A simple schematic of the inner solar system, the yellow star in the middle is the Sun, the gray circle is Mercury, the grayish-yellow circle is Venus, the blue circle is Earth, the red circle is Mars, and the orange circle is Jupiter. The green band between Mars and Jupiter is the asteroid belt.
What actually defines what we mean by a near-Earth asteroid? By definition, a near-Earth asteroid is one that can come within 0.3 AU of Earth. We use 0.3 AU as a baseline, because at closest approach between Earth and Venus, the planets are only 0.27 AU apart (when Venus is at Aphelion of 0.728 AU and Earth is at perihelion of 0.983 AU). This number is rounded up to 0.3 AU to make it easier to define NEOs.
As mentioned in a previous post, asteroids are numbered sequentially, i.e. in what order it was discovered. Typically, near-Earth asteroids are not numbered until they have been observed at opposition at least twice. Recall, opposition is when an object (in this case, an asteroid) is 180° away from the Sun in the sky.
An asteroid (brown circle) at opposition
It should be noted that although these asteroids do come close to the Earth, they do not necessarily cross the orbit of the Earth and are not potentially harmful to the well being of the inhabitants or our planet.
There are three types of near-Earth asteroids which will all be discussed in their own blog posts. We refer to these asteroids as Aten, Amor, and Apollo asteroids, with each group named after the first asteroid discovered to fall into that group.
- Aten asteroids are asteroids that have semi-major axes smaller than one AU
- Amor asteroids are asteroids that have semi-major axes larger than one AU
- Apollo asteroids are asteroids that have semi-major axes around one AU.
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