Each year, according to NASA, a car-sized chunk of rock hurtles through space on a collision course with our planet. Fortunately, thanks to Earth’s atmosphere acting as a natural shield, instead of crashing onto the ground, the asteroid burns up, producing an impressive light show, streaking across the sky as a meteor or fireball.
Unfortunately, other, much larger asteroids have the potential to be far more threatening than entertaining. “Asteroids come in all sizes,” says Michael Küppers, a planetary scientist for the European Space Agency (ESA). “The really big ones, like the 10km (6.21 mile) [wide] or so asteroid that we think led to the extinction of the dinosaurs, happen maybe once every 100 million years.”
The asteroid 2024 YR4, discovered in December 2024, has recently made headlines worldwide. At around 40-90m (131-295ft) wide, it is larger than a 12-story building. In January this year, the ESA calculated the rock’s trajectory and initially predicted a 1.2% chance of impact with Earth on December 22, 2032.
“Any asteroid we detect is generally a fragment of a much larger body that was formed at the birth of our Solar System,” says Alan Fitzsimmons.
This officially crossed the comfortable risk threshold for a near-Earth object – 1% – triggering investigations by several planetary defense organizations, as well as the US President and US Congress.
Luckily, YR4 is not large enough to cause species extinction but could still be a “city killer,” according to experts, if it lands in a densely populated area.
In February 2025, the risk briefly climbed to 3.1%, or one in 32. Fortunately, mass panic was avoided, and the risk has since reduced to a reassuring 0.001%. But where did this asteroid come from, and how concerned should we be about future scenarios?
NASA/Johns Hopkins APL After analyzing data from NASA’s DART mission, scientists concluded that it had successfully altered the asteroid’s orbit (Credit: NASA/Johns Hopkins APL)
A Distant Belt
When understanding asteroids, astronomers and scientists are still learning the rules of this potentially risky celestial game.
“Scientifically, there’s a huge amount we can learn from asteroids,” says Alan Fitzsimmons, an astronomer at Queens University Belfast and a member of NASA’s sky surveys that track near-Earth objects (NEOs), asteroids whose orbit brings them within 195 million km (121 million miles) of the Sun.
“Any asteroid we detect is generally a fragment of a much larger body formed at the birth of our Solar System,” says Fitzsimmons. “By studying their chemical makeup, we get an idea of the initial Solar System’s conditions, which have evolved over 4.6 billion years.”
These ancient rocky remnants, sometimes called minor planets, are often irregular, cratered, or spherical. They can spin slowly, quickly, or tumble. Usually solitary, they can be found in pairs or even have their own moon. NASA’s Jet Propulsion Laboratory tracks over 1.4 million asteroids, mostly in the asteroid belt between Mars and Jupiter, but millions more are too small to detect.
Most asteroids remain restrained within this belt by Jupiter’s gravity, but occasionally, another asteroid or Jupiter’s influence nudges some into orbits toward the inner Solar System.
Celestial Clues
Once an asteroid is ejected and potentially heading our way, the first challenge is detection.
“All an asteroid looks like in a telescope is a moving point of light against other points of light, which are stars, reflecting sunlight,” says Kelly Fast, NASA’s acting planetary defense officer.
Brightness indicates size, but color also affects brightness, as a small white asteroid might reflect more light than a larger dark one. This is why YR4’s size is estimated at 40-90m (131-295ft). More information leads to more precision.
Alamy The Chelyabinsk meteor was just 20m (66ft) in diameter (Credit: Alamy)
“A team at NASA and an ESA team will use the James Webb Space Telescope for infrared observations of YR4,” says Fast. These thermal emissions measurements “may help constrain that size range.”
YR4, like most asteroids, is from the asteroid belt. “Which part is hard to tell,” says Fitzsimmons. “One clue is a spectrum of its surface.” By examining light intensity over wavelengths, specific materials can be identified. “YR4 is a rocky asteroid deficient in lighter elements like carbon, suggesting it came from the inner asteroid belt,” says Fitzsimmons. “But exactly where, we don’t know – and may never know.”
Asteroids vary. “There are different populations,” says Fast. “Some are stony, some carbonaceous, and some have metallic content, depending on their parent bodies.”
Most asteroids are carbonaceous (C-type), containing carbon and appearing black. Others are silicaceous (S-type), a mix of metal and silicate crystals, or metallic (M-type), containing iron and nickel.
One asteroid of interest is 16 Psyche, a metal-rich rock in the main asteroid belt. Thought to be 95% nickel and iron, it could help understand Earth’s core. NASA’s Psyche mission is en route, arriving in August 2029.
Metal-rich asteroids like Psyche 16 are also valuable, containing resources worth approximately US$10 quadrillion. If asteroid mining becomes feasible, they could provide abundant resources.
Composition is key for defense. Iron-rich asteroids cause more damage than carbonaceous ones due to density and mass. The Moon, with its thin atmosphere, is littered with impact craters. While Earth is unlikely to be hit by YR4, there’s a 1.7% chance it could hit the Moon.
Damage also depends on structure. “Knowing more about the structure allows for more accurate calculations of what happens when it hits Earth’s atmosphere,” says Fitzsimmons. “Most asteroids below 10km are either heavily fractured solids or rock piles.”
Does this mean rubble pile asteroids are less dangerous? “Unfortunately not,” says Fitzsimmons. “At 17km per second, an asteroid is in the atmosphere for less than 10 seconds before impact.”
While the atmosphere protects, it doesn’t prevent all damage. “YR4 would likely only make it to a few kilometers above Earth’s surface, losing energy and exploding at altitude. We’d have an air blast, as devastating as a ground impact,” says Fitzsimmons.
We don’t know if YR4 is solid or a rock pile, but an air blast could be significant, considering the 2013 Chelyabinsk event. A fireball exploded 14 miles (22.5km) above Chelyabinsk, Russia, damaging over 4,000 buildings and injuring 1,200 people.
“The asteroid was around 20m (66ft) in size,” says Küppers. “This is about the limit size where you would start worrying if it hits populated areas.”
Coincidentally, on the day of the Chelyabinsk event, a UN committee was underway about defending Earth from asteroid impacts, resulting in the International Asteroid Warning Network and a Space Missions Planning Advisory Group.
Many missions have studied asteroids, starting with Galileo in the early 1990s. NASA’s Near Shoemaker orbited and landed on Eros in 2000 and 2001. Japan’s Hayabusa 2 mission visited Ryugu in 2018-2019, returning samples. The US OSIRIS-REx mission returned samples from Bennu in 2023. Recent analyses revealed amino acids. OSIRIS-APEX is now en route to Apophis.
These missions inform our understanding of the Solar System’s early magnetic field. But they also serve to protect our planet, as seen with concerns over YR4.
NASA When the OSIRIS-REx spacecraft return capsule was finally opened and its contents analysed, scientists discovered the ingredients for life (Credit: Nasa)
In 2022, NASA’s DART mission successfully deflected Dimorphos, a moonlet of Didymos, testing asteroid deflection. Does this mean future missions will focus on planetary defense?
“We’re very much interested in both,” says Fast, “and it’s a really good partnership. The planetary defense coordination office at NASA sits inside of the planetary science division because there is so much asteroid science involved in it. We want to protect the planet but we also want to study these amazing leftovers from the formation of the Solar System, both to understand the history of the Solar System and to see what the effects might be should they impact, or should we need to deflect one in space.”
