NASA discovered the issue today during a pre-landing Reaction Control System steering thrusters. The problematic front thrusters, which help control Endeavour after the deorbit burn, failed during testing, NASA said.

Bill Jeffs, a NASA spokesman, said the failed thruster simply won't be used during Friday's scheduled re-entry and landing. "They'll be able to land without it," he said. "The flight control system is in great shape."

According to NASA, the shuttle crew today also tested Endeavour's aerodynamic control surfaces that will be used once the shuttle enters the Earth's atmosphere. That system operated as expected.

The shuttle is scheduled to land at 10:48 a.m. EDT on Friday after spending 16-days aloft. The seven-person crew spent nearly 11 days docked with the International Space Station where they made five spacewalks and used to install the final piece of the Japanese laboratory, along with spare parts and six new batteries to hold power created by the station's solar arrays .

this mission was one of its most technically challenging ever as it called for a whopping five spacewalks and the use of three different robots - two of which were required to work in conjunction to literally hand off massive parts to each other. The robotic arm onboard the space station even made several Slinky-like "walks" down the backbone of the station.