Interstellar travel – the ability to move beyond our Solar System and out into the stars – has long been on humanity’s bucket list, but until now it’s been stumped by the seemingly insurmountable vastness of space.
Our nearest star, Proxima Centauri, is a staggering 4.24 light-years away (40 trillion kilometres, or 25 trillion miles). Even travelling at its current speed of around 56,000km/h, it would take NASA’s Voyager 1 spacecraft almost 73,000 years to cover such a distance.
By all accounts, that’s just too long to wait.
But on 23 April, NASA took one giant leap towards viable interstellar travel with the launch of its Advanced Composite Solar Sail System from a launchpad in New Zealand.
Experts and sci-fi fans alike are holding their breath to see whether this new system, which uses lightweight sails instead of rockets to propel the craft through space, could kick down the door to a new era of long-distance space travel.
How solar sails work
Most spacecraft, Voyagers 1 and 2 included, are pushed through space using thrusters, with the momentum of fuel being ejected pushing the spacecraft along.
These systems work really well, that is, until you run out of fuel. This puts a major constraint on speeds that can be achieved, especially given that fuel is heavy and spacecraft need a whole lot of it just to escape Earth's gravity.
But what if there was an external, near-unlimited supply of fuel out there? Well, as it turns out, there is – the Sun.
Light from the Sun (as with all light) is made up of particles called photons. And although these have no mass, they do have momentum. Using massive, reflective sails, spacecraft can actually absorb this momentum as the photons hit and bounce off them.
Down here on Earth you wouldn’t get very far using this energy – hence why no one has solar sailed across the Atlantic – but in space where there is no air resistance, the slight push from the Sun is all you need.
“Although each photon has just a tiny little bit of momentum, the Sun produces a whole lot of photons. And so over time, although you accelerate comparatively slowly, you get continuous acceleration for as long as you can see the light from the star,” Prof Patrick Johnson, author of The Physics Of Star Wars, tells BBC Science Focus.
Other than that, such systems work in a similar way to a typical sailing boat – you can even tack ‘upwind’ back towards the Sun if you fancy it.
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How fast could a solar-sailing interstellar explorer travel?
The ability of a solar sail system to accelerate its craft relies on three components: the size of the sail, the mass of the spacecraft and the distance from the Sun.
The first two components are relatively simple to address. All you need to do is make a solar sail that is as big as possible and make the whole thing reasonably light.
The distance from the Sun is a little trickier to navigate. Light intensity from a source like the Sun follows what’s known as an inverse-square law, meaning each time you double your distance from the Sun, the amount of sunlight hitting a given area falls by a quarter.
But scientists have come up with a few ingenious ways to get around the distance problem.
One is known as a slingshot manoeuvre, something a lot of spacecraft already do to navigate the Solar System. NASA estimates that by slingshotting a small satellite around the Sun at a distance of around two to five solar radii (1.4 million - 3.5 million kilometres), speeds over 108,000km/h could be achieved – about double what Voyager 1 is moving at.
Still, we can do better. In 2016, Breakthrough Initiatives announced its plans for a mission to the Alpha Centauri triple star system using powerful lasers that could give spacecraft an extra boost.
In theory, these spacecraft could reach speeds approaching a mindblowing 20 per cent of the speed of light (216 million km/h). That would put Alpha Centauri within reach in just 20 years.
“It’s amazing and ridiculous and awesome to think that we could get a human-made spacecraft to another star system in my lifetime,” Johnson exclaims. “As far as I know, we have all the technology we need; we just need the funding to be put towards it.”
Despite his excitement, Johnson notes that there are still a few details that need to be ironed out. “I don't know the implications of what might happen if we pointed our most powerful laser at one of these things. We might have to worry about the beam reflecting back to Earth and scorching a forest or something. Obviously, we don't want to do that.”
Could humans one day sail to another star?
In principle, this technology could one day enable humans to be an interstellar species, Johnson says, although we’re some way away from that.
The difference between sending a small unmanned satellite and what would need to be an intergenerational human mission is pretty stark.
“We’d need a much bigger ship, capable of bringing a century’s worth of infrastructure to produce food and medical supplies, plus all the other considerations.”
There would also be relativistic effects to take into account since time will move comparatively more slowly for humans travelling at a significant fraction of the speed of light. At 20 per cent of the speed of light, for example, “the effects would be small, but not non-negligible,” Johnson says.
According to Johnson, the biggest barrier is the cost. While monumental now, he hopes the costs could come down in the not-so-distant future once human colonies on the Moon and Mars are established.
What exactly is NASA’s solar sail mission?
NASA’s Advanced Composite Solar System is designed as a demonstration of solar sail technology to test a new lightweight boom made of flexible polymer and carbon fibre materials.
After reaching an orbit about 1,000 kilometres (600 miles) above the Earth, the 12U-sized CubeSat – a class of nano-satellites about the size of a microwave oven – will begin deploying its sails.
In a process that takes around 25 minutes to complete, the satellite will deploy sails spanning 80 square metres (just under half the size of a tennis court). Cameras on board will capture the deployment, so expect some epic footage in the coming days.
The large reflective sail will make the spacecraft easily visible from Earth – it might even be as bright as Sirius, the brightest star in the night sky.
“Seven metres of the deployable booms can roll up into a shape that fits in your hand,” the mission’s lead systems engineer, Alan Rhodes, said in a NASA post about the mission.
Once operational, the NASA team plans to test a series of manoeuvres with the sails designed to change the spacecraft’s orbit and gather valuable data for future solar sail missions.
The hope is that this boom technology could soon support basketball court-sized solar sails in the near future and that the technology resulting from the missions’s success could even be applied to sails the size of half a football field someday.
About our expert
Patrick Johnson is an associate teaching professor at Georgetown University’s Department of Physics and author of The Physics Of Star Wars. Earning his PhD at Washington University in St Louis, Johnson’s work on quantum mechanics has been published in the Physical Review journal.
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