7:55pm ET Update: After launching right on time, the Inmarsat spacecraft reached low-Earth orbit, and after two successive burns by the Falcon 9's second stage, the satellite deployed successfully into geostationary transfer orbit. This means SpaceX has now performed six successful missions in 2017, all in the last four months.

Original post: Tonight, SpaceX will attempt to launch its sixth Falcon 9 rocket of 2017. If successful, this take-off would put SpaceX on course to launch more than a dozen missions this year and, possibly, as many as eighteen. The 49-minute launch window opens at 7:21pm ET Monday (0:21am UK Tuesday), and the rocket will deliver an Inmarsat-5 F4 communications satellite to a geostationary transfer orbit. Weather is near ideal for a launch from Kennedy Space Center this evening, with a 90-percent chance of favorable conditions.

Because the satellite is so heavy—more than six metric tons—and going to a higher orbit, the Falcon 9 rocket won't have enough fuel to make a return attempt, even at sea. The company has not disclosed whether it will make another experimental attempt to recover the rocket's payload fairing.

A successful mission tonight by SpaceX would allow the company to demonstrate that it is making good progress toward its long-promised goal of flying the Falcon 9 frequently and working through a backlog of about 70 missions. Since SpaceX returned to flight on January 17 of this year, five Falcon 9 rockets have launched. After tonight, a seventh launch could come just two weeks later at the beginning of June, a cargo supply mission to the International Space Station.

Even with a static-fire accident that destroyed a rocket and its payload on September 1, 2016 was the company's most successful year in terms of overall launches with its workhorse Falcon 9 booster. Prior to the accident, SpaceX had made eight flights of the Falcon 9 rocket, a cadence of one launch per month.

The SpaceX webcast for tonight's launch should begin about 20 minutes before the window opens. The first and second stages will separate at 2 minutes, 49 seconds into the flight, and the second stage will burn until 8 minutes and 38 seconds after launch. The satellite is scheduled to deploy at 31 minutes, 48 seconds.