On Sunday, SpaceX CEO Elon Musk warned his Twitter followers that he had an idea that was “gonna sound crazy:” his company might use a “giant party balloon” to recover the upper stage of its Falcon 9 rocket from orbit.

How could a party balloon help SpaceX bring back a rocket that’s traveling thousands of miles per hour through space? Well, Musk has made bizarre SpaceX decrees before, which have later become reality: The company did launch his Tesla into deep space, after all.

And this balloon concept does have a history: for decades, NASA and other researchers have studied how to use inflatable structures to slow down spacecraft leaving orbit. Balloons are a lightweight tool that can change the shape and density of a spacecraft quickly, altering how that vehicle tumbles to Earth. A balloon can act like a big space brake and provide shielding from the enormous amount of heat a spacecraft experiences when plunging through the atmosphere. “It’s like when a Soyuz capsule is coming home,” Jonathan McDowell, an astrophysicist at Harvard and spaceflight expert, tells The Verge. “It needs a heat shield.”

Balloons have their own challenges. They can be hard to stabilize when falling through the sky, and they must be made of extra rigid, durable materials to survive. So, not exactly a party balloon, but one made to withstand incredibly high temperatures and rushing winds. But this approach could let SpaceX recover the last big piece of its Falcon 9 rocket intact.

Image: NASA

SpaceX has already mastered how to bring back most of its Falcon 9 rocket after launch. The vehicle’s first stage — or the 14-story body of the rocket that contains most of the engines and fuel — can return to a ground landing pad or a floating drone ship after takeoff. Once it starts to fall back to Earth, the stage reignites its engines a few times to control its descent. This slows the rocket down enough so that it can land upright on a flat surface.

But the second stage of the Falcon 9 — the top of the rocket that carries the payload into orbit — is way more difficult to recover. This part of the rocket travels much faster than the bottom portion of the vehicle. The Falcon 9’s first stage can reach up to 5,000 miles per hour before it breaks away and returns to Earth. But the second stage stays in space throughout the flight; to drop off a satellite in low Earth orbit, for instance, the second stage has to reach up to 17,000 miles per hour.

So whenever a second stage falls back to our planet, it’s coming in fast — and hot. The faster you descend, the more heating you experience. The upper stage may be as much as 27 times hotter than the first stage, according to Dave Akin, an aerospace engineer at the University of Maryland. (And if you look at the engines of a landed Falcon 9 first stage, they do get pretty charred.) The extreme temperature that the second stage experiences causes portions of the rocket to melt, and ultimately the whole thing breaks apart.

Image: SpaceX

Musk is looking for a way to slow the rocket without the vehicle getting too hot. Or at least that’s what his tweet indicated to Quinn Kupec, a sophomore aerospace major at the University of Maryland, who is working on high altitude balloons and heat shield technology. Kupec saw the balloon tweet and understood what Musk needed. “I thought, ‘An inflatable structure? That kind of sounds like a deployable heat shield,’” Kupec tells The Verge, referring to the technology he’s testing. He responded to Musk on Twitter. Musk replied, “Yeah, exactly! Would be great to hear your thoughts. We’re going to try a few approaches.”

It’s possible that SpaceX could deploy an inflatable balloon at the base of the upper stage, according to Kupec, providing a large shield between the rocket and the atmosphere. This would make the stage bigger at the bottom, and that could help spread all of the intense heat over a larger area. Thus the heat would be less concentrated.

But the big thing that a balloon can do is change how the second stage is affected by air resistance. Objects that are big in shape but lightweight for their size have a harder time overcoming air resistance, so they fall much more slowly. You can see this when you drop a piece of paper and bowling ball from the same height. The paper falls at a leisurely pace because it’s less dense and gets caught up in the air more easily.

The balloon, deployed at the bottom of the upper stage, could turn the rocket into more of a piece of paper than a bowling ball. That means the stage will start slowing down much more rapidly when it hits the upper atmosphere, so there are lower temperatures surrounding your spacecraft. “If I can come in with more area — if I get bigger and flatter — I’ll decelerate sooner and higher,” Akin, who is also Kupec’s teacher, tells The Verge. “And that will be less heat when I get down into the denser atmosphere.”

Now, a balloon isn’t totally necessary to do the trick. What the rocket needs is some kind of lightweight material that can be deployed quickly to change the shape of a spacecraft. Balloons are good for that because you just need a thin material and air to fill them up fast. But a lightweight upside-down umbrella could also provide a similar affect, something that Kupec and his team have tested out.

And it’s not just the University of Maryland that has looked into this, either. NASA has also tested out inflatable heat shields with programs like the Hypersonic Inflatable Aerodynamic Decelerator, or HIAD, and the Inflatable Re-entry Vehicle Experiment, or IRVE. And in 2000, Russia attempted to return the upper stage of a rocket from orbit using an inflatable heat shield. No one knows if it worked; the vehicle was never found.

But inflatable heat shields can get unwieldy when falling from high heights. At slower speeds, subsonic ones, the vehicle can become unstable. “It wants to flip over,” says Kupec. That’s not what engineers want, though. One way to fix this could be to deploy a balloon that completely engulfs the vehicle, turning it into a big beach ball instead.

It also may be tough for SpaceX to catch the inflated rocket on the way down. The vehicle could use its engine to land, like the first stage does. But the second stage’s engine is designed to work in space, not inside the atmosphere, so it may not be effective for that. Musk did indicate that SpaceX may try to catch the upper stage, with a “bouncy castle.” However, the company has been struggling with catching objects: SpaceX has been trying to catch part of the Falcon 9’s nose cone, using a boat with a giant net. Those efforts haven’t been successful yet, and catching a big inflated ball may be even more difficult than that. “The question is how controllable is it on the way down,” says McDowell.

It’ll be interesting to see what SpaceX comes up with (and what exactly Musk means by bouncy castle). Since Musk said the company would try different options, we may see multiple types of balloon-like structures on the Falcon 9. It just depends on what can be installed on the rocket for the lowest cost. “I’m sure he’s looking for a system that’s simple and lightweight,” says Akin. “Anything you carry all the way to orbit is payload you don’t get paid to carry.”

https://www.theverge.com/2018/4/18/17247748/spacex-elon-musk-giant-party-balloon-falcon-9-upper-stage-reusability