Our nearest stellar neighbor, Alpha Centauri, is just four light-years from Earth, and while that's pretty close in the grand scheme of things, we can't just pop over to borrow some milk. With our current technology, the journey would take tens of thousands of years, but there are plans for tiny unmanned probes that could trek there in as little as 20 years. Now, a German theoretical physicist has detailed a method to use magnetic sails as a braking system, to slow craft down to a cruising speed once they get there.

Last year, some ambitious space travel plans were laid out as part of Breakthrough Starshot, a program headed up by iconic physicist Stephen Hawking, Facebook CEO Mark Zuckerberg and entrepreneur Yuri Milner. The idea was to accelerate small spacecraft – roughly the size of a CubeSat – using a laser propulsion system like a light sail, and send them off towards Alpha Centauri.

This technique has the potential to get nano-spacecraft up to about 20 percent of the speed of light, allowing it to trek to the Alpha Centauri star system in mere decades rather than millennia. But acceleration is only half the equation: with no braking systems it'll be a flying visit, as the spacecraft zips through without being able to take in many of the sights.

Braking systems for this kind of craft are a challenge. Space and weight onboard are extremely limited, and with no friction in the interstellar medium it's hard to slow down an object moving at about 60,000 km (37,000 mi) per second.

So Claudius Gros, a professor of theoretical physics at Goethe University Frankfurt, has explained in a new paper how magnetic sails could be used to slow down these nano-spacecraft. The sail is made up of a superconducting loop that can be unfurled to a diameter of about 50 km (31 mi). A lossless current is induced in it to create a strong magnetic field in the middle of the loop, and charged particles passing through would be reflected off that magnetic field, gradually slowing the craft down.

Interstellar space is extremely empty, with a density of 0.005 to 0.1 particles per cm3, but by Gros' calculations, the system should be able to work even with a particle density that low. That said, the spacecraft would need to weigh less than 1,500 kg (3,300 lb) and be traveling relatively slowly: far slower, in fact, than the 20 percent of light speed originally proposed.

"Slow would mean in this case a travel velocity of 1,000 kilometers (620 mi) per second, which is only 0.3 percent of the speed of light, but nevertheless about 50 times faster than the Voyager spacecraft," says Gros.

Unfortunately, the ability to slow down might come at the cost of blowing out the travel times. While millennia-long journeys aren't much use for exploration, magnetic sails could be handy for things like the Genesis Project. Proposed by Gros last year, this project would see single-celled organisms sent to colonize distant planets, so slowing down to enter orbit a target is crucial. But of course, that project isn't without controversy, considering the efforts NASA and other agencies currently take to avoid contaminating celestial bodies with Earthly microbes.

The paper was published in the Journal of Physics Communications.