8am ET Wednesday update: Both rockets hit their instantaneous launch windows on Wednesday morning, with the Ariane 5 booster lifting off from Kourou, French Guiana under mostly sunny skies and the Falcon 9 rocket ascending from California through a thick fog layer. The upper stages of both rockets are now in their coast phases before deployment of their satellite payloads.

After the launches, attention turned toward SpaceX's attempt to recover its first stage and payload fairing. The atmosphere offshore, where the Just Read the Instructions droneship was stationed 235km away from the launch pad, had high wind shear. This means wind speeds and directions varied at different altitudes, making it a challenge to come back to the ground in a more or less straight path. This, combined with high seas, made for the "worst" conditions SpaceX has ever tried to land a rocket in, said launch commentator John Insprucker.

The cameras on board didn't capture the landing clearly, but afterward SpaceX said the rocket did, in fact, make a safe landing on the droneship. Less certain was the fate of the payload fairing amid the poor weather conditions. "This is an experimental attempt; we're still learning how to catch a fairing out of the air," Insprucker said.

Original post: The Western Hemisphere may host two launches within 15 minutes on Wednesday morning as both Arianespace and SpaceX prepare for satellite delivery missions. The launches are presently scheduled to occur between 7:25am ET (11:25 UTC) and 7:39am ET (11:39 UTC).

First up is Arianespace, with a mission launching from Kourou in French Guiana, over the Atlantic Ocean. This flight of the Ariane 5 ES rocket—the Ariane 5 fleet's third mission in 2018—will send four Galileo satellites into medium Earth orbit (at an altitude of 22,922km) for the European Commission. These satellites will form part of Europe's own global navigation system constellation.

Of note, this will be the 99th launch of the Ariane 5 rocket, which first flew in 1996. This rocket will continue flying until 2022, at which point Arianespace and the European Space Agency will phase out the booster in favor of the Ariane 6 rocket, designed for lower-cost operations in order to compete on the commercial launch market with SpaceX.

The Ariane launch livestream below should begin about 25 minutes before liftoff.

Speaking of SpaceX, its rocket has a launch time less than 15 minutes after the scheduled Ariane 5 liftoff. The Falcon 9 rocket will launch from the West Coast of the United States and seeks to deliver 10 Iridium NEXT satellites into a polar orbit 625km above the Earth.

This will be the third flight of the Block 5 version of the Falcon 9 rocket, and, if the vehicle launches on Wednesday, it would be the company's second launch in just over two days.

Wednesday's flight from Vandenberg Air Force Base in Southern California will also feature two separate recovery attempts. The Just Read the Instructions droneship will serve as a mobile landing platform for the Falcon 9's first stage, and the smaller Mr. Steven vessel—now equipped with a larger net—will attempt to catch one half of the rocket's payload fairing.

The Falcon 9 webcast should begin 15 minutes before the scheduled liftoff time—or just about the time the Ariane 5 rocket is taking off.