DULLES, Va. -- A commercial Cygnus spaceship finished a three-day pursuit of the International Space Station on Sunday, delivering nearly 2,800 pounds of supplies, fresh fruit, experiments and gifts to reinforce the orbiting laboratory's stockpiles.

The Cygnus spacecraft arrives at the International Space Station. Credit: NASA/JAXA/Koichi Wakata

Just a few hours after the Cygnus was firmly attached to the complex, the space station's crew opened up hatches leading to the spaceship's pressurized cabin and began unloading a potpourri of payloads.

One of the first items transferred into the space station was a colony of ants, an investigation developed by the University of Colorado at Boulder for grade school students to observe the ants' behavior and compare it to how they act in colonies on the ground.

Other experiments aboard the Cygnus spacecraft include a study of drug-resistant bacteria and investigations into how liquids slosh inside containers in microgravity and the behavior of fires in space.

One-third of the 2,780 pounds of cargo inside the Cygnus spacecraft is classified as science and research gear, including experiments designed and proposed by students.

The experiments represent the work of about 600 scientists on the ground, according to Tara Ruttley, NASA's space station program scientist. That represents the largest complement of scientific experiments delivered to the space station by any mission to date under the program's U.S. national laboratory designation.

As part of an effort to reach out to younger students, 23 experiments involving 8,675 elementary, junior high and high school students in the United States and Canada were also delivered aboard the Cygnus spacecraft.

And the Cygnus hauled 33 tiny CubeSats to the space station. During the spacecraft's stay, the astronauts will move the miniature satellites into the station for release in February from the Japanese Kibo laboratry using a specially-designed CubeSat deployer at the end of the Japanese robot arm.

Built and operated by Orbital Sciences Corp., the 16.8-foot-long Cygnus cargo craft launched Thursday on top of an Antares rocket from Virginia's Eastern Shore. After a series of rocket burns to adjust its orbit, the Cygnus closed in on the space station early Sunday guided by GPS and laser navigation data.

The spaceship held its approach just 30 feet below the complex, close enough for astronaut Mike Hopkins to use the space station's Canadian-built robot arm to snag the free-flying Cygnus resupply vessel at 6:08 a.m. EST (1108 GMT) and move it to a connecting port on the outpost's Harmony module.

"The ground crew and Orbital just did amazing job," Hopkins said. "The vehicle itself was just as solid as it could be."

At 12:17 p.m. EST (1717 GMT), the crew opened up hatches leading into the Cygnus spacecraft to begin unpacking.

For Orbital Sciences, it marked the first of eight cargo deliveries under a $1.9 billion contract with NASA, which turned to the private sector to resupply the space station after the space shuttle's retirement. NASA also has a resupply contract with SpaceX.

Follow Stephen Clark on Twitter: @StephenClark1.