Astronomers have found 30,000 near-Earth asteroids – and half in the last six years

Astronomers have found 30,039 near-Earth asteroids in the Solar System – with more than half spotted in the last decade.

The acceleration in discovery bodes well for detecting medium-sized asteroids a few hundred metres in diameter.

Many are still out there, waiting to be discovered, and at smallish sizes they have not been easy to find.

None of the near-Earth asteroids found so far are of concern for at least 100 years.

Some of the smaller objects will and do impact Earth – but the most common are also the smallest and have little effect, except for creating trails of shooting stars as they burn up in the night sky.

When it comes to large and potentially devastating asteroids larger than one kilometre across and above, the majority have been discovered and none show an impact risk for at least a century.

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Richard Moissl, ESA’s head of planetary defence, said: "The good news is that more than half of today’s known near-Earth asteroids were discovered in the last six years, showing just how much our asteroid eyesight is improving.

"As this new 30,000 detection milestone shows, and as new telescopes and methods of detection are built, it’s only a matter of time until we’ve found them all."

An asteroid is called a near-Earth asteroid (NEA) when its trajectory brings it within 1.3 Astronomical Units (au) of the Sun.

One au is the distance between the Sun and Earth, and so NEAs can come within at least 0.3 au, or 45 million kilometres, of our planet’s orbit.

Currently, near-Earth asteroids make up about a third of the roughly 1 million asteroids discovered in the Solar System.

Most of them reside in the asteroid belt between Jupiter and Mars.

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Asteroids have been catalogued by astronomers for more than two centuries since the very first asteroid, Ceres, was discovered in 1801 by Giuseppe Piazzi.

The first near-Earth asteroid, (433) Eros, was discovered nearly a century later in 1898.

The roughly 30-kilometre Eros asteroid was located by Carl Gustav Witt and Felix Linke at the Urania Observatory in Berlin and independently by Auguste Charlois at the Nice Observatory.

The stony asteroid’s orbit brings it to within around 22 million kilometres of Earth – 57 times the distance to the Moon.

Not only is Eros the first known NEA, but the first asteroid to be orbited by a spacecraft and the first to have a spacecraft land on it. Early calculations of the space rock’s orbit also enabled a precise determination of the-then imperfectly known distance between the Sun and Earth.

Naturally, large asteroids were discovered first as they are so much easier to see. They were thought of as minor planets, a term still used today.

As telescopes get more sensitive, we are finding many more asteroids at a great rate, even those down to tens of metres in size.

Gaia, ESA's space observatory on a mission to catalogue 1 billion stars in the galaxy, has also helped us better understand the asteroid risk.

"Because of Gaia, we know more about the stars in the galaxy which act as a backdrop to asteroid observations," said Tineke Roegiers, community support for the Gaia mission.

"Asteroid’s positions are obtained against these background stars, so the better one knows where the stars are, the more precisely the orbits of asteroids can be computed."

With the use of "Gaia's stars", even the orbits of already-known near-Earth asteroids have been improved, and some asteroids that were "lost" were found again.

"Of course, any asteroid discovered near Earth qualifies as a near-Earth asteroid, but many are found far from home," said Marco Micheli, astronomer at ESA’s Near-Earth Object Coordination Centre.

"New objects are observed over time, their movements are studied and with just a handful of data points from different nights their future positions can be predicted. Depending on the number and quality of observations, this can extend decades, even hundreds of years into the future."

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