When you are fast asleep at night, telescopes atop lofty mountains constantly look for space rocks that might fly close to Earth or even hit us.

Congress directed NASA to find and track the asteroids and comets that swoop into our neighborhood, meaning some 30 million miles from Earth. Thousands of large Near-Earth objects are thought to remain undiscovered.

The telescopes in Hawaii, Arizona, and beyond have spotted 95 percent of the giant monsters one kilometer wide or larger that would cause planetary destruction. 40 percent of the rocks are larger than 140 meters. These are large and threatening.

There are a lot of those out there waiting to be discovered, according to Larry Denneau, one of the researchers who heads the Asteroid Terrestrial-impact Last Alert System. Two percent of these rocks are found each year. About 500 such rocks are discovered annually.

"You need to know what's coming"

In the next century or so, there will be no known asteroid over 500 feet across. Astronomers say that the chances of a major impact in our lifetimes is very small. Impacts by objects around 500 feet in diameter occur every 10,000 to 20,000 years, and impact from a rock could be a half-mile across or larger. Something threatening could surprise us, like the asteroid that flew just 40,000 miles from Earth in 2019. That is why watching is important. We can prepare for an impact and move people out of the way, even though we might not be able to get an approaching rock away from our planet.

The director of the Catalina Sky Survey in Arizona told Mashable that you need to know what is coming and how hard it is going to hit.

Kansas City is home to half a million people and could be destroyed by a small asteroid. The surveys for large and small rocks are crucial.

Scientists at the three telescope surveys that discover most of these near-Earth objects talk to us about how they find relatively tiny unknowns in vast, starry skies.

Via Giphy

The asteroid is seen and tracked by the telescope survey in 2019. The credit is given to the ATLAS.


the Pan-STARRS telescope

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

The Panoramic Survey Telescope and Rapid Response System sits atop Haleakal, a volcano in Hawaii. The survey consists of two telescopes looking up at the sky and looking for unusual movement. A strong NEO candidate is a fast object.

The telescopes funded by NASA are great at picking up moving dots in the sky because they have some of the most powerful digital cameras in the world. A good consumer digital camera has 20 million bits. Pan-STARRS has been a leader in detecting near-Earth asteroids larger than 450 feet wide for the past decade.

The cameras take four pictures of the same spot in the sky over an hour, and then the software looks for movement in the frames. Pan-STARRS can find up to 5,000 previously unknown objects in the distant asteroid belt on a typical night. The Minor Planet Center is the world's official clearinghouse for cataloging objects in our solar system. Pan-STARRS will follow up on new discoveries. Potentially hazardous asteroids are those that come within five million miles of Earth.

NASA will give a warning to the White House and other government leaders if a rock larger than 30 feet is found to have a one percent chance of hitting Earth. NASA has never issued such a warning.

the Meteor Crater in Arizona

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

Slow rocks in the sky are problematic, because they are often faster moving than other objects in the sky.

A rock coming straight or nearly straight from our view is a slow- moving object. A few years ago, the asteroid 2019 OK swung just 40,000 miles from our surface, and it snuck up on us by moving very slowly.

Pan-STARRS may detect hundreds of new objects each year, but it would likely detect even more if it weren't for a number of challenges. Earth's large, extremely vivid moon makes detecting tiny, distant objects difficult or impossible. Survey nights are ruined by bad weather. Pan-STARRS has to compete with professional golf tournaments for the use of the only boom lift on Maui when a telescope goes offline. Having two operating telescopes makes a big difference in their productivity.

Other telescopes are still looking at the solar system when Pan-STARRS is not working.

Catalina Sky Survey

a Catalina Sky Survey telescope at night

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 are working together to find 90 percent of the space rocks.

The Catalina Sky Survey has three telescopes located in Arizona's Santa Catalina Mountains.

The Catalina Sky Survey is funded by NASA and takes a break when there is a full moon. We need to cover as much sky as possible.

"The more eyes on the sky, the better we are."

Pan-STARRS tends to find more larger NEOs than Catalina does, but Catalina tends to find more smaller ones. It is good that something under 140 meters is capable of destruction.

The Catalina Sky Survey started detecting hundreds more asteroids in 2016 and it is attributed to telescope improvements. Catalina looks at the night sky for objects moving against the stars. The Minor Planet Center gets anything interesting. New discoveries are often made.

There are still many asteroids to be found. He said that they would benefit from having better equipment.

a graph showing the number of near-Earth objects detected by NASA telescopes

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


inside the dome of an ATLAS telescope

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

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

When smaller objects come to Earth and whizz by, that is when we see them. Each night, the entire sky can be seen from the telescopes in Hawaii, Chile, and South Africa.

Denneau said that they were looking everywhere to find anything close to them.

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

The telescopes look for any moving dots in multiple images of the sky. New candidates are sent to the Minor Planet Center. Someone is always on call, and they are prepared for an emergency.

The telescopes can give a day, weeks, or months of notice. They can see something around 65 feet across a few days, and a 300 foot-wide rock can be seen weeks out. Small rocks that hit Earth have been spotted by ATLAS. The survey found a 13-foot-wide asteroid in June. Twelve hours later it exploded in the sky.

Soon, these three major survey operations will get help.

NASA will launch its Near-Earth Object Surveyor space telescope in the year of 2026. Out in space, the mission will reveal the compositions, shapes, and orbits of these rocks, and find thousands of unknown nearby objects. The giant Vera C. Rubin Observatory will come online in 2023. Among other goals, the telescope will take inventory of the millions of objects in our solar system, including many near-Earth objects.

The more eyes look at different parts of the sky, the more menacing rocks will be found. It will happen again if it is from a rock 100 feet, 200 feet, 300 feet, or 1,000 feet across. It might be in your lifetime. There are normal parts of space.

This is a continuous process that happens in the solar system.