You might only think of asteroids when you watch a science fiction movie or when the news says one might hit Earth. But globally, they are monitored by several organizations and observatories – and for many reasons.
Asteroids are rocky bodies left over from the formation of our solar system, about 4,6 billion years ago.
There are more than a million known asteroids, and most are in the main asteroid belt, which orbits the Sun between Mars and Jupiter.
But some of them sometimes come closer to Earth and could help us understand the origins of life, explains Monica Grady, professor emeritus of Planetary and Space Sciences at the Open University in the UK.
"Some of these asteroids have a lot of organic compounds in them that could be the building blocks of life," she says.
"One hypothesis is that life only arose on Earth because the ingredients for life were brought to Earth by asteroids."
Although most asteroids are harmless and pass by us without much fuss, some are worth noting.
"Occasionally there is a surge of interest in objects that approach Earth. They are closely monitored until their orbits are known enough to rule out a collision, and sometimes to predict collisions. Further out, we look for objects with unusual compositions," says Agata Rozek, a research associate at the School of Physics and Astronomy at the University of Edinburgh in the UK.
When it comes to their size, large asteroids present less cause for concern.
"We know exactly where they are and where they are going," explains Rozek.
"We have a very good understanding of the laws that govern their movement, and we study unusual cases to better understand them."
"Small, undiscovered asteroids are more likely to be a cause for concern until their orbits are known."
Despite that, these are the three main asteroids currently being tracked, according to scientists – and one additional one that is so important that NASA launched a separate mission to study it.
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1. Apophis
Named after the Egyptian god of chaos and destruction, Apophis was discovered in 2004.
It seemed like there was a slim chance of it hitting Earth, but NASA later said it was "confident that there is no risk of Apophis hitting Earth for at least another 100 years."
"We currently know that it will safely pass by Earth on April 13, 2029," says Rozek.
"Since its discovery, it has been the subject of extensive Earth observation campaigns and will pass very close, close to the distance at which we have geostationary satellites, and we think that the proximity to Earth could elongate the asteroid and change its shape."
According to NASA, Earth's gravity will also produce an attractive force that will change Apophis' orbit around the Sun and could even cause small landslides on the asteroid.
It has a whopping 340 meters in diameter – about the length of three football stadiums – and will pass within about 32.000 kilometers of Earth's surface.
Close enough to be seen with the naked eye.
2. 2024 YR4
Estimated by NASA to be 53–67 meters in size – about the height of a 15-story building – 2024 YR4 was discovered in 2024 and made headlines around the world recently when it appeared to have a slim chance of hitting Earth in 2032.
Researchers even estimated that the odds of it hitting Earth were one in 32, but NASA later ruled this out.
"One of the challenges that comes with observing asteroids that are on a collision course with Earth is determining how likely that is," says Grady.
"We need to continue to observe it to refine that orbit, that trajectory."
There is still a 3,8 percent chance that 2024 YR4 will hit the Moon, but NASA adds that even if that collision occurs, it will not alter the orbit of our natural satellite.
3. Didymos and Dimorphos
Didymos, which means twin in Greek, is an asteroid, and Dimorphos is a small moon that orbits it.
None are considered a threat to Earth, but they will pass relatively close to it.
In 2022, they were the target of NASA's Dual Asteroid Redirection Test (DART) – which sent a probe to hit Dimorphos, destroying itself in the process.
The goal was to test whether space rocks that could threaten Earth could be safely pushed out of the way.
Dimorphos and Didymos were carefully chosen for this.
Neither was on track to rendezvous with Earth before the demonstration, and a small change in their orbital relationship would not increase that risk.
"The mission impacted the moon Dimorphos, changing its orbit around Didymos in the first practical test of planetary defense," says Rozek.
"This change was primarily measured using observations from Earth. We are continuing to monitor it ahead of the arrival of the Hera mission next year to investigate the consequences of the collision."
4. Psyche
Described by NASA as "one of the most intriguing bodies in the main asteroid belt," Psyche was discovered in 1852.
She was named after the Greek goddess of the soul.
Psyche is very far away from us, orbiting the Sun between Jupiter and Mars, and is believed to be made of metal and stone.
Scientists think that most of the metal comes from the core of planetesimals, the building blocks of planets, and that studying it can reveal how Earth's core and the cores of other planets formed.
NASA launched a mission in 2023 to map and study it.
New discoveries
Earlier this month, the Vera Rubin Observatory announced that its new telescope had discovered more than 2.000 new asteroids and seven near-Earth objects in just 10 hours.
Approximately 20.000 asteroids are discovered annually in all other observatories on Earth and in space combined.
"If you want to map the entire night sky, you need to have a very, very wide field of view. And that's what this telescope at the Vera Rubin Observatory has," says Professor Grady.
The observatory says it expects to discover millions of new asteroids in the first few years of the project, giving scientists even more asteroids to observe and find more clues about our solar system.
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