New York — Despite a rocky start, two unmanned spacecraft have succeeded in their first autonomous satellite refueling demonstration while orbiting high above Earth.

The Autonomous Space Transport Robotic Operations (ASTRO) satellite, one of two spacecraft flying the Orbital Express refueling mission, successfully pumped vital hydrazine fuel into its NextSat counterpart as part of Scenario 0-1, the first in a series of increasingly challenging tests.

“The first Orbital Express demonstration, Scenario 0-1, was very successful,” Jan Walker, a spokeswoman for the U.S. Defense Advanced Research Projects Agency (DARPA), which is overseeing the mission, said in a written update April 4 . “ASTRO transferred just under 32 pounds (14 kilograms) of hydrazine to the NextSat client, meeting the scenario objective.”

Walker said the robotic arm-equipped ASTRO first delivered the hydrazine during the early hours of April 1 via a fluid hookup, and then transferred an additional 8.6 kilograms of propellant to NextSat a day later. NextSat was due to return propellant to ASTRO April 4 , she added.

In addition to refueling demonstrations, ASTRO also is designed to perform tasks such as autonomous undocking, proximity operations and re-docking, as well as use its robotic arm to install a battery on NextSat.

Orbital Express mission managers believe the technology on board ASTRO and NextSat could allow future military reconnaissance satellites to keep station over targets on Earth by providing a renewable propellant source. The technology also could help extend the lifetimes of general-use satellites by allowing in-flight repairs, equipment replacements or upgrades, mission managers have said.

The Orbital Express mission, however, did suffer a glitch shortly after launch.

Incorrect installation of a pitch momentum wheel aboard ASTRO led the spacecraft to react opposite of what its onboard navigation software intended, DARPA officials said.

“It was a combination of hardware and software issues,” Walker said of the glitch, adding that flight controllers relied on NextSat’s navigation systems to orient ASTRO’s solar panels toward the sun until new software could be uploaded. “With the updated software, [ASTRO’s] commands result in the expected pitch wheel motion.”

ASTRO already has employed a camera on its robotic arm to snap about 1,600 images of itself and NextSat, currently mated to one another .

ASTRO’s April 1 refueling operations with NextSat were performed under a low-autonomy level, requiring the spacecraft to receive a total of 23 approvals from flight controllers on Earth, Walker said. Later this week, ASTRO will transfer a battery from its bay to a corresponding slot aboard NextSat, she added.

The separation ring joining the two spacecraft together will be jettisoned April 16 to mark the beginning of rendezvous and capture activities, Walker said.

The Orbital Express mission launched March 8 as part of the U.S. Air Force’s Space Test Project-1 (STP-1) mission, with ASTRO and NextSat vehicles successfully reaching orbit alongside four other small satellites.

The $300 million mission is expected to run about 91 days. Boeing’s Phantom Works led the 952-kilogram ASTRO vehicle’s development, while Ball Aerospace oversaw construction of the 226-kilogram NextSat for DARPA.