The Cygnus spacecraft, which started its journey on September 18, has just docked with the International Space Station. It’s the second belonging to a private company to dock at the International Space Station after the SpaceX Dragon. The success of this mission was vital to Orbital Sciences to get a contract with NASA for eight cargo transport missions to the Station but the two spacecraft are very different, not only in appearance.

The Cygnus spacecraft can carry up to about two tons (about 4,400 pounds) of cargo. In the future the use of a version capable of carrying up to 2,700 kg (about 6,000 punds) of cargo is expected. It’s still a small spacecraft compared to the Dragon, which can carry up to six tons (more than 13,000 pounds) of cargo.

A fundamental difference between the two spacecraft is that whan it comes back to Earth the Cygnus gets destroyed in the atmosphere so it can only carry cargoes the International Space Station wants to get rid of while the Dragon can safely splash down in the sea. For this reason, the Dragon can also be used to bring cargo back to Earth, typically scientific experiments, test samples and other materials, up to about three tons (more than 6,500 pounds).

The missions that include the transport of goods to the International Space Station are already optimized to take advantage of the characteristics of each of the existing cargo spacecraft. With the addition of Cygnus, there will be a wider range of choice especially for the Americans, who after being forced to rely on foreign spacecraft can choose between two American ones.

The Cygnus spacecraft carries various supplies to the International Space Station including food and clothing. However, there’s nothing essential because this is a test mission therefore it’s the one that has the greatest probability of being interrupted for some problems that can not be solved.

The date originally scheduled for the docking was September 22 but when the Cygnus spaceship came 30 km (almost 20 miles) from the International Space Station a direct communication was established and a problem was found. The GPS navigation data exchanged between Cygnus and the Station were rejected by the Cygnus computer causing the abort of the approach.

An analysis made by Orbital Sciences led to the discovery of the use of a different date format by the computers of the Cygnus spaceship and the one of the Station. It seems absurd and yet computers built in various countries sometimes end up using formats or different measurement units causing communication problems.

Luckily, today’s spacecraft are equipped with computers whose software can be updated even in orbit. The Cygnus spacecraft can stay in orbit for weeks so there was the time to send it an updated program and the new docking attempt was postponed to allow the crew of the International Space Station to deal with the arrival of new crew members.

Today the communication systems worked properly. The Cygnus spacecraft approached the International Space Station step by step, demonstrating that it can operate in a manner appropriate to move safely near the Station. Once arrived at 10 meters away, astronaut Luca Parmitano, assisted by his colleague Karen Nyberg, caught it with the Canadarm2 robotic arm. With a slow maneuver, Parmitano moved the Cygnus until it docked.

It took a week longer than expected but Orbital Sciences has shown it can deal with unexpected problems. In the end, the mission of the Cygnus spacecraft has been accomplished and maybe in December 2013 it will start its first official mission among those included in the contract with NASA that will now be activated.

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