How scientists find the big asteroids that can threaten Earth

When you're fast asleep at night, telescopes atop lofty mountains continually sleuth out unknown space rocks that might fly close to Earth, or even potentially hit us.

Congress directs NASA to find and track the asteroids and comets that swoop into our cosmic neighborhood, meaning some 30 million miles from Earth's orbit around the sun. They're aptly named "near-Earth objects," or NEOs, and thousands of sizable ones are thought to remain undiscovered.

Specialized telescopes in Hawaii, Arizona, and beyond have spotted around 95 percent of the behemoths one kilometer (0.6 miles) wide or larger that would trigger planetary devastation. Yet astronomers have only found 40 percent of the rocks 460 feet (commonly referenced as 140 meters) or bigger. These are still relatively large, menacing objects.

"There are a lot of those out there waiting to be discovered," Larry Denneau, one of the researchers who heads the ATLAS survey, or Asteroid Terrestrial-impact Last Alert System, told Mashable. "140 meters is take-out-a-large-city size." By mid-May 2022, surveys for the 460-foot-plus rocks had spotted around 10,000 of an estimated population of 25,000 such near-Earth objects. At the current pace, two percent of these rocks are found each year. That's about 500 such rocks discovered annually.

Fortunately, no known asteroid over 460 feet across will threaten Earth in the next century or so. The chances of a major impact in our lifetimes is, as far as we know, extremely small, astronomers say. To illustrate, impacts by objects around 460 feet in diameter occur every 10,000 to 20,000 years, and a "dinosaur-killing" impact from a rock perhaps a half-mile across or larger happens on 100-million-year timescales. But something threatening could surprise us, like the unexpected football-field-sized asteroid that swung just 40,000 miles from Earth in 2019. That's why watching is critical. We might not be able to nudge an approaching rock away from our planet — that's an ambitious space endeavor that takes years of planning — but we can prepare for an impact and move people out of the way.

"You need to know what's coming, when it's coming, and how hard it's going to hit," Eric Christensen, the director of the NEO-seeking Catalina Sky Survey in Arizona, told Mashable.

Crucially, even a smaller asteroid, some 100 to 170 feet across, could destroy a place like Kansas City, home to half a million people. So the surveys for rocks large and "small" are vital.

We spoke with scientists at the three telescope surveys that discover most of these near-Earth objects about how they detect relatively tiny unknowns in vast, starry skies.

Via Giphy

The near-Earth asteroid 2019 MO as viewed and tracked by the ATLAS telescope survey in 2019. Credit: ATLAS

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Pan-STARRS

A Pan-STARRS telescope above the clouds atop Maui's Haleakalā. Credit: Rob Ratkowski

Pan-STARRS, or the Panoramic Survey Telescope and Rapid Response System, sits atop Hawaii's dormant volcano Haleakalā at some 10,000 feet. The survey is comprised of two telescopes peering up at the sky and looking for unusual movement, particularly things moving fast or slow. A fast object implies a closer object, meaning a strong NEO candidate.

The telescopes, largely funded by NASA's Near Earth Observation Program, are superb at picking up moving dots in the sky because they're equipped with some of the most powerful digital cameras in the world, containing either 1.4 or 1.5 billion pixels. (A good consumer digital camera might have some 20 million pixels.) With its keen eyesight, Pan-STARRS found 253 of the 456 near-Earth asteroids larger than 460 feet wide discovered in 2021, and has been a leader in these detections for the past decade.

The cameras snap four pictures of the same spot in the sky over an hour, and then software reveals any telltale movement in those frames. On a typical night, Pan-STARRS might find a whopping 3,000 to 5,000 previously unknown objects in the distant asteroid belt, with five to 10 of them being potential near-Earth objects. The high-velocity candidates are promptly sent to the Minor Planet Center, which is the world's official clearinghouse for cataloging objects in our solar system. Pan-STARRS will then follow up on new discoveries, to pin down the rocks' orbits. Some NEOs are ultimately labeled "potentially hazardous asteroids," meaning they come within about five million miles of Earth.

If it turns out a rock larger than 30 feet wide has greater than a one percent chance of hitting Earth, NASA will give an official warning to the White House and other government leaders, who will then assess the situation and inform the public about any potential strike (hopefully it misses Earth or drops into the expansive oceans). NASA, however, has still never issued such a warning.

A 100 to 170-foot-wide object left a crater 0.75 miles across in the Arizona desert some 50,000 years ago. Credit: NASA / USGS

Though faster moving objects in the sky are often NEOs, it's the slow rocks that are particularly problematic. "You need to watch for the slow-moving ones," Richard Wainscoat, an astronomer at the University of Hawaii who leads the Pan-STARRS survey, told Mashable. "You have to be a little careful of those."

From our view on Earth, a slow-moving object might actually be a rock coming straight, or nearly straight, at Earth. A salient example is the asteroid 2019 OK which, as noted earlier, swung just 40,000 miles from our surface a few years ago. "That snuck up on us by moving very slowly," Wainscoat said.

Pan-STARRS may detect hundreds of new near-Earth objects each year, but it would likely detect even more if it weren't for a number of challenges, both natural and unnatural. Earth's oversized, extremely vivid moon brightens the sky and can make detecting relatively tiny, distant objects difficult, or impossible. Bad weather ruins survey nights. And when a telescope goes offline and needs repairs, Pan-STARRS has to compete with professional golf tournaments for the use of the only boom lift on Maui. Having just one operating telescope makes a sizable, months-long dent in their productivity. "It makes a big difference having two operating telescopes," Wainscoat explained. "We can survey twice as much sky."

Fortunately, when Pan-STARRS is hampered by weather or repairs, other telescopes are still sleuthing the solar system.

Catalina Sky Survey

A Catalina Sky Survey telescope, with its dome opened, viewing space at night. Credit: Catalina Sky Survey

There is no competition between the different surveys. They're working together to meet NASA's congressionally-directed mandate to find 90 percent of all space rocks 460 feet wide or larger.

"The more eyes on the sky, the better we are," said Christensen, of the Catalina Sky Survey, whose three telescopes are located in Arizona's Santa Catalina Mountains.

The Catalina Sky Survey, funded by NASA, is on the NEO hunt around 27 days a month, only taking a break when a luminous fuller moon hinders observing. "They can appear anywhere in the sky," explained Christensen. "The name of the game is sky coverage. We need to cover as much sky as we can."

While Pan-STARRS tends to find more NEOs larger than 460 feet wide (or 140 meters), Catalina tends to find a bit more NEOs overall. That's good, because something under 140 meters is quite capable of devastation. "I think 130 meters would also be pretty bad," noted Pan-STARRS' Wainscoat.

Beginning in 2016, the Catalina Sky Survey started detecting hundreds more asteroids each year, an improvement Christensen attributes to telescope improvements. Like Pan-STARRS, Catalina scours the night sky for objects moving against the background of stars. They send anything interesting to the Minor Planet Center. Often, new discoveries are made.

Christensen acknowledges there are still plenty of near-Earth asteroids to find. To speed discoveries up, they could use improved equipment. "We would benefit from having better instrumentation, and larger telescopes," he said.

A graph showing the number of NEOs detected by different telescope surveys. Credit: NASA / Center for Near Earth Object Studies

ATLAS

Inside the dome of an ATLAS telescope. Credit: ATLAS / Larry Denneau

While Pan-STARRS or Catalina peer deeper into the solar system for any movement, the ATLAS survey, or Asteroid Terrestrial-impact Last Alert System, acts more like a huge floodlight looking closely around Earth.

"When smaller objects come to Earth and whizz by, that's when we see them," said ATLAS' Denneau. The ATLAS telescopes — two in Hawaii, one in Chile, and one in South Africa — can scan the entire sky each night.

"We're looking everywhere all the time to find anything close to us," Denneau said.

Similar to the other surveys, the ATLAS telescopes take multiple images of the sky, looking for any moving dots. They send interesting new candidates to the Minor Planet Center. And they're prepared for an emergency. "Someone's always on call," Denneau explained. "Someone always has eyes on the data coming out of the telescopes."

The telescopes can ultimately give a day, weeks, or months of notice. They can spot something around 65 feet across a few days out; a 300 or so foot-wide rock can be detected weeks out. ATLAS has spotted small, fortunately harmless rocks that indeed did hit Earth. For example, in June 2019 the survey detected the 13-foot-wide asteroid 2019 MO. Just twelve hours later it exploded in the sky near Puerto Rico.

These three major survey operations will soon get help.

In 2026, NASA will launch its much-anticipated Near-Earth Object Surveyor space telescope (NEO Surveyor), which will orbit Earth. Out in space, the mission will reveal the compositions, shapes, and orbits of these rocks, and find thousands of still-unknown nearby objects. What's more, in 2023 the giant Vera C. Rubin Observatory, located over 8,700 feet up in Chile's Cerro Pachón ridge, will come online. The over 27-foot-wide telescope will deeply survey the sky, and, among a variety of goals, take inventory of the millions of objects in our solar system, including many near-Earth objects.

As Christensen noted above, the more eyes viewing disparate parts of the sky, the more potentially menacing rocks we'll find. A big impact — whether it's from a rock 100 feet, 200 feet, 300 feet, or 1,000 feet across — will happen again. It could be in your lifetime, or not. Collisions are a normal part of space.

"This is just a continuous process that happens in the solar system," Christensen said.