Halley’s visions: How far have we come?

23 Mar 2023

The existence of the Oort cloud is the answer to the questions people have always asked about comets: what are comets and where do they come from? Back in the fourth century BC, Aristotle believed that comets were clouds of luminous gas in the upper layers of the Earth’s atmosphere.

In the first century, the Roman philosopher Seneca proposed the idea that comets are celestial bodies that travel along their paths in the firmament. Only fifteen centuries later, this hypothesis was confirmed by the Danish astronomer Tycho Brahe by comparing observations of the comet from 1577 from several places in Europe.

If the comet is relatively close, then it will have a different position in the sky in relation to the stars from every point of view. Brahe could not detect any differences in position, so he concluded that the comet was somewhere beyond the Moon’s orbit.

Exactly how far away comets are from us became clear only when astronomers began to determine their orbits. The first catalog of comets was made by the English astronomer Edmond Halley back in 1705, which included 24 objects.

Halley realized that the orbits were probably very elongated ellipses going around the Sun. Edmond Halley understood that comets travel great distances and may even be in interstellar space. This first simple idea of a cloud of comets between stars helped to conclude the existence of the Oort cloud two centuries later, because the right steps were taken from the beginning.

Halley also noticed that the comets of 1531, 1607 and 1682 had very similar orbits and that their occurrences were approximately 76 years apart.

From there, he concluded that it was the same comet and predicted its next appearance in 1758, which unfortunately he did not get to see. He died in 1742, before the appearance of the comet that bears the name Halley’s Comet in his honor and which was last found among the inner planets in 1986.

Short, medium or long-period?

Since Halley’s time, astronomers have split comets into two groups according to the time it takes for a comet to complete an orbit around the Sun.

Long-period comets, such as Hyakutake or Hale-Bopp, have orbital periods longer than 200 years. In contrast, short-period comets have orbital periods shorter than 200 years.

In the last decade, astronomers divided short-period comets into two subgroups: comets of the Jupiter family, such as Encke’s comet, which have periods shorter than 20 years, and Halley-type comets, or medium-period comets, with periods from 20 to 200 years.

The boundary between long- and short-period comets corresponds to 34.2 AU (astronomical units). Of course, it is possible for Jupiter to change the path of a long-period comet so that it becomes short-period, but only about 10% of short-period comets in the Jupiter family are Oort cloud comets.

Medium- and long-period comets come from the Oort Cloud, while comets of the Jupiter family come from the Kuiper Belt, the belt of asteroids beyond Neptune’s orbit.

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