In April 2016, billionaire Yuri Milner announced the $100 million Breakthrough Starshot project to visit Alpha Centauri, the nearest star system to Earth. An expedition to Alpha Centauri is exciting for several reasons. For one thing, this would be the first trip made by a probe to another star system. But even cooler than that is what we’d explore when we got there–Proxima Centauri b, the closest exoplanet to Earth. Proxima b orbits the small red dwarf, Proxima Centauri, at a distance ten times closer than that between Mercury and the Sun. However, because Proxima Centauri is more than 600 times dimmer than the sun, Proxima b may lie within the red dwarf’s habitable zone, the area around the star warm enough for the planet to possess liquid water on its surface — and thus, perhaps,
bits the small red dwarf, Proxima Centauri, at a distance ten times closer than that between Mercury and the Sun. However, because Proxima Centauri is more than 600 times dimmer than the sun, Proxima b may lie within the red dwarf’s habitable zone, the area around the star warm enough for the planet to possess liquid water on its surface — and thus, perhaps, life as we know it.
The aim of the Starshot project is to send a tiny spacecraft propelled by an enormous rectangular photon sail to the Alpha Centauri star system, where it would fly past the Earth-like planet Proxima Centauri b. The four red beams emitted from the corners of the sail depict laser pulses for communication with the Earth. Credit: Planetary Habitability Laboratory, Univesity of Puerto Rico at Arecibo
Although Alpha Centauri is the closest to our planet, it’s not exactly in our backyard. The star system is located roughly 4.37 light-years away or more than 25.6 trillion miles. Conventional rockets are nowhere near efficient enough to cross this vast distance within a human lifetime. For some perspective, at the space shuttle’s maximum speed of about 17,600 mph, it would have taken roughly 165,000 years to reach the system.
Given that rockets aren’t able to cut it, Breakthrough Starshot’s plan is to get there using “laser sailing,” a strategy that involves equipping chip-sized spacecraft with thin, highly reflective sails and utilizing powerful lasers to push thousands of these probes outward per year. The project aims to build a ground-based high-altitude 100-gigawatt laser array to launch swarms of probes at 20 percent the speed of light for 20-year journeys to Alpha Centauri.
A major challenge that the program has always faced is how to slow these projectiles down once they reached their destination. Unless there is a laser array at the other end that can slow them down, the probes would likely traverse Alpha Centauri within hours given their high speeds, said astrophysicist René Heller at the Max Planck Institute for Solar System Research in Göttingen, Germany, and his colleague Michael Hippke, an information technology specialist.
In a new study, Heller and Hippke have proposed a novel idea to solve this problem: using the Alpha Centauri system itself to decelerate the Starshot probes. Each spacecraft would redeploy their sails as they approached the system so that light from the Alpha Centauri stars would slow their advance. This braking force would increase as the probes approached the stars. The probes would next fly to Alpha Centauri A, and then reach Alpha Centauri B within a few days, after which they would make their way to Proxima Centauri. During each probe’s encounter with these stars, the gravitational pull of each star would deflect the spacecraft outward to their next destination. The scientists detailed their findings online Jan. 30 in The Astrophysical Journal Letters.
In this scenario, the probes would take about 95 years to reach Alpha Centauri A and B, and then another 46 years to reach Proxima Centauri. By the time the probes arrive at the red dwarf, Heller and Hippke suggest they will have slowed down enough to enter into orbit around the star and get a closer look at our nearest exoplanet.
This new strategy does call for some changes to Breakthrough Starshot’s current plan. For example, it suggests that probes should travel only at roughly 4.6 percent the speed of light — any faster, and Alpha Centauri’s light would not be enough to slow them down, and they would simply overshoot the system. The scientists also noted that their strategy could use light from the sun to launch probes at Alpha Centauri instead of a giant, expensive laser array. The probes would have to get very close to the sun to get the right momentum, but after such a boost, they could make it to Alpha Centauri in about 115 years.
The researchers are now discussing their concepts with the Breakthrough Starshot initiative. “Our new mission concept could yield a high scientific return, but only the grandchildren of our grandchildren would receive it,” Heller said in a statement released earlier today. “Starshot, on the other hand, works on a timescale of decades and could be realized in one generation. So we might have identified a long-term follow-up concept for Starshot.”
Although Breakthrough Starshot faces many other daunting technical challenges, the researchers remain optimistic. “Many great visions in the history of mankind had to struggle with seemingly insurmountable obstacles,” Heller said in the statement. “We could soon be entering an era in which humans can leave their own star system to explore exoplanets using fly-by missions.”