SpaceX finally landed a rocket at sea — a huge step toward making spaceflight cheaper

Elon Musk's private spaceflight company SpaceX pulled off a stunning feat Friday afternoon — launching one of its Falcon 9 rockets into space and then, minutes later, successfully landing the rocket's main component onto a drone ship floating in the ocean for reuse.

Here's a GIF of the touchdown. The rocket was nearly dead on target when it landed upright. That's impressive considering it faced 50 mph winds and the platform is just the size of a football field:

This is the first time SpaceX has successfully landed a rocket onto a floating platform, after four previous failures. Last December, the company brought a Falcon 9 back down onto land for the first time, at Cape Canaveral. But ocean landings are likely to be more common in the future, so this was a crucial step. They're also much trickier:

Onboard view of landing in high winds pic.twitter.com/FedRzjYYyQ

— SpaceX (@SpaceX) April 9, 2016

The Falcon 9 was launched in order to resupply the International Space Station and deliver an inflatable house that NASA will use for research purposes (no, really!). That payload is now in orbit and will reach the station Sunday morning, SpaceX said.

The point of recovering the first stage of the rocket, meanwhile, is ultimately to try and make future spaceflight far, far cheaper.

Why SpaceX is so keen on developing reusable rockets

SpaceX's Falcon 9 rockets consist of two parts. There's the 138-foot-tall first stage, which burns fuel for a few minutes, lifting the spacecraft to a height of 50 miles, before disconnecting and falling back to Earth. Then there's a smaller, 49-foot-tall second stage, which burns for another five minutes, carrying the craft into orbit before also separating.

In traditional launches, those initial rocket stages would simply break apart after they'd done their job and plop unceremoniously into the ocean, never to be used again. Musk has long argued this is a senseless waste, akin to junking a brand-new 747 after a single flight to London.

So, instead, Musk's company is trying to make spaceflight cheaper by recovering the rocket for reuse. As the first stage of the Falcon 9 falls to Earth, it fires up engines that guide it down for a controlled landing:

SpaceX

The appeal is obvious: Building a new Falcon 9 rocket costs $54 million, but the rocket itself only burns about $200,000 worth of fuel when it goes into orbit. So if SpaceX can reuse the rocket's first stage, that would vastly lower the cost of all sorts of missions:  commercial satellite launches, collaborations with NASA, or even space tourism.

The hitch is that landing the first stage without damaging it is incredibly tricky. As the rocket descends, decelerating to a speed of just 4.5 miles per hour, steerable fins affixed to the side guide it to its final destination. When it approaches the platform, a set of legs then unfolds from the bottom of the rocket, and the whole thing lands, fully upright.

The fins, affixed to the outside of the first stage. (SpaceX)

Or at least that's the dream. But a number of previous SpaceX landing attempts have faltered. It was only last December that the company finally brought one of its Falcon 9 rockets back down on land.

And that still left an ocean landing, the ultimate prize. SpaceX has estimated that two-thirds of all future rocket landings are likely to occur on water. That's particularly dicey, since you have to hit a smaller target that's bobbing about in the vast ocean. There is help, however — the floating drone platform can reposition itself to "catch" the rocket.

SpaceX's past efforts at an ocean landing fell short, as the rockets either came in too hard, tipped over, or even exploded. This time, they got it just right at last:

At a press conference afterwards, Musk said that the first stage of the Falcon 9 could be re-used 10 to 20 times — and some parts could be recycled thousands of times. That, in turn, could potentially bring the cost of space missions down by orders of magnitude. Musk also said that SpaceX still needs to get to the point where these landings are routine. "We will be successful when it becomes boring," he quipped.

Musk's company isn't the only one working on reusable rockets. Jeff Bezos's private spaceflight company, Blue Origin, has successfully landed three launched rockets so far, though those have only gone into suborbital space, not all the way into orbit as SpaceX's have.

Further reading: Check out the card stack Joseph Stromberg did on private spaceflight, which puts SpaceX's efforts in broader context.

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