OA-6 Cygnus berthed to International Space Station
After three days of travel to the International Space Station (ISS), the fifth Orbital ATK Cygnus spacecraft to visit the outpost was captured and berthed to the station early Saturday morning, delivering 7,756 pounds (3,518 kilograms) of food, supplies, and experiments.
Once the spacecraft was about 33 feet (10 meters) below the station, NASA astronaut Tim Kopra, the commander of the orbiting laboratory, took control of the space station’s robotic arm to move in and grab the free-flying Cygnus. Capture took place at 5:51 a.m. CDT (10:51 GMT) when the cargo freighter and ISS were about 250 miles (402 kilometers) over the south Indian Ocean.
“We’re really honored to bring aboard the S.S. Rick Husband to the International Space Station,” said Kopra. “It recognizes a personal hero of so many of us and this will be the first Cygnus honoree who was directly involved with the construction of this great station.”
After capture, ground teams spent the next two hours maneuvering the spacecraft to just below the Unity module of the orbiting lab. Once Cygnus was only inches away from the Earth-facing Common Berthing Mechanism (CBM), fine-tuning of the alignment began.
Getting the spacecraft below the CBM into the Ready To Latch (RTL) position took a little longer than usual. Ground teams told the crew that because the robotic arm was fully stretched out, the procedure had to take place a little slower.
During final RTL alignments, the space station’s ground track took it away from video downlink. This caused the operators on the ground controlling the robotic arm to pause for about 30 minutes.
About an hour behind the timeline, berthing finally took place at 9:52 a.m. CDT (14:52 GMT) while the station was flying over the Pacific Ocean just west of Mexico.
Throughout the morning, Kopra was assisted by British astronaut Tim Peake who called out Cygnus positioning data over the radio as the spacecraft got ever-closer to the ISS.
When the spacecraft was 2.5 miles (4 kilometers) away from the outpost, the BT-4 engine briefly fired to push Cygnus toward ISS in an approach vector called R-Bar—approaching from below the outpost. Once the cargo ship was within a mile (1.6 kilometers), the crew verified that they were able to command the craft if necessary.
The crew had the ability to give three commands to Cygnus if problems had come up: HOLD, which would stop the vehicle’s approach; RETREAT, which would send the spacecraft back to its previous hold point; and ABORT, which would command the cargo ship to perform a burn to escape the vicinity of ISS.
Once Cygnus was about 820 feet (250 meters) away, the spacecraft held its position to give teams on the ground time to verify everything was still proceeding normally. Additionally, the cargo ship was switched to Proximity Navigation Mode using data from the Triangulation and LIDAR Automated Rendezvous and Docking (TriDAR) system, a laser-based 3-D sensor used to collect 3-D data of ISS. This allowed for the system to know its range and relative velocity.
Just before 5 a.m. CDT (10:00 GMT), Cygnus continued moving toward the ISS and entered the “Keep Out Sphere”—an imaginary sphere with a radius of 656 feet (200 meters). At 98 feet (30 meters), the spacecraft held its position again, as planned. This gave teams time to review data and verify the cargo ship was ready for grappling by the space station’s robotic arm.
Then, at about 5:30 a.m. CDT, Cygnus moved in closer to about 10 meters—in range of the robotic arm. The spacecraft and ISS both went into free drift before Kopra commanded the arm to move toward Cygnus to capture the vehicle.
Cygnus launched a little more than three days prior at 11:05 p.m. EDT on March 22 (3:05 GMT on March 23) aboard a United Launch Alliance (ULA) Atlas V. The booster was in the 401 configuration—a four-meter fairing, no solid rocket boosters, and a single RL10C-1 engine in its Centaur upper stage.
Everything appeared to go well. However, during ascent, the first stage encountered a velocity deficit. The Russian-built RD-180 engine shut down five seconds prematurely.
SpaceFlight Now reported that the rocket’s Centaur upper stage burned a little more than a minute longer than the originally planned 14 minutes. This compensated for the first stage’s shortfall.
It is not yet known exactly what caused the shortfall, but as a precaution, ULA delayed by a week the next Atlas V, which is scheduled to launch the fifth Mobile User Objective System (MUOS-5) satellite, in order to give their team more time to understand the anomaly.
Cygnus was the first of three planned cargo ship arrivals in the next two weeks. A new Progress spacecraft will launch on March 31 and is scheduled to dock on April 2. Then a SpaceX Dragon will fly atop a “full thrust” Falcon 9 on April 8, before being berthed to the Harmony module on April 10.
Together, some 12 tons (10.8 metric tons) of cargo will be delivered in arguably the busiest time in the space station program’s history.
The hatch between the station and Cygnus is scheduled to be opened tomorrow. The crew will then begin the long process of unloading the thousands of pounds of cargo on board the spacecraft.
“As we accomplish our fifth Cygnus berthing to the ISS, we celebrate the completion of a primary mission objective for OA-6,” said Frank Culbertson, President of Orbital ATK’s Space Systems Group via a company-issued release. “Our flexible Cygnus spacecraft has a lot of work left to do. Following its stay at the ISS, and for the first time, we will undertake three experiments [on board] the unmanned spacecraft.”
The vehicle is expected to remain attached to ISS for about two months (55 days) before being loaded with trash and unberthed on May 20. After departing the vicinity of the outpost, Cygnus will remain in orbit for eight more days to conduct the Saffire experiment.
Video courtesy of NASA
Derek Richardson has a degree in mass media, with an emphasis in contemporary journalism, from Washburn University in Topeka, Kansas. While at Washburn, he was the managing editor of the student run newspaper, the Washburn Review. He also has a blog about the International Space Station, called Orbital Velocity.