PARIS and MADRID — An unplanned abort maneuver performed by the H-II Transfer Vehicle-3 (HTV-3) as it departed the International Space Station (ISS) last September has been traced to friction between the Japanese cargo vessel’s grapple fixture and the space station’s robotic arm, which nudged the vessel off course during release.

NASA spokesman Joshua Byerly says the HTV-3 abort was caused by an interaction between the grapple fixture on the vehicle and the Space Shuttle Remote Manipulator System (SSRMS) “due to the relative positions of the HTV and the ISS.” Byerly says the interaction “created rates on the HTV vehicle that indicated the vehicle would leave its designed departure corridor and thus the vehicle initiated a preplanned abort per our joint safety requirements.”

The Sept. 12 abort, which occurred about a minute before HTV-3’s planned departure, sent the capsule speeding away from the space station at 1.2 meters (4 ft.) per second, rather than the intended 8 ft. per minute planned during the first in a series of departure burns, according to NASA briefing slides presented by ISS Manager Michael Suffredini to the NASA Advisory Council’s human spaceflight and operations committee during a Nov. 14 public meeting.

A spokesman for the Japan Aerospace Exploration Agency (JAXA) said an “unexpected impulse caused by SSRMS was applied during the release and it made HTV take a different trajectory.”

The spokesman said it is possible a similar impulse occurred during one or both of the first two HTV missions to the space station but did not trigger an abort maneuver. He said slight variations in the location, attitude and timing of each HTV vessel visiting the station could result in friction with the SSRMS and ultimately lead to a departure abort.

“We think small differences in the hardware spec and operations have an impact on the interface force,” he said. “Those differences between the flights are very small, and within requirements, and a combination of them might cause a deviation to HTV’s Fault Detection, Isolation & Recovery criteria,” resulting in an abort.

By design, Byerly said, the preprogrammed burns do not put the ISS at risk. He also said NASA has assessed other free-flyer spacecraft, “and this issue is not applicable to them.”

In addition to HTV, the Space Exploration Technologies (SpaceX) Dragon cargo capsule and Orbital Sciences Corp.’s Cygnus vessel are designed to berth with the ISS. Developed under NASA’s Commercial Orbital Transportation Services (COTS) program, Dragon has already completed two cargo runs to the space station, including the first of 12 planned under a separate $1.6 billion fixed price contract with NASA. Orbital plans to launch Cygnus atop the company’s new Antares rocket under COTS later this year.

In the meantime, NASA and JAXA have not set a date for launching HTV-4. According to Suffredini’s November briefing slides, the vehicle was scheduled to launch to the ISS in mid-July this year. For now, NASA and JAXA are assessing options for addressing SSRMS interaction with future HTV vehicles. Both JAXA and NASA say the mission is likely to occur this summer, and that NASA’s review will be completed in time to support any procedural changes necessary for HTV-4.