WASHINGTON — After getting a two-week reprieve to attempt one last high-wire space maneuver, the Pentagon’s Orbital Express satellites were decommissioned July 21-22, bringing the successful satellite-servicing and robotics demonstration to an end.
The two spacecraft comprising Orbital Express — the Boeing-built Autonomous Space Transport Robotic Operations (ASTRO) servicing spacecraft and the Ball Aerospace & Technologies-built NextSat — were launched together in early March on an Atlas 5 rocket. The experiment, sponsored by the U.S. Defense Advanced Research Projects Agency (DARPA), was a pathfinder for future, long-lasting satellites capable of being serviced in orbit. Over the last four and a half months, ASTRO demonstrated its ability to approach NextSat with limited interaction from the ground, grapple the spacecraft with its robot arm and transfer fuel and hardware.
U.S. Air Force Lt. Col. Fred Kennedy, DARPA’s Orbital Express program manager, said in a July 24 interview that the two satellites were decommissioned after successfully completing one last maneuver that demonstrated ASTRO’s ability to locate and home in on NextSat from long range with an assist from the ground-based U.S. Space Surveillance Network. ASTRO then switched over to onboard sensors to complete the rendezvous. Decommissioning was to have begun July 5, but DARPA postponed that action while senior U.S. Air Force leaders considered what else, if anything, to do with the satellites.
DARPA Director Tony Tether ultimately directed his team to attempt the long-range maneuver as part of Orbital Express’ so-called end-of-life scenario. The final demonstration entailed putting enough distance between the satellites that ASTRO’s sensor suite lost track of NextSat, and thus required input from the Space Surveillance Network to locate its companion satellite and close in. “The main thing we were trying to get out of the modified end-of-life scenario was an indication of whether or not we could go out to extremely long ranges beyond sensor range then pick up an externally supplied navigation [source]… then lock that down onto the ASTRO vehicle and then have it go in and guide itself … until it started picking up NextSat with its sensors again,” Kennedy said. “And what do you know, we were able to do exactly that.”
On July 16, the Orbital Express flight team at Kirtland Air Force Base, N.M., commanded ASTRO to separate from NextSat and fall back beyond sensor range. Kennedy said ASTRO lost sight of NextSat at 310 kilometers but kept going all the way out to more than 400 kilometers. The entire transit took roughly 12 hours. At that point, the controllers, mostly Boeing employees, fed ASTRO NextSat’s general coordinates from the Space Surveillance Network. Kennedy said ASTRO found NextSat right away, set a course and began to head back in. By early morning July 18, ASTRO had closed within 1,000 meters of NextSat and trailed it for the next 24 hours, never coming closer than 500 meters.
Kennedy said the team wanted to show that ASTRO could maintain a lock on a target satellite, and loiter some distance out before proceeding toward rendezvous. On July 19, ASTRO moved from its position behind NextSat to 500 meters in front, the predetermined rendezvous point.
Kennedy said there was no reason to send ASTRO any closer since it successfully had conducted multiple autonomous rendezvous and docking maneuvers over the course of the mission. After completing the final rendezvous, ASTRO fell back 30 kilometers behind and 15 kilometers above NextSat before firing its thrusters once again to put even more distance between itself and its orbital dance partner. The Orbital Express team took advantage of this disposal maneuver to give ASTRO one more test, which Kennedy said the spacecraft passed with flying colors. With a tweak from the ground, ASTRO’s sensors were able to track NextSat out to 600 kilometers, a result Kennedy called impressive.
By late evening July 21, NextSat’s solar arrays were turned away from the sun and its computer was shut down. The following afternoon, ASTRO likewise was turned off after jettisoning its remaining propellant. By that time, the distance between the two spacecraft was more than 1,000 kilometers and growing. The satellites were separated to eliminate the chance they will accidentally collide as their orbits — nearly 500 kilometers up — decay in the years ahead.
ASTRO, the heavier of the two spacecraft, will stay in orbit another 12 to 15 years, while NextSat is expected to re-enter the atmosphere in three to five years. Neither satellite was launched with enough propellant to permit a deorbit burn for a controlled re-entry. While proclaiming Orbital Express “a fantastic success,” Kennedy acknowledged there were some nail-biting moments early in the mission. “I was concerned early on, especially when we encountered some serious problems right after launch, that we were in a heap of trouble,” Kennedy said. A problem with ASTRO’s momentum wheel delayed the start of on-orbit operations following launch, forcing ground controllers to rely on NextSat’s navigation system to keep ASTRO’s solar arrays oriented toward the sun until a software fix could be sent up. Had the two satellites not been launched in a mated configuration, the Orbital Express mission might have been over before it began.
Then in May, after a computer glitch disrupted ASTRO’s navigation system during a rendezvous and capture maneuver, the Orbital Express team took a nearly month-long time out to study the anomaly before resuming operations. While Orbital Express went on to accomplish its objectives, there was interest from some quarters in doing more with the satellites.
Tether e-mailed June 29 more than 20 U.S. government officials — including Air Force Secretary Michael Wynne and Air Force Gen. Kevin Chilton, commander of Air Force Space Command — telling them Orbital Express would be turned off within a matter of days despite NASA’s expressed interest in using the spacecraft for some of its own test objectives.
“It was hoped that [Orbital Express] would continue for NASA missions,” Tether wrote. “However the Air Force is unable to support any further Orbital Express missions; rationale unknown at least to me but offers of paying the ground station cost for the next three weeks were rejected.”
NASA spokesman Dwayne Brown said at the time there was no dispute with the Air Force over its decision to decommission the satellites. NASA, he said, simply opted not to pay for an Orbital Express mission extension, which would have focused on demonstrating techniques useful to future Mars sample return missions. But Doug McCuistion, director of NASA’s Mars Exploration Program, said several days later the space agency was willing to pay the cost of an extension, but was told that the Air Force needed the Orbital Express team to vacate its operations center at Kirtland Air Force Base to make way for a “strategic activity.”
McCuistion said July 9 that NASA had wanted use of Orbital Express hardware from mid-July to September. During that time, he said, NASA would have used ASTRO to capture the U.S. Naval Academy’s MidStar-1 spacecraft. The tiny experimental satellite, located in a higher orbit, was more representative of a martian soil-carrying canister than NextSat, he said. NASA also wanted to try out some sample return-related algorithms that had been planned for the now-canceled 2009 Mars Telecommunications Orbiter.
Kennedy declined to comment on discussions between the Air Force and NASA but said he was not aware of any pressure on the Orbital Express team to clear out of the Kirtland facility.