Understanding various populations of Near Earth Asteroids
The asteroids that cross the orbit of the Earth are called Near Earth Asteroids. A subset of these are potentially hazardous asteroids that can potentially strike the Earth in the future and cause regional or widespread devastation because of their sizes. These asteroids are classified into different groups based on the parameters of their orbits.
The Solar System, from the point of view of asteroids, is not an orderly arrangement of planets at increasing distances from the Sun, but a bustling neighbourhood. Most of the asteroids are housed in the Man Belt between Mars and Jupiter. Some are made up of primordial material leftover from the birth of the Sun, others are fragments of planetesimals that were battered to bits in the chaotic infancy of the Solar System. There are groups of asteroids distributed throughout the Solar System, including in the same neighbourhood around the Sun as the Earth.
The Near Earth Asteroids are classified according to their orbital parameters as Apollos, Athens, Amors, Atiras and Arjunas. Each of these groups are named after a representative member of the group, which is typically the first asteroid discovered from that group. Asteroids are numbered sequentially in the order in which their discoveries are confirmed, starting with 1 Ceres, 2 Pallas, 3 Juno and so on. 1862 Apollo was first discovered in 1932. These are asteroids with orbits larger than that of the Earth, but ones that intersect with Earth’s trajectory around the Sun.
Groups of Near Earth Asteroids
These are now the largest group of asteroids known to have orbits that intersect with the Earth. Most of these asteroids cross the orbit of Earth twice every lap around the Sun. There are over 17,000 asteroids known in the Apollo group, making them the most populous among all the Near Earth Asteroids. The asteroid 99942 Apophis for example, is a part of this group and is expected to approach within 31,000 kilometres of Earth in April 2029. The asteroid will zip past safely, but is close enough to rattle nerves and sharpen our focus on these cosmic interlopers.
On the fringes of our orbit we encounter the Amors, that come close but do not cross the orbit of the Earth. They remain beyond one astronomical unit, the distance between the Earth and the Sun. There are roughly 10,000 Atens group asteroids known. The group is named after 1221 Amor, discovered in 1932. This is a fairly large population, with roughly 10,000 members known, including 433 Eros, a potato-shaped rock that was observed from up close by NASA’s NEAR Shoemaker mission. While Amors would not normally collide with the Earth, gravitational nudges from Mars could cause them to drift into more threatening orbits over time.
| Group | Orbit Characteristics | Named After | Example | Known Count |
|---|---|---|---|---|
| Apollo | Crosses Earth’s orbit, semi-major axis > 1 AU | 1862 Apollo | 99942 Apophis | ~17,000 |
| Aten | Crosses Earth’s orbit, semi-major axis < 1 AU | 2062 Aten | 2016 HO3 | ~2,000 |
| Amor | Approaches Earth, perihelion 1.017–1.3 AU | 1221 Amor | 433 Eros | ~10,000 |
| Atira | Entirely inside Earth’s orbit, aphelion < 0.983 AU | 163693 Atira | 2020 AV2 | ~30 |
Then there are the Atens, named after 2062 Aten first spotted in 1976. These asteroids have orbits tighter than that of the Earth, yet at the outer edges of their orbits, they can approach close to the Earth. The Atens spend most of their time well within the orbit of the Earth, only occasionally crossing the planet. Members of the Atens group include 469219 Kamoʻoalewa, an elongated quasi-satellite of the Earth that tags along the planet in its orbit around the Sun. The tighter orbits of the Atens results in more frequent close encounters.
And then there are the elusive Atiras, the most mysterious of the Near Earth Asteroid groups, and the one with the fewest known members. This group is named after 163693 Atira, first spotted in 2003. These are rare gems entirely within the orbit of the Earth, with few venturing beyond 0.9 AU. Most of these asteroids are difficult to spot and lost in the glare of the Sun. Fewer than 30 Atira asteroids are known so far. Their orbits are in a private loop entirely detached from that of the Earth. Most of these are discovered only after they hurtle past. The difficulty in spotting them makes them seem more hazardous.
Finally there is the Arjuna group, named after a legendary archer from the Indian epic Mahabharata. The Arjuna asteroids have orbital parameters similar to that of the Earth, with an average distance of one AU. They have nearly circular orbits, and are aligned with the plane of the Earth’s orbit around the Sun, and orbit the Sun about once a year as well. Many of the Arjuna asteroids have slipped into a 1:1 mean-motion resonance with the Earth, and these can often appear as mini-moons, such as 2024 PT5 that traced a horseshoe orbit in the skies of the Earth over a period of two months, and 2023 FY3 that is spinning rapidly and has been a quasi-moon multiple times in the past.
Influences and processes
These Near Earth Asteroids are time capsules, remnants from the infancy of the Solar System that never coalesced into planets, or the shattered fragments of differentiated planetesimals. They may have been nudged out of the Main Belt by the gravitational tug of war between Jupiter and the Sun, or by the slow but persistent push from the pressure of solar radiation, called the Yarkovsky Effect. Some like Eros bear the scars of a violent past, and others like Apophis are a faint threat in the distant future. Observing and discovering these Near Earth Asteroids is important to unravel the evolutionary history of the Solar System, and to keep the citizens safe from any potential impactors. This is why it is important to know who is dropping by, the endless close approach stories can help us understand our neighbours better, and are not just clickbait headlines.