SpaceX the Alan Notes

SpaceX launched more than two tons of cargo to the International Space Station — and also conducted an experiment in rocket recovery.

The company’s Falcon 9 rocket lifted off into the cloudy skies over Cape Canaveral Air Force Station at 3:25 p.m. ET, sending a robotic Dragon cargo capsule into orbit.

The Dragon held nearly 5, 000 pounds (2, 200 kilograms) of supplies and equipment for the station, ranging from the legs for a space robot to a lettuce-growing experiment. This is the third cargo delivery under the terms of SpaceX's 12-flight, $1.6 billion contract with NASA.

In addition to sending up the Dragon, SpaceX tested a landing system that could make it possible for future Falcons to return autonomously to a landing pad for reuse.

After the launch, Musk reported via Twitter that the first stage executed a good re-entry burn and was able to stabilize itself on the way down. However, the rough seas were a problem. "I wouldn't give high odds that the rocket was able to splash down successfully, " he said.

Later updates were more positive: "Data upload from tracking plane shows landing in Atlantic was good! Several boats enroute through heavy seas, " Musk tweeted. "Flight computers continued transmitting for 8 seconds after reaching the water. Stopped when booster went horizontal."

Recovering and reusing rockets are key parts of Musk's strategy for reducing the cost of spaceflight and eventually sending colonists to Mars. The South African-born Musk, whose net worth is estimated at more than $9 billion, is in charge of SpaceX as well as the Tesla electric-car company and the Solar City power-generation venture.

Success after weeks of delay

This space station resupply mission had been delayed for more than a month, due to setbacks ranging from contamination concerns, to a radar outage, to a helium leak on the rocket's first stage. On Friday, however, the countdown went smoothly all the way to the end.

A dark-colored plume rose up along the side of the Falcon 9 during liftoff, but Musk said the plume presented no concerns for the Dragon's flight.

A dark plume sprays up the side of SpaceX's Falcon 9 rocket during Friday's launch, leaving a black stain behind. SpaceX CEO Elon Musk said the stain came from dirty water that had been spread over the Cape Canaveral pad before liftoff. NASA

"Essentially, what happened is we splashed dirty water on ourselves, " he told NBC News. "A little embarrassing, but no harm done."

SpaceX also had some "slight initial challenges" with the Dragon's thrusters, but the spacecraft was now on track for its trip to the space station, Musk said.

"This is a happy day, " Musk said. "Most important of all is that we did a good job for NASA. Everything else is secondary to that."

Spacewalk timing affected

The timing of the launch affected the timing of an emergency spacewalk to replace a backup computer on the station's exterior that failed a week ago.

The computer, which is formally known as a multiplexer-demultiplexer, routes commands to several essential systems on the station's exterior — including the robotic rail car, cooling system and solar arrays. The primary computer is working fine, and the crew is in no immediate danger. But just in case something goes wrong, NASA wants the backup computer to be working as well.