An asteroid about 2 kilometres in size will pass Earth tonight, about 6.3 million kilometres away from the planet.

Classed as a potentially hazardous object (PHO) — meaning it is bigger than 140 metres in size and within 5 million miles (8.04 million km) of Earth's orbit — the asteroid known as (52768) 1998 OR2 will pass harmlessly by, with its closest approach happening 7:56pm AEST.

First spotted in 1998, the Arecibo Observatory in Puerto Rico has been tracking the space rock ever since.

"Although this asteroid is not projected to impact Earth, it is important to understand the characteristics of these types of objects to improve impact-risk mitigation technologies," the observatory's head of planetary radar, Anne Virkki, said in a statement.

The asteroid is projected to come even closer to Earth in 2079, when it will be just 1.8 million km away from Earth.

It is for this reason that the Arecibo Observatory tracks the asteroid, to ensure plans can be made in future should 1998 OR2 venture too close.

"The radar measurements allow us to know more precisely where the asteroid will be in the future, including its future close approaches to Earth," said Flaviane Venditti, a research scientist at the observatory.

"In 2079, asteroid 1998 OR2 will pass Earth about 3.5 times closer than it will this year, so it is important to know its orbit precisely."

The observatory announced the asteroid's approach last week, saying images of the rock looked similar to a face mask worn during the coronavirus pandemic.

"The small-scale topographic features, such as hills and ridges, on one end of asteroid 1998 OR2 are fascinating scientifically," Dr Virkki said.

"But since we are all thinking about COVID-19, these features make it look like 1998 OR2 remembered to wear a mask."