HOUSTON - While astronauts set a record for using robotics in space, NASA engineers on Friday focused on a slow leak aboard space shuttle Discovery that if it worsens could cause a first-of-its-kind shutdown of one of three hydraulic systems during Monday's landing attempt.

John Shannon, the shuttle program's deputy manager, said the problem was unlikely to affect the shuttle's return to Earth, but engineers were closely monitoring the leak in the pipeline of an auxiliary power unit that controls hydraulic steering and braking maneuvers.

It is leaking at a rate of "about six drops per hour," and could be leaking harmless nitrogen or flammable hydrazine fuel, Shannon said.

The leak is more likely nitrogen, but there is no way of knowing that, so NASA is treating the problem as if the leak were fuel, he said. If it is fuel, the current rate is still 100,000 times slower than what would cause a fire, he said.

So if nothing changes, the shuttle will land normally, Shannon said.

Just in case, NASA will turn on the power unit with the leak early Sunday as part of its normal testing and then see if the leak rate changes. If it does, NASA may burn off the hydrazine and shut down the power unit before the shuttle returns to Earth to eliminate any fire hazard, Shannon said.

If that happens, the shuttle would land with just its two other power units for the first time in the spacecraft's history.

The shuttle is certified to land normally with two power units, with the only change requiring pyrotechnics to lower landing gear, Shannon said. The shuttle could land with only one power unit, but that would be more difficult, he said.

Also Friday, the shuttle and the international space station set an informal record for robotic activity during a mission. Discovery's crew used the shuttle's robotic arm for yet another examination of the shuttle heat shield.

It was just another day in space for Discovery's robot operators Stephanie Wilson and Lisa Nowak, whom Mission Control called the "robo chicks."

Hours later, minor problems with the space station's robotic arm put Wilson and Nowak behind schedule on their big inspection of the day, looking at the shuttle's left wing for damage from dust-sized meteorites and small pieces of space junk.

The inspection used a laser sensor attached to the extension that doubles the length of the 50-foot robotic arm.

When Mission Control praised Discovery for getting the inspection finished, commander Steve Lindsey answered: "Robo chicks take care of it again."

Engineers will not know until Saturday if the shuttle gets its expected all-clear for landing. The shuttle undocks from the space station that day, and the robotic arm will be hauled out again to look at the left wing and the nose cap ¡ª something easier to do when detached from the space station.

When asked about being dubbed robo chicks, Nowak and Wilson laughed.

"We've actually learned a lot while we've been up here," Nowak told CBS News. "We trained for a couple of years before we got up here."

Nowak and Wilson performed flawlessly at the task, which is like controlling a video game stick and being an air traffic controller at the same time, said NASA robotics operator Simon Aziz.

"It's a piloting skill," Aziz said. "You're flying the joystick and the operator is trying to absorb information off all the screens."