There are a staggering number of asteroids floating around the solar system. People have already counted a number of them at least 1.4 million of them—and there are probably more we haven't seen yet.
Most of the asteroids we have observed orbit the sun in the solar system. main asteroid belt between Mars and Jupiter, but a large fraction of them, known as Near Earth Objects or NEOs, have orbits that cross Earth's orbit around the sun. Some of these NEOs could eventually hit Earth, with consequences ranging from a pleasant meteor shower to the extinction of an entire species (sorry, dinosaurs).
While there are many asteroids, space is also mind-bogglingly large. So how often do these asteroids actually come near our planet?
The short answer: It depends on the size of the asteroid. Just as smaller earthquakes happen more often than The Big One, small asteroids pass us more often than catastrophically large chunks of rock.
According to asteroid expert and MIT professor Richard BinzelEarth encounters more than ten tons of dust every day. That’s almost twenty grizzly bears’ worth of dust falling on Earth every day. These tiny dust particles can’t do us any harm, though: they simply burn up in Earth’s atmosphere, creating the kind of meteors you could wish on. (And if you want to catch a glimpse of some of them, the upcoming annual Perseid meteor shower is a great chance to see them!)
If we scale up, rocks from the size of a marble to a bowling ball “get swept up a few times a day, creating bright streaks called bolides,” Binzel explains. “Once you get to the size of a few beach balls, those arrive multiple times a year, occasionally yielding fragments called meteorites.”
Objects the size of a truck hit Earth only two to three times per century, and – fortunately for humans – most of those impacts occur over the ocean, because about 71% of the earth's surface is covered by waterOne of these entered the Earth's atmosphere quite recently, over Chelyabinsk, Russia in 2013. It exploded a few miles above the surface, but the shock was still powerful enough to break a few windows. An asteroid about twice the size of the Chelyabinsk meteor exploded in 1908 over another part of Russia known as the Tunguska eventcausing an entire forest to be razed to the ground.
The largest asteroids, those larger than 140 meters (460 feet), about the size of the Washington Monument, are the rarest. “In this size range, their impact would have enough force to cause significant local or regional damage, including tsunami risk if they hit the ocean,” Binzel says. “Fortunately, these impacts are quite rare on human timescales, averaging about once every 25,000 years. That’s a probability of less than one percent per century, but still not zero over the course of a human lifetime.” Binzel says we’ve already found and tracked about 40 percent of the asteroids in this size range.
For comparison, the asteroid that wiped out the dinosaurs was thousands of meters in diameter—an extremely rare event. We know that in the near future a fairly large asteroid (a little over 300 meters in diameter) is coming our way, known as ApophisBased on observations, astronomers predict it will come uncomfortably close in 2029, 2036, and 2068. Fortunately, the risk of it actually hitting Earth is quite low.
Astronomers are also preparing for a potentially threatening asteroid. In fact, the NASA Double Asteroid Redirect Test (DART) mission collided with the asteroid Dimorphos in 2022, demonstrating the technology needed to divert an asteroid from a collision course.
One of the biggest problems for so-called “planetary protection” is identifying the threats in the first place. Just a few weeks ago, two large asteroids (2024 MK and 2011 UL21about 150 and 2,300 meters wide respectively) swung past the edge of the Earth. However, asteroid 2024 MK was only discovered a week before it flew between Earth and the moon, which feels a little worrying.
Discovering all the little bits of rock flying through the solar system is a tricky task. “The Earth and telescopes on it are a constantly moving platform in our orbit around the sun. Asteroids also orbit the sun, and to be detected, the asteroid has to swing past the night side of the Earth in a direction that a telescope happens to be pointing,” Binzel explains. “Sometimes the two orbital dances are not favorable for decades.”
To find as many asteroids as possible, we need large telescopes and cameras that can monitor a large part of the sky every night. A pair of facilities in the making will take on this task, namely the Vera Rubin Observatory on earth and the NEO Surveyor satellite in space. Rubin is a large observatory currently under construction in Chile, and is expected to Discover millions more asteroids as scans the entire sky every three nights. NEO Surveyor is a NASA space telescope set to launch in 2027 and specifically designed to hunt for asteroids full-time for five years in the hope of identifying more than 90% of potentially hazardous asteroids.
“I would be concerned if we weren't taking the challenge of asteroid exploration seriously,” Binzel added. “But ultimately, NASA and its funding sources have the adult responsibility to do the research necessary to ensure that our asteroid future is safe.” If all goes well, astronomers will be able to say for sure whether large impacts are expected in the next century. And if we find something dangerous, it might be time to invest in DART's successor.
