If you're going to send a nano-spacecraft powered by photons across the galaxy – travelling at a fraction of the speed of light, no less – you better have a good idea on how to slow that sucker down when it reaches its ultimate destination.
These kinds of concerns are weighing on the minds of some of the planet's most ambitious astrophysicists, who are wrestling with how they can make a project like Breakthrough Starshot, announced last year, actually deliver on its fantastic promise.
If you missed it, the basic gist is to use lasers fired from the surface of Earth to propel a tiny, stamp-sized spacecraft to Earth's nearest neighbouring star system, Alpha Centauri, located a scant 4.37 light-years away.
But while Alpha Centauri might be the closest solar system to our own, astrophysicist René Heller from the Max Planck Institute for Solar System Research in Germany thinks it wouldn't necessarily be the quickest trip for humanity's diminutive ambassador to make.
Heller and his team suggest that visiting Sirius – the brightest star in Earth's night sky – would actually make for a quicker journey, even though it's almost twice as distant as Alpha Centauri, at about 8.6 light-years away.
But how could it be quicker to get to Sirius if it's further away? The answer lies in the various hypotheses about how you could propel and then slow down these tiny (and still very hypothetical) spacecraft.
Breakthrough Starshot aims to zing its mini-spaceship to Alpha Centauri at speeds of up to 20 percent the speed of light, which would see it make the trip in 20 years.
But the problem with this mind-boggling endeavour is that at such a breakneck speed, it would be impossible to slow the craft down once it reaches its destination.
In other words, as currently envisaged – and assuming researchers can figure out how to make the craft survive the perils of the journey – it would strictly be a fly-by mission, with no opportunity to conduct anything but the most fleeting research on the Alpha Centauri system, before sailing on into the void.
Which would be a shame, given that Alpha Centauri plays host to our nearest Earth-like planet, Proxima b – even though scientists are divided on just how habitable that tantalising alien world might (or might not) be.
To devise an alternative mission concept that could make it to Alpha Centauri and slow down long enough to smell the alien roses when it gets there, Heller and fellow researchers proposed using a different kind of vessel earlier in the year.
Instead of being propelled by lasers like Breakthrough Starshot, their tiny craft would feature a solar sail, that would enable it to catch photons from the Sun to send it on its way to Alpha Centauri.
Then, once it reaches the end of its journey, the same trick would work to slow the craft down: the sail would be deployed to catch radiation from Alpha Centauri's stars, applying a braking force to the probe.
It's a cool idea, but the slower speeds involved – not to mention the considerable breaking time – would mean the solar sail would take about 140 years to settle into an orbit around Proxima Centauri, the host star of Proxima b.
Compared to Breakthrough Starshot's 20 years, such a difference in time could be a deal-breaker for some.
"Our main constraint in defining the Starshot concept was to visit Alpha Centauri within our lifetime," astrophysicist and Starshot chief scientist Avi Loeb from Harvard University told National Geographic in February.
But now, Heller's team have revised their hypothesis in a new study, and propose that an optimised trip could see improved acceleration and deceleration times on such an interstellar journey.
That could shave 20 years off the travel time to Alpha Centauri, the researchers say, but the greater brightness of Sirius – which shines 16 times as bright as Alpha Centauri – means a journey to the Sirius system would only take about 69 years, in their calculations.
The research hasn't as yet been peer-reviewed by other scientists, but for his part, Loeb calls the idea "innovative and interesting" – although he points out that the solar sail technology would have to be extremely refined to enable the plan to work.
"[T]he concept requires an extremely thin sail if the goal is to reach a fraction of the speed of light," he told Abigail Beall at New Scientist.
Heller and team acknowledge that this is true, but say that if scientists can figure out how to build the right kind of ultra-large but ultra-thin sails, the Universe is within our grasp.
"We need a very light, solid, temperature-resistant, and highly reflective sail material that can span an area of several hundred metres squared," Heller told New Scientist.
"If this works out, then humanity can really go interstellar."
The study has been published on pre-print website arXiv.org.