How far is the asteroid belt from the Sun?

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In the 18th century, observations made on all known planets (Mercury, Venus, Earth, Mars, Jupiter and Saturn) led astronomers to discern a pattern in their orbits. Eventually, this led to the Titius-Bode Law, which predicted the amount of space between planets. In accordance with this law, there appeared to be a noticeable gap between the orbits of Mars and Jupiter, and the investigation of it led to a major discovery.

Eventually, astronomers realized that this region was overrun by countless smaller bodies which they named “asteroids”. This in turn led to the term “asteroid belt,” which has since come into common use. Like all the planets in our solar system, it orbits our sun and has played an important role in the evolution and history of our solar system.

Structure and composition:

The asteroid belt consists of several large bodies, as well as millions of smaller sizes. Larger bodies, such as Ceres, Vesta, Pallas, and Hygiea, make up half of the total mass of the belt, with almost a third represented by Ceres alone. Beyond that, more than 200 asteroids over 100 km in diameter and 0.7 to 1.7 million asteroids with a diameter of 1 km or more.

Ceres compared to asteroids visited to date, including Vesta, Dawn’s mapping target in 2011. Credit: NASA / ESA / Paul Schenck

In total, the mass of the asteroid belt is estimated to be 2.8 × 1021 to 3.2 × 1021 kilograms – which is equivalent to about 4% of the mass of the Moon. While most asteroids are made up of rocks, a small portion of them contain metals such as iron and nickel. The remaining asteroids are made up of a mixture of these, as well as carbon-rich materials. Some of the more distant asteroids tend to contain more ice and volatiles, including water ice.

Despite the impressive number of objects contained in the belt, the asteroids of the main belt are also distributed over a very large volume of space. As a result, the average distance between objects is approximately 965,600 km (600,000 miles), which means that the main belt consists largely of empty space. In fact, due to the low density of materials within the belt, the chances of a probe hitting an asteroid are now estimated to be less than one in a billion.

The main population (or nucleus) of the asteroid belt is sometimes divided into three areas, which are based on what are known as the “Kirkwood Gaps”. Named after Daniel Kirkwood, who in 1866 announced the discovery of gaps in the distance of asteroids, these gaps are similar to what we see with the ring systems of Saturn and other gas giants.

Origin:

Originally, the asteroid belt was seen as the remnants of a much larger planet that occupied the region between the orbits of Mars and Jupiter. This theory was originally suggested by Heinrich Olbders to William Herschel as a possible explanation for the existence of Ceres and Pallas. However, this assumption has since been shown to have several flaws.

On the one hand, the amount of energy required to destroy a planet would have been staggering, and no scenario has been suggested that could explain such events. Second, there is the fact that the mass of the asteroid belt is only 4% that of the Moon (and 22% that of Pluto). The chances of a cataclysmic collision with such a small body are highly unlikely. Finally, the significant chemical differences between asteroids do not suggest a common origin.

Today, the scientific consensus is that, rather than fragmenting from a home planet, asteroids are remnants of the early solar system that never formed a planet. During the first million years of the solar system’s history, gravitational accretion caused clusters of matter to form from an accretion disk. These clusters gradually came together, finally undergoing a hydrostatic equilibrium (becoming spherical) and forming planets.

However, in the region of the asteroid belt, the planestesimals were too strongly disturbed by the gravity of Jupiter to form a planet. As such, these objects would continue to orbit the Sun as they did before, with a single object (Ceres) having accumulated enough mass to undergo hydrostatic equilibrium. Occasionally they collided to produce smaller fragments and dust.

Asteroids also melted to some extent during this time, allowing the elements they contained to be partially or completely differentiated by mass. However, this period would have been necessarily short due to their relatively small size. It probably ended around 4.5 billion years ago, a few tens of millions of years after the formation of the solar system.

Although they date from the ancient history of the solar system, asteroids (as they are today) are not samples of his primordial self. They have undergone considerable evolution since their formation, including internal heating, surface melting due to impacts, spatial alteration due to radiation, and bombardment by micrometeorites. Therefore, it is believed that the asteroid belt today contains only a small fraction of the mass of the primordial belt.

Computer simulations suggest that the original asteroid belt could contain a mass equivalent to that of Earth. Mainly because of gravitational disturbances, most of the matter was ejected from the belt a million years after its formation, leaving behind less than 0.1% of the original mass. Since then, it is believed that the size distribution of the asteroid belt has remained relatively stable.

When the asteroid belt first formed, temperatures at a distance of 2.7 AU from the Sun formed a “snow line” below the freezing point of water. Essentially, planetesimals formed beyond this radius may have accumulated ice, some of which may have provided a source of water for Earth’s oceans (even more so than comets).

Distance from the Sun:

Located between Mars and Jupiter, the belt is between 2.2 and 3.2 astronomical units (AU) from the Sun – 329 million to 478.7 million km (204.43 to 297.45 million mi ). It is also estimated to be 1 AU thick (149.6 million km, or 93 million mi), which means it occupies the same distance as what is between the Earth and the Sun.

Asteroids of the Inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars.  Credit: Wikipedia Commons
Asteroids of the Inner Solar System and Jupiter: The donut-shaped asteroid belt is located between the orbits of Jupiter and Mars. Credit: Wikipedia Commons

The distance of an asteroid from the Sun (its semi-major axis) depends on its distribution in one of three different zones based on the “Kirkwood spaces” of the belt. Zone I is between the Kirkwood 4: 1 and 3: 1 resonance deviations, which are approximately 2.06 and 2.5 AU, respectively (3 to 3.74 billion km; 1.86 to 2 , 3 billion mi) from the Sun.

Zone II continues from the end of Zone I to the 5: 2 resonance gap, which is 2.82 AU (4.22 billion km; 2.6 mi) from the Sun. Zone III, the outermost section of the belt, extends from the outer edge of Zone II to the 2: 1 resonance space, located at approximately 3.28 AU (4.9 billion km; 3 billion of mi) from the Sun.

While many spacecraft traveled through the asteroid belt, most transited there en route to the Outer Solar System. It is only in recent years, with the Dawn mission, that the asteroid belt has been a focal point of scientific research. Over the next several decades, we might find ourselves sending spaceships out there to mine asteroids, harvest minerals and ice for use here on Earth.

We have written extensively on the asteroid belt here at Universe Today. Here is what is the asteroid belt ?, How long does it take to get to the asteroid belt ?, How far is the asteroid belt from Earth ?, Why is the asteroid belt ?, Why is the asteroid belt ?, Why is the asteroid belt? asteroids a planet? and Why isn’t the asteroid belt Threatening the spaceship.

For more, see NASA’s Lunar and Planetary Science page on asteroids and the Hubble asteroid site press releases.

Astronomy Cast also some interesting asteroid episodes, like Episode 55: The Asteroid Belt and Episode 29: Asteroids Make Bad Neighbors.

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