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 ability to detect potentially risky asteroids is rapidly improving.
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, 45 million km, of our planet’s orbit.
Currently, near-Earth asteroids make up about a third of the roughly one million asteroids discovered so far in the solar system. Most of them reside in the asteroid belt between Jupiter and Mars.
Asteroids have been cataloged 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 one hundred years later, on 13 August 1898.
The roughly 30 km Eros 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 stony asteroid’s orbit brings it to within around 22 million km of Earth—57 times the distance of 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.
How to un-Earth a near-Earth asteroid
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 and at a great rate, even those down to tens of meters in size.
Ground-based survey telescopes such as the Catalina Sky Survey in Arizona, in the United States, discover new asteroids every week. They are designed to scan large sections of the sky, looking for new objects moving in front of the backdrop of “motionless” stars.
More focused, large telescopes, such as the European Southern Observatory’s Very Large Telescope (VLT), can then be used for follow-up observations, helping us better understand a “new” asteroid’s path, size and even composition.
Gaia, ESA’s space observatory on a mission to catalog one billion stars in the galaxy, has also helped us better understand the asteroid risk.