So far 30,000 asteroids have been discovered near the Earth

With NASA’s DART mission recently succeeding in deflecting an asteroid from its course, you might think our planet is in order when it comes to defending itself from oncoming asteroids. But there are a lot of asteroids out there, and looking for potentially dangerous asteroids is ongoing work.

According to the European Space Agency (ESA), there are now more than 30,000 known near-Earth asteroids in our solar system. A near-Earth asteroid is defined as one that approaches Earth at one point in its orbit, as many asteroids have highly elliptical orbits that bring them closer to the sun at some times than at others. Astronomers use a measure called the Astronomical Unit (AU), which is the distance between the Sun and the Earth, and asteroids close to Earth are those that are within 1.3 AU of the sun.

Artist’s impression of asteroid 21 Lutetia. ESA – C. Carreau

Not all asteroids near Earth are a threat to the planet, as many are small enough to burn in Earth’s atmosphere while others travel in a way that doesn’t intersect with Earth’s orbit. If an asteroid is larger than 460 feet in diameter and has passed through Earth’s orbit, it is classified as a potentially dangerous object and will be monitored by astronomers.

But to decide which asteroids are potentially dangerous, they first need to be identified, and that’s no easy feat. Tools like ESA’s Gaia, a space observatory on a mission to map all the stars in the Milky Way, can help by providing information on background stars seen when searching for asteroids.

“Thanks to Gaia, we know more about the stars in the galaxy that form the backdrop for asteroid observations,” explained Tineke Roegiers, community support for the Gaia mission, in a statement. “The positions of the asteroids are obtained relative to these stars in the background, so the better you know where the stars are, the more precisely you can calculate the orbits of the asteroids.”

Once an asteroid has been spotted, it must be observed multiple times to determine its orbit and see if it will approach Earth. “Obviously, any asteroid discovered near Earth qualifies as a near-Earth asteroid, but many are far from home,” said Marco Micheli, astronomer at ESA’s Near-Earth Object Coordination Center. “New objects are observed over time, their movements are studied and, with only a handful of data points from different nights, it is possible to predict their future positions. Depending on the number and quality of the observations, this can span decades, even hundreds of years into the future. “

If you feel like you’ve heard of more dangerous asteroids today than before, it may seem like the threat from the skies is growing, but in reality, we’ve just gotten a lot better at spotting them.

“The good news is that more than half of the near-Earth asteroids known today have been discovered in the past six years, showing how much our view of asteroids is improving,” said Richard Moissl, ESA’s head of planetary defense. “As this new milestone of 30,000 detections shows and as new telescopes and detection methods are built, it is only a matter of time before we have found them all.”

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