Russian Supply Ship Docks at Space Station on 2nd Try
This view from a camera on the exterior of the International Space Station shows the unmanned Progress 37 cargo ship and Soyuz crew-carrying capsule on July 2, 2010 after another unmanned cargo ship - Progress 38 - failed to dock as planned.
Credit: NASA TV.

An unmanned Russian cargo ship that missed its first chance to dock at the International Space Station last week tried again Sunday ? this time successfully linking up with the orbiting lab.

The Progress 38 cargo ship docked flawlessly at 12:17 p.m. EDT (1617 GMT) while flying without the safety net of a remote control system that allows cosmonauts inside the space station to take command of incoming supply ships using a joystick should they veer off course.

The successful docking comes two days after the Progress 38 cargo ship's first failed docking attempt on Friday, in which the spacecraft sailed clear past the space station after aborting its delivery run.

"No doubt a sigh of relief on the part of the Russian control team," NASA commentator Rob Navias said in a NASA TV broadcast after today's smooth docking.

The orbital rendezvous occurred 220 miles (354 km) above Earth as both spacecraft flew over the point where the borders of China, Kazakhstan, Mongolia and Russia meet. The Progress 38 supply ship launched Wednesday and delivered 2.5 tons of new equipment, fresh food and other supplies for the space station's six-person crew.

"Congratulations on the successful Progress docking," flight controllers at Russia's Mission Control center near Moscow told the station crew.

Docking glitch traced to interference

Russian engineers suspect it was interference between the Progress vehicle remote control system on the International Space Station and a TV camera on the space freighter that forced the incoming spacecraft to abort its first approach on Friday.

Instead of docking to an aft berth on the station's Russian Zvezda module, the Progress 38 cargo ship flew by at a range of about 3 miles (3 km) to a point about 186 miles (300 km) in front of the space station until another attempt could be made.

For Sunday's docking, Russian flight controllers opted to forgo any use of the remote control system, called the Telerobotically Operated Rendezvous Unit (TORU). Instead, they checked and rechecked the Progress 38's autonomous Kurs navigation system, as well as its backup system, to make sure the spacecraft was completely ready to dock itself at the space station.

"Everything was tested extensively yesterday, including the primary and backup systems," Navias said.

Like Russia's crew-carrying Soyuz spacecraft, the unmanned Progress freighters are designed to fly themselves to the space station using the Kurs navigation system. Soyuz spacecraft can be flown manually by cosmonauts from inside the vehicles, while Progress can be guided in using the TORU system.

Both spacecraft have a long track record for reliability, though some have had to be guided in by cosmonaut every now and then. The last Progress to be docked under cosmonaut control arrived at the station on May 1.

"It is a bit surprising," former NASA astronaut Leroy Chiao, a former space station commander, told SPACE.com of the recent Progress malfunction. "These vehicles are very reliable, and the operations for getting them to stations are quite mature."

Holiday delivery

Sunday's cargo ship arrival at the International Space Station also occurred on the Fourth of July holiday in the United States.

As both a weekend and a U.S. holiday, the station crew had expected to have the day off, but worked through the Progress 38 ship's arrival instead. Of the six people living on the space station, three are Americans and three are Russians.

One of the station's American crewmembers, NASA astronaut Douglas Wheelock, sent July 4th greetings to Earth to mark Independence Day in space.

"In celebration of freedom and liberty?Happy 4th from the [space station's] Russian Segment!" Wheelock wrote in a post on Twitter, where he is chronicling his missing under the name Astro_Wheels.

The station crew will open the Progress 38 cargo ship later today to begin unloading supplies, NASA officials said.

Packed aboard the spacecraft are 1,918 pounds (nearly 870 kg) of propellant for the station, 110 pounds (nearly 50 kg) of oxygen, 220 pounds (99 kg) of water and 2,667 (1,209 kg) pounds of dry cargo ? which includes spare parts, science equipment and other supplies.