Spinning through space: The astronomers watching the skies for asteroids

Sue Nelson explores how asteroids can inform our understanding of the Earth's past – or threaten our future.

Each year, according to Nasa, a chunk of rock the size of a car hurtles through space on a collision course with our planet. Fortunately, thanks to the Earth's atmosphere acting as a natural shield, instead of crash landing on the ground, the asteroid burns up and produces 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, which was discovered in December 2024, has been making headlines around the world recently. At around 40-90m (131-295ft) wide, it is larger than a 12-storey building. In January this year, the ESA calculated the rock's trajectory and initially predicted that there was a 1.2% chance of impact with Earth on 22 December 2032.

This officially crossed the comfortable threshold of risk for a near-Earth object – 1% – and triggered the need for investigation by several planetary defence organisations, as well as the US President and US Congress. 

Luckily YR4 is not large enough to be capable of making our species extinct but it could still be a "city killer", according to some experts, if it is at the upper end of the estimated size range and landed in a heavily populated area. 

In February 2025, the risk of the asteroid hitting the Earth briefly climbed to 3.1% or one in 32. Luckily, humanity managed to avoid mass panic and the risk has since reduced to a more reassuring 0.001%. But where did this asteroid come from in the first place? And how concerned should we be about a similar scenario emerging in the future? 

When it comes to understanding asteroids, astronomers and scientists are still being dealt the cards in order to understand 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 one of Nasa's sky surveys that searches and tracks Near Earth Objects (NEOs). These are 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 that was formed at the birth of our Solar System" says Fitzsimmons. "So by studying their chemical make-up we get some idea of the conditions in the initial Solar System as since then, over 4.6 billion years, it has evolved dynamically." 

These ancient rocky remnants are sometimes referred to as minor planets. Often irregular and cratered, they can also be spherical. They can spin slow, fast or tumble. Usually solitary, they can sometimes be found in pairs (binary or double asteroids) and some even have their own moon. Nasa's Jet Propulsion Laboratory's website keeps tabs on their numbers and at the last count there are over 1.4 million in our Solar System. The majority are located within the asteroid belt between Mars and Jupiter but millions more are too small to be detected. 

Most of the time these asteroids remain restrained within that interplanetary belt by Jupiter's gravitational field, unable to coalesce into a larger body. But occasionally another asteroid or the influence of Jupiter's gravity can nudge some into another orbit around the Sun, towards the inner Solar System.

Once an asteroid has been ejected from its usual path and is potentially heading our way, the first challenge is to detect it. 

"All an asteroid looks like in a telescope is a point of light against other points of lights that are stars, except it's moving," says Kelly Fast, Nasa's acting planetary defence officer, "and it's reflecting sunlight."

The brighter the asteroid, the bigger it will be. But its colour also affects the brightness as a small white asteroid might reflect more light than a much larger dark one. It's not an exact science, which is why YR4's size is estimated at 40-90m (131-295ft) wide. The more information we have, however, the more precise that number will become.

"A team at Nasa and an ESA team will be using the James Webb Space Telescope to take infrared observations of YR4," says Fast. These measurements of an asteroid's thermal emissions "may be helpful for constraining that size range", she adds.

YR4, in common with most asteroids, is from the asteroid belt. "Which part of the belt is hard to tell," says Fitzsimmons. "One clue that we have is a spectrum of its surface." By examining the intensity of light emitted over a range of wavelengths, specific materials on the surface of the asteroid can be identified. "YR4 is a rocky asteroid that's deficient in lighter elements such as carbon, which tells us that it probably came from the inner asteroid belt," says Fitzsimmons. "But exactly where we don't know – and we may never know."

Asteroids are not all the same either. "There are different populations," says Fast. "Some are stony, some are carbonaceous and some have metallic content depending on the parent bodies they came from."

While there are rare types of asteroids, more than three out of four are carbonaceous, or C-type, and contain carbon, appearing black as coal. The rest are mostly S-type or silicaceous – a mix of metal and silicate crystals – or metallic or M-type asteroids, which contain predominantly iron and nickel. 

One asteroid scientists are particularly interested in is  – a large, metal-rich rock the shape of a potato, which inhabits the Solar System's main asteroid belt. It's thought this asteroid could be up to 95% nickel and iron, which is similar to the Earth's core. Studying it will therefore increase our understanding of how our own planet formed. Nasa's Psyche mission is currently on its way there and is due to arrive in August 2029.

In terms of metal content, asteroids like Psyche 16 are also potentially extremely valuable – it's thought to contain resources worth approximately US$10 quadrillion (£7.8 quadrillion). In the future, if asteroid mining ever became necessary as well as economically and technically feasible, it's thought they could provide an abundance of resources.

Knowing an asteroid's composition is key for defence reasons too. An iron-rich asteroid, for example would do more damage on impact than carbonaceous ones as it is more dense, has more mass and would have more energy if it smashed into a moon or planet. The Earth's Moon, as can be seen, is littered by asteroid impact craters since it only has a thin atmosphere, or exosphere, which cannot protective it. Although the Earth is unlikely to receive an impact by YR4, there remains a 1.7% chance it could hit our Moon.

The level of damage also depends on the asteroid's structure. "If you know more about the structure of these objects," says Fitzsimmons, "then you can calculate more accurately what happens when it hits the Earth's atmosphere.Most asteroids below 10km [6.2 miles] across are almost all either heavily fractured solid objects or rock piles – smaller fragments of asteroids grouped together mostly by gravity."

Does this mean a rubble pile asteroid would be more likely to burn up in the atmosphere and therefore be less dangerous? "Unfortunately not," says Fitzsimmons. "If an asteroid is hitting the Earth at 17km per second (over 38,000 mph), it's in the atmosphere for less than 10 seconds before it reaches impact."

While the Earth's atmosphere acts as a protective barrier, that still doesn't prevent all damage. "Something the size of YR4 would probably only make it to a few kilometres above the Earth's surface. It will lose its energy and explode at altitude. Although we wouldn't get a crater, we would have an air blast and that could be just as devastating as if it hit the ground," says Fitzsimmons. 

At the moment we don't know if YR4 is one solid object or whether it's a rock pile, but an air blast could have been significant considering what happened in 2013. A fireball – which is what an asteroid is called once it enters Earth's atmosphere – exploded about 14 miles (22.5km) above the city of Chelyabinsk in Russia. Eyewitnesses described the light as brighter than the Sun, and the blast's shockwave damaged over 4,000 buildings and injured 1,200 people.

"The asteroid was around 20m (66ft) in size," says Küppers. "This is maybe about the limit size where you would start worrying if it hits populated areas."

Coincidentally, on the day of the Chelyabinsk event, a United Nations committee was underway in Vienna about defending Earth from future asteroid impacts. The meeting resulted in the International Asteroid Warning Network, chaired by Nasa, and a Space Missions Planning Advisory Group for space agencies around the world to collaborate.

Fortunately, there have already been a number of missions which have studied asteroids, starting with Galileo when it flew past two asteroids in the early 1990s. In 2000, Nasa's Near Shoemaker became the first spacecraft to orbit an asteroid, Eros, and – a year later – to land on one too. The Japanese Huyabusa 2 mission visited the C-type asteroid 162173 Ryugu in 2018 and 2019 and even sent back a sample to Earth in a hermetically sealed container in 2005.

The US OSIRIS-REx mission (Origins, Spectral Interpretation, Resource Identification and Security–Regolith Explorer) collected dust and rocks from asteroid Bennu and returned them to Earth in 2023. Earlier this year, the first in-depth analysis of the samples' minerals and molecules were published. It included finding 14 of the 20 amino acids used by life to make proteins on Earth. The spacecraft, renamed OSIRIS-APEX, is now on its way to explore asteroid Apophis. 

All these missions, and more, have allowed scientists to study the composition and particle size of several asteroids, as well as the magnetic properties of samples to inform our understanding of the Solar System's early magnetic field. But there is another reason for studying asteroids, brought home by the concerns over YR4: to protect our planet.

In 2022, Nasa's Dart (Double Asteroid Redirection Test) mission deliberately aimed itself at a harmless – from the Earth's point of view – binary asteroid system consisting of asteroid Didymos (780m, or 2,559 ft) and its smaller orbiting asteroid or moonlet, Dimorphos (160m, or 525ft). The spacecraft used itself as a kinetic impactor, targeting Dimorphos at around 22,530km (14,000 miles) per hour. This was Nasa's first test to see if it could deflect an asteroid's path and it was a success. After impact, Dimorphos' orbit was altered. Does this mean, in future, that Nasa asteroid missions will focus on planetary defence rather than understanding more about the science? 

"We're very much interested in both," says Fast, "and it's a really good partnership. The planetary defence 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."

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