A handout artist shows a grey impression irregularly-shaped asteroid, rocky objects that often pass safely past Earth. AFP

A large asteroid will safely zoom between Earth and the Moon on Saturday, a once-in-a-decade event that will be used as a training exercise for planetary defence efforts, according to the European Space Agency.

The asteroid, named 2023 DZ2, is estimated to be 40 to 70 metres (130 to 230 feet) wide, roughly the size of the Parthenon, and big enough to wipe out a large city if it hit our planet.

At 19:49 GMT on Saturday it will come within a third of the distance from the Earth to the Moon, said Richard Moissl, the head of the ESA's planetary defence office.

Though that is "very close", there is nothing to worry about, he told AFP.

Small asteroids fly past every day, but one of this size coming so close to Earth only happens around once every 10 years, he added.

The asteroid will pass 175,000 kilometres (109,000 miles) from Earth at a speed of 28,000 kilometres per hour (17,400 miles per hour). The moon is roughly 385,000 kilometres away.

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An observatory in La Palma, one of Spain's Canary Islands, first spotted the asteroid on February 27.

Last week, the UN-endorsed International Asteroid Warning Network decided it would take advantage of the close look, carrying out a "rapid characterisation" of 2023 DZ2, Moissl said.

That means astronomers around the world will analyse the asteroid with a range of instruments such as spectrometers and radars.

The goal is to find out just how much we can learn about such an asteroid in only a week, Moissl said.

It will also serve as training for how the network "would react to a threat" possibly heading our way in the future, he added.

Scientifically interesting

Moissl said preliminary data suggests 2023 DZ2 is "a scientifically interesting object", indicating it could be a somewhat unusual type of asteroid. But he added that more data was needed to determine the asteroid's composition.

The asteroid will again swing past Earth in 2026, but poses no threat of impact for at least the next 100 years -- which is how far out its trajectory has been calculated.

Earlier this month a similarly sized asteroid, 2023 DW, was briefly given a one-in-432 chance of hitting Earth on Valentine's Day 2046.

But further calculations ruled out any chance of an impact, which is what normally happens with newly discovered asteroids. Moissl said 2023 DW was now expected to miss Earth by some 4.3 million kilometres.

Even if such an asteroid was determined to be heading our way, Earth is no longer defenceless.

Last year, NASA's DART spacecraft deliberately slammed into the pyramid-sized asteroid Dimorphos, significantly knocking it off course in the first such test of our planetary defences.