Halley’s Comet is one of the most famous comets in history and there are many reasons for that.
As you may already know, Halley’s Comet is visible to the naked eye from Earth every 75-76 years. But scientists didn’t know that comets could describe an orbit until the study of Halley’s Comet.
Halley’s Comet rotates around the Sun in a retrograde motion —that is, opposite to Earth’s orbit— across a circumference of about 7.6 billion miles (12.2 billion kilometers), covering a distance from around the orbit of Mercury to the orbit of Neptune in those 75-76 years. In spite of this, it is called a short-period comet —a name that designates a comet with an orbit that lasts less than two centuries. The period varies because of the gravitational effects of the planets. Halley's period has been as short as 74.42 years and as long as 79.25 years.
Interesting facts about Halley's Comet
- Halley's Comet's official name is 1P/Halley.
- Scientists estimate that Halley has been in its present orbit for at least 16,000 years. On its last pass, it was estimated that Halley lost around one-one thousandth of its mass. Its current mass is estimated at around 2.2×1014 — so it doesn't seem to be in danger of disappearing, splitting in two, or getting pulled out of the solar system anytime soon.
- Some researchers have estimated that Halley's comet may have described 2,000 to 3,000 orbits already. Since the average periodic comet is theorized to complete about 1,000 trips around the Sun, that would make Halley quite long-lived, in comet terms.
- At aphelion in 1948, Halley was moving at just 0.91 km/sec (2,000 mph). However, as comets get closer to the sun, their speed increases. By the time of perihelion in 1986, Halley's was traveling at approximately 122,000 mph. That is 54.55 km/sec relative to Earth's movement.
- Halley's dimensions are about 9.3 by 5 miles (15 kilometers by 8 kilometers). It is one of the darkest, or least reflective, objects in the solar system. It has an albedo of 0.03, which means that it reflects only 3% of the light that falls on it.
- The nucleus of the comet is a mixture of water and other volatile ices, mixed with rocky and carbon-rich dust. On the comet's last flyby, about 70 percent of the surface of the nucleus was covered by a dark insulating “crust” that prevented the water ice underneath from sublimating. However, the remaining 30 percent produced bright jets of gas and dust in a tail around 31 million miles (50 million kilometers) long. Earth passes through this debris stream each year, resulting in the Orionid and Eta Aquarid meteor showers in October and May.
History of Halley's Comet
The history of Halley's Comet starts way before the birth of Edmond Halley, the English astronomer after whom the comet was named, and even before the invention of the first telescope in 1608.
According to the European Space Agency, Comet Halley was likely spotted by Chinese astronomers in 240 B.C, as explained in The Records of the Grand Historian, a book written between 109 B.C. and 91 B.C. by Han Dynasty historian Sima Qian. In the book, the comet is described as a “broom star”.
Some have suggested that a comet seen in Ancient Greece between 467 and 466 BCE may have been Halley. Babylonian records found on ancient clay tablets reveal what may have been sightings in 164 B.C. and 87 B.C.
Halley may also have been the comet described by Roman historian Dion Cassius as a “fearful star” in 218 CE. In fact, the appearance of comets, in general, was often treated as deadly omens. It has been suggested that Halley's comet was seen right before the defeat of Attila the Hun at the Battle of Chalons in 451, and Halley's comet was seen six months before the Norman Conquest of England in 1066 and is depicted in the Bayeux Tapestry.
Who discovered Halley’s Comet?
Halley’s Comet was observed many times in history, but no one knew that it was actually the same comet. It was English astronomer Edmond Halley who first proposed the idea. In 1705, Halley used Isaac Newton's theories of gravitation and planetary motions to calculate the orbits of various comets. He found similarities in the orbits of comets recorded in 1531, 1607, and 1682, and suggested that these were actually the same comet.
Halley, who was also a physicist and a mathematician, calculated that this comet would reappear in 1758. Although Halley did not live long enough to see his prediction proved true (he died in 1742), shortly after the comet reappeared in 1758, it was named in Halley’s honor by French astronomer Nicolas-Louis de Lacaille.
When did Halley's Comet last appear?
Halley’s Comet appeared most recently in 1986. Four years before its appearance, astronomers imaged the comet with the Hale Telescope at Palomar Observatory in California, when it was still beyond the orbit of Saturn. Five spacecraft were sent to fly past the comet in March 1986: two Japanese spacecraft (Sakigake and Suisei), two Soviet spacecraft (Vega 1 and Vega 2), and a European Space Agency spacecraft (Giotto) that passed just 370 miles (596 km) from the comet’s nucleus. Altogether, the multiple probes that were sent to study Halley’s Comet were referred to as the Halley Armada.
The technology of that time allowed scientists to obtain high-quality images of Halley’s Comet, especially in comparison to the earliest images, taken in 1910. These close-up images allowed scientists to determine that Comet Halley’s core is a solid mass mainly made of dust and ice - a "dirty snowball".
NASA also planned to observe the comet from low Earth orbit through two Space Shuttle missions, but these were canceled due to the Challenger disaster
Where is Halley’s Comet now?
Halley’s Comet is currently visible in the constellation of Hydra, the largest in the southern hemisphere, at a distance of more than 5,200 million kilometers (around 34.8 astronomical units) from Earth.
Halley's Comet's next visit to Earth is “scheduled” to take place in July 2061 when it will reach its next perihelion transit —that is, the point nearest to the Sun. At that time, Halley’s Comet will be located at about 44 million miles (71 million kilometers, or 0.49 astronomical units) from our planet. While this is further than at the last perihelion in 1985-86, the comet will be better positioned for observation, because it will be on the same side of the Sun as Earth at the same time as it reaches perihelion. In 1985-86, the comet was not visible until about two months later.
What would happen if Halley's comet hit Earth?
Scientists estimate that Halley’s Comet has a diameter of 6.8 miles (or 11 kilometers). According to geological evidence, the comet or asteroid that killed the dinosaurs about 65-66 million years ago is estimated to have had a diameter of about 6 miles (or 10 kilometers). It is thought to have left a crater of 110 miles (or 180 kilometers) near the current Yucatán Peninsula in southeastern Mexico.
But the dinosaurs weren’t immediately wiped out by the impact. Apparently, the impact also produced wildfires, earthquakes, and tsunamis, and released several aerosols into the atmosphere, such as sulfate (which caused acid rain) and soot (which blocked out sunlight and created an impact winter).
If Halley’s Comet hit Earth, we would not only have to worry about the explosion (and the release of tremendous energy from the impact) but also about the environmental damage which could lead to the extinction of most forms of life on Earth.
The good news is that the closest that Halley's Comet has ever been to Earth was 3.07 million miles or 4.94 million kilometers away, in 837, and scientists do not foresee any closer approach of Halley’s Comet in the next several hundred years. We are simply not in the way.
However, we will get a spectacular view of Halley’s Comet from Earth in 2061 and can see its remnants every year during the Eta Aquarids and the Orionid meteor showers.