UPDATE (Apr. 13, 2015 at 20:35 UTC): Today's launch was scrubbed due to weather; storm clouds got too close (10 nautical miles) from the launch pad. The next attempt will be tomorrow, Tuesday, April 14, at 20:10 UC (4:10 p.m. Eastern).
SpaceX is scheduled to launch a Falcon 9 rocket today at 20:33 UTC (4:33 p.m. Eastern) from Florida. The primary goal: Send a Dragon capsule loaded with two tons of supplies to the International Space Station. The secondary one: Land the first stage booster on a barge floating in the Atlantic Ocean.
I suspect most people will be most interested in the booster landing attempt. The Falcon 9 rocket has two stages. Every kilo you send to orbit means you need fuel to lift it, so many rockets use staging to save fuel; the heavy bottom half is jettisoned at some point, carving a lot of weight off the rocket.
Usually the first stage is discarded; dropping into a watery grave in the ocean or burning up on re-entry. But SpaceX wants to save money by reusing that booster stage. The Falcon 9 booster saves just enough fuel to slow down after the initial launch (and the second stage is safely away). It then drops down, deploys fins on the bottom to help steer it, and—should all go well—lands vertically on a floating platform (technically, the autonomous spaceport drone ship; it has onboard computers that allow it to position itself under the returning booster automatically).
SpaceX tried this in January, with, um, less-than-perfect results. That was due to the fins running out of hydraulic fluid while the booster was still aloft, a shortcoming that has been corrected. This second attempt* will hopefully go better. Mind you, in that first attempt the booster slowed and descended correctly, the barge positioned itself, and everything went right except for the one (catastrophic) problem. Fixing that should go a long way to a successful landing.
If it works, it will be the first time in history a booster will have been recovered in this way.
The main mission is to get a Dragon capsule to ISS. This mission, Commercial Resupply Service 6 (or just CRS-6), will deliver cargo and supplies to the crew on the station. One piece of hardware going up in the Dragon is the Arkyd 3 Reflight, a very small satellite by the company Planetary Resources that will test technology that will be used in future asteroid reconnaissance missions.
Right now, the weather for the afternoon is iffy. If the launch is scrubbed, a second attempt will be made Tuesday at 20:10 UTC (4:10 p.m. Eastern).
Incidentally, SpaceX recently released a bunch of super-hi-res footage of launches and landings from previous missions. It’s pretty cool. This should keep you sated until this next launch.
*Kinda sorta second, that is; during a February launch the ocean was too rough to land on the barge, so SpaceX landed the booster vertically in the ocean. I consider that a practice run.