Saturday, August 7, 2021

Origin of near earth asteroids

Near earth asteroids are asteroids that have been pushed by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth’s neighborhood. They are basically divided into three groups defined by their orbits with respect to that of Earth, namely Atens, Apollos and Amors. 

The Atens spend most of their time within Earth’s orbit. The Apollos spend most of their time further from the Sun than Earth, and the Amors always stay outside Earth’s orbit. In this article we will discuss two major mechanism of origin of near Earth asteroids. So let us start.

First Mechanism

Due to collision with newly forming planets or by ejection from the solar system by gravitational interaction with larger bodies, many of these meter- to kilometer-sized near-Earth objets located in the inner Solar System at the time of its formation were removed

The life of an asteroid as a near-Earth object is relatively short, about 2 to 6 million years, before they got lost due to an impact with Earth or Moon, or any other inner planet or by being ejected from the inner Solar System.

Cratering studies show that the impact rate has remained unchanged for the last 3 billion years and this is the reason why the number of near-Earth asteroids have been reasonably constant over the same period. Most near earth objects have their origins in the Main Belt. You can also read – Asteroid nomenclature

The collision of two such asteroids could result in one or both, or fragments of either, entering one of the unstable regions known as Kirkwood Gaps. A Kirkwood gap is a gap in the distribution of the semi-major of the orbits of main-belt asteroids. So this is one of the mechanism of origin of near-Earth asteroids. Let us discuss another mechanism.

Second Mechanism

The second mechanism that we are going to discuss next is more probable than the first mechanism. Let us see what it is.

Asteroids with diameters of 20 km or less have their orbits changed gradually and thus moved into the unstable regions of the Main Belt due to the Yarkovsky effect. Let us see what Yarkovsky effect is.

As asteroids rotate they absorb the Sun’ heat and then reradiate it. This re-emission causes a change in the orbit of the asteroid. For the case of asteroid which is spinning in the same direction in which it orbits, also known as prograde motion, the force acts to speed up the asteroid, thus causing it to move outwards from the Sun.

The force on an asteroid which is spinning in the opposite direction to which it orbits, also known as retrograde, acts in the opposite direction, and it causes the asteroid to slow down, causing it to spiral inwards towards the Sun. The force due to Yarkovsky effect is very small in magnitude.

Using radar observations of asteroid, it was found that a 0.5-km-diameter asteroid shifts its orbit by only 15 km in the 12 years due to the Yarkovsky effect. So you can guess how weak this force is. Now if we consider that an asteroid made its way into Kirkwood Gap, the orbit of that asteroid will be strongly influenced by Jupiter’s gravity.

The greatest effect of such a perturbation due to Jupiter’s gravity is to increase the eccentricity of the asteroid’s orbit so that it eventually became a Mars crosser. After that it’s orbit will be perturbed by Mars, and then its new orbit would bring it in to the inner Solar System as an near earth asteroid.

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