Officials in charge of NASA's Juno space probe orbiting Jupiter last weekend delayed a crucial maneuver due to a main engine malfunction, they said Wednesday.

In addition, the ship's computer systems automatically went into safety mode early Wednesday (5:47 GMT). Unrelated to the main engine, the switchover was due to a malfunction of two helium valves in the fuel pressurization system.

But officials sought to downplay any serious concerns.

Safety mode turns off instruments and some non-critical spacecraft components, and confirmed the spacecraft was pointed toward the sun so solar arrays power up.

"At the time safe mode was entered, the spacecraft was more than 13 hours from its closest approach to Jupiter," said Rick Nybakken, Juno project manager from NASA's Jet Propulsion Laboratory in Pasadena, California.

"We were still quite a ways from the planet's more intense radiation belts and magnetic fields. The spacecraft is healthy and we are working our standard recovery procedure."

Weighing 3.6 tonnes, Juno is to swing within some 2,600 miles (4,200 kilometers) of the solar system's largest planet, the closest any spacecraft has passed.

The next close fly-by is December 11, with science instruments activated.

Juno first swept close to Jupiter when it entered orbit around the planet in July after a nearly five-year voyage to help study the solar system's origins.

The probe will be examining Jupiter's many layers to measure their composition, magnetic field and other properties. Scientists hope to learn the source of the planet's fierce winds and whether Jupiter is made entirely of gas or has a solid core.