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How to slow down an interstellar spacecraft at Alpha Centauri

Artist's impression of the planet Proxima b orbiting Proxima Centauri.

This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface. Image & Caption Credit: ESO / M. Kornmesser

While one of the most notable challenges for future interstellar travel is how to send a probe to other stellar systems relatively fast, another issue that needs to be resolved is how to successfully slow a spacecraft down once it gets there. Recently, two German researchers have posed a solution addressing this problem, presenting a method that could decelerate an interstellar craft sent to our neighboring system Alpha Centauri.

Although the Alpha Centauri star system is located about 4.37 light-years away from the Earth, there is a concept of sending a fleet of tiny spacecraft which could reach this system within 20 years. The initiative, known as Breakthrough Starshot, aims to develop an ultra-light light sail that can be accelerated to 20 percent of the speed of light, allowing it to get there relatively fast. However, as traveling with a velocity of about 133 million mph (215 million km/h) could result in overshooting the star, there is a need to develop a method ensuring a successful deceleration of an interstellar probe.

That is why René Heller and Michael Hippke of the Max Planck Institute for Solar System Research in Göttingen, Germany, have lately studied a concept of slowing down a spacecraft at Alpha Centauri by utilizing the radiation and gravity of stars in this system. They are convinced that photon pressures of the stellar triple consisting of Alpha Centauri A, Alpha Centauri B, and Proxima Centauri can be used together with gravity assists to decelerate an incoming fleet of solar-sail-based craft.

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.

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. Image Credit: Planetary Habitability Laboratory, University of Puerto Rico at Arecibo

“I think our concept of slowing down interstellar spacecraft using the stellar photons might become part of a range of methods that at some point will be taken into account for real mission planning like for Breakthrough Starshot or follow-up missions. So I would say, we delivered one piece of the puzzle,” Heller told

The scientists refer to their technique as a photogravitational assist. It would allow multiple stellar flybys in the Alpha Centauri system and deceleration of a sail into a bound orbit. Moreover, it could also enable sample return missions to Earth.

According to a paper published by Heller and Hippke, the spacecraft’s solar sail would be redeployed upon arrival so that it would be optimally decelerated by the incoming radiation from the stars. Furthermore, once at Alpha Centauri, the probe would not only be repelled by the stellar radiation but also would be attracted by the star’s gravitational field. The calculations required to test the feasibility of the method were based on a space probe weighing less than 100 grams in total, which is mounted onto a 100,000-square-meter sail.

In particular, the plan is to use the stellar pressure from Alpha Centauri A to brake and deflect the probe toward Alpha Centauri B, where it would arrive after just a few days. Afterward, the sail would be slowed again and catapulted toward Proxima Centauri, where it would arrive after 46 years. After last year’s discovery of an Earth-like planet orbiting Proxima Centauri, such a photon accelerated/decelerated probe could deliver crucial information about this nearby alien world.

“Photogravitational assists allow visits of three stellar systems and an Earth-sized potentially habitable planet in one shot, promising extremely high scientific yields,” the paper reads.

What is noteworthy is that the photogravitational assist could also be performed in the Solar System. The scientists note that a spacecraft could be accelerated to interstellar velocities using solar photons rather than using additional expensive technologies such as ground-based laser launch systems.

“The Sun as a star could equally well be used to steer a photon sail. Solar System missions would actually be a natural intermediate step to test the technologies before we step out to other stars,” Heller said.

Currently, the researchers are working on improving their technique that will allow them to save much of the flight time to the star system with a full stop at Proxima b. This concept might even put interstellar space exploration within the period of a human lifetime. They also plan to extend their technique to other nearby stars.




Tomasz Nowakowski is the owner of Astro Watch, one of the premier astronomy and science-related blogs on the internet. Nowakowski reached out to SpaceFlight Insider in an effort to have the two space-related websites collaborate. Nowakowski's generous offer was gratefully received with the two organizations now working to better relay important developments as they pertain to space exploration.

Reader Comments

I think it a spacecraft entering a star system at those speeds would be very quickly destroyed by collisions with dust particles.

I think probe will be destroyed as invader and swift retaliation will be the result

Laurence Klein

In addition, there’s Dr. Robert Zubrin’s ‘magsail’ braking concept.

I wonder how many years it would take to achieve interstellar travel if we diverted the trillions of dollars each year that goes into the upkeep/renewal of the global nuclear arsenal

@Jim, we only spend an average of ~20 billion per year to maintain our nuclear arsenal, and plans to modernize the nuclear payloads and delivery systems are estimated at $36 billion per year.

See CBO publication 49870.

I don’t know JIM, but I don’t think it matters since so long as one country has nuclear weapons, other countries will want them as well to balance power.

Too bad you can’t convince everyone to give them up at once, if you did maybe the nukes could be repurposed for interstellar travel via nuclear pulse propulsion, so long as they wait until Earth is behind them to fire it up..

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