Whitson and her NASA crewmate, Shane Kimbrough, were also supposed to install four protective shields over the port where the PMA-3 gateway used to be attached.
The PMA-3 relocation gets the ISS ready for the new International Docking Adapter-3 set to be delivered on a future SpaceX Dragon cargo mission.
As the drama unfolded, Peggy Whitson set a record for the most spacewalks by a woman-eight-and the most accumulated time spent spacewalking-well over 50 hours.
No other details were immediately available about how the shield, which weighs 18 pounds (8 kilograms) and measures 63.6 by 23.4 by 2.6 inches (162 by 59 by 7 centimeters) was lost.
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She completed the walk with her partner, current Space Station commander Shane Kimbrough, as they were preparing a docking station at the International Space Station where they reside.
Covering the access point for the docking port was critical, Huot added, to protect that area of the module from what NASA calls "MMOD" (micro-meteoroids or orbital debris) - or, in layman's terms, "basically any space junk, any space rocks that might impact it".
Spacewalkers have lost objects before but usually the items are small, like bolts. Mission Control instructed the astronauts to retrieve the cloth cover just removed from the docking port, and try to fashion it over the gaping hole left by the lost shield.
The relocated docking port will serve as one of two parking spots for commercial crew capsules under development by SpaceX and Boeing. It was disconnected during a spacewalk last Friday by Kimbrough, the space station's commander and a six-time spacewalker. Whitson has been in orbit on the station since November 2016.
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Russian cosmonaut Anatoly Solovyev holds the all-time spacewalking record, with 16 separate spacewalks and more than 82 hours of cumulative spacewalking time.
It was Whitson's eighth spacewalk, also a record for a female astronaut.
Whitson and Pesquet are officially scheduled to carry out another spacewalk April 6, but needed equipment is awaiting launch aboard an Orbital ATK Cygnus cargo ship that has been grounded by work to fix hydraulic lines in the freighter's Atlas 5 rocket.
According to the AP, Whitson is scheduled to return to Earth in June but may extend her time in space for an additional three months, until September.
Spacewalking astronauts lose piece of shielding
Both spacewalkers then worked to install four protective shields over Tranquility's outboard port where PMA-3 had been attached. The spacewalk began at 7:29 AM (local time) when both Whitson and Shane Kimbrough switched their spacesuits to battery power.