illustration of a slice of  the earth


Tracking the asteroids that menace Earth

The discovery of an asteroid the size of a small shipping truck mere days before it passed Earth on Jan. 26 - an object that posed no threat to humans - highlights a blind spot in our ability to predict those that could actually cause damage, astronomers say.

The asteroid is the fourth-closest of all past and future Earth approaches in the 300 years from 1900 to 2200, according to data from NASA's Centre for Near Earth Object Studies (CNEOS).

Near-Earth Objects (NEOs) are predominantly asteroids, but a small fraction are comets. So far, astronomers have found more than 31,000 NEOs, ranging from a few metres to a few kilometres in diameter. The graphic below shows the closest passes of objects in the past and predicted in the future, scaled by the object’s minimum estimated diameter.

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NASA for years has prioritized detecting asteroids much bigger and more existentially threatening than 2023 BU, the small space rock that streaked by 2,200 miles from the Earth's surface, closer than some satellites. If bound for Earth, it would have been pulverized in the atmosphere, with only small fragments possibly hitting the ground.

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But 2023 BU sits on the smaller end of a size group, asteroids 5 to 50 meters in diameter, that also includes those as big as an Olympic swimming pool. Objects that size are difficult to detect until they wander much closer to Earth, complicating efforts to brace for one that could affect a populated area.

The probability of an Earth impact by a space rock, called a meteor when it enters the atmosphere, of that size range is fairly low, scaling according to the asteroid's size: a 5-meter rock is estimated to target Earth once a year, and a 50-meter rock once every thousand years, according to NASA.

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But with current capabilities, astronomers can't see when such a rock targets Earth until days before it happens. "We don't know where most of the asteroids are that can cause local to regional devastation," said Terik Daly, a planetary scientist at the Johns Hopkins Applied Physics Laboratory.

The roughly 20-meter meteor that exploded in 2013 over Chelyabinsk, Russia, is a once-every-100-years event, according to NASA's Jet Propulsion Laboratory. It created a shockwave that shattered tens of thousands of windows and caused $33 million in damage, and no one saw it before it entered Earth's atmosphere.

Some astronomers consider relying only on statistical probabilities and estimates of asteroid populations an unnecessary risk, when improvements could be made to NASA's ability to detect them.

"How many natural hazards are there that we could actually do something about and prevent for a billion dollars? There's not many," said Daly, whose work focuses on defending Earth from hazardous asteroids.


Most asteroids can be found orbiting the Sun in the main asteroid belt between Mars and Jupiter. The rocky rubble is left over from the early formation of our solar system about 4.6 billion years ago.

The orbits of these asteroids can be altered by encounters with other space objects, Mars, or the powerful gravity of Jupiter. This can sometimes knock asteroids out of the main belt and send them hurling into new orbits, sometimes crossing those of other planets. There are four types of near-Earth asteroid orbits.

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Ground based telescopes keep an eye on the night sky, taking images at certain intervals and studying them to see whether an object is moving faster compared to the background stars. This parallax effect can help find an asteroid.

Asteroid being detected in night sky

Computers check the detected object against a database of known objects, and if no match is found, it is added to a global list of objects to confirm. Space organizations and amateur astronomers then share additional observations, which help refine the object’s projected path.


One major upgrade to NASA's detection arsenal will be NEO Surveyor, a $1.2 billion telescope under development that will launch nearly a million miles from Earth and surveil a wide field of asteroids. It promises a significant advantage over today's ground-based telescopes that are hindered by daytime light and Earth's atmosphere.

So far, two major ground-based NASA-funded sky survey projects in the United States – Catalina Sky Survey in Arizona and The Panoramic Survey Telescope and Rapid Response System in Hawaii – have discovered about three-quarters of all known Near-Earth Asteroids (NEAs), significantly increasing the pace of asteroid discoveries over the past two decades.

The new telescope will help NASA meet a goal assigned by Congress in 2005: detect 90% of the total estimated amount of NEAs larger than 140 meters, or those big enough to destroy anything from a region to an entire continent.

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"With Surveyor, we're really focusing on finding the one asteroid that could cause a really bad day for a lot of people," said Amy Mainzer, NEO Surveyor’s principal investigator. "But we're also tasked with getting good statistics on the smaller objects, down to about the size of the Chelyabinsk object."

NASA has fallen years behind on its congressional goal, which was ordered for completion by 2020. The agency proposed last year to cut the telescope's 2023 budget by three quarters and a two-year launch delay to 2028 "to support higher-priority missions" elsewhere in NASA's science portfolio.

Asteroid detection gained greater importance last year after NASA slammed a refrigerator-sized spacecraft into an asteroid to test its ability to knock a potentially hazardous space rock off a collision course with Earth.

The successful demonstration, called the Double Asteroid Redirection Test (DART), affirmed for the first time a method of planetary defense.

"NEO Surveyor is of the utmost importance, especially now that we know from DART that we really can do something about it," Daly said.

"So by golly, we gotta find these asteroids."


Data as of February 1, 2023. Impact scenarios may vary slightly depending on projectile density, impact velocity and angle, and target density.


Centre for Near Earth Object Studies (CNEOS), NASA; Earth Impact Effects Program, Imperial College London; Jet Propulsion Laboratory, NASA; European Space Agency; Britannica.

Additional work by

Jackie Gu

Edited by

Gerry Doyle