‘This is futuristic stuff’: Will we one day sail through space on light alone?

We’ve all seen the sci-fi movies and perhaps expect that one day space travel will be taken for granted. But what are the realities of travelling such vast distances in space? One possibility would be to sail, like a space-bound Francis Drake, writes Steven Cutts

An artist’s depiction of the Japanese Ikaros spaceprobe in flight (Andrzej Mirecki/CC BY-SA 3.0)

As far as space travel goes, the 2020s are an exciting time. Elon Musk is cutting the cost of blast-off from Cape Canaveral. Low Earth orbit has never been so alluring but once we actually get there, what could we do next and how could we possibly afford it?  

Well, we’ve all seen Star Trek and, in the end, we’re all expecting to see interplanetary travel become routine. If you look at it optimistically, we might be able to get four people back onto the Moon by about 2025. All of this, nearly 60 years after Neil Armstrong achieved exactly the same thing in 1969.  

But what if we wanted to go further than that and with more than four people? What if we wanted to explore the asteroids and the planets in our solar system using real people who were looking to make it affordable and routine? How could we do that?

It isn’t easy. Both the space shuttle and the new generation of manned spacecraft now coming off the production line can only fly into low Earth orbit, a mere 250 miles above our heads. Bear in mind that the Moon is 250,000 miles away and that Mars is never anything less than 50 million miles away. Worse still, deep space is a particularly hostile environment, much more hostile than territory that we have already occupied with our first space stations.  

There may be an answer. For most of the last hundred years, visionary space scientists have been talking about sails. Rather than taking vast amounts of fuel into low Earth orbit and preparing to ignite our fiery engines, we will instead launch some vast space sail, unfold the thing in the airless environment of space and then hook our fearless astronauts to the rigging.

Solar sails are propelled by the pressure created by sunlight. If you can capture enough sunlight, you can achieve a very impressive speed without actually burning any fuel. Better still, you can fly in and out of the solar system as often as you like with the same sail. They have no moving parts and they never wear out. People meeting up with a solar sailing ship in Earth orbit would only need to bring their food and oxygen. As in the days of Sir Francis Drake, a sailing ship requires no fuel.  

Konstantin tsiolkovsky, pioneer scientist in the field of rockets and space travel, with his all-metal airship models in his workshop in Kaluga in 2013 (Sovfoto/Universal Images Group/Rex)

The concept of light pressure has been recognised for more than a hundred years and the legendary Russian engineer Konstantin Tsiolkovsky proposed a solar sailing ship decades before anyone could have possibly built one. Conceptual studies on the solar sail have continued ever since with each generation of forward thinking engineers coming up with a different design. During the 1970s the celebrity astronomer Carl Sagan did his best to popularise the idea but was unable to persuade Nasa to actually build one. Various other technology demonstrators came and went but it wasn’t until 2010 that the Japanese space agency launched Ikaros, a square, not terribly large solar sail that was intended to fly by Venus.  

Incredibly, it worked. At a mere 14 by 14m the square sail on Ikaros had a cross section of 196 square metres. Most people assume that a solar sail could only fly outwards towards more distant planets. Scientists have long understood that we could tack around the solar system simply by altering the angle of the sail to the Sun. Remember that the intensity of sunlight falls off rapidly as you fly further from the Sun so the Japanese plan to fly Ikaros to Venus (which is closer to the Sun than our own planet) actually improved the performance of their machine.  

At the beginning of the 20th century, scientists got their head around the idea that light is a stream of very small particles called photons. Needless to say, nobody has ever seen one in the usual sense of the word but we can infer their presence. Each photon streams out from the sun at the speed of light and comes with a tiny bit of momentum. When lights hits a mirror and bounces off, it actually transfers its momentum to that object. Again, the amount of pressure created in this process is mind bogglingly small, so small that most of us never notice the pressure of light on our own bodies and it wasn’t until well into the 20th century that anybody had accurately measured the impact of a photon. Nevertheless, a satellite in geostationary orbit will eventually drift in to face the continuous impact of sunlight and requires multiple course corrections to stay in position. And remember that a telecommunications satellite isn’t designed to capture the pressure of sunlight, it’s just out there and it finds the pressure impossible to resist.

If we wanted a very large solar sail, something capable of sending a significant cargo to a colony on Mars, then we’d need a sail that was at least 1,000m along each border

Since the pressure from the light is so tiny, the only way to pick up a significant force is to build a huge solar sail with a vast surface area. On the same note, I should probably mention that the sail itself will then be incredibly heavy, so heavy that almost the entire weight of our proposed spacecraft will be the sail. Luckily, outer space itself is pretty damned big and the actual environment is so different from the one we know here on Earth that it is possible to build structures in space that would be impossible here.  

If we built a sail that was 800m wide and 800m long it would pick up a force of just under 5 Newtons, or about one half of one kilogram (In imperial measurements this is about 1lb).

For a structure of this size, one pound of thrust doesn’t sound that exciting. Remember that if we wanted to use this as a means of propulsion we’d also have quite a lot of rigging that linked out cargo to the actual sail and that the weight of both the cargo and the connecting cables would further slow the space craft down. In practise, we’d have to build the rigging out of very fine fibres, much thinner and much lighter weight than the finest fishing line.  

The secret to solar sail science is that the thrust is constant. So long as the Sun keeps shining, the sail will continue to deliver thrust every second of every day of every year. Think about the kind of thrust we get out of a rocket engine. It’s big and spectacular, but it’s all over in a few minutes. If we give ourselves six months to get to Mars then the thrust created by a solar sail is potentially very useful and might enable us to fly through space at incredibly high speeds without ever having to stop and look for a petrol station.  

So just how far could we push this kind of technology?

If you’ve ever tried to wrap a sandwich in cling film you’ll know how difficult it is to mess with a very thin sheet of material. They can easily snag and stick to one each other and the engineers who have looked at solar sail as a concept have long struggled to figure out a way to build one here on Earth, box it up into a small package, and then try to deploy it in the vacuum of space.  

The Ikaros concept relied on inflating a perimeter tube with gas, rather like a self-inflating life raft. The actual cross-sectional area of the sail was modest in comparison to some of the more futuristic designs that people have talked about with but at least the Japanese actually proved the concept and managed to reach Venus.  

If we wanted a very large solar sail, something capable of sending a significant cargo to a colony on Mars, then we’d need a sail that was at least 1,000m along each border and we’d need a material that was 100 per cent reflective and ultra-thin. In pursuit of this objective, various labs across the world have attempted to manufacture such super-thin and highly reflective materials with a laminate of aluminium on plastic being the most popular.  

Materials that are less than 100th the thickness of a sheet of paper have been successfully produced in the lab. During the 1970s, an early Nasa idea involved attaching several rolls of sail material to a spacecraft like a series of carpets. Once in space, the spacecraft would begin to spin and the sails would unroll from the centre by centrifugal force. Ongoing spin would maintain their structural integrity and the force of sunlight captured by each sail would propel the space craft forward.

For the beginning of the journey we could fire lasers at the sail, adding to the natural pressure created by the sun

But in the future, we could probably do better than that. Assuming we had managed to build a significant industrial base in outer space, new and exotic technologies may come into play. Imagine that we could create a large ring, similar to the sort that we might duck in a bucket of soapy water to blow bubbles. Instead of soapy water we fill the ring with a film of wax. Working in the airless environment of space, we proceed to boil a canister full of aluminium until it actually evaporates and forms a gas. Next, we spray the gaseous metal onto the wax from a paint gun. Wait a while for it to condense on the wax and we will have created a veneer of aluminium so thin that it could completely change the future of spaceflight.  

Next, we move the whole thing into a sealed box and heat it to a more modest temperature until the wax evaporates and is recycled, leaving a layer of aluminium that is solid and less than 50 atoms thick. Hey presto, we have just manufactured an incredibly thin space mirror that weighs practically nothing. Better than that, it is almost totally reflective. The material used to build Ikaros weighs 10g per square metre. Our new sail weighs less than 1g per square metre but achieves the same thrust. If we could mass produce them and link them together as a mosaic, we could build a sail with astonishing flight characteristics. Something at least 10 by 10km, constructed by a team of ant-like robots and capable of sending a large manned space craft to Mars and back in a matter of weeks.  

Needless to say, this is futuristic stuff. Several space agencies have looked at sail technology and for the time being they’re focusing on materials we could make here on Earth and unfold in the weightless vacuum of space. When the first sailed spacecraft undocks from the ISS it will seem to linger just a few feet from its launch pad for an agonising length of time. Acceleration for a solar sail is a slow and lengthy process that continues over many months, but in time they could achieve the escape velocity of the Earth (7 miles per second) and more, dramatically reducing the journey time to Mars and the other planets.

Other scientists are being even more farsighted. If you find acceleration an issue with sunlight, why not augment it using powerful lasers? If we put the sail in behind the actual spacecraft then crew and cargo would be shielded from the light. For the beginning of the journey we could fire lasers at the sail, adding to the natural pressure created by the sun, further increasing the acceleration. A few scientists have even proposed this kind of technology for exploring other stars, claiming that we might be able to accelerate exquisitely crafted robotic ships to a significant fraction of the speed of lights by egging them on with lasers.

Solar sailing isn’t quite a reality, but it isn’t going to go away fast and, in the end, the allure of free propulsion will be difficult to resist.