'Planet killer' near-Earth asteroid spotted, the largest in 8 years

Astronomers have spotted three near-Earth asteroids that were lurking undetected within the glare of the sun. One of the asteroids is the largest potentially hazardous object to Earth to be discovered in the last eight years.

The asteroids belong to a group found within the orbits of Earth and Venus, but they're incredibly difficult to observe because the brightness of the sun shields them from telescope observations.

To avoid the sun's glare, astronomers leaped at the chance to conduct their observations during the brief window of twilight. An international team spied the space rocks while using the Dark Energy Camera located on the Víctor M. Blanco 4-metre Telescope located at the Cerro Tololo Inter-American Observatory in Chile.

Their findings published Monday in The Astronomical Journal.

One of the asteroids, called 2022 AP7, is 1.5 kilometres (0.9 miles) wide and has an orbit that could bring it within Earth's path in the future, but it's difficult for the scientists to know when.

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"Our twilight survey is scouring the area within the orbits of Earth and Venus for asteroids," said lead study author Scott S. Sheppard, an astronomer at the Earth & Planets Laboratory of the Carnegie Institution for Science, in a statement.

"So far we have found two large near-Earth asteroids that are about 1 kilometre across, a size that we call planet killers."

The other two asteroids, 2021 LJ4 and 2021 PH27, are on much safer orbits that don't pose a risk to Earth.

Astronomers are intrigued by 2021 PH27, however, because it's the closest known asteroid to the sun. As the space rock moves closer to our star, its surface reaches temperatures hot enough to melt lead.

Asteroid-hunting astronomers face quite a challenge if they want to find space rocks within the inner solar system -- which includes Mercury, Venus, Earth, Mars and the main asteroid belt. In order to avoid the harsh light of the sun, they only have two 10-minute windows each night to sweep the area with ground-based telescopes.

During twilight, astronomers still face the complications of a bright sky in the background due to the sun. And in order to search the inner solar system, their telescopes must focus near the horizon, which means they have to peer through Earth's thick atmosphere and its blurring effects.

If things sound complicated for ground-based telescopes, observations of the inner solar system are impossible for space-based telescopes like Hubble and James Webb because the sun's heat and intense light could fry their instruments, which is why both space observatories are pointed away from the star.

The Dark Energy Camera's wide-field capability helped astronomers overcome their observational challenges and they were able to sweep vast stretches of the night sky in detail.

"Large areas of sky are required because the inner asteroids are rare, and deep images are needed because asteroids are faint and you are fighting the bright twilight sky near the Sun as well as the distorting effect of Earth's atmosphere," Sheppard said. "DECam can cover large areas of sky to depths not achievable on smaller telescopes, allowing us to go deeper, cover more sky, and probe the inner Solar System in ways never done before."

Near-Earth objects are asteroids and comets with an orbit that places them within 30 million miles (48.3 million kilometres) from Earth. Detecting the threat of near-Earth objects that could cause grave harm is a primary focus of NASA and other space organizations around the world.

No asteroids are currently on a direct impact course with Earth, but more than 27,000 near-Earth asteroids exist in all shapes and sizes.

While NASA recently proved it can successfully alter the motion of an asteroid in space with September's Double Asteroid Redirection Test, or DART mission, astronomers first must find space rocks that pose a threat to our planet. Instruments like the Dark Energy Camera, as well as future space-based observatories like the Near Earth Object Surveyor, can pinpoint previously unknown asteroids.

Studying and understanding asteroid populations will also help scientists learn the distribution and dynamics of space rocks -- like how the heat of the sun can fracture and fragment them over time.

"Our DECam survey is one of the largest and most sensitive searches ever performed for objects within Earth's orbit and near to Venus's orbit," Sheppard said. "This is a unique chance to understand what types of objects are lurking in the inner Solar System."