NEW YORK — After almost five years on the ground, NASA’s Space Shuttle Endeavour is being primed for launch following a major overhaul to upgrade and refit the 100-ton space plane.

The shuttle and its STS-118 astronaut crew are slated to launch Aug. 8 on a construction mission to the international space station (ISS). The upcoming spaceflight will mark Endeavour’s first flight since late 2002, following several years’ worth of maintenance and modifications.

“It’s like a new space shuttle,” Wayne Hale, NASA’s shuttle program manager, said of Endeavour, adding that the orbiter has been inspected from stem to stern. “It’s like driving a new car off the showroom floor.”

During Endeavour’s down time, engineers inspected some 241 kilometers of wiring, enhanced its avionics interface, and added a new power transfer and engine monitoring systems, among other upgrades.

Engineers are working to fix a cabin leak traced to a pressure relief valve behind the toilet, but not a part of the plumbing, inside Endeavour’s crew module. As of Aug. 1, NASA decided to replace the valve with one from the Atlantis shuttle. The work should not prevent Endeavour from its scheduled liftoff.

Endeavour is NASA’s youngest space shuttle. It was commissioned in 1987 as a replacement for its sister ship, Challenger, which was destroyed in January 1986, shortly after liftoff, killing its six-astronaut crew, which included “Teacher in Space” Christa McAuliffe

Endeavour rolled out of its then-Rockwell International—now Boeing—hangar in Palmdale, Calif., in 1991, to join NASA’s shuttle fleet.

Endeavour now is poised to make its 20th launch into space with NASA’s STS-118 mission to deliver cargo, spare parts and a new piece of starboard-side framework to the ISS. Commanded by veteran astronaut Scott Kelly, the mission also features the first flight of teacher-turned-spaceflyer Barbara Morgan, who served as McAuliffe’s backup in 1986.

Standing out among Endeavour’s nearly 200 modifications is a trio of systems making their first operational appearance on a NASA shuttle flight.

Endeavour will be NASA’s first shuttle to carry a Station-Shuttle Power Transfer System (SSPTS), which is designed to allow the shuttle to siphon electrical power from the station’s 120-volt grid via a docking port connection. The system then converts that power to feed the orbiter’s 28-volt system. If successful, the new power transfer system will allow the STS-118 astronauts to conserve Endeavour’s own fuel cell supplies.

“[A]ssuming that it works, we’ll be able to fly a 14-day mission, so we can add three extra days to our flight,” Kelly said in a NASA interview. Endeavour also sports the first fully activated Advanced Health Management System to watch over the shuttle’s three main engines during launch, as well as a three-string GPS for pinpoint navigation during landings, NASA said.

The health management system is designed to monitor vibrations in each of the high-pressure fuel and oxidizer turbopumps—which rotate 34,000 times and 23,000 times per minute, respectively. The turbopumps feed Endeavour’s three main engines with the nearly 2 million liters of propellant required for the 8.5-minute launch into space. If an engine’s turbopumps vibrate too much, the new system is designed to shut it down.

“An engine would be shut down before it could progress to any catastrophic situation,” Hale said of the monitoring system.

The three-string GPS system, which was tested in part on a shuttle flight last year, replaces Endeavour’s 1950s-era Tactical Air Navigation, or Tacan, system that gradually is being phased out worldwide, he added.

“We’ve got a far superior system, far safer, far more accurate to fly our big glider back home with,” Hale said.

In addition to testing new shuttle technology, Endeavour now is equipped with hardware already installed aboard its sister ships Discovery and Atlantis.

Endeavour has been outfitted with a “glass cockpit,” a series of flat screen, full-color multi-functional electronic displays that present flight data to the orbiter’s astronaut crew.

“Endeavour was the last orbiter to get that modification,” TassosAbadiotakis, NASA’s vehicle flow manager for Endeavour, told Space News. Atlantis first flew with the upgrade during its STS-101 mission in 2000, followed by Discovery in 2005 during NASA’s STS-114 flight.

Like Discovery and Atlantis, Endeavour now is equipped to carry a 15-meter sensor boom, a vital extension of the orbiter’s robotic arm that allows astronauts to scan the orbiter’s heat shield in flight for signs of damage, Abadiotakis said.

The sensor boom was added as a safety measure following the February 2003, loss of seven astronauts aboard Shuttle Columbia, which broke up during re-entry after the orbiter’s heat shield had been damaged by fuel tank debris.

The orbiter’s wing leading edge sensors, also a post-Columbia safety measure designed to record any impacts from debris or micrometeorites, sport a new voltage booster to extend their in-flight operations, NASA said.

A team of up to 200 shuttle workers helped upgrade Endeavour, and performed vital wiring and structural inspections to once more prepare the spacecraft for flight.

“I would say that it’s better than when it first rolled out of the barn in Palmdale,” Abadiotakis said of Endeavour. “We basically reset the vehicle, the clock, back to zero.”