Spacewalking astronauts gave the International Space Station's big robot arm a new hand Thursday.

NASA Commander Randy Bresnik and Mark Vande Hei floated out around 8 a.m. ET and stayed out for nearly seven hours to replace one of two Latching End Effectors (LEE) on the Canadarm2. It's the first of three NASA spacewalks planned over the next two weeks. 

The latching mechanism on one end of the 17.7-metre Canadarm2 malfunctioned in August. It had to be replaced before the arrival of an Orbital ATK supply ship in November. 

Canadarm2 Earth

The Canadarm2, which is part of the International Space Station's Remote Manipulator System, is pictured with the Earth in the background during a daytime orbital pass on April 16, 2017. (NASA)

Hustling through their work, the spacewalkers unbolted the old mechanism and promptly installed the spare. Initial testing by ground controllers indicated success.

"All right, gentlemen, we show a good arm," Mission Control radioed.

"That is great news, Houston," Bresnik said. "Much rejoicing."

Canadarm2 in use for 16 years

Canadarm2 has two identical LEEs that are used to grab visiting spacecraft, as well as provide data and telemetry to the rest of the Canadian-built Mobile Base System. This bundle of latches — more than a metre long — also attaches to grapple fixtures outside the space station. That way, the arm can move like an inchworm across the sprawling structure. 

The Canadian-built arm has been in orbit for 16 years. The two latching mechanisms, one on each end of the arm, have been used nearly 400 times, and engineers attribute the recent trouble to wear and tear. 

Canadarm2

In this photo from 2005, astronaut Stephen K. Robinson is seen anchored to a foot restraint on the Canadarm2. (NASA)

The latching mechanism on the opposite end will be replaced early next year.

It was the first spacewalk for Vande Hei, a rookie astronaut who arrived at the orbiting outpost a few weeks ago. 

"Congratulations, my friend, on becoming the 221st human to exit in your own personal spacecraft into the void of space," said Bresnik, a veteran spacewalker. 

As the duo worked, they marvelled over the views of Earth below and the full moon above. They'll venture back out on Tuesday to lubricate the new mechanism and do other chores.