Right now, space travel as we know it is plagued by a few stubborn problems, most of which boil down to striking a balance between speed and time. The faster you travel, the more energy you need. Without gravitational assists, that means more fuel will be required — and that has to be carried somehow. Faster speeds — required to reach long distances in a reasonable timeframe — also mean more difficulty slowing down.
So far, New Horizons is the fastest spacecraft ever in terms of launch velocity. Its launch speed was about 58,000 km/hr. After a flyby of Jupiter, New Horizons picked up a gravitational boost — but even still its speed was only 48,000 km/hr as it approached Pluto.
It was thanks to those gravitational flybys that Voyager 2 became the fastest spacecraft to leave our solar system. After getting boosts from the four outer planets, Voyager 2 achieved a speed of about 60,000 km/hr on its way out. To put that into context, a trip to Alpha Centauri — the closest star system to ours — would take about 78,000 years at this pace.
Furthermore, a craft making the trip would need to slow down when it arrived. Deceleration would require yet another series of gravitational flybys, which are only possible around large planets—none of which appear to orbit Proxima Centauri, the smallest star in the Alpha Centauri system. So, if there’s no possibility of a gravitational assistance, the crafts would have to bring along enough fuel to slow the craft down—which is about as much as you’d need to achieve top speed to begin with. That makes the craft heavier, and therefore harder to launch — and then you’re back to the drawing board. In short, the whole process has been long been unfeasible — until now.
Researchers are working on a way to use light to overcome these obstacles. One issue with sending a spacecraft like Voyager to another star is its mass. The Breakthrough Starshot project aims to launch extremely lightweight spacecraft — weighing only grams —into space accelerated by an array of lasers, at a mere percent of light speed. A spacecraft that slight could be slowed down using just the light of Alpha Centauri. Building such small spacecraft is outside our capabilities at the moment, but the concept is becoming more developed.
Researchers set forth the idea in the Astrophysical Journal. This approach seizes upon the fact that light exerts a minuscule amount of force on anything it hits. So, a lightweight enough craft with a large enough sail should be able to use this force to slow itself down. The idea isn’t new, but such a practically possible iteration of the concept is.
In January of 2016, NASA’s Juno spacecraft became the most distant solar-powered probe in the history of space exploration — an achievement made possible by improved solar cells and a more energy-efficient spacecraft.
Another light and sail application — the crowd-funded LightSail 2 spacecraft of the Planetary Society — completed its final end-to-end systems tests successfully. It is now set to launch in late 2017. LightSail 2’s large, reflective sails will test whether solar sailing is a feasible propulsion method for exploring further into the depths of space.
Developments like the Breakthrough Starshot could allow us to reach farther into space than ever before, gathering important data in our hunt for extraterrestrial life, habitable planets, and discoveries we haven’t even considered yet. Instead of a quick flyby of the Proxima system taking 95 years, a spacecraft would reach Alpha Centauri A in the same 95 years — and then slowly journey past Alpha Centauri B and Proxima Centauri, collecting data until system failures ended the trip.
These plans are still in their nascent stages, but they are based in sound science. It seems likely that small, ultralight spacecraft will be engaged in scientific missions, orbiting planets around other stars, sometime in the future.