ANU looks to reach Alpha Centauri with laser propulsion

In 2016, Professor Stephen Hawking and Yuri Milner unveiled the Breakthrough Starshot program as part of the Breakthrough Initiatives. The Starshot program is a $100 Million research and engineering program aiming to demonstrate proof of concept for a new technology, with the hope of developing a feasible method of propelling an uncrewed space vehicle at 20% of the speed of light. The goal of the technology and the program is to send this vehicle to Alpha Centauri, a triple star system located just over four light-years from Earth.  

The biggest challenge with travelling four light-years is the time it would take to travel that distance with the standard propulsion systems available today. To reach Proxima Centauri, which is the closest star to the Sun, it would take approximately 78,000 years for the New Horizons probe to get there, if it were travelling in that direction. New Horizons is currently travelling at around 58500 km/h. It is hoped that a laser-based system would be able to propel a space vehicle to such a speed that it could cover the distance in around 20 years.

The Breakthrough Starshot program aims to break down the huge task into more manageable engineering challenges, but it is still a very ambitious project. All of these challenges should be able to be solved using technology either already available or likely to attainable in the near future. 

The program focuses on using laser technology to propel an ultra-light nanocraft, attached to a lightsail up to 60,000 km/s. The challenges in developing such a system range from creating batteries that are under 150mg to the development of a material suitable to create a sail that would need to be several meters across and not melt whilst being heated by the powerful laser required to propel the spacecraft to one fifth the speed of light. 

The team at ANU have been working on a way to focus millions of lasers and get them working together to push the sail and its nanocraft towards its end destination of Alpha Centauri with funding from the Breakthrough Initiatives. 

"To cover the vast distances between Alpha Centauri and our own Solar system, we must think outside the box and forge a new way for interstellar space travel," said the study's lead author, Australian National University astrophysicist Dr Chathura Bandutunga.

According to Dr Paul Sibley, another of the study's authors, “the three big challenges of the whole project [can be] broken down into is building the laser array, [which is] what we looked at. Making the sailcraft itself, which needs to be really small, really light, but also sturdy enough and large enough that it can operate as well as have the instruments on it, which is its own big challenge. And communicating with the sailcraft once it's there because it's a small craft and needs to be able to send data back.”

The team at ANU has been investigating whether it is feasible to use a ground laser propulsion system to accelerate a spacecraft so that it could travel the distance required within a generation, around 30 years. The team is composed of Dr Chathura Bandutunga, Dr Paul Sibley, and Dr Robert Ward, from the Centre for Gravitational Astrophysics, Research School of Physics, and Professor Michael Ireland, from the Research School of Astronomy and Astrophysics.  

According to Sibley, “the real crux of the problem is, we need a lot of lasers in order to give enough push to this lightsail”. In fact, what the team have in mind is something similar to the Square Kilometer Array that is being built in Western Australia. 

“It's not possible to just get one really really big laser, it can't be done. So what you have to do is join multiple lasers together and have them work together well,” said Dr Sibley. 

Whilst there are a number of options for where the laser could go, on the moon or on a satellite, the sheer size and power requirements make an Earth-based system a more practical option. 

“[I]t's funny to use the word easier for this sort of array, but I think it probably will be. So one of the big things is the amount of resources. [It is all] based on Earth [so] you don't need to shift that up into space, which also takes its own orbit. So like the International Space Station that uses maybe around 100 kilowatts of power, and if you need to reach gigawatts of power, you need thousands of times more, which is its own challenge again as well,” said Dr Sibley. 

"The Breakthrough Starshot program estimates the total required optical power to be about 100 GW - about 100 times the capacity of the world's largest battery today,"  said Dr Ward. 

"To achieve this, we estimate the number of lasers required to be approximately 100 million,” he continued. 

But an Earth-based array, also has its challenges and it is these challenges the team set out to overcome. 

"Unless corrected, the atmosphere distorts the outgoing laser beam, causing it to divert from its intended destination," said Professor Ireland. "Our proposal uses a laser guide star. This is a small satellite with a laser that illuminates the array from Earth orbit. As the laser guide star passes through the atmosphere on the way back to Earth, it measures the changes due to the atmosphere.”

"We have developed the algorithm which allows us to use this information to pre-correct the outgoing light from the array,” he added. 

“The outgoing beam is pre-distorted so that once it reaches the sailcraft having passed through the atmosphere, it's converged on to a single point, or indeed whatever shape being we need,” said Dr Bandutunga.

Once the sailcraft is placed into the correct orbit and the solar sail deployed, the laser would be turned on for approximately 10 minutes. That 10 minutes would be enough to propel the spacecraft to one-fifth the speed of light.  

“You start with your laser, you point it up at the sail and we turn on the laser for 10 minutes and that 10 minutes is the window that we have to engage with the sailcraft. That limit is set mostly by the sailcraft moving so fast, it’s moved so far away that you can't put enough laser power on it anymore. It's pretty well on its way to Mars by the end of 10 minutes,” said Dr Sibley. 

"Once on its way, the sailcraft will fly through the vacuum of space for 20 years before reaching its destination. During its flyby of Alpha Centauri, it will record images and scientific measurements which it will broadcast back to Earth,” said Dr Bandutunga.

The idea behind the technology is that once one vehicle is on its way you will be able to send another and another. “[I]t is a single shot that you get, but because the laser is based on Earth you can then have another shot and send a whole train of these [sailcraft] out. So  it's less risky than just sending one and hoping [that it survives the 20-year journey],” said Dr Sibley

Once the sailcraft reaches its destination, it will take another 4 years to get the information back. 

“If you wanted to have a little phone call with the sailcraft, that's for years for each message,” said Dr Sibley, “and that’s each direction, so an 8-year return trip,” added Dr Bandutunga

We know a bit about Alpha Centauri (it has three stars), including a pair called “A” and “B” which are similar to the Sun, and “C”, also known as Proxima, which is a red dwarf star that orbits around the binary pair. Proxima has at least one planet, orbiting within its habitable zone, Proxima Centauri b. But there is a huge amount we don’t know about Alpha Centauri. 

According to Dr Sibley, Alpha Centauri is a good destination to attempt to reach. 

"[I]t's the closest star that's not in the Solar system that we can reach,” he said. “We can only get very few bits of pixels of information from some of the world's best telescopes. So if we were actually able to send a probe there, we can get higher resolution images and actually see what's on those planets.”

“And it tells us not only what's in that Solar system, but also it gives you a bit more insight into what other systems are like all across the Galaxy, and that’s just something we don't have access to if we're limited to just our Solar system,” added Dr Sibley.

“It feels a little bit like we are [working] where science fiction ends and this stuff begins and I think that's a cool place to be working in,” he concluded.

The next step for the project is to test some of the techniques in linking multiple arrays together and trying out the atmospheric correction technique to prove their research.