París, Nov 20, 2025.- Astronomers recently discovered the 40 000th near-Earth asteroid! These space rocks range from a few metres to a few kilometres in size and are on orbits that bring them relatively close to Earth. Each new discovery is both a reminder of our planet’s vulnerability and a testament to how far the field of planetary defence has advanced in just a few decades.
An asteroid is a rocky leftover from the formation of the Solar System more than four billion years ago. Most of them orbit the Sun between Mars and Jupiter. A near-Earth asteroid (NEA) is one whose orbit brings it within roughly 45 million kilometres of Earth’s orbit – close enough for planetary defence teams to keep a watchful eye on it.
The first NEA, Eros, was discovered in 1898. For decades, discoveries came slowly, until dedicated survey telescopes in the 1990s and 2000s began to find hundreds of new NEAs every year. In November 2025, the total number of identified NEAs surpassed 40 000, with around 10 000 of these discovered in the last three years alone.
“The number of discoveries is rising exponentially, from one thousand at the beginning of the century to 15 000 in 2016 and 30 000 in 2022. As the next generation of telescopes enter operation, we expect the number of known NEAs to continue to grow at an even higher pace,” says Luca Conversi, manager of ESA’s Near-Earth Object Coordination Centre.
“Inaugurated this year, the Vera C. Rubin Observatory in Chile, while not dedicated exclusively to asteroid surveys, will discover tens of thousands of new NEAs and other asteroids. Meanwhile, ESA’s Flyeye telescopes, designed with a wide, insect-like view of the sky, will catch asteroids that slip past current surveys.”
Just the tip of the iceberg
Every time a new NEA is discovered, astronomers use all of the available observations to predict its path years, decades, and even centuries ahead. Dedicated software systems calculate whether the object has any chance of impacting Earth within at least the next century.
These predictions and risk assessments are then updated and refined each time a new observation is made. At ESA, this work is carried out by the Near-Earth Object Coordination Centre (NEOCC), part of the Planetary Defence Office.
Almost 2000 NEAs have a non-zero chance of impacting Earth sometime in the next hundred years. However, most of these objects are very small, pose no significant danger, and their impact probabilities are typically much lower than one percent.
Importantly, the largest NEAs – those larger than one kilometre across – are also the easiest to spot, and many of them were among the first to be found. These objects would cause global effects if they struck Earth, but the scientific community is confident that the vast majority have already been found.
Today’s focus is on finding and tracking mid-sized asteroids, between about 100 and 300 metres wide. Much harder to spot, they would cause serious regional damage if they were to hit our planet. There is still work to be done as current models suggest that we have discovered only about 30% of these object.
Europe must be able to predict and prevent asteroid impacts
Thankfully, none of the 40 000 known NEAs are cause for concern for the foreseeable future.
However, ESA is not just waiting around for the day we do detect a hazard. The Agency’s Planetary Defence team also oversees the development of Europe’s asteroid mitigation capabilities through missions, such as Hera.
Hera is currently in space and enroute to the asteroid Dimorphos, where it will study the aftermath of the impact carried out by NASA’s DART spacecraft in 2022. By examining in detail how the DART impact changed the structure and trajectory of Dimorphos, Hera will help turn asteroid deflection into a reliable way to protect Earth.
ESA is also actively planning the Ramses (Rapid Apophis Mission for Space Safety) mission to the 375-m asteroid Apophis and accompany it through a safe but exceptionally close flyby of Earth in 2029.
Hunting asteroids in the infrared, ESA’s NEOMIR (Near-Earth Object Mission in the Infra-Red) will enable us to detect impact hazards similar to the Chelyabinsk event in advance for the first time. Targeted for launch in the mid-2030s, NEOMIR will close a large blind spot on the dayside hemisphere of Earth as bright sunlight prevents the detection of asteroids with optical, ground-based telescopes.
What began with the discovery of Eros in 1898 has grown into a global effort that has successfully identified tens of thousands of near-Earth asteroids. Each new addition, of which there are sure to be many thousands in the years ahead, improves our understanding of the Solar System’s history and strengthens our ability to keep our planet safe.