U.S. and European astronauts Don Pettit and Andre Kuipers, as part of a temporarily downsized International Space Station crew, are preparing for the arrival of the SpaceX Dragon cargo capsule, which is awaiting a tentatively scheduled April 30 liftoff on the first attempt at a U.S. commercial resupply mission to the orbiting science lab.

NASA and SpaceX mission managers are scheduled to gather at NASA’s Johnson Space Center April 16 for a flight-readiness review to assess the preparations and settle on an official launch date.

Commercial resupply missions to be carried out by SpaceX and Orbital Science Corp., which is preparing for a similar test flight later this year, are an essential part of NASA’s post-shuttle era support strategy for the six-person orbital outpost and its research agenda.

If current scheduling holds, the SpaceX Falcon 9 rocket will lift off with Dragon from Cape Canaveral on April 30 at 12:22 p.m. EDT. If the unpiloted freighter clears a series of inflight communications and navigational system checkouts, the Dragon could be positioned 30 ft. below the station’s U.S. segment on May 3, ready for Pettit and Kuipers to grapple it with the Canadarm2 robotic arm.

Once the 15-ft.-long Dragon and its minimal cargo of food, clothing and a few other items is in the arm’s grasp, Pettit and Kuipers will berth the capsule to the station’s Harmony module for an 18-day stay.

“We have a very operationally oriented task that we are doing intensive training for now, so we can get this task done expertly,” Pettit told reporters during an April 11 news briefing.

The two men stepped-up their onboard training this week, with Pettit as the primary arm operator and Kuipers as the secondary operator.

Pettit says they are putting the 58-ft.-long robot arm through its paces from the main control console in the Cupola observation deck as well as running daily simulations of the grapple in the station’s U.S. lab module using a computer and hand controller.

“The setup allows us to fly track-and-capture trajectories just like we would if we were in the simulator back in Houston [at NASA’s Johnson Space Center],” Pettit says. “It’s really a neat capability. I have it set up all the time.”

Initially, NASA astronaut Dan Burbank, the station’s current commander, was to lead Dragon’s track and capture, with Pettit as the secondary operator and Kuipers in a supporting role. However, that changed with a series of delays in the Dragon launch associated with software development and testing and a January ground-test problem with what was to be the next Russian Soyuz crew transport capsule. Dragon’s launch was delayed from February to March, then April.

Burbank and cosmonauts Anton Shkaplerov and Anatoly Ivanishin found their mission extended from mid-March to late April by the ground-test issue that required the Russian space agency to prepare a new Soyuz capsule to launch their replacements. They are scheduled to descend to Earth onboard the Soyuz 28 spacecraft on April 27, three days before the Dragon mission’s earliest launch opportunity.

The departure leaves Pettit, Kuipers and Russian Oleg Kononenko to staff the station until three U.S. and Russian replacements lift off for the station on May 15 and dock two days later.

Station cosmonauts are responsible for Russian Progress and European Automated Transfer Vehicle cargo mission dockings. NASA crew are trained in the track and capture of U.S. and Japanese cargo vessels.