 SpaceX has finally done it. After launching its Falcon 9 into space, the company successfully landed the majority of the rocket on a floating drone ship at sea. It's the second time SpaceX has landed its rocket post-launch and the first time the company has pulled off an ocean landing. This is another big milestone for SpaceX. In December, the company made history when it successfully landed its Falcon 9 for the first time at a ground-based landing site at Cape Canaveral, Florida. Sure, Blue Origin has also launched and landed its own rocket. That vehicle can't really be compared to the Falcon 9. And now, SpaceX has shown it is capable of landing its rockets both on solid ground and at sea. But why land on a floating drone ship when you can land on land? It's definitely easier to touch down a rocket on a large, expansive ground rather than a ship floating on moving water. Plus, all the previous times SpaceX has tried to land the Falcon 9 at sea, the results have been… explosive. The reason has to do with fuel. When the Falcon 9 reaches a certain altitude in space and separates from the top of the vehicle, it uses leftover fuel to reignite its engines and come back to Earth. The engines are lit in a series of burns that get the vehicle in the right position for entering Earth's atmosphere and landing. Different landings require different amounts of fuel to pull off. It's because the Falcon 9 doesn't go straight upward, but follows a curved path up and away from the launch pad. For a ground landing, the rocket needs extra fuel to slow down on that arc, completely flip around, and then retread all the horizontal and vertical distance it has covered to make it back to solid ground. But for ocean landings, the drone ship can position itself downrange of the rocket, or in the ideal place to catch the vehicle, on a more natural path back to Earth. That decreases the distance the rocket needs to travel and the amount of fuel needed to maneuver the Falcon 9 for landing. Not all of SpaceX's missions are even capable of performing ground landings, since some use up way more fuel than others. Rockets that launch heavy payloads or go to a high orbit need extra speed for the initial launch, and extra speed requires extra fuel. So for Falcon 9 vehicles that accelerate much more rapidly during ascent, there's not as much fuel left over for the landing. That's when the drone ship is the best, if not only, option for recovery. SpaceX expects to land about one-third of its rockets on land, and the rest in the ocean, so it's definitely going to need to perfect those sea landings in order to recover and reuse more rockets in the future. The company's president, Gwynne Shotwell, expects reusing the Falcon 9 will lead to a 30 percent reduction in launch costs. Those savings will only start to benefit SpaceX with the more rockets they land, and the faster that they can land them. Want to learn more about reusable rockets? Check out our video about why you shouldn't compare SpaceX to Blue Origin. Or if you just need more space in your life, check out our chat with astronauts living and working on the International Space Station.