Near the Yucatán peninsula in Mexico, the colossal Chicxulub meteorite crater is probably the most famous of its kind. It’s a souvenir from when a 15km space rock hit the Earth, around 66 million years ago.

NASA’s Centre for Near-Earth Object Studies (CNEOS) at the Jet Propulsion Laboratory in California reports 1,746 ‘potential ‘future Earth impact events’ according to their Sentry Earth Impact Monitoring System. But these numbers only tell half the story.

The orbits of all known Potentially Hazardous Asteroids (PHAs): over 1,400 as of 2013
The orbits of all known Potentially Hazardous Asteroids (PHAs) circa 2013. Image credit: NASA/JPL-Caltech

What are near-Earth objects?​

Let’s back up. What even are near-Earth objects? The short answer is they’re comets and asteroids whose orbit brings them into Earth’s neighbourhood.

Most near-Earth Objects (NEOs) are asteroids rather than comets, making them near-Earth asteroids (NEAs). They’re further divided into four groups: Atira, Aten, Apollo and Amor: learn more about those groups here.

‘Near Earth’ in this context means the closest point of the object’s orbit around the Sun (the perihelion) is less than 1.3 au. 1 AU is the distance from the Earth to the Sun, 149,598,000 kilometres, so ‘near’ is quite relative.

This distance is measured from when the object is closest to the Sun, so these space rocks spend most of their time rather far away from Earth, where they’re hard to observe.

How many near-Earth asteroids are there?

The ESA’s Near-Earth Objects Coordination Centre lists a total of 35,149 near-Earth objects, with 1,626 making its “risk list”.

Dr Richard Moissl, head of the ESA’s planetary defence office, says the biggest challenge with communicating information about NEO threats is what he calls the “low-likelihood variable-risk” scenarios. “There is a tendency to think towards the catastrophic part of the spectrum of impacts (i.e. the Chicxulub event),” Richard says. “For us, the main area of concern is asteroids in the range of ~20-100m in diameter. Here, the overall (still numerically low) risk levels are the highest.”

Impact events of the size of 1908’s Tunguska meteor are rare, Richard says, but we still need to improve our knowledge to know with enough advance warning time about any potential impacts.

Dr Paul Chodas, director of the Centre for Near-Earth Object Studies, says they “do get a few TS [Torino Scale] 1 cases every year, but they go away after more observations effectively rule out the potential impact.”

Organisations like these track near-Earth objects not because of a high likelihood of impact, but because of the potential consequences.

The ESA is more concerned with asteroids between 20 and 100m in diameter, than with unlikely catastrophic impacts.
The ESA is more concerned with asteroids between 20 and 100m in diameter, than with unlikely catastrophic impacts.

What's the risk from potentially-hazardous asteroids?​

Most NEAs aren’t a threat to Earth, and their orbits mean they never will be. Astronomers are more concerned with the ones that might cross paths with Earth: potentially hazardous asteroids (PHA). For something to be “potentially hazardous”, they need to have an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 au or less and an absolute magnitude (H) of 22.0 or less.

A PHA around 20m in diameter, like the 2013 Chelyabinsk meteor, is capable of causing strong airbursts, and injuries from shattering glass. Something the size of Tunguska, closer to 40m, could destroy a city. Anything larger than 1km could end civilisation.

Australia doesn’t have a dedicated body for studying/tracking these asteroids, such as the ESA’s Near-Earth Objects Coordination Centre or NASA’s Centre for Near-Earth Object Studies. However, the Australian Space Agency has direct contact with the ESA for impact and close-approach notifications and warnings.

Ground-based telescope surveys are currently our best tool for detecting asteroids. Dr Chodas says how much notice we have of an asteroid depends on many factors. The most important factor, he says, is the intrinsic brightness of the object. The farther out we can detect the object, the longer the warning.

The telescope technology protecting Earth

Astronomers use many different observatories and telescopes around the world search for undiscovered near-Earth objects, and investigate known objects.

The ESA collaborates with three telescopes in Australia, including UWA’s Zadko Observatory in Gingin, the 6Roads observatory in Youndegin, and Las Cumbres Observatory’s Siding Spring Observatory near Coonabarabran, in New South Wales.

And there are more advances in telescope technology to come. The ESA’s first NEOSTEL “Flyeye” Telescope will form part of a proposed future network of NEO-tracking telescopes. Each instrument will have a view similar to an insect’s compound eye, able to detect everything down to about 40m in diameter, and typically three weeks before a potential impact.

The Vera C. Rubin Observatory expected to see first light in 2025. Using the 8.4m Simonyi Survey Telescope the 3200-megapixel LSST Camera, it will detect asteroids at much larger distances than we can now. Dr Chodas says it will provide a major step up in our discovery capabilities.

Earth-based telescopes are limited by our atmosphere, and NASA is currently working on the Near Earth Object Surveyor mission space telescope, the first of its kind specifically designed to hunt and categorise potential asteroid threats and will be able to detect NEOs that ground-based telescopes might miss.

The sky is a vibrant yellow, pink, and purple over the Rubin Observatory
Vera C. Rubin Observatory will capture the cosmos in exquisite detail. Image credit: Rubin Obs/ NSF/AURA

How do you stop an asteroid?​

In the unlikely event of a threat from an asteroid impact, the actions needed vary depending on the object’s size. Dr Moissl explains with objects of an estimated diameter of 20m or larger, civil protection authorities need to start actively preparing, according to their respective plans.

For objects estimated at 50m and above, the Space Mission Advisor Group will study space-based reconnaissance and deflection options and give the UN their recommendations.

Dr Chodas has previously said five years of preparation was necessary for deflecting a potentially hazardous asteroid, but he clarifies that’s “an absolute minimum for the types of deflection that are called for in our scenarios.”

Scenarios involving a larger asteroid, or a more difficult orbit, would require a longer warning, perhaps 10 years. “In our latest asteroid impact exercise, we simulated a 14-year warning, which most participants thought was adequate time,” Dr Chodas says.

In 2022, NASA crashed its DART spacecraft into asteroid Dimorphos. DART’s impact successfully changed Dimorphos’ orbit, demonstrating how a mission like it might deflect an asteroid. But deflection techniques, including ‘kinetic impactors’ like DART or nuclear explosive devices, may also unintentionally disrupt or fragment an asteroid in a largely unpredictable fashion.

produce a more controlled deflection, Dr Chodas says, but may only be effective against asteroids smaller than a couple hundred meters.

an artist's impression of a near-earth asteroid with Earth in the distance
NASA reports 10,907 known near-Earth objects 140m or larger in diameter.

Cooperation helps humanity​​

Dr Moissl says international collaboration is vital for planetary defence. “We are all literally sitting in the same boat,” he says, “so it makes perfect sense for us all to join forces.”

On 16th of June 2024, astronomers detected Asteroid 2024 MK. Between 120 and 260 m across, it will pass within the orbit of the Moon on the 29th of June.

“It poses no risk to our planet,”Dr Moissl says, “but the fact that it was discovered so late highlights the need to continue improving our ability to detect potentially hazardous objects in our cosmic neighbourhood.”


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A cartoon drawing of handsome bald man wearing a black hoody is looking thoughtfully up at the starry night sky
Jay Chesters is a freelance journalist, feature writer, and award-winning author with a particular passion for stargazing and astronomy. Jay enjoys any opportunity to share stories or pass on what they know.

Jay Chesters


Stargazers Club WA and Astrotourism WA acknowledge and pay tribute to the Traditional Custodians of Country throughout Western Australia. We recognise Aboriginal and Torres Strait people as the world's first astronomers and their continuing connection to lands, sky, waters and communities. We offer our respect to them, their cultures, and to Elders both past and present.