In the Solar System, we have now discovered 30 039 near-Earth asteroids orbiting the Sun on a path that brings them close to Earth’s orbit.
In the Solar System, we have now discovered 30 039 near-Earth asteroids orbiting the Sun on a path that brings them close to Earth’s orbit. The majority of these were discovered in the last decade. Demonstrating how rapidly our ability to detect potentially dangerous asteroids is improving.
When an asteroid’s trajectory brings it within 1.3 Astronomical Units (au) of the Sun. It is classified as a near-Earth asteroid (NEA). Because 1 au is the distance between the Sun and Earth, NEAs can approach within 0.3 au, or 45 million kilometres, of our planet’s orbit.
When an asteroid’s trajectory brings it within 1.3 Astronomical Units (au) of the Sun, it is classified as a near-Earth asteroid (NEA). Because 1 au is the distance between the Sun and Earth, NEAs can approach within 0.3 au, or 45 million kilometres, of our planet’s orbit.
When an asteroid’s trajectory brings it within 1.3 Astronomical Units (au) of the Sun, it is classified as a near-Earth asteroid (NEA). Because 1 au is the distance between the Sun and Earth, NEAs can approach within 0.3 au, or 45 million kilometres, of our planet’s orbit.
The roughly 30 km Eros asteroid was discovered independently by Auguste Charlois at the Nice Observatory and Carl Gustav Witt and Felix Linke at the Urania Observatory in Berlin. The stony asteroid’s orbit brings it within 22 million kilometres of Earth – 57 times the distance between Earth and the Moon.
Eros is not only the first known NEA, but also the first asteroid orbited by a spacecraft and the first asteroid to have a spacecraft land on it. Early calculations of the space rock’s orbit also allowed for a precise determination of the Sun-Earth distance, which was previously unknown.
Un-Earthing a Near-Earth Asteroid
Large asteroids were naturally discovered first because they are much easier to see. They were considered minor planets, a term that is still used today. We are discovering many more and at a rapid rate as telescopes become more sensitive, even those as small as tens of metres in size.
Every week, new asteroids are discovered by ground-based survey telescopes such as the Catalina Sky Survey in Arizona, USA. They are designed to scan large areas of the sky for new objects moving against the backdrop of’motionless’ stars.
Large telescopes with greater focus, such as the European Southern Observatory’s Very Large Telescope (VLT), can then be used for follow-up observations, allowing us to better understand the path, size, and even composition of a ‘new’ asteroid.
Gaia, the European Space Agency’s space observatory on a mission to catalogue one billion stars in the galaxy, has also contributed to a better understanding of the asteroid risk.
“Because of Gaia, we know more about the stars in the galaxy which act as a backdrop to asteroid observations,” explains Tineke Roegiers, community support for the Gaia mission.
The use of ‘Gaia’s stars’ improved the orbits of already-known near-Earth asteroids, and some “lost” asteroids were discovered again.
ESA’s asteroid risk list
Of course, any asteroid discovered near Earth qualifies as a near-Earth asteroid, but many are found far from home,” explains Marco Micheli, Astronomer at ESA‘s Near-Earth Object Coordination Centre.
New objects are discovered over time, their movements are studied, and their future positions can be predicted using only a few data points from different nights. Depending on the number and quality of observations, this could last decades or even hundreds of years.”
The Near-Earth Object Coordination Centre (NEOCC) of the European Space Agency (ESA) is based in ESRIN, Italy, and houses the Agency’s asteroid experts and risk assessors. The team uses its global network of telescopes to collect observations of newly discovered asteroids and determine their impact risk, while also pursuing ‘old’ asteroids that have not yet been deemed safe.
Will any of these asteroids strike Earth?
Currently, 1 425 asteroids with a ‘non-zero’ chance of impact are under their watchful eye, organised in the NEOCC’s Asteroid Risk List, which is constantly updated and open to the public. You can also subscribe to the ESA’s monthly ‘Asteroid Newsletter,’ and the asteroid news will be delivered directly to your inbox
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