As scientists from Project Icarus work on ideas to make interstellar travel a reality Paul French asks: how on Earth are they going to do it?
In 1973 the British Interplanetary Society launched Project Daedalus, aiming to establish whether interstellar travel might be possible. Five years later, the project team concluded that it would be feasible, by using current or credible extrapolations of existing technology, to launch an interstellar probe that could reach another solar system on timescales of a normal human lifetime.
Now the society, in collaboration with the US non-profit Icarus Interstellar, is reaching the end of another project, known as Icarus, which has sought to build on the work of Daedalus and bring interstellar travel closer to reality.
Travelling across light years
The main challenge facing the Icarus team is obvious: with the nearest star system, Alpha Centauri, more than four light years away, how can you build something that will get there within the life span of the people involved in the project?
“One of the biggest challenges is creating the energy required,” says Icarus project leader Rob Swinney. “The nearest star is four light years away. If you could travel at the speed of light it would take four years to get there, but to even go at even a fraction of that speed takes a phenomenal amount of energy.
“Chemical rocket powered engines don’t cut it so the Project Icarus team has been looking into designing an unmanned probe that would use fusion technology. That would allow us to go at ten percent of the speed of light, which would mean we could get to the Alpha Centauri in 44 years. Fusion reactors don’t exist yet but the science is well understood and the engineering solution is probably only decades away.”
Project Icarus was launched by the British Interplanetary Society in 2009 in conjunction with the Tau Zero Foundation. For the first couple of years, the conglomeration of over 30 scientists and engineers investigated the problems associated with interstellar travel.
“Since then we’ve been working on creating a credible design for an unmanned craft that can overcome those problems,” explains Swinney. “At the moment we have four different possible designs and two possible engine types. We’re currently trying to narrow it down to one design.”
“One of the biggest challenges is creating the energy required”
The need for speed
Back in the 1970s the Project Daedalus team identified inertial confined fusion (ICF) as the best way of propelling their probe quickly enough to negate the issues of time and distance to the nearest star. The Icarus team has sought to build on and refine this approach.
“The Daedalus ICF design basically involves using an electron beam to hit a pellet of fuel and a magnetic field to draw it out of the exhaust,” Swinney explains. “The Daedalus team discarded lasers because the technology wasn’t that advanced then, but it has come on in leaps and bounds since. That’s why we’ve decided to base our designs around laser ignition. So you’d put fuel pellets into the reaction chamber, hit them with a laser and use superconductors to create a strong magnetic field to force the plasma out of the exhaust. Some of the energy is then captured to ‘bootstrap’ the next cycle.”
The other potential engine design the researchers are looking into uses a Z-pinch concept. Swinney explains: When a lightning rod on a building is hit by a lightning strike and a large current is discharged, you’d expect it to be smashed. However, the huge current creates a magnetic field around the rod that creates an inward force so strong it actually crushes the rod. We’re looking into whether we could use that force to squeeze a plasma stream enough to fuse the fuel rather than the pellets and laser.”
On a project as complex as Icarus it is almost inevitable that as one door opens, another closes. Managing to solve the issue of creating enough energy to send the probe interstellar at a reasonable speed creates a range of other headaches.
“Another major problem is the mass of the probe,” says Swinney. “The Daedalus probe had an all up mass of over 54,000 tonnes with a payload of 450 tonnes and we want to make Icarus smaller but if anything it is likely to be bigger with a smaller payload.
“A reason for this is that Daedalus was a fly-through probe. Our intention is to decelerate Icarus into orbit around the target star, which requires even more fuel and adds even more mass onto the probe.”
Handling the heat
For engines to work effectively they must create an enormous amount of heat. This is a hard enough problem to solve for conventional spacecraft, says Swinney, let alone one creating enough energy to fly interstellar missions.
“Heat is hard to get rid of in the vacuum of space but you need to do it if you don’t want to fry your equipment,” he explains. “Most spacecraft currently use radiators to radiate energy into space but that would be harder for us if we’re using fusion reactors because they’ll generate even more heat. Adding in more radiators to deal with this could add significant weight to the probe.
“One theory we’re exploring to overcome this is to use liquid droplet radiation. Essentially we’d pump liquid drops into space, collect them once they’ve cooled and re-use them as part of the cooling process.”
“Heat is hard to get rid of in the vacuum of space”
There are lots of tiny dust particles far out in space. As high-speed collisions could potentially prove fatal, shielding is an important aspect of any interstellar probe design.
“If you were to hit dust particles whilst travelling at ten percent of the speed of light, they could easily destroy your machine,” says Swinney. “Project Daedalus looked into the idea of firing particles out the front of the probe that could vaporise the dust. However, they also designed a shield to go on the front of their probe and we concluded that it would be enough to protect it, so will incorporate that into our design.”
Navigating deep space
Navigation is surprisingly simple for solar system missions. NASA has a deep space network that allows spacecraft to know how far from Earth they are, and also uses star sensors for attitude control. However, all that will go out the widow once you exit the solar system.
“Navigation will be different,” says Swinney. “The nearest star is beyond the deep space network and it will be harder to navigate because the local stars that appeared fixed before will move. However, with some clever algorithms we think we’ll be able to take this into account and build a system that can find its way around.”
If you’ve ever complained about a mobile phone signal in a remote part of the world, spare a thought for a probe designed to go interstellar. At four lights years from Earth, how do you hope to beam a signal back?
“Transmission rates get slower and slower for probes in the outer solar system,” says Swinney. “The problem for us is once you get out to the nearest star, how do you transmit back to Earth?”One idea we’re looking at is gravitational lensing. Basically, you can use a heavy object to bend light and see things further away.
“The sun has its own gravitational lensing point. We think we may be able to use it to magnify a transmitter and boost it back to the deep space network. That could be one of the first precursor missions – to send a probe out to the sun’s gravitational bending point and see if it works.”
“Transmission rates get slower and slower for probes in the outer solar system”
Looking to the future
Project Icarus has inspired further study into interstellar travel. Icarus Interstellar is a non-profit organisation launched in the US to help manage Project Icarus and other related projects and in 2012, the US Defense Advanced Research Projects Agency funded the 100 Year Star Ship project with the intention of making the capability for human interstellar flight a reality within 100 years.
“There’s now a community across the world looking into this,” says Swinney. “I suspect that there will be half a dozen or so problems that will drop out of Icarus. We’d then hope to influence people with money like the national agencies into investing in some precursor missions that could help to solve those problems.”
Fusion technology is decades away, but sending a probe could happen sooner than we think. The Japanese Space Agency, for instance, is currently flying a probe around the solar system using solar sails, which are covered in reflective material and use the sun’s light for propulsion.
“The problem with fusion is the amount of fuel,” Swinney explains. “Solar sails could take away that problem but the force they produce is tiny so another thing we’re looking into is the possibility of a beam-driven sail. It might be possible for small payloads and that technology is much closer than fusion. If you could combine it with a nuclear-electric engine it might be possible to send a probe within the next ten years.”
So, will people one day be able to travel between the stars? “Personally, I think they will,” Swiney says. “We’re not that far away from living and working in the solar system. I think from there we’ll progress further out. We underestimate how much we can achieve. Just over a hundred years ago we were building planes out of old bicycle parts but 60 years later we put a man on the moon.”
Images courtesy of Icarus Interstellar by Nick Stevens/Robert van der Veeke/Adrian Mann