To infinity and beyond: the technologies that could take us interstellar

Ryland Grace has a big problem. He’s just woken up on a spaceship with a serious case of amnesia. To make matters worse, that spaceship is in a star system 11.9 light years from Earth – that’s more than 100 trillion kilometres away. How did he get there? 

So opens the recent sci-fi blockbuster Project Hail Mary, based on the book by Andy Weir. (Very mild spoilers ahead.) 

In that story, the key is a fuel called astrophage, which can store huge amounts of energy in the tiniest fractions of a gram. The incredible energy density of this miracle fuel can accelerate Grace and his spaceship to almost light speed, making interstellar travel possible within a human life span. On today’s existing spacecraft, that same journey would have taken hundreds of thousands of years.

Astrophage is the stuff of science fiction. But if we wanted to make the same journey, what near- or distant-future technology might take us there? What would be the best kind of fuel? Before we can start answering these questions, we’ll need a little rocket science.

Up, up and away

There’s only one way to escape the clutches of Earth’s gravity – with an incredibly controlled and sustained explosion.

For most of the history of rocket science, that has meant loading your launch vehicle with a mixture of a fuel and an oxidiser, often liquid hydrogen and liquid oxygen, igniting it, and standing well back. Liquid hydrogen has long been a rocket fuel of choice. That’s because of its lightness, and the fact that it burns extremely hot (producing faster exhaust gases).

Different propellants provide different amounts of thrust, which is measured using a quantity called specific impulse. Simply put, specific impulse measures how efficiently an engine produces thrust from fuel.

The amount of thrust you need from your propellant – that’s the term for a mixture of fuel and oxidiser – to get into orbit (and beyond) depends on your ultimate destination. The magic number is delta-v, defined as the change of velocity of your spacecraft. So, a low Earth orbit (LEO) mission, which would cover satellites or spacecraft orbiting no more than a few thousand kilometres above the Earth’s surface, won’t need as big a delta-v as a mission to the Moon or beyond.

Those who want to travel to space, but don’t need to reach as high as LEO, can hitch a ride on Virgin Galactic’s spaceplane. Before 2024, that would have been a vehicle called SpaceShipTwo. “It’s essentially an aircraft with a rocket engine attached,” says David Mackay, Virgin Galactic’s chief pilot, who flew SpaceShipTwo three times between 2019 and 2021, and was the first Scot to enter space.

SpaceShipTwo is carried to around 45,000 feet by its mothership, a dual-fuselage plane called WhiteKnightTwo, before being released to complete its journey into space. “For a handful of seconds, you’re a glider,” says Mackay. Everything falls silent. “Then we light the rocket motor and you suddenly get this big push in the back as we accelerate very quickly indeed.”

SpaceShipTwo runs on a solid fuel and a liquid oxidiser, Mackay explains. “It has about the same level of simplicity as a solid rocket motor, but with safety in that we can shut it down at any time.”

After 60 seconds, you’re flying at three times the speed of sound, and then, as the motor shuts down and the spaceplane climbs to an altitude of almost 90 kilometres, you experience weightlessness. As pilot, Mackay remains strapped into his seat, though his arms drift upwards. At this altitude, it’s possible to see so much of the curvature of the Earth that you get a sense of its scale, and also see the thin layer of atmosphere enveloping the planet. “It was shocking how thin it is,” Mackay says. “You look at that and you think that’s what’s keeping everything on planet Earth alive, and it’s beautiful.”

That sustained inferno that transforms thousands of tonnes of liquid propellant into millions of newtons of thrust is the only way we can leave Earth. When we’re out in the vacuum of space, however, our propulsion needs are quite different. We need an engine that can keep pootling along for millions, even hundreds of millions of kilometres. For such missions, you need an electric propulsion system.

It’s electrifying

Many of the spacecraft orbiting Earth have electric propulsion systems, which can squeeze years of service out of a relatively small amount of propellant.

In electric propulsion systems, a gas, usually xenon or krypton, is ionised by electricity, and the ions are accelerated out of the engine to create thrust. Xenon and krypton are chosen as they’re relatively massive (which according to F = ma, provides more thrust), and are fairly easily ionised. Although these systems produce only millinewtons of thrust, over the course of an entire mission, it adds up to a healthy delta-v.

“It’s very efficient,” says Emily Longhi, chief of operations for Mars Space, which makes bespoke electric propulsion systems for satellites and other spacecraft. One of Mars Space’s thrusters is onboard BepiColombo, a mission to Mercury that’s due to arrive at the Sun’s nearest planetary neighbour before the end of the year.

“That’s the main reason that it’s chosen for a lot of missions – you can get the most [specific impulse],” she adds. This is important for years-long projects where space for propellant is at a premium. NASA’s Dawn mission, which sent an orbiter on a multi-stop trip to dwarf planet Ceres and giant asteroid Vesta, used just 425 kilograms of xenon on its 6 billion kilometre journey.

To save propellant on long journeys through space, many missions use gravity assist trajectories, where the spacecraft picks up some angular momentum (or a bit of a kick) from the planets it swings past. Such trajectories rely on optimal alignment of the planets, which can mean waiting around. In fact, Voyager 2 could only take its tour of the outer four planets thanks to an alignment that happens every 175 years.

Spacecraft with electric propulsion systems are under continuous thrust, making it possible to use unconventional, and crucially, shorter, paths from Earth to our destination, says Aaron Knoll, associate professor in spacecraft engineering at Imperial College London. This opens new opportunities and greater flexibility for crewed space travel. 

“If you’re planning a manned space exploration mission to Mars, you want to try to cut down your transit time as much as possible,” he says. “You also don’t want to be bound by the fact that you have to wait for the orbits to synchronise exactly for your return trip.” 

With their impressive efficiency, electric propulsion systems are excellent for satellites and spacecraft that need to run for a long time. Though if we want to go yet further, there are engines that can provide even more of a kick.

Going nuclear

For missions where covering distance is important, nuclear propulsion systems provide a good option.

There are two main types of nuclear propulsion: nuclear electric propulsion (NEP), where thermal energy from a nuclear reactor is converted into electrical energy to power an electric propulsion thruster; and nuclear thermal propulsion (NTP), where a nuclear fission reactor heats a liquid propellant, such as hydrogen.

NEP systems are best for very high delta-v missions where efficiency matters more than thrust, and where continuous thrust is required over many years, says Davina Di Cara, a senior electric propulsion engineer at ESA. This would include deep space robotic exploration, such as to the outer planets, and large cargo missions to the Moon and Mars. For missions where speed isn’t as important, but you want to go far, NEPs are your pals.

NASA plans to put an NEP propulsion system to the test with Space Reactor-1 Freedom, which will launch to Mars in 2028. 

If keeping transit times down is crucial, however, such as for crewed missions to Mars and beyond, NTPs are the way to go.

While nuclear propulsion systems can potentially take us to the edges of the Solar System, the fuel is still going to run out, making covering interstellar distances impractical. So what if we want to go really far? With fuel being the main limitation, what if we didn’t even need to carry it at all?

Sailing on sunlight

For the longest journeys, instead of packing propellant for our trip, finding ways of harnessing energy en route will be the key.

Out in space, there’s one clear source of energy. The sun is pumping out billions upon billions of photons every second. These “quantum packets of energy”, when bouncing off a large flat structure, say, a reflective sail, exert a tiny amount of pressure, explains Colin McInnes, James Watt Chair, professor of engineering science at the University of Glasgow and a Royal Academy of Engineering Chair in Emerging Technologies.

This is the principle behind solar sail technology, which uses this pressure from the Sun to propel spacecraft. “The pressure is extremely small, so a solar sail has to be large and ultra-lightweight, but over a period of weeks, months or years, that very small pressure is constantly pushing the solar sail,” McInnes explains.

Solar sails are made of Mylar or another type of film called Kapton, which are coated in aluminium to make them reflective. It’s the same kind of material as a space blanket, only thinner.

“Solar sails could be used to enable high-energy missions, which are essentially impossible for chemical or ion propulsion,” McInnes says. As long as you have photons pushing the sail, you can “keep on going for as long as you want,” he says.

As the sun’s light grows weaker, however, you’re going to need another source of light, such as a laser, to keep the spacecraft accelerating. That was the concept behind Breakthrough Starshot, a proposed mission to reach the nearest star system, Alpha Centauri, 4.3 light years away, in just 20 years.

The idea was to propel tiny spacecraft, weighing no more than a paperclip, to 20% the speed of light by pointing high-powered lasers at them. The tiny probes were fitted with sails that open to about 16 square metres.

While Breakthrough Starshot has been put on hold indefinitely, NASA is two years into a mission testing the feasibility of solar sails for deep space missions. Its Advanced Composite Solar Sail System aims to review how well the sail’s shape and design works.

For interstellar missions, solar sails look to be the best option. But how soon could human beings ever cover such huge distances?

A long way to go?

As the saying goes, prediction is very difficult, especially if it’s about the future. On 9 October 1903, an article ran in the New York Times predicting that human flight was up to ten million years away. Less than ten weeks later, Orville and Wilbur Wright achieved the first ever powered flight in a heavier-than-air aircraft. As this tale illustrates, who’s to know what’s right around the corner? 

“I think we’re at the stage that Leonardo da Vinci was, where he was drawing a helicopter,” says sci-fi author and astrophysicist Alastair Reynolds. “He had the right general idea, but he didn’t know about electricity. He didn’t know about the internal combustion engine.”

What would we need to go interstellar? Reynold continues: “If we separate out the extreme physics like warp drives, wormholes and tachyons, and consider what we can do with the constraints of physics as we know today, I think interstellar travel is probably technically achievable if you’re prepared to accept relatively long crossing times.”

Knoll adds: “Innovations in propulsion might come along that we haven’t anticipated that could change the way we do interstellar spaceflight.”

Stephanie Barron, nuclear power engineer at ESA, says: “Interstellar travel is something we can only dream of at this point – and the hurdles that will need to be overcome should not be underestimated.” She adds: “The overall cost is such that it would likely require a global effort.”

As far into the future as interstellar space travel might be, efforts made today may translate into gains for life on Earth. “Many of these technologies feed back into energy systems, nuclear safety, and high-performance engineering on Earth,” says Jamila Mansouri, head of propulsion, aerothermodynamics and flight vehicle engineering division at ESA.

“It’s important in its own right to think about these big ideas for the future,” McInnes says. “It stretches our imaginations, gives us a sense of where current technologies could develop, and where they could ultimately go.”

Indeed, maybe covering the physical distance isn’t the most important thing. Rather, it’s how far the magnitude of the challenge can push our thinking about what might be possible.