Nasa's Artemis I rocket sits on launch pad 39-B at Kennedy Space Center in Cape Canaveral, Florida, on Friday. AFP
The 32-story tall Space Launch System (SLS) rocket and its Orion capsule were due for blastoff from Cape Canaveral, Florida, at 2:17pm EDT (1817 GMT), kicking off NASA's ambitious moon-to-Mars program Artemis program 50 years after the last Apollo lunar mission.
The previous launch bid on Monday ended with technical problems forcing a halt to the countdown and postponement of the uncrewed flight.
Tests indicated technicians have since fixed a leaky fuel line that contributed to Monday's canceled launch, Jeremy Parsons, a deputy programme manager at the space center, told reporters on Friday.
Two other key issues on the rocket itself â€” a faulty engine temperature sensor and some cracks in insulation foam â€” have been resolved to Nasa's satisfaction, Artemis mission manager Mike Sarafin told reporters Thursday night.
Weather is always an additional factor beyond Nasa's control. The latest forecast called for a 70% chance of favorable conditions during Saturday's two-hour launch window, according to the US Space Force at Cape Canaveral.
If the countdown clock were halted again, Nasa could reschedule another launch attempt for Monday or Tuesday.
Dubbed Artemis I, the mission marks the first flight for both the SLS rocket and the Orion capsule, built under Nasa contracts with Boeing Co and Lockheed Martin Corp, respectively.
The Artemis I unmanned lunar rocket sits on the launch pad at the Kennedy Space Center. AFP
It also signals a major change in direction for Nasa's post-Apollo human spaceflight program, after decades focused on low-Earth orbit with space shuttles and the International Space Station.