Japan's fourth H-2 Transfer Vehicle will leave the International Space Station on Wednesday, and the astronauts in charge of releasing the unmanned cargo carrier will use a new technique to keep the HTV steady and avoid the recurrence of a hair-trigger abort that expedited the departure of a previous mission.

The HTV 4 spacecraft is pictured with the Canadian robot arm and Dextre. Credit: NASA

Filled with trash and other unneeded gear, the HTV will be released from the space station's 58-foot robotic arm at about 12 p.m. EDT (1600 GMT) Wednesday. Using a control panel inside the space station, astronauts will command the HTV to retreat from the complex a few minutes later, beginning a preprogrammed sequence of two separation burns with the ship's rocket thrusters.

Space station flight engineers Karen Nyberg and Luca Parmitano closed the HTV's hatches Tuesday to prepare for the departure.

The robotic cargo freighter delivered 3.6 tons of supplies and experiments to the space station Aug. 9 after a five-day transit from a Japanese launch pad to the orbiting outpost.

The astronauts unpacked food, spare parts, experiments and other equipment from the HTV's pressurized compartment, while ground controllers put two robotic arms and a two-armed robotic handyman to work outside the space station to handle the spacecraft's cache of external cargo.

The Canadian-built Dextre robot stowed a main bus switching unit, utility transfer assembly, and an experiment package sponsored by the U.S. Defense Department on platforms mounted on the space station's truss.

Dextre retrieved an older U.S. military experiment - part of the Air Force's Space Test Program - and placed it on the HTV's exposed cargo pallet, then the platform was put back inside the cargo craft Aug. 30.

The trash and experiment box packed inside the HTV will be destroyed during re-entry over the Pacific Ocean on Saturday.

Perched on the end of the station's Canadian robot arm, the HTV will be removed from its berthing port on the Harmony module early Wednesday and maneuvered to a location about 30 feet below the complex.

When astronauts get the go to release the 33-foot-long spacecraft, they will follow a new plan devised after trouble encountered when the third HTV left the space station in September 2012.

After its release from the robot arm, the HTV 3 spacecraft began to drift outside of a predetermined box. Its on-board computers sensed the unplanned movement.

"When we released it with the arm, it imparted a moment on the spacecraft, which caused the spacecraft to translate a little bit," said Mike Suffredini, NASA's space station program manager.

"The abort itself was required because of the moment put on it with the arm. The big thing about the abort was it used the main engines, which put a higher plume and heating load out there close to ISS. Both of those issues have been dealt with between then and now," Suffredini said.

The HTV 3 spacecraft went on to a normal re-entry after the abort, but managers wanted to understand what went wrong before the next mission.

Engineers suspect the HTV was nudged by the robot arm as it started to move soon after the arm let go of the cargo freighter. The arm grasps objects with snare wires, which latch on to a pin-like grapple fixture on modules and visiting vehicles. If the begin arm starts to move with the grapple fixture nearby, the arm's end effector can contact the grapple fixture, Suffredini said.

"When you start to move the arm, it sometimes moves a little bit as it starts to move because it's settling to its zero spot," Suffredini said. "While the arm is really close, if there's any movement, you can impart a moment on the lobes of the grapple fixture."

After an investigation by NASA, Canadian and Japanese officials, controllers established a new plan.

"The first thing we're going to do is release it and not pull back immediately," Suffredini said.

The extra time will ensure the robot arm's end effector is free of the grapple fixture on the HTV before the crew begins moving the arm. The HTV will naturally drift down away from the space station, and there is a slide-like device on the robot arm that will eventually force the HTV and the arm apart.

"All we're going to do is release the pin and let the spacecraft try to float out," Suffredini said. "If it doesn't float out, it will be pushed out."

According to Suffredini, engineers also upgraded the robot arm's control software to limit its unexpected movement when astronauts begin operating the arm.

The space station's other cargo supply vehicles which use the robot arm, such as SpaceX's Dragon spacecraft and the Orbital Sciences Corp. Cygnus logistics vehicle, are not susceptible to the same problem.

Suffredini said the geometry of the grapple fixtures on the Dragon and Cygnus spacecraft make those vehicles less prone to contact between the robot arm and the fixture's pin.