NASA said the suit worn by NASA astronaut Terry Virts has a history of "sublimator water carryover." Water in the sublimator cooling component can condense when the suit is repressurized after a spacewalk, causing a small amount of water to push into the helmet, NASA said.
NASA said International Space Station managers had "a high degree of confidence" in the suit.
On the upcoming spacewalk, Virts and Barry Wilmore will install antennas to provide data to visiting vehicles and deploy 400 feet of cable along the edge of the station.
Virts said he first noticed traces of fluid and dampness in his helmet Wednesday while he was waiting for the crew lock cabin to repressurize in the International Space Station.
He and Wilmore had been outside the space station for nearly seven hours working on the station's robotic arm and performing some maintenance.
Virts immediately alerted fellow astronaut Samantha Cristoforetti about the water and she alerted Mission Control in Houston.
Cristoforetti helped Virts out of his helmet and examined it. She confirmed the presence of moisture, mostly in the helmet absorption pad, or HAP, describing it as "wet and cold."
At the request of Mission Control, Anton Shkaplerov used a syringe to draw as much water as he could from the top of the helmet. Water had collected in the white plastic at the top and around both ear cups. Shkaplerov estimated there was 15 milliliters of water in the helmet.
That's a far cry from the amount of water that accumulated in Luca Parmitano's suit during a spacewalk in July 2013. Between 1 and 1.5 liters of water backed up in the suit and helmet, prompting fears Parmitano could drown in his own helmet. The spacewalk was cut short and NASA implemented some changes to its suits, including the addition of absorbent padding in helmets.
There was another intrusion of water into the helmet during another spacewalk on December 24, 2013. NASA commentator Rob Navias said it was noticed at the same point in the mission as Wednesday's leak -- during the repressurization of the crew lock, the area from which astronauts enter and exit ISS while in space.