Updated @ 10:10am, December 18: SpaceX has delayed the launch of CRS-5 until January 6, 2015. There was an anomaly during a test firing of Falcon 9’s rockets, and so SpaceX has pushed the launch back, to give it enough time to do a second test firing.

On Friday, December 19, SpaceX will attempt one of the most exciting maneuvers in the entire history of space travel: The first stage of the Falcon 9 rocket will try to land on a giant platform in the middle of the Atlantic Ocean. The Falcon 9 rocket, after launching a Dragon capsule into space (it’s heading to the International Space Station), will attempt a controlled descent back to Earth. Usually, the first stage just falls into the ocean, never to be used again; in this case, if the soft landing is successful, the rocket can be reused, massively cutting the cost of space launches.

At first glance, the mission — CRS-5, SpaceX’s fifth resupply mission to the International Space Station — is just a standard launch. Look closely, though, and you’ll see that the Falcon 9 rocket is slightly different: There are four hypersonic grid fins placed around the top of the first stage. These fins will be stowed during ascent, but after separation from the second stage, as the rocket begins to fall back to Earth, they will pop out and provide precision control of the rocket’s roll, pitch, and yaw.

The other new part of the mission, of course, is the giant ocean platform (pictured top), which the CRS-5 rocket will attempt to land on. The autonomous spaceport drone ship (really, that’s what SpaceX calls it) measures approximately 90 meters by 50 meters (300 feet by 160 feet), but beyond that we don’t know a whole lot. Presumably, for safety reasons, it is unmanned. I imagine it’s quite hard to design a ship/barge that can hold its position precisely enough for ocean landings — plus, SpaceX also has a tentative plan to use it for launches, too, which are even more exacting.

How the launch and soft landing will go down

So, here’s how it should go. Assuming the weather is fine, the Spacex CRS-5 mission will launch from Cape Canaveral at around 1pm Eastern time on December 19. About three minutes after launch, the first stage of the Falcon 9 rocket (i.e. most of it) will complete its burn and separate from the second stage. The second stage will ignite and carry a Dragon capsule to the International Space Station. The first stage, which will be travelling at around 3,000 miles per hour, will start falling back towards the Atlantic Ocean.

Here’s where things get complex. There will be a small amount of rocket fuel still in the tank (about 10%) — but given how the rocket is now very light, it won’t need a lot of fuel to control its descent. There will be a series of three rocket burns after separation: An initial boost-back burn to stop the rocket going any further down range; later, with the assistance of Earth’s atmosphere, there will be a supersonic retro propulsion burn that slows the rocket right down, to around 550 mph; and then finally, when the landing legs deploy and the ocean platform is in sight, there will be a landing burn that brings the rocket’s vertical speed down to just a few miles per hour.

According to SpaceX, the entire procedure has a “50% at best” chance of success. SpaceX has tried each of the individual steps, either on previous Falcon 9 launches or on its F9R test rocket (videos above), but now it’s time to string them all together and hope for the best. As far as we know, this will also be the first outing for the autonomous spaceport drone ship — if I had to guess the reason behind SpaceX’s low confidence, it’s probably because of the ocean platform. (Will its placement be accurate enough? Will it get hit by an ocean swell just before landing?)

If everything goes to plan, SpaceX will be one (very big step) closer to realizing the dream of reusable spacecraft. One of the main reasons that space launches are so is that most of the rocket — which costs millions of dollars — simply falls into the ocean, or sits around in orbit as space debris, never to be used again. Ultimately, SpaceX wants to bring both the first and second stages all the way back to the original launch pad, refuel them, and then send them back up into space again. A few years ago it sounded like a very lofty goal — now, it’s sounding surprisingly realistic.

Now read: SpaceX unveils Dragon V2, the world’s first commercial manned reusable spaceship