After weeks of delay, the countdown for the launch of Orbital Sciences' resupply mission to the International Space Station began early Thursday ahead of a 1:07 p.m. blast off.

The shipment of nearly 3,000 pounds of cargo was originally scheduled to leave Dec. 18 aboard the aerospace firm's Antares rocket. It was delayed, however, when a pump module on one of the space station's cooling loops shut down Dec. 11 after reaching its reset temperature limit. The module was replaced during two spacewalks carried out by NASA crew members Mike Hopkins and Rick Mastracchio. The first, which took place Dec. 23, lasted more than 5 hours, while the second - carried out two days later - lasted more than 7 hours.

Rescheduled for Jan. 7, the mission was once again put on pause, this time out of fear of unusually high levels of space radiation following a massive solar flare.

According to NASA, the X1.2-class solar flare unleashed Tuesday was associated with an Earth-directed coronal mass ejection, a solar event that can hurl billions of tons of particles into space. Solar flares are classified according to their strength, with A being the weakest and X the strongest. The number also relates to strength - an X2 being twice as intense as an X1, and an X3 three times as intense.

Engineers for the private aerospace firm conducted an overnight analysis of radiation levels that exceeded constraints imposed on the Antares rocket. In the end, officials decided to postpone the launch in order to better examine the radiation's potential effects on the rocket's electronic system. The Cygnus cargo capsule, meanwhile, was well equipped to handle the onslaught.

In addition to science equipment and crew provisions, the cargo includes spare parts and 23 student experiments on topics ranging from amoeba reproduction to calcium in salamanders' bones.

The launch will mark Orbital's first following a successful trial carried out back in September as part of a $1.9 billion contract with NASA.