When should we try to visit other stars?

We're feeling some wanderlust right now

In 1977, NASA launched the Voyager 1 space probe to study the outer solar system and beyond. It provided incredible data and images of Jupiter in 1979 and Saturn in 1980 but was never to return home. Voyager 1 is already far beyond the orbits of our planets and is hurtling through interstellar space at 40,000 mph making it the fastest object we’ve ever created. In 1990, astronomer and author Carl Sagan asked for the satellite to be turned so that one last photograph could be taken of Earth. In a book about the now-famous photograph, titled The Pale Blue Dot, Sagan described astronomy as “a humbling and character-building experience.” All of human history took place on the dot in the following image, which really puts things into perspective.

The image conveys the sheer scale of our solar system. From far beyond the orbit of Saturn, Earth is just a dot. Our planet feels incredibly vast since none of us will truly explore it all, yet the Earth is nothing compared to the entire solar system. In that same sense, the solar system is nothing compared to the vastness that Voyager 1 will witness. Eventually, in the far distant future, the space probe will encounter another solar system somewhere in our galaxy. The distances to other stars are almost beyond comprehension.

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy

Nearly 40 years into its journey, Voyager 1 still has a long way to go before it truly leaves the solar system. It will be another 300 years before it passes through the Oort cloud. Voyager 1 doesn’t have an actual destination but let’s imagine it’s heading straight for the closest system to our own, the Alpha Centauri system of 3 stars about 4 light-years away. This distance is changing and within approximately 25,000 years will be closer to 3 light-years away. Remember that a light-year is a measurement of distance, not time. It would be lovely if we could reach the stars within 3 years!

After 26,000 years, the star system will begin to move further away from us again. If Voyager was on its way and hoping to reach the system when it’s at its closest, it would take approximately 75,000 years. That’s the fastest spacecraft we’ve ever made. In reality, Voyager isn’t heading to Alpha Centauri so it could be millions of years before it reaches another system assuming it even lasts that long. But what about future spacecraft? Alpha Centauri makes sense as the first system to try and visit if we’re ever going to leave the confines of our own system. This raises an interesting question: how long will it be until we reach other star? Could we send a probe there faster than Voyager 1? Could we ever send humans? Could we someday be an interstellar species?

Probing the stars

Voyager 1 got a huge speed boosts by using the gravity of both Jupiter and then Saturn. The much newer New Horizons spacecraft could be faster but has only used Jupiter’s gravity so it could never catch up with Voyager. Right now it’s travelling 8.7 miles every second, which seems pretty fast! Sadly it would still take 100,000 years to get to the Alpha Centauri system. If we’re ever going to reach the nearest stars faster than that, we’re going to need to use some sort of propulsion system combined with the gravitational assist from nearby planets.

NASA's Dawn spacecraft, using ion propulsion. Image © NASA

The best propulsion technology we currently have is ion propulsion, which is being by NASA’s Dawn spacecraft. Dawn is designed to reach the tiny dwarf planets and is currently orbiting Ceres and sending back spectacular images. Having its ion propulsion thruster means it can escape the orbit of Ceres and continue to another dwarf planet. If we combined a power ion propulsion thruster with gravitational assist using the sun itself, we could cut the journey from 100,000 years to something nearer to 20,000 years. It isn’t encouraging for anyone wanting to see close-up images of our nearest neighbours any time soon.

To boldly go…

Imagine if instead of probes we had huge spaceships capable of carrying thousands of millions of humans to our nearest star. Maybe it’s unrealistic to think that a civilisation could survive that long regardless of whether they’re on a spacecraft or not. But this is a thought experiment so just roll with it. Let’s assume we have a spacecraft that can support a civilisation for the incredibly long journey to our nearest star.

If the ship uses current technology, such as ion propulsion combined with gravity assist from the sun, many generations will pass on the ship before it eventually gets to its destination. It will take tens of thousands of years. Perhaps we’ll develop better propulsion before the journey begins.

The most far-fetched but real research taking place is NASA’s Alcubierre Drive, which is entirely theoretical for now. The drive is supposed to let spacecraft travel faster than the speed of light but without actually travelling faster than the speed of light. Yes you read that correctly. No local speed rules would be broken; instead the ship would travel by contracting space in front of it and expanding space behind it. Nothing can travel faster than the speed of light in normal space, but the Alcubierre Drive would warp spacetime itself. Yeah, there goes your mind.

It’s probably safe to assume we won’t ever get a real FTL drive working. Our ship is going to have to use something a bit more grounded in reality. NASA and other space agencies are already looking at improvements such as solar sails that use the radiation pressure from stars to accelerate. This technology is exciting for small probes but couldn’t possibly bring a huge, civilisation-carrying ship to another solar system.

Russia is currently working on a rocket that uses nuclear fission to get to Mars within 6 weeks. That’s so fast that it would it get to Alpha Centauri in about 10,000 years. It’s not ideal news for wannabe interstellar tourists but it shows that progress is being made. Unfortunately the main reason their spacecraft is so fast is because it’s so light. In order to move an entire civilisation we would need incredible energy to reach those speeds and it just isn’t something we can currently do.

When should we leave?

For argument’s sake, let’s assume that we’ve developed technology that could get us to the Alpha Centauri system within tens of thousands of years. When is the best time to actually make the journey? The obvious answer seems to be now; it will take so long to get there it makes sense to leave as soon as possible. Once we have a spacecraft capable of carrying a civilisation, we should send is straight away. There’s a problem with this approach though as it ignores what we’ve just been saying about the development of new technologies. It’s very likely that within tens of thousands of years we will have developed dramatically faster propulsion technologies than those used on the ship that has already left.

This is called “wait calculation” and makes the question quite tricky. The earlier you leave, the sooner you can be there. However, our propulsion technologies are improving so quickly that it’s likely we’ll have a faster ship ready soon after the first ship has begun the journey. If the technology advances quick enough, newer ships might even overtake the original ship. The argument here is that we should hold off and invest our energy and resources into new propulsion technologies rather than waste time working on spacecraft that will be overtaken anyway.

This just raises the question all over again: when do we leave? If there’s always a risk that we’ll come up with a better technology, we’ll forever be putting off the journey. Some scientists feel that we should hold off and work on propulsion until we’re getting such incremental improvements that it’s clear we’ve reached our limits and then send the ship. This would put all resources into development and not waste time or money on a ship that will be overtaken anyway.

There are also scientists who feel the complete opposite approach might be the best one to take; we should go as soon as it’s possible even if the journey is slow. In the end, future propulsion technologies might be pointless to use if it turns out such high speeds have negative effects on the travelling civilisation itself. The faster you travel, the more dangerous it is when you collide with tiny debris. The fast speeds could affect human health and we wouldn’t know until we had already begun. With that in mind, it might pay to send a slower ship first. If the technology improves then we can try to overtake it but if there are other problems with travelling at such high speeds, at least we will have made a start with the slower ship.

Really though?

I’m going to call it. We’ll never make it. I think we’ll develop the technology necessary to explore the solar system, but not other systems. Our best bet of reaching another star system while humans are still around long enough to appreciate it would be with tiny nanoprobes. Being so small, nanoprobes would take very little energy to accelerate and could technically accelerate constantly using solar power. Researchers at the University of Michigan are working on thrusters that fire out nanoparticles a bit like how the Large Hadron Collider fires particles around itself and smashes them together.

Nanoprobes will never be able to carry anything useful and we obviously won’t be able to hitch a lift. But it’s believed that nanoprobes could reach speeds near the speed of light meaning they might be our best bet at sending something to other stars during a human lifetime. If we get really good at building nanorobots then perhaps a whole fleet of these tiny probes could travel elsewhere in the galaxy and combine to make some sort of communications receiver so that we could learn more about the distant location. We would surely have to send fleets anyway as it’s likely such tiny probes would frequently be lost to collisions with high-speed dust particles.

If it really is impossible to reach other stars, perhaps because species die out before they can develop the technology to get there, it could be one explanation for why we appear to be alone in the universe. How tragic would it be if intelligent alien life existed out there but we’re all marooned on isolated islands in the cosmos?

For now, we’ll make more progress observing our nearest neighbours rather than travelling there. The Hubble space telescope has taught us so much about the cosmos, and we’re about to replace it with a much more powerful telescope that will be able to look back in time to the earliest days of the universe. As our telescopes improve, we might someday be able to view exoplanets in greater detail. If our new telescopes did allow us to find somewhere similar to Earth, that might be the push we need to start taking the journey more seriously.

Main image © 20th Century Fox