Halley's Comet: Visitor from the Depths of the Solar System

Halleys Comet
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Comet Halley (officially known as P1/Halley) is the most famous known comet in our skies. It returns to Earth's skies every 76 years (or so), and leaves behind a trail of dusty debris that forms the annual Orionid Meteor shower each October.  If you didn't see it during its last appearance in the 1980s, its next one will begin in 2060, so mark your calendars! 

People have observed Comet Halley throughout history.

However, it wasn't until the year 1705 that astronomer Edmund Halley calculated its orbit and predicted its next appearance. He used Isaac Newton's recently developed Laws of Motion plus some observational records, and stated that the comet — which appeared in 1531, 1607 and 1682 — would reappear in 1758.

He was right — it showed up right on schedule. Unfortunately, Halley did not live to see its ghostly appearance, but astronomers named it after him to honor his work.  

Comet Halley and Human History

Comet Halley has a large icy nucleus. As it nears the Sun it brightens up and can be seen for many months at a time. Its first known apparition occurred and was recorded by the Chinese in the year 240 BCE. It may have been recorded in the year 467 BCE in ancient  Greek records. One of the more interesting "recordings" of the comet came after the year 1066 when King Harold was overthrown by William the Conqueror at the Battle of Hastings.

It is depicted on the Bayeux Tapestry, which chronicles those events and prominently displays the comet over the scene. 

In 1456, on a return passage, Halley's Comet was excommunicated as an agent of the devil by Pope Calixtus III, but it didn't do any good; the comet came back! During this same apparition, while Turkish forces laid siege to Belgrade (in today's Serbia), the comet was described as a fearsome celestial apparition "with a long tail like that of a dragon" which was perceived by some as being in the form of "a long sword advancing from the west ...


Modern Observations of Comet Halley

During the 19th and 20th centuries, the comet's appearance in our skies were greeted by scientists with great interest. In 1985 and 1986, amateur and professional astronomers worldwide united to observe it as it passed close by the Sun. Their data helped astronomers fill in the story of what happens when a cometary nucleus passes through the solar wind. At the same time, spacecraft explorations revealed the lumpy nucleus of the comet, sampled its dust tail, and studied very strong activity in its plasma tail. 

During that time, five spacecraft from the USSR, Japan, and the European Space Agency journeyed to Comet Halley. ESA's Giotto obtained close-up photos of the comet's nucleus, Because Halley is both large and active and has a well-defined, regular orbit, it was a relatively easy target for Giotto and the other probes. 

Although the average period of Halley's Comet's orbit is 76 years, it's not that easy to calculate the dates when it will return by simply adding 76 years to 1986. Gravity from other bodies in the solar system will affect its orbit. Jupiter's gravitational pull has affected it in the past, and could do so again in the future when the two bodies pass relatively near each other.

Over the centuries, Halley's orbital period has varied from 76 years to 79.3 years. Currently, we know that this celestial visitor will return to the inner solar system in the year 2061 and move close to the Sun ion July 28th of that year. Then it will make a slow return to the outer solar system before heading back for the next close encounter some 76 years later.

Since the time of its last appearance astronomers have been avidly studying other comets.The European Space Agency sent the Rosetta spacecraft to Comet 67P/Churyumov-Gerasimenko, which went into orbit around the comet's nucleus and sent a small lander to sample the surface. Among other things, the spacecraft watched numerous dust jets "turn on" as the comet got closer to the Sun. It also measured the surface color and composition, "sniffed" its smell, and sent back many images of a place most people never imagined they would see.


Edited by Carolyn Collins Petersen.