Diagram of the orbits of 2,200 near-Earth asteroids and the planets of the inner solar system. The double NEA Didymos is highlighted.  NASA/JPL-Caltech
Diagram of the orbits of 2,200 near-Earth asteroids and the planets of the inner solar system. The double NEA Didymos is highlighted.  (NASA/JPL-Caltech)

Thousands of Asteroids Orbit Near Earth. NASA's Next-Gen Spotter Helps Protect Us

Thousands of Asteroids Orbit Near Earth. NASA's Next-Gen Spotter Helps Protect Us

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Of NASA’s many launches, those with a fiery blastoff and a gleaming aerial ascent into space get the most love from space fans — but not all missions fly on rockets.

Take Sentry II. It's not a spacecraft or even a ground-based telescope. It’s a computer algorithm designed to forecast future asteroid near-misses and collisions with Earth. To do that, it must search through mountains of asteroid orbital data collected by many observatories. In December, NASA launched Sentry II on its mission.

The inaugural “flight” of a computer program may not sound flashy, but Sentry II’s mission to sift through tens of thousands of known asteroid orbits looking for those that could pose a threat to us is both an enormous task and vitally important to predicting potential calamity.

Keeping an eye on traffic

Think of it like this: You’re driving your car on a crowded freeway, sharing the road with hundreds of other cars all moving along with you, changing lanes, speeding up, slowing down. You constantly monitor other cars through your windshield, out the side windows and in your rearview mirror.

Animation of known near-Earth asteroids and the inner solar system. (NASA/JPL-Caltech)

Many of the cars you can safely ignore: those two or three lanes over, slower traffic falling behind, or that speeder up ahead disappearing into the distance. Nothing to worry about at the moment.

Then there are the cars you watch more closely, those whose speed and trajectory you judge may bring them close. To avoid a collision, you want as much advance warning as possible.

Now imagine your task is to monitor 27,000 cars and predict potential near-car encounters years into the future. That’s like what the Sentry algorithms do. 

Sentry II's fresh perspective

The original Sentry algorithm, in operation since 2002, did its job very well, projecting asteroids’ paths and gravitational interactions with the sun and planets a century into the future. But Sentry I had limitations, and humans had to step in and do some of the math themselves. 


For example, it could not account for changes in an asteroid’s trajectory from influences such as heating effects by sunlight; these are nongravitational influences that cause small but constant deviations.

Sentry I also had difficulty plotting how much the course of an object would deviate when it swung close to Earth and our planet’s gravity. 

Like a Gen-Z youngster with technical savvy that puts older generations to shame, Sentry II takes these influences into account with finesse, coming swiftly to more accurate projections and predictions. This high-powered mission is well equipped to take on the rising tide of orbital data flooding in from around the world. 

Near-Earth asteroids

Since the first asteroid was discovered on New Year’s Day in 1801, generations of astronomers have found many, many more space rocks lurking in the dark. 

Graph showing all near-Earth asteroids discovered each year, by mission or survey, since 1995. (NASA/Center for Near-Earth Object Studies)

Over the last two centuries, larger and more sensitive telescopes, and more recently spacecraft, have probed for the faint, elusive dots of light crawling against the starry backdrop of space. In the last few decades, our awareness of them has risen exponentially.

To date, astronomers know of more than 27,000 near-Earth asteroids, and discover about 3,000 more every year. Of these, 9,948 are larger than 460 feet across, and 889 are larger than a half mile in diameter, large enough to inflict regional or global devastation should they impact our planet — so the importance of tracking their orbital motions and projecting their paths into the future is plain. 

Radar image of 2021 PJ1, the 1,000th near-Earth asteroid detected by radar observations since 1968. 2021 PJ1 is a 65- to 100-foot asteroid that passed about 1 million miles from Earth in August 2021. (NASA/JPL-Caltech)

Asteroids that cross Earth’s orbit and present the possibility of collision are, obviously, the ones to keep the closest eye on. Those larger than 460 feet (about the length of one-and-a-half football fields) are considered potentially hazardous objects — the cars on the freeway that can make your heart jump. 

What would happen if an asteroid hit Earth?

It’s happened before: a major asteroid or comet impact with Earth. Sixty-six million years ago, a 6-mile-wide asteroid hit the northern top of the Yucatan Peninsula and wrought global devastation that contributed to the extinction of the dinosaurs, along with 75% of all species living at the time. 

On average, an impact on the scale of the dinosaur-killer happens about every 100 million years. The good news is, pretty much all asteroids even approaching that size have been found and their orbits mapped out very accurately, and none of them is projected to come near Earth. Their orbits are fairly stable and can be accurately predicted far into the future. 

Illustration of near-Earth asteroid 2020 QG, an SUV-sized rock that passed within 1,830 miles of Earth's surface in August 2020. (NASA)

But smaller asteroids are another matter. Their orbits are more strongly influenced by gravitational interactions with planets and other effects, which complicates predictions of their future whereabouts. They’re also harder to detect, so there are many yet unknown. 

Smaller asteroids also are more numerous, and they both pass near and impact Earth more frequently. 

In 1908, a collision by a large object caused an aerial explosion over Siberia that flattened forests for miles around and sent tremors through Earth that were detected by seismographs in London.

In 2013, a 20-meter object exploded in Earth’s atmosphere above the Russian city of Chelyabinsk. The shockwave shattered windows and caused some brick structures to collapse, and though there were no fatalities, over 1,400 people were injured by indirect causes.

Smaller chunks of rock burn up in Earth’s atmosphere or hit the ground on a daily basis, and though their effects may be minor, they usually go undetected until only hours before impact, if at all, coming at us with little or no advance warning. 

Like a fender bender in traffic, even the smallest meteorite impact could spoil someone’s day, or worse, so keeping tabs on the traffic around Earth is important. Knowing what’s coming at you is the first step in predicting, and possibly avoiding, a collision.