While there may now be , and the core of the Falcon Heavy rocket that launched it may be , the twin Falcon 9-based boosters that helped make it all happen are safe and sound back on Earth. And not only did they look good, they made two big bangs doing it.

I need to rephrase a little here. Technically they didn’t make “two” big bangs, they made six bangs, because each booster assembly breaks the sound barrier when it’s coming back to Earth in triplicate. You can hear it as a “boom-ba-BOOM” a little ways after you can see the boosters themselves re-ignite for landing, as the crowds recording have to stand far away enough that if something goes wrong they’re not incinerated or crushed. While the boosters themselves may be falling back to Earth quickly enough to break the sound barrier, the noise itself still has to follow the usual physics of sound in a medium:

The roar you hear immediately after the sonic booms is from the rocket engines themselves, which fire up to slow the boosters down enough for a relatively soft landing so that they’re available for re-use.

The Space Shuttle was famous for making a loud double BANG as it re-entered the Earth’s atmosphere, but back in 2016 learned why the Falcon 9 makes a triple boom:

“[The] first boom is from the aft end (engines),” said John Taylor, SpaceX’s Communications Director. “[The] second boom is from the landing legs at the widest point going up the side of the rocket. [The] third boom is from the fins near the forward end.”

Just don’t stand too close.