The final hours of the countdown are underway at the equator as Sea Launch prepares to return its commercial launch business to service after a year-long stand-down due to an on-pad rocket explosion and rough seas.

Credit: Sea Launch

Blastoff of the Zenit 3SL rocket is set for 1149 GMT (6:49 a.m. EST) Tuesday morning. The three-stage booster will propel the Thuraya 3 mobile communications satellite into orbit, with separation of the 11,381-pound spacecraft scheduled for the 99-minute point in the mission.

The 72-hour countdown kicked off early Saturday after the Odyssey launch platform and the Sea Launch Commander control ship dropped anchor at the launch site in the equatorial Pacific Ocean.

A previous trip to the equator in November ended without a launch due to persistently strong ocean currents and high winds. The adverse conditions forced managers to give up on the campaign and return to the company's California home port for the holidays.

This time around, the ocean currents have subsided enough to allow the launch team to press on with the final countdown, according to the Sea Launch.

"The team is seeing a significant improvement in conditions, particularly in the strength of the currents," said Paula Korn, Sea Launch spokesperson. "We are currently working no issues."

After liftoff, the Zenit rocket will pitch over and head east, gaining altitude and shedding its spent first stage about two-and-a-half minutes into the flight. During the second stage burn, the payload fairing will be jettisoned three minutes and 41 seconds into the mission. The second stage's RD-120 engine will shut down and the rocket body will separate eight minutes and 20 seconds after launch at an altitude of 112 miles.

The Block DM-SL upper stage, burning liquid oxygen and rocket-grade kerosene, will ignite moments later to push the Thuraya 3 payload into a preliminary low-altitude parking orbit. The stage will fire for four minutes and 19 seconds during the first of two scheduled burns.

Coasting for more than one hour, the upper stage and its cargo will traverse South America, the Atlantic Ocean and Africa before reigniting above the Indian Ocean.

Producing nearly 18,000 pounds of thrust during a nearly six-and-a-half minute firing, the stage will reach the launch's targeted egg-shaped geosynchronous transfer orbit stretching from a low point of 457 miles to a high point of about 22,327 miles. The rocket will aim for a deployment orbital inclination of 6.2 degrees.

Spacecraft separation is expected approximately one hour and 39 minutes after liftoff.