When an asteroid approaches within 1.3 astronomical units (au) of the Sun, we term it a near-Earth asteroid (NEA). The average distance between the Sun and Earth is 1 au, therefore near-Earth asteroids (NEAs) can approach within 0.3 au or 45 million kilometers.
At present, around a third of the roughly one million asteroids found so far in the Solar System are considered to be near-Earth asteroids. They are concentrated in a band between Jupiter and Mars called the asteroid belt.
Since the discovery of Ceres in 1801 by Giuseppe Piazzi, astronomers have been compiling lists of asteroids. On August 13, 1898, over a century later, the first Earth-approaching asteroid, (433), was detected.
Carl Gustav Witt and Felix Linke of the Urania Observatory in Berlin and Auguste Charlois of the Nice Observatory separately discovered the about 30-kilometer-wide asteroid named Eros. Through its orbit, the stony asteroid comes to within around 22 million km of Earth, which is about 57 times the distance to the Moon.
Eros is not just the first asteroid to have a spacecraft orbit it or land on it, but it is also the first known Near-Earth Asteroid. The initial orbital calculations for the space rock also allowed for accurate measurement of the Sun-Earth distance, which was only vaguely known at the time.
A near-Earth asteroid's un-Earthing procedure
The largest asteroids were the first to be spotted because of their sheer size. They were classified as what is now known as minor planets. Increases in telescope sensitivity have allowed us to discover an increasing number of galaxies, some as small as tens of meters in diameter.
New asteroids are found weekly by ground-based survey telescopes like the Catalina Sky Survey in Arizona, USA. Their purpose is to search for new objects moving against the background of “motionless” stars across wide swaths of the night sky.
#PlanetaryDefense means we monitor the skies and search for potentially hazardous near-Earth objects every night. We recently hit a new milestone of more than 30,000 near-Earth #asteroids of all sizes discovered!
— NASA Asteroid Watch (@AsteroidWatch) October 14, 2022
This allows for more targeted studies with larger telescopes like the Very Large Telescope (VLT) at the European Southern Observatory, which can shed light on the ‘new' asteroid's orbit, size, and even chemical makeup.
The European Space Agency's Gaia space observatory, which is on a mission to catalog one billion stars in the galaxy, has also contributed to our improved understanding of the asteroid threat.
Gaia's community supporter, Tineke Roegiers, notes that the mission has improved our understanding of the stars in the galaxy, which serve as a backdrop for asteroid observations.
Since asteroids' coordinates are determined in relation to these reference stars, pinpoint accuracy in calculating their orbits is directly proportional to the accuracy with which their positions can be determined.
Even the orbits of already-known near-Earth asteroids have been improved thanks to the usage of ‘Gaia's stars,' and several asteroids that had been “lost” have been rediscovered.
The ESA's Asteroid Threat Catalog
As ESA astronomer Marco Micheli puts it, “of course, any asteroid detected near Earth qualifies as a near-Earth asteroid,” but many are found far from home.
“New objects are detected over time, their motions are examined, and their future positions can be predicted with just a handful of data points from different nights.” This may go back hundreds of years or more, depending on the amount and quality of observations.
ESA's asteroid specialists and risk analysts are based out of the Near-Earth Object Coordination Centre (NEOCC) in ESRIN, Italy. In order to assess the threat posed by newly discovered asteroids, the team activates its global network of telescopes to obtain views of them. This includes tracking down ‘ancient' asteroids that haven't been cleared for travel.
The NEOCC is keeping an eye on 1,425 potentially hazardous asteroids, all of which are listed in the Asteroid Risk List which is updated regularly and is available online for anybody to view. A monthly “Asteroid Newsletter” is published by the European Space Agency, so interested parties can sign up to receive information about asteroids as it becomes available.
Do any of these asteroids pose a threat to Earth?
For the next hundred years at least, none of the near-Earth asteroids detected so far pose a threat to Earth. Smaller objects do, in fact, occasionally collide with Earth, but these collisions often leave behind only brief trails of shooting stars before they burn up in the atmosphere.
More than 90% of potentially catastrophic asteroids with diameters of 1 kilometer or more have been found, and none of them appear to pose an impact risk for at least another century. We may take our time studying the potential effects and planning a deflection mission for the ones that won't be felt for a while.
ESA's state-of-the-art “Flyeye” spies have several little eyes. Using a fly-eye telescope, scientists will search the sky for any new asteroids that may have appeared. It will divide the sky into smaller sections and analyze each one using a compound, fly-inspired eye.
ESA's state-of-the-art “Flyeye” spies have a lot of small eyes. The search for new asteroids will be made using a fly-eye telescope. It will divide the sky into smaller sections and analyze each one using a compound eye inspired by the fly.
The asteroids that are a few hundred meters in diameter are currently being prioritized. There is still a great number waiting to be found, and because they are on the smaller side, they are a little more elusive.
More than half of today's known near-Earth asteroids were discovered in the last six years, illustrating how much our asteroid eyesight is improving, says Richard Moissl, ESA's Head of Planetary Defense.
With the advent of new telescopes and detection methods, it's only a matter of time till we've found them all, as demonstrated by the recent 30,000 detection milestone.