We have discovered 30,039 near-Earth asteroids in the Solar System, most of which were discovered in the last decade.
Near-Earth objects are asteroids and comets with orbits around the sun that bring them to within 120 million miles (195 million kilometers) of the Sun, which means they can circulate through the Earth’s orbital neighborhood.
An asteroid is called a near-Earth asteroid (NEA) when its trajectory brings it within 1.3 Astronomical Units (au) of the Sun. 1 au is the distance between the Sun and Earth, and so NEAs can come within at least 0.3 au or 45 million kilometers (28 million miles) of our planet’s orbit.
When it comes to size, most near-Earth objects are asteroids that range in size from about 10 feet (a few meters) to nearly 25 miles (40 kilometers) across. Most of them reside in the asteroid belt between Jupiter and Mars.
It is good to know that most near-Earth asteroids have orbits that don’t bring them very close to Earth and therefore pose no risk of impact. However, a few of them – called potentially hazardous asteroids (PHA) – require more attention.
Potentially Hazardous Asteroids (PHAs) are currently defined based on parameters that measure the asteroid’s potential to make threatening close approaches to the Earth, according to NASA. But NASA and the ESA assure us that none, so far are a concern, for at least one hundred years.
There are currently 1,425 asteroids with a ‘non-zero’ chance of impact being watched very closely. The Near-Earth Object Computing Center (NEOCC) publishes an Asteroid Risk List that is constantly updated and freely available for anyone to see, according to the European Space Agency.
The first near-Earth asteroid
Asteroids have been cataloged by astronomers for more than two centuries since the very first asteroid, Ceres, was discovered in 1801 by Giuseppe Piazzi.
Nearly 100 years later, on August 13, 1898, Eros, the first near-Earth asteroid, was discovered by Carl Gustav Witt and Felix Linke at the Urania Observatory in Berlin and independently by Auguste Charlois at the Nice Observatory.
The NEO was given the moniker 433 Eros. It has a mean diameter of about 16.8 kilometers (10.4 miles), and is the second-largest NEO we know about. Eros was also one of the first asteroids visited by a spacecraft, the first one orbited, and the first one soft-landed on. NASA spacecraft NEAR Shoemaker entered orbit around Eros in 2000 and landed in 2001.
As you can imagine, there are many telescopes around the Earth. They are designed to scan large sections of the sky, looking for new objects moving in front of the backdrop of ‘motionless’ stars.
More focussed, large telescopes, such as the European Southern Observatory’s Very Large Telescope (VLT) in Chile, can then be used for follow-up observations, helping us better understand a ‘new’ asteroid’s path, size, and even composition.
