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

comet halley
Comet Halley as seen in March 1986. NASA International Halley Watch, by Bill Liller.

Everyone's heard of Comet Halley, more familiarly known as Halley's Comet. Officially called P1/Halley, this solar system object is the most famous known comet. It returns to Earth's skies every 76 years and has been observed for centuries. As it travels around the Sun, Halley leaves behind a trail of dust and ice particles that form the annual Orionid Meteor shower each October. The ices and dust that make up the comet's nucleus are among the oldest materials in the solar system, dating back to before the Sun and planets formed some 4.5 billion years ago.

Halley's last apparition began in late 1985 and extended through June of 1986. It was studied by astronomers around the world and even was visited by spacecraft. Its next close "flyby" of Earth won't happen until July 2061, when it will be well placed in the sky for observers. 

Comet Halley has been known about for centuries, but 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, just as other comets do. As it nears the sun, it brightens up and can be seen for many months at a time. The first known sighting of this comet occurred in the year 240 and was duly recorded by the Chinese. Some historians have found evidence that it was sighted even earlier, in the year 467 BCE, by the ancient Greeks. 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.The battle 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 Pope Calixtus III determined it was an agent of the devil, and he attempted to excommunicate this naturally occurring phenomenon. Obviously, his misguided attempt to frame it as a religious issue failed, because the comet came back 76 years later. He wasn't the only person of the time to misinterpret what the comet was. During the 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." One anonymous writer suggested it was "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 was greeted by scientists with great interest. By the time the late 20th century apparition was about to start, they had planned extensive observing campaigns. In 1985 and 1986, amateur and professional astronomers worldwide united to observe it as it passed close by the Sun. Their data helped 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. 

Comet Halley's Schedule

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 will pass its closest to the Sun on July 28th of that year. That close approach is called "perihelion." 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.

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Greene, Nick. "Halley's Comet: Visitor from the Depths of the Solar System." ThoughtCo, Aug. 27, 2020, thoughtco.com/halleys-comet-visitor-from-afar-3072470. Greene, Nick. (2020, August 27). Halley's Comet: Visitor from the Depths of the Solar System. Retrieved from https://www.thoughtco.com/halleys-comet-visitor-from-afar-3072470 Greene, Nick. "Halley's Comet: Visitor from the Depths of the Solar System." ThoughtCo. https://www.thoughtco.com/halleys-comet-visitor-from-afar-3072470 (accessed June 4, 2023).