Article Source: IMechE
It has been 50 years since humanity set foot on the Moon.
Since then we might have expected a continued drive to explore the cosmos, but limitations of money, technology and political will have put the brakes on the space race. In the 50 years since visiting the Moon we have not stepped onto Mars or explored much beyond our solar system.
But that might all be about to change. With rapid improvements in propulsion technologies, a new generation are daring to dream again, this time not of a Moon shot, or even a Mars shot, but a star shot.
One project aiming for the stars is Breakthrough Starshot. Announced in 2016 by Russian venture capitalist Yuri Milner and physicist Stephen Hawking, the project would propel tiny ‘nano-craft’ attached to light sails up to a fifth the speed of light using powerful arrays of Earth-based lasers. The ambitious goal would be to travel the four light years to our nearest stellar neighbour, Alpha Centauri, in just 20 years. From there the craft would beam back images and data on, among other things, the star system’s Earth-like planet, Proxima B.
“The idea is to leave the fuel behind, unlike any rocket that was used in the past,” says Avi Loeb, physicist at Harvard University in Massachusetts and chairman of Breakthrough Starshot. “The amount of power one needs is comparable to what is used for the lift-off of today’s shuttles,” says Loeb. “The difference here is you are delivering that to a single gram of material. The electronics would also weigh less than a gram and that part is already here – we do have a camera, navigation device and communication device that could be packed into a single gram.”
According to the plan, the nano-craft would be delivered into orbit where the light sails – measuring at current estimates about the size of a person – would unfurl. An array of lasers based on Earth would then propel the craft up to 100 million miles an hour within a few minutes. Unfortunately, this won’t be happening next year or even next decade as several technological hurdles stand in the way.
“At the moment we are focusing on three challenges,” says Loeb, “one is the sail, the other is the laser and the third would be communication.”
The laser itself would need to be immensely powerful – around 100GW, according to Loeb. This would be too expensive to achieve with a single laser but research into creating laser arrays made up of many less-powerful lasers looks promising. The challenge is to combine these lasers in a coherent fashion so that their power adds up in a synchronised – or collimated – beam.
Communication is another issue. Not only will it take four years to send data back from Alpha Centauri, the great distance means it will be challenging to focus the beam on a small patch of Earth. “You cannot focus the beam better than the separation between the Earth and the Sun,” says Loeb, “so it will be a very broad beam that will be highly diluted. What we need is a telescope or observatory that will collect that information and analyse it and that is a great challenge.”
A laser array on Earth would fire together to create a powerful beam…
propelling the craft up to 100,000,000mph
One of the most immediate challenges is the sail itself which needs to combine a large number of factors to be successful, such as strength, durability, lightness and efficient reflection. “We need the material to reflect 99.99% of the laser,” says Loeb, “otherwise it will heat up and evaporate.” Diamond is one candidate, while so-called metamaterials – materials engineered at the nano-scale – might also be promising possibilities.
One of the biggest challenges has been creating sails that are stable enough that they don’t spin uncontrollably or fall off the laser beam entirely. At first rather unwieldy spherical-shaped sails seemed a promising solution. But research at Caltech has shown that nano-engineering the surface of the material could provide the stability required. “If you think of a plastic bag on a garden hose, the chances are it’s going to fly off to one side or the other,” says Ognjen Ilic, former researcher with the Caltech team, now an assistant professor at the University of Minnesota. “What we can do is essentially engineer the way the object scatters light such that it wants to stay pinned to the beam of light and can be accelerated in a stable way.”
The key is to create specific nano-scale patterns on the surface of the material which effectively encode their own stability, mimicking different macro-scale geometries, such as a sphere. The technique also works if the light source is a long way away – several million miles in fact. Although it is still at the theoretical stage, Ilic believes it could be a promising solution to the problem of stability.
Breakthrough Starshot might still be up in the air, but another proposed contender for interstellar travel is even more contentious. If correct it might even require the physics textbooks to be rewritten.
The EM Drive, first proposed by engineer Roger Shawyer in 2001, takes a conical chamber and pumps it full of microwaves which bounce around inside the cone. If the geometry of the cone is designed so that it matches the frequency of the waves, resonance will occur which boosts the activity inside the cone further.
According to Shawyer’s description, these waves exert a force against the walls of the chamber and produce thrust. Because the device requires no fuel, needing only electricity to operate, it could be extremely light and therefore break free of the rocket equation – the effective brakes put upon rockets by the mass of their fuel.
Closed box that moves
The only problem is that such thrust should be impossible according to basic laws of physics such as the conservation of momentum. “The thrust is coming out of nowhere,” says Martin Tajmar, head of space systems at Dresden University of Technology in Germany, who is currently testing the EM Drive. “In our understanding there is no radiation leaving the system so it’s a closed box – a closed box that moves. Any physicist will tell you that’s nonsense.” It would be, as another commentator observed, like Han Solo powering the Millennium Falcon by headbutting the dashboard.
This hasn’t stopped Shawyer, who has designed an interstellar probe based on the EM Drive which, he claims, would travel at two thirds the speed of light, reaching Alpha Centauri in just 10 years. NASA has also got involved, building its own version of the EM Drive which, it claims, has produced observable amounts of thrust in tests. This is where Tajmar got involved. “I didn’t take it seriously,” he says, “until I saw a video where this EM Drive was mounted on a test stand which showed rotation.”
Tajmar decided to test it himself using the hyper-sensitive vacuum-encased equipment at his Dresden lab. Initial tests showed that thermal drift could account for some of the thrust. The microwave radiation caused the chamber to heat up and expand, so shifting the centre of gravity which could produce a false thrust signal.
The first generation of Roger Shawyer’s EM Drive
Another possible source of the thrust was the magnetic field produced by the electronics powering the system. “This can interact with the Earth’s magnetic field,” explains Tajmar, “and then you can get a force or torque in your system which is coming from the electronics and not necessarily the thrust itself.” Tajmar’s team has removed the electronics from the chamber, replacing them with RF waves coming from outside. This should remove the source of error, but the new system requires tinkering. Tajmar hopes to have a device that can prove or disprove the EM Drive by the end of the year.
If the former happens the implications for space travel – and physics itself – could be immense. Tajmar however is not optimistic for the EM Drive’s chances. “This is such an extraordinary claim that you must, as Carl Sagan said, have extraordinary proofs,” he says. “You must be really super sure on that. Without that, it’s just a fairy tale.”
So what are the chances of going interstellar this century? The Breakthrough Starshot project has a proposed timeline – a decade to develop the technology, a decade to build a prototype, and a decade to prepare the real system. Loeb recognises this is ambitious but, even if it doesn’t live up to the highest hopes, it will still be a vast improvement on rocket technology. “If we reach only a tenth of the speed of light,” he says, “we can get to Pluto within a week. That is much faster than the New Horizons spacecraft which took almost a decade.”
Compared with NASA’s Moon shot it would certainly be cheaper, costing tens of millions of dollars, according to Ilic, rather than the tens of billions spent on the space race. Given the same critical mass as was put behind the Moon missions, Ilic believes we could achieve a star shot in this generation.
Loeb is more cautious. “It was not obvious that we could land a man on the Moon but 50 years ago we did,” he says. “There was some risk in that but much less of a risk because rockets were used, and the technology was known. Here the technology has to be developed. So my hope is that, even if it doesn’t work out the way we envision it, we will have some by-products that will be very useful for other purposes.”
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