, longer-range spaceflight for minimum investment.


For archived prototype video of pulsed propulsion using conventional explosives, watch this video »

The Partial Test Ban Treaty of 1963 is largely attributed to the cancellation of Project Orion (due to the obvious design flaw that huge amounts of radioactive waste would be pumped into space), but what kind of velocities could a nuclear pulse propulsion spaceship attain? Some estimates suggest a ballpark figure of 5% the speed of light (or 5.4×107 km/hr). So assuming a spacecraft could travel at these speeds, it would take a Project Orion-type craft approximately 85 years to travel from the Earth to Proxima Centauri.

In conclusion, if you were hoping to travel to the nearest star within your lifetime, the outlook isn’t very good. However, if mankind felt the incentive to build an “interstellar ark” filled with a self-sustaining community of space-faring humans, it might be possible to travel there in a little under a century if we developed nuclear pulse technology. So your descendents may touch down on a planet closely orbiting Proxima Centauri, but unless we make a breakthrough in interstellar travel (and science fiction becomes more like science fact), we’ll be stuck with long-term, pedestrian transits for the foreseeable (and distant) future…

Sources:
NASA
ESA SMART 1
NASA Helios 2

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Hello! My name is Ian O'Neill and I've been writing for the Universe Today since December 2007. I am a solar physics doctor, but my space interests are wide-ranging. Since becoming a science writer I have been drawn to the more extreme astrophysics concepts (like black hole dynamics), high energy physics (getting excited about the LHC!) and general space colonization efforts. I am also heavily involved with the Mars Homestead project (run by the Mars Foundation), an international organization to advance our settlement concepts on Mars. I also run my own space physics blog: Astroengine.com, be sure to check it out!

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