30,000 near-earth asteroids discovered and rising

An asteroid is called a near-Earth asteroid (NEA) if its trajectory brings it within 1.3 astronomical units (AU) of the Sun. 1 AU is the distance between the Sun and Earth, and therefore NEAs can be at least 0.3 AU, 45 million km, from our planet’s orbit.

Near-Earth asteroids currently make up about a third of the approximately one million asteroids discovered in the solar system to date. Most of them are 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 by Giuseppe Piazzi in 1801. The first near-Earth asteroid, (433) Eros, was discovered almost a hundred years later on August 13, 1898.

The approximately 30-kilometer-wide asteroid Eros 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 orbit of the stony asteroid brings it up to around 22 million km from Earth – that is 57 times as far as the moon.

Eros is not only the first known NEA, but also the first asteroid to have been orbited by a spacecraft and the first on which a spacecraft landed. Early calculations of the orbit of the space rock also allowed an accurate determination of the then only incompletely known distance between the Sun and Earth.

How to get a near earth asteroid out of the earth

Of course, large asteroids were spotted first since they are so much easier to see. They were considered minor planets, a term still used today. As telescopes become more sensitive, we are finding many more and at great speed, even those up to 10 meters in size.

Ground-based survey telescopes like the Catalina Sky Survey in Arizona, USA, discover new asteroids every week. They were designed to scan large parts of the sky looking for new objects moving against a background of “stationary” 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 the orbit, size and even composition of a ‘new’ asteroid.

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

“Thanks to Gaia, we know more about the stars in the galaxy that serve as a backdrop for asteroid observations,” explains Tineke Roegiers, community support for the Gaia mission.

“The positions of asteroids are determined against these background stars. So the better you know where the stars are, the more accurately you can calculate the orbits of asteroids.”

With the use of “Gaia’s Stars” even the orbits of already known near-Earth asteroids have been improved and some “lost” asteroids have been rediscovered.

ESA asteroid risk list

“Of course, any asteroid discovered near Earth is considered a near-Earth asteroid, but many are found far from home,” explains Marco Micheli, astronomer at ESA’s Near-Earth Objects Coordination Center.

“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 the observations, this can reach decades, even hundreds of years into the future.”

ESA’s Near-Earth Objects Coordination Center (NEOCC) in ESRIN, Italy, houses the agency’s asteroid experts and risk assessors. The team is activating its network of telescopes around the world to obtain observations of newly discovered asteroids and determine their impact risk, while also hunting “old” asteroids that are not yet considered safe.

Currently under their watchful eye are 1,425 asteroids with a “non-zero” impact probability, organized in the NEOCC’s Asteroid Risk List, which is constantly updated and freely available to everyone. You can even sign up for ESA’s monthly ‘Asteroids Newsletter’ and get the asteroid news direct.

Will one of these asteroids hit Earth?

Currently, none of the near-Earth asteroids discovered to date have been of concern for at least a hundred years. Some of the smaller objects will and will hit Earth – but the most common are also the smallest and have little effect other than creating trails of shooting stars as they burn up in the night sky.

When it comes to large and potentially devastating asteroids, 1km+ in diameter, most have been discovered and none have shown impact risk for at least a century. For those that might have an impact later, we have enough time to study them and prepare a diversionary mission.

The medium-sized asteroids with a diameter of a few hundred meters are currently in the foreground. Many are still out there waiting to be discovered, and at smaller sizes, they’re not quite as easy to find.

“The good news is that more than half of the near-Earth asteroids known today have been discovered in the last six years, showing how much our vision for asteroids is improving,” said Richard Moissl, Head of ESA’s Planetary Defense Division.

“As this new milestone of 30,000 discoveries shows, and new telescopes and detection methods being built, it’s only a matter of time before we find them all.”