The U.S. military's groundbreaking Orbital Express mission was shut down this weekend after concluding a series of tests to demonstrate autonomous satellite servicing techniques during more than four months in space.

Military officials said controllers decommissioned the mission's two spacecraft Saturday and Sunday.

"I think that this is a resounding success," said Air Force Lt. Col. Fred Kennedy, the Orbital Express program manager for the Defense Advanced Research Projects Agency, which sponsored of the $300 million mission.

Kennedy oversaw a team of nearly 40 controllers at Kirtland Air Force Base near Albuquerque, N.M. Most of the operations team members were contractors from Boeing Co., which built the ASTRO satellite.

"We met all of our mission success criteria, which I have to tell you I didn't expect we would do on day one of this mission," he told Spaceflight Now in a recent interview.

Orbital Express completed a final test last week, which began July 16 by separating the ASTRO and NextSat satellites to a distance of about 250 miles. ASTRO, which serves as the mission's servicing craft, honed in on NextSat using ground-supplied navigation data during a challenging three-day chase of the client satellite.

ASTRO's navigation sensors reacquired NextSat and the satellite automatically closed within one kilometer of its target. After flying in formation behind NextSat for 24 hours, ASTRO maneuvered in front of the client satellite and then performed a separation burn to permanently separate the two craft.

"The end-of-life maneuver demonstrated a capability for long-range rendezvous and track," said an update posted on the DARPA Web site.

Engineers turned off NextSat by pointing the craft's solar panels away from the sun and switching off the computers early Saturday.

ASTRO was shut down Sunday after jettisoning leftover propellant and decommissioning its flight computers, according to DARPA.

Orbital Express received a brief reprieve earlier this month when DARPA director Tony Tether asked the ground team to hold off decommissioning the satellites. The postponement allowed senior Pentagon leaders an opportunity to review additional demonstration scenarios, but none were identified other than the long-range rendezvous experiment.

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, according to DARPA.

Lessons were learned

If requested, data from the mission will be made available to other government agencies, including other military branches and NASA, Kennedy said.

Kennedy said it will be up to the other agencies and the private sector to determine whether an operational servicing system is put in orbit. Other organizations were heavily involved in the mission, he said.

Boeing and Ball Aerospace and Technologies Corp., which built NextSat, were the primary contractors for the mission. NASA built a navigation sensor for Orbital Express and representatives of the Air Force 1st Space Operations Squadron visited the Kirtland control center throughout the mission, Kennedy said.

"DARPA's here to take the technical excuse off the table for these particularly hard activities," Kennedy said. "Until you put up an Orbital Express, a lot of folks will simply say, 'Well, we don't think it's technically feasible to do servicing, so we're just not going to investigate it.'"

Other organizations will have to study the fiscal viability of such an endeavor and weigh the costs and benefits of setting up an in-space servicing system.

"All we can do is offer up the opportunities and say, 'We have a good idea here and we think it's something you ought to consider.' The question is to what degree will it be incorporated in the future, and at this point, we don't know," Kennedy told Spaceflight Now.

Kennedy said the mission immediately proved the value of satellite servicing. ASTRO's pitch momentum wheel was installed incorrectly on the ground and engineers had to upload new software to allow the mission to continue. In the meantime, NextSat was charged with controlling the orientation of the two satellites.

"The biggest lesson that I've learned from all of this is that it's extremely valuable to have a satellite around that can help you in times of trouble," he said. "We saw that right away."

Kennedy outlined a number of potential demands for a fleet of repair satellites and fuel depots stationed in orbit. Spacecraft running out propellant, suffering from power and communications problems, and victims of launch failures could be key users of such a system, he said.

"We invest too many billions of dollars in these systems just to watch them turn themselves off within 24 hours of getting to orbit," Kennedy said. "That's just completely unacceptable, and I think what DARPA has demonstrated here is that we don't have to do it that way."

Kennedy said Earth observation spacecraft circling the planet in polar orbit and communications satellites in the geosynchronous belt could be the beneficiaries of servicing constellations.

"If were to place a system like this in a Sun-synchronous or potentially a geosynchronous orbit, I think you would be able to handle the vast majority of missions that might find value in doing something like this," he said.

Complex demonstrations tested

The mission completed a series of increasingly ambitious scenarios since April, beginning with fuel transfers between ASTRO and NextSat. ASTRO then used its robot arm to move spare equipment between the two satellites.

The mission conducted its first unmated operations in early May, when the spacecraft separated to a distance of about 30 feet.

A subsequent demonstration May 11 was automatically aborted due to a serious problem with ASTRO's sensor flight computer.

"It's an autonomous vehicle, so it took itself out to 120 meters separation from our client spacecraft NextSat, and stayed there," Kennedy said. "That's what it was designed to do."

The abort is designed to allow controllers to investigate the situation and decide on a solution. The problem recurred several more times and the two satellites coasted nearly four miles apart because ASTRO lost critical navigation data, Kennedy said.

Engineers opted to use a spare sensor computer that was designed to be a replacement unit as part of the mission's robot arm operations. The backup has been the primary sensor computer since May, but the original computer has showed no more signs of problems, he said.

"We're still a bit mystified about exactly what happened," Kennedy said. "We were able to pin down where it happened inside the computer and exactly what process got hung up, but exactly what caused it has still eluded us."

ASTRO recovered from the computer failure and performed a circumnavigation of NextSat in June, followed by a rendezvous scenario from a distance of two-and-a-half miles that culminated with a capture of NextSat by ASTRO's robot arm.

The mission's grand finale was a five-day scenario ending July 2 that included a rendezvous from a distance four miles, a circumnavigation of NextSat, and additional fuel, battery and computer transfers between the two satellites.