Update: SpaceX did it! They delivered Dragon to orbit and then made the first ever water landing on an autonomous drone ship. Ars will have a full report on this historic landing later this evening.

Original story:With an instantaneous launch window that opens and closes at 4:43pm ET (9:43pm BST) today, SpaceX will attempt to send its Falcon 9 rocket and Dragon spacecraft soaring into space.

While there is some intriguing cargo aboard the Dragon spacecraft, notably the expandable Bigelow habitation module, most of today's suspense will come after launch when SpaceX attempts to land its first stage booster on a drone ship off the Florida coast.

According to today's flight profile, the first stage main engines will cutoff at 2 minutes, 30 seconds after launch. The main engines will separate about 4 seconds later, and the first stage boostback burn will begin 4 minutes after launch. The landing burn should occur at 8 minutes, followed by an experimental landing attempt at around 10 minutes.

Technical problems have foiled previous attempts to land the Falcon on an autonomous drone ship. But having learned from those failures, and with good weather today and sizable margins of propellant, SpaceX has the best chance it has ever had of successfully sticking a historic sea-based landing. For economic reasons it is important to land rockets at sea, because rockets usually use most of their fuel just getting payloads into orbit. (Note: After this Elon Musk tweet in response to this story, let me clarify that I meant "economic" in the sense that it is more difficult to close a business case for reusable flight if you can't land rockets at sea, because that's where most of them will have to come down.)

The live video below should begin about 20 minutes before the launch window opens.

Should SpaceX successfully launch and land its first stage booster later today Ars will have full coverage of that event and its implications for spaceflight.