Explained | Why did NASA cancel the Moon rocket launch?

NASA has scrubbed a test flight of its powerful new rocket, in a setback to its plan to send humans back to the Moon and eventually to Mars, but may shoot for another launch attempt on Friday.

"We don't launch until it's right," NASA administrator Bill Nelson said after an engine issue forced the cancellation of Monday's flight from the Kennedy Space Center in Florida.

What is the mission about?

The goal of the mission baptised Artemis 1 after the twin sister of Apollo is to test the 98-metre Space Launch System (SLS) rocket and Orion crew capsule that sits on top.

The mission is uncrewed -- mannequins equipped with sensors are standing in for astronauts and will record acceleration, vibration and radiation levels.

This is NASA's first Moon mission in nearly 50 years after Apollo 17 landed the last humans on the satellite in December 1972.

What are the mission parameters?

The Orion capsule is to orbit the Moon to see if the vessel is safe for people in the near future.

During the 42-day trip, Orion will follow an elliptical course around the Moon, coming within 60 miles (100 kilometres) at its closest approach and 40,000 miles at its farthest -- the deepest into space by a craft designed to carry humans.

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One of the mission's primary objectives is to test the capsule's heat shield, which at 16 feet in diameter is the largest ever built.

On its return to Earth's atmosphere, the heat shield will have to withstand speeds of 25,000 miles per hour and a temperature of 5,000 degrees Fahrenheit (2,760 degrees Celsius) -- roughly half as hot as the Sun.

The dummies aboard the spacecraft will record acceleration, vibration and radiation levels.

The craft will also deploy small satellites to study the lunar surface.

The Artemis program aims to establish a lasting human presence on the Moon with an orbiting space station known as Gateway and a base on the surface.

Gateway would serve as a staging and refuelling station for a voyage to the Red Planet that would take a minimum of several months.

Why was the launch cancelled?

Blastoff had been planned for 1233 GMT (6:00 pm IST) but was cancelled because of a temperature problem with one of the rocket's four RS-25 engines.

NASA said a test to get one of the main engines to the proper temperature range for blastoff was not successful.

Overnight operations to fill the orange-and-white rocket with more than three million litres of ultra-cold liquid hydrogen and oxygen were briefly delayed by a high risk of lightning.

Earlier, a potential leak was detected during the filling of the main stage with hydrogen, causing a pause. After tests, the flow resumed.

NASA engineers later detected the engine temperature problem and put a hold on the countdown before scrubbing the launch altogether.

What does NASA have to say about the scrub?

"We don't launch until it's right," NASA administrator Bill Nelson said after an engine issue forced a cancellation of the launch from the Kennedy Space Center in Florida.

"This is a very complicated machine," Nelson said. "You don't want to light the candle until it's ready to go."

Delays are "part of the space business," Nelson said, expressing confidence that NASA engineers will "get it fixed and then we'll fly."

"Our commitment to the Artemis Program remains firm, and we will return to the Moon."

Veteran NASA astronaut Stan Love told reporters he was disappointed but "not really surprised."

"This is a brand new vehicle," Love said. "It has a million parts. All of them have to work perfectly."

(With agency inputs)