Although there are many known comets, some have become more famous than others for various reasons. Everyone knows these kites.
In 1705 Edmond Halley predicted, using Newton's laws of motion, that the comet seen in 1531, 1607 and 1682 would return in 1758. The comet returned as predicted, and subsequently named in his honor.
The average period of Halley's orbit is 76 years, but the dates of its reappearances cannot be calculated exactly. The gravitational force of the major planets alters the period of the comet in each orbit. Other effects, such as the reaction of the ejected gases during the passage near the Sun, also play an important role in the alteration of the orbit.
The orbit of the Halley is retrograde and inclined 18º with respect to the ecliptic. And, like that of all comets, highly eccentric. The nucleus of Halley's comet measures approximately 16x8x8 kilometers.
Contrary to previous assumptions, the Halley's core is very dark, blacker than coal and one of the darkest objects in the solar system.
The density of the Halley core is very low: about 0.1 grams / cm3, indicating that it is probably porous, perhaps due to the large amount of dust left after the ice has sublimated.
Halley is almost unique among comets, since it is both large and active, and has a regular and well-defined orbit. But, although famous, it may not be representative of comets in general.
Halley's comet will return to the inner solar system in 2061.
The Hale-Bopp is a periodic comet that returns every 3,000 years and that approached Earth in 1997, causing great expectation. Alan Hale in New Mexico and, independently, Thomas Bopp of Arizona, discovered the comet that now bears the name of both. Soon after the discovery it became clear that this comet could be one of the brightest in recent years.
Comet Hale-Bopp was at that time one of the brightest stars in the sky, reaching a magnitude -0.8, which means that the comet was brighter than any object in the night sky at that time, with the exception of Moon, Sirius and the planet Mars.
Despite its brightness, comet Hale-Bopp did not get too close to Earth. In its maximum approach it was 194 million kilometers away, that is to say a little further from us than the Sun.
It is believed that the nucleus of the comet is relatively large, about 40 kilometers according to estimates, since it is not possible to directly see the nucleus. However, more than the nucleus, the determining factor for the comet's brightness is the coma, the gas and dust envelope that surrounds the comet's nucleus.
When approaching the Sun part of the comet is sublimated. Some comets develop several tails, and in particular in the Hale-Bopp it was possible to observe two tails, one of gas and one of dust. The tail of comet Hale-Bopp, difficult to observe from the cities, reached several million kilometers in length.
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