One of the very important keys to making interstellar travel possible in space is to build something that can go fast — very fast. Another thing that’s not so obvious? Efficiently hitting the brakes.

Although the goal behind space travel is to travel long distances as quickly as possible, the design has to be based on the mission. If you’re trying to just travel to the cosmological equivalent of Bumfuck, Nowhere, stopping quickly isn’t as important — you don’t even have to build a braking mechanism for your spaceship.

But that’s not exactly the point of space travel. You want to go somewhere — either because you’re trying to study a system from afar, or you’re trying to land on a new world and explore it on the surface.

In either case, you have to make sure you can slow your spacecraft so that you don’t just skip by it in the blink of an eye (or worse, crash into something.) If you’re just doing a flyby — like what the New Horizons probe is doing out in the Kuiper belt with Pluto and other worlds — you still need to go slow enough to actually collect worthwhile data. If you’re trying to enter a planet’s orbital space, then you definitely need to make sure you’re moving slowly enough that you don’t simply burn up in that world’s atmosphere — or crash into the surface like an asteroid with no sense of sanctity.

Aircraft that travel through Earth’s skies use drag to slow down. There are no gases that you can take advantage of to slow down.

So how do you brake? One technique engineers employ, called aerobraking, takes advantage of gravity. Basically, a spacecraft should change its velocity as it enters an elongated elliptical orbit at its destination. This happens by combining a reverse propulsion system (i.e. shooting fire out the front of the spacecraft) with the planet’s own gravity and atmosphere. If the atmosphere is thick, then a single orbital pass should be efficient for slowing the spacecraft down. If it’s thin or nonexistent, then several orbital passes will work to slow the spacecraft down well enough so it finally enters a stable orbit around the planet or moon being investigated.

But this isn’t easy. For example, achieving a final, stable orbit around Mars takes an additional six months after a spacecraft has already reached the red planet. If your propulsion system is chemical-based, then thinner atmospheres mean you have to waste more fuel to slow down and aid the aerobraking process. Those costs are much higher if you’re trying to land on the surface itself.

And when it comes to renewable spacecraft propulsion systems — which are still in development — braking mechanisms are even less well thought out. For example, let’s look at the Breakthrough Starshot initiative, which plans to send nanocraft out to Alpha Centauri at about one-fifth the speed of light, using a light beam that pushes a spacecraft’s solar sails forward.

Breakthrough Starshot

Solar sails could be a fantastic form of spacecraft propulsion for lightweight vehicles. You just rely on the power of the sun to move you forward. But then you have a bigger question to contend with — how do you slow down? Like a normal sail, the idea would be to allow the sail’s shape to reconfigure itself such that it could also use the power of the sun to slow down.

That’s much easier said than done. After all, if your plan is to travel to a new star system, you won’t have real time control of the spacecraft’s sail. You also have to deal with another star’s light interacting with the sail. Moving towards that system means you probably are heading towards that star (or stars) head first.

Other experts are trying to modify the aerobraking system in a way that takes advantage of emerging forms of technology. One of the most bizarre ideas is the magnetosphere — a project just funded as part of NASA’s next found of Phase II awards through its NASA Innovative Advanced Concepts Program. Proposed by Redmond, Washington-based company MSNW, the plan is to create a magnetized plasma shield around a spacecraft that would interact with the atmosphere of a destination planet and help reduce the vehicle’s speed even more than a conventional aerobraking system working alone would. The concept works kind of like an invisible parachute.

Of course, this idea is totally conceptual right now. MSNW plants to use their $500,000 grant to advance the research into making the magnetosphere work, but who knows if they’ll even get close to achieving a working prototype.

In the meantime, braking continues to be an overlooked design consideration when it comes to spacecraft development. There’s no doubt speed is essential, but it’s important to remember that it’s just like when we drive cars here on Earth: going fast only leads to doom if we can’t also slow down to a stop.

Photos via Breakthrough Initiatives, NASA/JPL/Corby Waste