Asteroids are a class of objects in space. Also called planetoids or minor planets, they orbit the sun. While they are smaller than even tiny Mars, space asteroids can still seem quite large – some even have their own moons. Today, we know that a major asteroid belt is located between the orbits of Jupiter and Mars, while other asteroids occur throughout the universe.

At the end of the 18th century, a group of 24 astronomers gathered, marshalled by Baron Xavier von Zach, to search for a “missing planet”. The planet’s existence had been hypothesized but never proven. To find it, they had to hand-draw astronomical charts for all the stars, over and over again. Using these charts, the astronomers hoped to spot any object that appeared to be moving. Unfortunately, they were not to succeed.

The missing “planet”, now known as the asteroid Ceres, was discovered accidentally by Guiseppe Piazzi of the Palermo observatory in Sicily. After tracking its movements over several nights, his colleague was able to calculate that the asteroid was located between Mars and Jupiter. Piazzi named this asteroid after the Roman goddess of agriculture. Today, it is the largest known asteroid, with a diameter of 900 to 1000 km.

Over the next several years, three other asteroids were discovered, the last one being Vesta in 1807. Astronomers searched for a further eight years, but no traces of extra asteroids were found.

In 1845, Astraea was found by Karl Ludwig Hencke. Less than two years later, he found a sixth asteroid, Hebe. His success inspired other astronomers to join the hunt. Since then, at least one new asteroid has been discovered each year, with the exception of 1945, when World War II made asteroid-hunting seem unimportant.

By 1891, hand-drawn charts were being replaced with photography as a way of detecting asteroids. Through this method, the number of known asteroids soared. Max Wolf, who pioneered the use of photography in astronomy, discovered a whopping 248 asteroids in an age where they hitherto knew of 322. Many astronomers were uninterested in asteroids, believing them to be less exciting than other discoveries (planets, comets, and more).

To discover asteroids through photography, a region of the sky is photographed twice, one hour apart. Many series of photos may be taken. Pictures are then analyzed. Any moving planetoids would be in a slightly different position from one photo to the next.

Once a moving body is detected, its location is measured using a digitizing microscope. This provides its position relative to other planetary bodies. The body is now known as an “apparition”, pending further identification.

Dr. Brian Marsden is an astronomer at the Minor Planet Centre. Reports of possible asteroid discoveries are sent to him. He enters the time and location of sightings into a computer to determine whether the apparition has a single orbit. If so, it receives a number.

The first person to observe an asteroid gets the honor of naming it once it is numbered. All names must be approved by the International Astronomical Union.

Today, satellites and improved technology have led to the discovery of thousands of asteroids. Astronomers and others find around 5000 each month. Currently, there are over 330,000 minor planets registered. Of those, 129,436 have orbits that are well-enough defined to warrant a permanent registration number.

The main asteroid belt (shown in white) is located between the orbits of Mars and Jupiter. Courtesy Wikipedia.
Left to right: 4 Vesta, 1 Ceres, Earth’s Moon. Courtesy Wikipedia.