A SpaceX Dragon cargo ship during berthing last year at the International Space Station. SpaceX plans to launch its fourth operational station resupply mission early Saturday. SpaceX

SpaceX engineers loaded last-minute equipment and research gear aboard a Dragon cargo ship Friday, setting the stage for launch atop a Falcon 9 rocket early Saturday on a two-day flight to the International Space Station.

Loaded with more than 5,100 pounds of cargo, including 20 mice in a compact habitat, spacesuit batteries, an experimental 3D printer, IMAX cameras and an instrument to measure ocean wind speeds, the Dragon cargo craft was scheduled for liftoff from pad 40 at the Cape Canaveral Air Force Station at 2:14 a.m. EDT Saturday, roughly the moment Earth's rotation carries the pad into the plane of the space station's orbit.

Assuming an on-time launch, the Dragon cargo ship is expected to rendezvous with the space station early Monday, pulling up to within about 30 feet and standing by while the lab's robot arm locks on around 7:30 a.m. and pulls the capsule in for berthing at the Earth-facing port of the forward Harmony module.

If all goes well, the spacecraft will remain attached to the station until around Oct. 18, returning to a Pacific Ocean splashdown, bringing some 3,800 pounds of research samples and equipment back to waiting scientists and engineers.

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For the first time humans will be able to build tools in space. The SpaceX Dragon cargo craft, scheduled to launch on Saturday, will carry a 3D p...

Hans Koenigsmann, vice president of mission assurance at SpaceX, told reporters Friday the rocket and capsule were in good shape and ready for flight, but forecasters predicted a 50 percent chance of rain and thick clouds that could force a 24-hour delay. The outlook for Sunday calls for a 70 percent chance of acceptable weather, and a third launch opportunity may be available next Tuesday if needed.

If the Dragon is not off the ground by then, SpaceX will have to stand down until Sept. 28 to make way for the launch of a Russian Soyuz spacecraft carrying three fresh crew members to the outpost. The Soyuz TMA-14M spacecraft, carrying commander Alexander Samokutyaev, flight engineer Elena Serova and NASA astronaut Barry "Butch" Wilmore, is scheduled for launch Sept. 25 from the Baikonur Cosmodrome in Kazakhstan.

Joining station commander Maxim Suraev, European Space Agency astronaut Alexander Gerst and NASA flight engineer Reid Wiseman, the combined six-member Expedition 41 crew faces a particularly busy Fall schedule, unloading the SpaceX cargo ship, repacking it with samples and equipment for return to Earth, capturing an Orbital Sciences Cygnus cargo craft in mid October and carrying out three spacewalks Oct. 7, 15 and 22.

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SpaceX readies Dragon for launch to space station