What’s Out Tonight?

A general ASTRONOMY site to get you started exploring the night sky


Facts about the Asteroid Belt 

• Ceres, the first asteriod discovered (in 1801 by the Italian astronomer Piazzi) is also the largest of the asteroids. It has a diameter of 584 miles and orbits the Sun every 4.6 years at a distance of 257 million miles. Ceres is the only spherical asteroid and is technically a dwarf planet.

Diameters: Range from a few feet to 584 miles in diameter. Total Mass of whole Asteroid Belt: 3% that of our Moon 
Densities: 1.3 to 3.5 where water = 1 
Gravity: Enough for a spacecraft to orbit the largest. 
Albedos: Most reflect 4% to 30% of sunlight but some reflect as much as 70%. 

Distances from Sun: Varies from 175 to 375 million miles Revolutions about Sun: Varies from 3.5 to 6.5 years 

Atmospheres: None of the asteroids have atmospheres. 

Moons: Some larger asteroids have small moons (other asteroids) orbiting them. 

Observing with a small telescope: It is best to locate the position of the largest asteroids with a planetarium software program, then try to locate them with your telescope. They will appear star-like, just like the word “asteroid” implies. Right. Vesta,the third largest asteroid with a length of 318 miles, as imaged by the spacecraft Dawn.

Photos of Ceres (top) and Vesta (bottom). 
Both of these images were by spacecraft.

Images of Pallas (top) and Hygiea (bottom) taken by telescopes from Earth.

More about Asteroids and the Asteroid Belt

The asteroid belt lies between Mars and Jupiter. It is composed of a billion or so chunks of rock that vary in size from a few feet to Ceres, the largest at 584 miles in diameter. The total mass of all the asteroid belt is about 3% the mass of our Moon but about half of this mass come from just 4 asteroids—Ceres, Vesta, Pallas and Hygiea. Distances from the Sun vary from around 175 to 375 million miles, so orbits range from 3.5 to 6.5 years. None of the asteroids have atmospheres.

Most have odd shapes and resemble potatoes (Ceres is the only one that is spherical in shape). They are pitted with craters, formed when the asteroids struck one another. Their colors range from reddish and light brown to dark gray.

Asteroids vary in composition, from silicates (sand, quartz and other rock-type materials) to metals such as nickel and iron. They represent remnants left over from the formation of the solar system and are not material from a planet that exploded.

Not every asteroid lies in the asteroid belt. Today, public interest in asteroids focuses on those that could possibly collide with Earth. There may be up to 700 “Apollo-Amor” asteroids that present collision hazards. This category has lengths of over 1/2 a mile and crosses the orbits of either Mars or Earth. Astronomers are working to find all of these. It is estimated that one large “Apollo-Amor” asteroid might strike the Earth every 250,000 years, but none are expected to do so in the foreseeable future. The orbits of these asteroids change because of the gravitational influence of Jupiter.

History of Asteroids and the Belt

On January 1, 1801 the first asteroid, Ceres, was discovered by Giuseppe Piazzi. And, at the time of its discovery, it was called and considered a NEW planet! Yes, a planet like Mercury, Venus, Mars, Jupiter and Saturn. Then in 1802, a second asteroid (planet), Pallas was discovered. However, William Hershel, a famous astronomer of the time (he discovered Uranus) suggested that these objects should not be called planets, but should be considered a new class of objects, objects that he called Asteroids, meaning “star-like.” But, for 20 or so years after the discovery of Ceres, the asteroids were referred to as planets. 

Then between 1845 and 1850, numerous new asteroids were discovered. By the early 1850s, the asteroids were no longer considered planets but a special class of objects that were very small, numerous and had orbits confined between Mars and Jupiter. Does this not sound like the same thing that happened to Pluto! 

Before the 1800s, many scientists believed that there had to be some mathematical or geometrical system that described the placement of the

Giuseppe Piazzi, 1746–1826

planets in the heavens (The heavens were considered a reflection of God’s work, and, as such, a perfect system.). Kepler (1571–1630) worked on finding such a geometry and came up with a set of nesting geometric solids (like cubes, pyramids, etc) that approximated the placements of the planets. There was also a simply numerical progression put forth by Bode in 1772 that seemed to accurately

described the distances of the six known planets from the Sun (see below). But according to Bode’s Law, as it became to be known, there was no planet between Mars and Jupiter, as predicted by his law. So, in 1781, when William Herschel discovered Uranus, its distance from the Sun matched the distance as predicted by the “law.” Scientists then gave renewed credence to the law, with serious thought and effort put into finding the missing planet between Mars and Jupiter. Before a group of 24 astronomers could find an asteroid, the Italian astronomer, Giuseppe Piazzi, not part of the group, found the first asteroid, by chance. The idea, which unfortunately persist to this day, that the asteroid belt represents a planet that exploded, started after the second asteroid was discovered in 1802. Since the first two asteroids were very small and Bodes’s law seemed definitive about a “missing” planet, it was postulated that this “planet” may have blown up in the past. Of course, this is not the case!

Bode’s “Law”

Johann Elert Bode, a German astronomer (1747–1826), noticed that the distances of the six known planets from the Sun, expressed in units of Earth’s distance—called an Astronomical Unit, followed the simple pattern as indicated below:

Add 4 to each of the numbers in the sequence 0, 3, 6, 12, 24, 48, 96, 192, 384, then divide each of the results by 10 and you get the distances of the planets from the Sun in Astronomical Units, that is: 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, 19.6, 38.8. Compare these to the actual values in the next column!

Actual Values

0.4 Astronomical Units 
Venus0.7 Astronomical Units 
Earth1.0 Astronomical Units 
Mars1.5 Astronomical Units 
Asteroid  Belt2.77 AU for Ceres Jupiter5.2 Astronomical Units 
Saturn9.5 Astronomical Units 
Uranus19 Astronomical Units 
Neptune30 Astronomical Units

When Bode first came up with this series, Uranus had not been discovered and according to the law, there was a missing planet between Mars and Jupiter. It is uncanny how close these numbers come to the actual values listed to the left. 

The orbit of Neptune does not fit Bode’s Law. Today, it is unequivocally believed that Bode’s Law is a mere coincidence. One can probably find a sequence to roughly approximate anything.