SpaceX will be going back to sea landings for a while, despite successfully landing its Falcon 9 rocket on solid ground in December, a company spokesperson confirmed today. That means after the next few Falcon 9 rocket launches, the vehicles will attempt to land upright on one of the company's autonomous drone ships in the ocean.

At first glance, the decision doesn't seem to make a lot of sense. Landing on the ground seems like a much easier task than hitting a tiny ship bobbing on the ocean's surface. Plus, SpaceX hasn’t had much luck in landing its rockets at sea. The past two attempts have ended in explosions.

At first glance, the decision doesn't seem to make a lot of sense

But the decision is more complicated than that. For instance, SpaceX has to first get clearance from the Federal Aviation Administration to do ground landings; the company has to ensure that the rocket won't harm anyone or any outside property. SpaceX wasn't able to get clearance for its next mission on Sunday, January 17th, which will launch NASA's Jason-3 satellite into space from California’s Vandenberg Air Force Base.

"For Jason-3 we didn’t receive environmental approval for a land landing in time for the launch, so we are doing it on the drone ship (ASDS)," a spokesperson told America Space.

As for the other upcoming launches, the decision mostly comes down to fuel. Rockets launch up and away from their pad to get into space — remember, rockets don’t go straight up, they follow a parabolic arc. Once the Falcon 9 reaches space and separates from the top of the vehicle, it decelerates and flips around to make the journey back. To return to a ground landing port, the rocket needs extra fuel to reverse the vertical and horizontal distance it has covered. But a floating drone ship can travel away from the launch site and set up shop right underneath the rocket. In this scenario, the rocket doesn't need as much fuel to travel back to its landing target: it mostly just goes straight down.

Fuel requirements vary, even for the same rocket. The acceleration the Falcon 9 must undergo matters: the faster the rocket is going to exit Earth, the more fuel it's going to need to burn to decelerate and return. The cargo matters too — the heavier it is, the more fuel the rocket needs. Slowing down a heavier rocket requires a lot more energy, and thus, more fuel. So for launches that are fast or heavy, an ocean landing is more feasible.

For launches that are fast or heavy, an ocean landing is more feasible

Once you know that, SpaceX’s barge landing attempts make more sense. The company will be launching an SES-9 communications satellite into geostationary orbit sometime in January. A Falcon 9 has to go a lot faster to deliver payloads into geostationary (GEO) orbit — a much higher altitude than lower Earth orbit (LEO). After that, SpaceX will be launching a heavy payload to the International Space Station for NASA, though it's unclear if that launch will be better suited for a barge landing or a ground landing. The Jason-3 could have been better suited for a ground landing since it's only going into lower Earth orbit, but SpaceX didn't secure the necessary clearance in time.

So as exciting as that ground landing was last year, it's going to be a while before we see it again.

Correction: SpaceX's next launch will occur on Sunday, January 17th. A previous version of this article mistakenly stated February. A line was also removed concerning acceleration speeds.