Over 30,000 near-Earth asteroids have been discovered; here is what you need to know

For an asteroid to be classified as a near-Earth Asteroid (NEA), its trajectory should bring it within 1.3 Astronomical Units (AU) of the Sun. One AU is the dance between the Sun and our planet. According to the European Space Agency (ESA), over 30,000 NEAs have been discovered so far, with most of the discoveries happening in the last decade.
About a third of the around one million asteroids we have discovered so far in the solar system are NEAs, and most of these live in the asteroid belt between Jupiter and Mars. Astronomers have been cataloguing asteroids since the discovery of Ceres, the very first asteroid discovered, Giuseppe Piazzi in 1801. Around a hundred years later, on August 13, 1898, 433 Eros, the first NEA was discovered.

“We have now discovered 30 039 near-Earth asteroids in the Solar System – rocky bodies orbiting the Sun on a path that brings them close to Earth’s orbit. The majority of these were discovered in the last decade, showing how our ability to detect potentially risky asteroids is rapidly improving,” said the ESA in a statement.
Detecting near-Earth asteroids
As can be expected, the earliest discovered asteroids were the largest ones since they are much easier to see. But with modern advances in telescope technology, we are finding many more at a much faster rate, with some of these asteroids having sizes in the tens of metres.
Ground-based survey telescopes, which are designed to scan large sections of the sky, discover new asteroids every week. They do this looking for new objects that are moving in front of a backdrop of stars that is relatively motionless. One good example is the Catalina Sky Survey in Arizona.
After an asteroid is discovered, more “focussed,” larger telescopes, like the Very Large Telescope in Chile, can be used to make further observations. This helps astronomers understand the asteroid’s, path, and size and can even give clues about its composition.

But not all of these asteroids are near-Earth asteroids. Many have trajectories that don’t bring them close to Earth and many of those that do are so small that they will burn up when they enter our planet’s atmosphere. But according to NASA, asteroids larger than 140 metres in size, and with a chance of coming within 0.05 AUs of Earth are classified as potentially hazardous asteroids or PHAs.
This is where missions like Gaia are really helpful. ESA’s Gaia is a mission to catalogue one billion stars in the galaxy. Thanks to Gaia, scients know more about the stars that serve as a backdrop to the asteroids. This, in turn, makes it easier to get asteroid’s positions against these background stars.
Will any of these asteroids hit Earth?
Thankfully, none of the near-Earth asteroids that have been discovered so far pose any concern, at least for the next hundred years. Even though some small objects will and sometimes do impact our planet, they have little to no effect. The worst they do is create trails of “shooting stars” as they burn up on entry into our atmosphere.
Also, according to the ESA, a majority of the largest and potentially catastrophic asteroids measuring larger than one kilometre have been discovered and none of them are a risk for the century. The hope is that missions like NASA’s DART pave the way for a future where even if asteroids pose a threat to our planet, we would have the technology to deflect them so they are no longer a threat.