The ‘planet killer’ asteroid in motion

The space rock had been hidden by the glare of the sun, suggesting that more large asteroids are in a region of the solar system that is difficult to study from Earth

Astronomers searching for modest-sized asteroids that could vaporize a city or larger beasts that could sterilize Earth’s surface have spotted a potential new threat. But there is no immediate need for concern – it will be many generations before it poses a danger to our planet.

Detection of unexplored space rocks relies on spying on sunlight reflecting off their surfaces. But some asteroids occupy corners of the sky in which the glare of the sun smothers them and, like embers fluttering past a thermonuclear bonfire, they disappear from view.

Last year, in hopes of finding asteroids cloaked in excessive sunlight, an international team of astronomers co-opted a camera primarily designed to study the universe’s notoriously elusive dark energy.

In an announcement made earlier this month based on a survey first published in September in The Astronomical Journalthe researchers announced the discovery of three new projectiles drowned in light.

One of them, 2022 AP7, is about 1.6 kilometers long, and its orbit intersects with Earth’s path around the sun, coming as close as 7 million kilometers to Earth itself – uncomfortably close by cosmic standards (though much further away than Earth’s moon).

That makes 2022 AP7 “the largest potentially hazardous asteroid discovered in the past eight years or so,” said Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, DC, and author of the study.

After the asteroid was discovered in January, other observatories studied its motion and other astronomers retrospectively identified it in older images. This dataset has made it clear that it will not visit Earth in the next century, and possibly much longer.

“There is an extremely low probability of an impact in the foreseeable future,” said Tracy Becker, a planetary scientist at the Southwest Research Institute who was not involved in the study.

But the gravitational pull of objects around the solar system – including our own planet – ensures that asteroids passing Earth don’t dance the same way forever. Asteroid 2022 AP7 is no exception.

“Over time, this asteroid will get brighter and brighter in the sky as it begins to cross Earth’s orbit closer and closer to where Earth really is,” Dr Sheppard said.

It’s possible that “in the next few thousand years this will become a problem for our descendants,” said Alan Fitzsimmons, an astronomer at Queen’s University Belfast who was not involved in the study.

What if, in the most unlucky of timelines, 2022 AP7 finally impacts Earth?

“It’s what we call a planet killer,” Dr Sheppard said. “If this hit the Earth, it would cause planetary destruction. It would be very bad for life as we know it.”

But since we are safe for many generations, the orbit of this asteroid is not its most remarkable feature. “The interesting thing about 2022 AP7 is its relatively large size,” said Cristina Thomas, a planetary astronomer at Northern Arizona University who was not involved in the study. Its existence suggests that other elephantine asteroids, shrouded in sunlight, remain surprisingly unknown.

Today, astronomers searching for potentially dangerous asteroids – those that approach at least 7.3 million kilometers from Earth and are too large to be incinerated without incident by our atmosphere – are focusing on the search for rocks about 140 meters in diameter.

There are most likely tens of thousands, and less than half have been identified. They could wreak nationwide destruction. Such threats have motivated NASA and other space agencies to develop planetary defense missions like Dart, the spacecraft that successfully adjusted the orbit of a small, non-threatening asteroid in September.

Most of the asteroids a kilometer long and longer – far less common, but capable of global devastation – have already been discovered. But “we know some are yet to be found,” said Professor Fitzsimmons.

Several no doubt sneak near Mercury and Venus. But it is “incredibly difficult to discover objects inside Earth’s orbit with our current discovery telescopes”, Professor Thomas said. For most hours of the day, the sun blinds terrestrial telescopes and objects can only be hunted for the few minutes around dusk.

To overcome this limitation, the astronomers who detected 2022 AP7 relied on the dark energy camera of the 4-meter Víctor M Blanco Telescope in Chile. Not only can it examine wide swaths of the sky, but it’s also sensitive enough to find faint objects engulfed in sunlight.

So far, the camera has found two other near-Earth objects: a planet-killer whose orbit never crosses Earth’s but is closer to the sun than any other known asteroid, blazing its surface at temperatures extreme enough to liquefy the lead ; and a smaller boulder, the size of a country killer, which poses no risk.

Twilight survey capabilities will eventually be eclipsed by NASA’s Near-Earth Object Surveyor mission. Launching later this decade, this Earth-orbiting infrared observatory will stare at the sun’s glare and find most dangerous asteroids that other surveys have missed.

“We want to do everything possible not to be surprised,” Professor Thomas said. That’s why these surveys exist: to find asteroids impacting Earth several lifetimes in advance so that, through energy prods or nuclear blasts, we can send these monsters back into the shadows.