Three new potentially dangerous asteroids have been discovered, including a large one that is 1.5 km long.

A 1.5 kilometer wide asteroid is no joke. Even a much smaller one, about the size of a house, can explode with more force than the first nuclear weapons. When an asteroid is larger than 1 km in diameter, astronomers call it “killer planets.” The impact energy released by a planet killer hitting Earth would be devastating, so it is extremely important to know where these asteroids are and where they are headed.

Our defense capability against asteroid strikes is in its infancy, so early notification of asteroids that could cross Earth’s orbit is critical. We will need time to prepare.

Imagine a time a few hundred years ago when the Solar System was largely mysterious to us compared to now. Earth has craters, so we know something fell on it. But we didn’t know much more than that.

Earth’s best-known impact crater is probably Chicxulub Crater, the physical evidence of the impact that led to the extinction of the dinosaurs. But it was not easily visible.

Other craters are more visible, like this one in Arizona. It’s called Barringer Crater, or, less fancifully, Meteor Crater.

So we know that space rocks hit Earth. And we know they’ve also hit the Moon, Mercury, and Mars. We now know that no planet has been safe from impacts in the long history of the Solar System.

We now have an entire scientific enterprise aimed at finding and cataloging the Solar System’s asteroid population. In 1994 the US Congress told NASA to get busy and find near-Earth objects (NEOs) larger than 1 km in diameter. There are about 25,000 asteroids larger than 140 km in diameter near our planet’s orbit. The size of 140 km was chosen because one that big can destroy an entire city.

When the US Senate gave NASA the mandate to find asteroids in 1994, the Senate wanted the agency to find 90% of asteroids larger than 140 meters. But as of 2022, scientists believe we’ve only found about 40% of them.

This study adds two more to the list and a third smaller one.

An international team of astronomers using the Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile has spotted three asteroids lurking in the inner region of the orbits of Earth and Venus. They presented their findings in an article in The Astronomical Journal. It’s “A Deep and Wide Twilight Survey for Asteroids Interior to Earth and Venus,” and the lead author is Scott Sheppard, an astronomer at the Carnegie Institution for Science’s Earth and Planets Laboratory.

The inner Solar System is a very difficult environment for searching for asteroids. There are only two ten-minute windows each night where astronomers can search the region. The Sun’s glare creates a bright background glow that confuses observations. Earth’s atmosphere adds another layer of difficulty by distorting and obscuring observations.

“Our twilight survey scans the region within the orbits of Earth and Venus for asteroids,” Shepard said. “So far, we have found two large near-Earth asteroids that are about 1 kilometer in diameter, a size we call planet killers.”

The asteroid of concern is a 1.5km one called 2022 AP7. Its orbit intersects that of Earth, so it is a potential threat. The other two are called 2021 LJ4 and 2021 PH27, but their orbits are completely inside Earth’s orbit, so they pose no threat. The researchers behind this paper believe there are more asteroids in this region, although scientists have only discovered about 25.

“There are probably only a few NEAs of similar sizes to be found, and these undiscovered large asteroids probably have orbits that keep them inside the orbits of Earth and Venus most of the time,” Sheppard said. “Only about 25 asteroids with orbits completely inside the Earth’s orbit have been discovered to date, due to the difficulty of observing near the Sun’s glow.”

Spotting tiny objects like these asteroids in the Sun’s glow, when there are only two 10-minute observing periods each night, is beyond the capabilities of most telescopes. It takes a unique tool to work effectively in these conditions. The instrument of choice was the Dark Energy Camera (DEC) at the Cerro Tololo Inter-American Observatory in Chile.

DEC was not developed to hunt asteroids. Researchers created it to chase a different, more elusive goal: Dark Energy, the mysterious force driving the expansion of the Universe. DEC needs to observe hundreds of millions of galaxies to fulfill its mission. To do this, DEC is both deep and wide: it can look deep into the Universe while simultaneously recording a wide field of view. Only in this way can it record the movements and conditions of galaxies in the early age of the Universe.

This ability to observe deep and wide at the same time means DEC is well-suited for finding asteroids near the Sun.

“Large areas of sky are required because inner asteroids are rare, and deep images are required because asteroids are faint, and you’re fighting the bright twilight sky near the Sun as well as the distorting effect of Earth’s atmosphere,” Sheppard said. “DCam can cover large areas of the sky at depths unattainable with smaller telescopes, allowing us to go deeper, cover more sky, and probe the inner Solar System in ways never before possible.”

One way astronomers classify asteroids like 2022 AP 7 is by a metric called Earth MOID, which stands for Minimum Earth Orbit Intersection Distance. It is a way of assessing the collision potential between astronomical objects. In this case, the two objects are the Earth and the asteroid. The MOID of 2022 AP 7 is only 0.0475 au. That’s only about 4.4 million kilometers or 2.7 million miles. Not much in astronomical terms.

With an MOID this small, 2022 AP 7 is definitely a Potentially Hazardous Asteroid (PHA.) Its MOID and size mean it meets the definition. It is probably the largest PHA found in about eight years.

This research isn’t just about finding dangerous asteroids so we can prepare to defend Earth from future impacts. It goes to the heart of understanding our Solar System. Since asteroids in the inner Solar System are so difficult to detect, our theoretical models of object populations are based on what is easiest to see. This creates a bias in our models, and DEC helps mitigate this bias.

The Dark Energy Camera was built to perform the Dark Energy Survey, which was completed in 2019. Now astronomers are turning the camera’s power to other things. As this study shows, this effort is producing substantial results.

“Our DECam survey is one of the largest and most sensitive searches ever conducted for objects within Earth’s orbit and near the orbit of Venus,” Shepard said. “This is a unique opportunity to understand what kinds of objects are hidden in the inner Solar System.”

2022 AP 7 is not yet an immediate threat to Earth. As a potentially hazardous asteroid, its orbit will cross Earth’s orbit at some point in the future. But we don’t know when.

More: