To ship a spacecraft to a different star, 1,000,000 lasers must work collectively
In 2016, Russian-American billionaire Yuri Milner founded Breakthrough Initiatives, a nonprofit dedicated to researching some of the universe’s most enduring mysteries. The most important of their scientific endeavors is Breakthrough Starshot, a proof-of-concept prototype that combines a light sail, nano-vehicle, and directional energy propulsion (also known as a laser) to create one spacecraft capable of driving the next Stern (Alpha Centauri) in our lifetime.
This, of course, poses all sorts of engineering and engineering challenges, not least the amount of energy needed to accelerate the spacecraft to relativistic speeds (a fraction of the speed of light). Fortunately, scientists at the Australian National University (ANU) recently came up with a design for a directional energy array made up of millions of individual lasers positioned above the earth’s surface.
The paper describing their research (conducted with support from Breakthrough Initiatives) was recently published in the Journal of the Optical Society of America B. The team was led by Dr. Chathura P. Bandutunga, a research fellow from the ANU Center for Gravitational Astrophysics (CGA) and included members of the ANU’s ARC Center for Engineered Quantum Systems and the Mount Stromlo Observatory.
The plan for Breakthrough Starshot calls for a gram-scale nano-vehicle equipped with tiny sensors, engines, a camera, and a radio antenna. This nano-vehicle would be pulled by a meter-scale light sail measuring 4 x 4 m (13 x 13 ft), accelerated by a 100 gigawatt (GW) laser array. This could enable the space probe to reach speeds of up to 20% of the speed of light (0.2 ° C) and make the journey to Alpha Centauri in just 20 years.
Together, the ANU team combined expertise in many areas of optics and astronomy, from fiber optic sensors and optical phased arrays to astrophysics and gravitational wave instrumentation. For their study, Dr. Bandutunga and her colleagues different ways of creating a laser array that can generate 100 gigawatts (GW) of optical continuous wave power.
In the end, they found that the best option is to rely on 108 ground-based arrays to act together. Like Dr. Bandutunga recently said in an ANU press release
“In order to overcome the huge distances between Alpha Centauri and our own solar system, we have to think outside the box and break new ground for interstellar space travel. Along the way, the sail will fly through the vacuum of space for 20 years before reaching its destination. During its flyby of Alpha Centauri, it will take pictures and scientific measurements, which it will send back to Earth. ”
Artist’s impression of the laser array used to accelerate starshot. Credit: breakthrough initiatives
Dr. Robert Ward, a co-author of the paper, is also the founding scientist who developed the ANU node for this project. According to Ward, a 100 GW array is not an easy task as it is about 100 times the capacity of the largest batteries in the world today. “To do this, we estimate the number of lasers needed to be around 100 million,” he said. In addition, these lasers would have to act as a single one, focusing on a sail of light no more than 16 m2 (139 ft2) in size.
Another big challenge is measuring the drift of each laser. “We use a random digital signal to scramble the measurements from each laser and decode each one individually in the digital signal processing,” said Dr. Sibley from the ARC Center for Engineered Quantum Systems. “In this way, we can only find the dimensions that we need from a huge mess of information. We can then break the problem up into small arrays and link them together in sections. ”
Then there was the challenge of atmospheric distortion, which is inevitable when relying on ground-based arrays. For this reason, the ANU design requires the use of a beacon satellite (i.e., a guide laser) that is stationed in orbit around the earth and acts as a guide, bringing the entire laser assembly together. As Professor Michael Ireland of the ANU Research School of Astronomy and Astrophysics (another co-author) explained:
“If this is not corrected, the atmosphere will distort the exiting laser beam and cause it to be deflected from its intended target. Our proposal uses a laser guide star. This is a small satellite with a laser that illuminates the array from orbit. While the laser guide star crosses the atmosphere on its way back to earth, it measures the changes caused by the atmosphere. We have developed the algorithm that enables us to use this information to pre-correct the outgoing light from the array. ”
Star Shot approaches Alpha Centauri and shows how relativistic velocities cause background stars to be redshifted. Credit: breakthrough initiatives
Of course, there is still a lot to be done, which Dr. Bandutunga compared the Starshot himself with the later voyage. The next step is to test some of the basic elements of the mission architecture in a controlled laboratory environment. According to Dr. Bandutunga developed an algorithm to correct atmospheric distortions and investigated various ways of combining small arrays into larger arrays.
“The ANU’s work was to see if this idea would work at all,” he said. “The aim was to find out-of-the-box solutions, to simulate them and to determine whether they are physically possible. Although this proposal was made by the ANU team, international work continues to find unique and clever solutions to other parts of the problem. It will be exciting to bring these solutions together to bring the project to life. ”
The science behind Breakthrough Starshot has advanced significantly over the past five years. Although no target date for the start of the maiden voyage has been announced, Yuri Milner has suggested in the past that a mission could be completed by 2036. This means that humanity could get its first glimpse of a neighboring star system in the 2060s. This could include the first close look at a potentially habitable exoplanet.
Further reading: ANU