Military officials will quietly put the revolutionary Orbital Express mission to sleep this week after three months of highly successful demonstrations to test the concept of robotic satellite-to-satellite refueling and in-space repairs.

An artist's concept shows the Orbital Express satellites flying above Earth. Credit: Boeing

The mission wrapped up its final task Friday when the ASTRO servicing spacecraft used its Canadian-built robot arm to reach out into space to snag NextSat, a craft posing as both a supply depot and a client spacecraft in the mission.

The crowning moment of the $300 million mission came after a daring rendezvous scenario that separated the two satellites by seven kilometers, or more than four miles.

Orbital Express is managed by the Defense Advanced Research Projects Agency, the Pentagon's primary research and development unit.

After completing mission operations, controllers immediately began planning the decommissioning of the two satellites.

Engineers at the Orbital Express control center at Kirtland Air Force Base in Albuquerque, N.M., will be responsible for turning off the satellites later this week.

The spacecraft are scheduled to fly apart Thursday and separate to a distance of up to 1,000 kilometers, or about 600 miles. Leftover propellant on ASTRO will be dumped overboard to safe the satellite, and engineers expect to decommission the computers by Saturday, said Jan Walker, DARPA spokeswoman.

Documents released to the media before launch expressed the possibility of extending the mission beyond the original nine scenarios planned for DARPA. Tentative plans called for the U.S. Air Force Space Command to use Orbital Express for up to four additional scenarios, according to the documents.

"We had discussed the possibility of NASA or the Air Force perhaps conducting additional experiments following the end of the DARPA demonstration mission," Walker said.

Walker said both satellites have more available lifetime, but NASA and the Air Force opted not to participate in further tests using the spacecraft. Both craft were certified for a life of up to one year.

"Once we determined that NASA and the Air Force had no additional experiments that they wished to conduct, we decided to decommission the satellites," Walker told Spaceflight Now.

Atmospheric drag estimates predict NextSat - the lighter of the two satellites - will reenter Earth's atmosphere in three to five years. It may take up to 15 years for the heftier ASTRO to fall back to the planet, Walker said.

Orbital Express closed out its four-month mission by packaging all its previous tasks into a single three-day scenario incorporating long-range rendezvous, proximity operations, a robot arm capture, and mated commodity transfers.

"It's like if you go out to your car, push a button, and the car autonomously drives to the service station, the fuel tank autonomously comes in and refuels you, and then (the car) autonomously takes you back to your garage," said Paul Geery, Boeing's Orbital Express program manager, before the mission's March 8 launch.

A view from ASTRO looking at the NextSat spacecraft. Credit: DARPA

The final scenario began early last Wednesday, when ASTRO autonomously flew to a point about seven kilometers behind NextSat.

ASTRO, which was built by Boeing Phantom Works, used on-board equipment to chase down NextSat over the next two days. ASTRO relied on optical and infrared imaging sensors for much of the approach, which included several stops at varying distances from NextSat.

ASTRO circled NextSat to perform a thorough visual inspection of the spacecraft before performing the final stages of the rendezvous. Once in range, ASTRO's robot arm grappled NextSat and docked the two satellites together after a brief intervention by ground controllers to correctly align the docking ports.

After rejoining in space, ASTRO again employed its robot arm to move a spare battery between the two spacecraft. Propellant transfers were also performed to complete the scenario.

The final scenario built on earlier accomplishments this spring, which were structured in a schedule of increasingly challenging tasks. This "crawl-walk-run" approach provided the mission team a series of incremental steps leading to the grand finale last week.

The two satellites first transferred propellant and a spare battery and computer while mated. This demonstration was followed by tests of the robot arm and docking mechanism.

In May, Orbital Express began rendezvous demonstrations, beginning with an hour-long close-range test that separated the two satellites by about 30 feet. During the next scenario, ASTRO was nearly crippled by a major failure in its sensor computer, which processes data gathered by the craft's rendezvous instruments, including cameras, an advanced video guidance sensor and a laser rangefinder.

The two satellites drifted nearly four miles apart before controllers could devise a trajectory to guide ASTRO back to NextSat.

"After that, it was all ASTRO's show," said Air Force Lt. Col. Fred Kennedy, DARPA's Orbital Express program manager.

ASTRO autonomously executed the rendezvous profile as it flew itself within 400 feet of NextSat. After reaching the docking corridor, ASTRO automatically docked with NextSat with no input from the ground, according to a DARPA statement.

Following a review of May's computer anomaly, DARPA officials gave approval to continue the unmated portions of the mission last month.

ASTRO completed a demanding circumnavigation of NextSat on June 16, followed by a two-and-a-half mile rendezvous on June 22, which ended with a robotic arm capture by ASTRO.

Orbital Express successfully achieved a number of space industry firsts during its mission. The mission featured the first fully autonomous capture and servicing of a satellite without client assistance, the first self-directed transfer of propellant using U.S. technology, and the first automatic capture of another satellite using a robot arm.