Near Earth Asteroids

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.

 

Besides the asteroid belt and the Greek and Trojan Asteroids, there are also asteroids that come close to Earth. These asteroids are called near-Earth asteroids, or near-Earth objects (NEOs). You have probably have heard of these asteroids on the news, telling us a new one has been discovered and it will make a near approach to Earth sometime in the future.

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.

Asteroids

Asteroids are believed to be remnants from the formation of the Solar System. The majority of these objects are found in the space between Mars and Jupiter called the asteroid belt.



As seen in the above image, there are also a few asteroids that are 60° ahead of Jupiter called "Greek" asteroids and some that are 60° behind Jupiter called "Trojan" asteroids. Not seen, but will be discussed are the three-A asteroid groups: Amor, Apollo, and Aten. These are near-Earth asteroids that are a concern for scientists and are searched for and catalogued by astronomers.

In our previous post about Phobos and Deimos, we learned a little about C-type and D-type asteroids. There are two other main types (and no, they are not A-type and B-type) called S-type and M-type. As mentioned before, C-type asteroids (and to an extent, D-type) have strong carbon lines in their spectra. S-type asteroid are stony asteroids, made of silicates, with densities similar to Earth's density, and make up about 17% of all asteroids. M-type asteroids are metallic, with the most common metal found in them iron and some having nickel. This is one reason why mining asteroids might be a lucrative business in the future, when travelling easily from Earth to the outer reaches of the Solar System will be achieved.

253 Mathilde - C-type Asteroid

NASA

15 Eunomia - S-type Asteroid

Astronomical Institute of the Charles University

16 Psyche - M-type Asteroid

Astronomical Institute of the Charles University

 



 Asteroids, generally, come in many sizes, with the largest being 1 Ceres at 950 km in diameter. However, most asteroids are only a few miles in size and are irregularly shaped. It is typically believed that asteroids are planetessimals (baby planets) that were not able to accrete into a planet because of the presence of Jupiter. Measurements of the mass of the asteroid belt show that there is only enough mass there to create a planet with the quarter of the size of a planet.

Asteroids have two parts to their names, a number designation and a proper name. The first few asteroids were just given names, but as more and more were discovered, astronomers started placing a number in front to give an indication of the sequence of discovery. So 1 Ceres was the first asteroid discoved and 253 Mathilde was the 253rd. As of 2013, there are several hundred thousand asteroids named with more than a million probably out there. Many of those asteroids are in the asteroid belt, but as mentioned above, there are thousands leading and trailing Jupiter in its orbit and thousands that are near Earth's orbit. We will learn more about Amor, Aten, and Apollo asteroids in a future post.

Most asteroids orbit independently around the Sun, but there are some that mutually orbit each other.  The most well known pair is Ida and Dactyl, where Dactyl was the first satellite discovered around an asteroid.  The Solar System is a strange place.