10 fun facts as Halley’s Comet makes its big comeback

On December 9, 2023, Halley’s comet achieved aphelion, reaching its greatest distance.

This 1910 photograph of Halley’s comet represents the best display seen by human eyes since the development of photography as applied to astronomy. Although comets are usually visible near perihelion, or closest approach to the Sun, they can be billions of times fainter near aphelion, when farthest away.

These 10 fantastic facts celebrate its impending return.

This simulated sky view shows the skies over London, England in the spring of 1066: when Halley’s comet returned. Although this event was recorded in numerous places, its identification with the return of Halley’s comet would require several hundred years to pass.

1.) Its first recording was 240 B.C.E.

This ancient tablet is more than 2000 years old, and records the event of Halley’s comet as follows: “In the 7th year of Emperor Qin Shihuang of the Warring States, a broom star first appeared in the east, then it appeared in the north.”

Spotted in China, records describe a “Broom Star.”

This Babylonian tablet records the appearance of Halley’s comet dating to late September, 164 BCE. There is evidence that Halley’s comet dates to prehistoric times, but this and the Chinese record of one orbit prior are the first reliable, verifiable records of Halley’s comet as seen from Earth.

2.) Halley’s questioning Newton led to the Principia.

There may never be another Einstein or another Newton, but we can all learn to utilize their equations under the right physical circumstances. We can become excellent at physics the same way they did: by solving problems quantitatively.

Newton offhandedly told Halley a central, ~1/r² force law would create elliptical orbits, then proved it.

The orbits of the planets in the inner Solar System aren’t exactly circular, but are elliptical, as are the orbits of all bodies gravitationally bound to the Sun. Planets move more quickly at perihelion (closest to the Sun) than at aphelion (farthest from the Sun), conserving angular momentum and obeying Kepler’s laws of motion, which were put on a more solid, generalized mathematical footing by Newton. Once the quantitative relationships between orbital period and distance were uncovered, it became possible to know the distance from the Earth to the Sun.

3.) Halley identified 3 prior returns.

This diagram shows Halley’s comet’s orbit, neglecting the gravitationally perturbative effects of the planets. Halley’s comet spends most of its time near aphelion, near the orbit of Neptune and beyond, but plunges into the inner Solar System once every 74-79 years.

Previous comet arrivals in 1531, 1607, and 1682 portended Halley’s 1705 prediction.

The original publication of Edmond Halley’s wherein he first identified the thrice-recorded comets of 1531, 1607, and 1682 as the same object: Halley’s comet, with a predicted return in 1758.

4.) Its orbit is variable.

In Newton’s theory of gravity, orbits make perfect ellipses when they occur around single, large masses. However, in general relativity, there is an additional precession effect due to both the curvature of spacetime and the fact that the planets are in motion with respect to the Sun, and this causes the orbit to shift over time, in a fashion that is sometimes measurable. Mercury exhibits the largest such effect within our Solar System, precessing at a rate of an extra 43″ (where 1″ is 1/3600th of one degree) per century due to this additional effect. Meanwhile, any comets passing through the Solar System are affected by the gravity of the planets inside, which often perturbs their periods and orbital trajectories.

The other planets, especially Jupiter, perturb cometary orbits. Halley’s periodicity varies from 74-79 years.

The animation depicts a mapping of the positions of known near-Earth objects (NEOs) at points in time over the past 20 years and finishes with a map of all known asteroids as of January 2018. Although Jupiter absorbs many asteroids and comets, it can also redirect them, potentially further endangering the Earth.

5.) Halley never observed its return.

While many remarkable comets grace the skies both regularly and irregularly, the greatest comet of Halley’s life, in 1680, was not related to his namesake comet at all. Halley never saw the return of his comet, but it might have looked similar to 2020’s comet NEOWISE, shown here.

Halley calculated a 1758 return, but died in 1742.

Painted by Isaac Whood in ~1720, this painting of a likeness of Edmond Halley is considered one of the best likenesses of the 17th-18th century astronomer.

6.) Its return led to the Messier catalog.

This image of an archaeoastronomy panel from the Peñasco Blanco trail shows a crescent Moon, a 10-pointed star identified with the Crab Supernova of 1054, and, at the bottom, a concentric circle symbol with a flame-like extension: surmised to be a comet, possibly the reappearance of Halley’s Comet in 1066.

Searching for the comet in 1758, a deep-sky nebula confounded Charles Messier. His catalog prevented future confusion.

Through an 18th century-quality telescope, comets, nebulae, and other extended objects are not readily distinguishable from one another. It makes blindly hunting for a slow-moving comet very difficult, and the search for Halley’s Comet in 1758, coupled with the accidental re-discovery of the Crab Nebula, sparked Charles Messier to develop his famous catalog.

7.) Its orbital speed varies tremendously.

Even before we understood how the law of gravity worked, we were able to establish that any object in orbit around another obeyed Kepler’s second law: it traced out equal areas in equal amounts of time, indicating that it must move more slowly when it’s farther away and more quickly when it’s closer. For orbits that depart significantly from perfect circles, such as the Moon orbiting the Earth, the speed at which the Moon appears to migrate across the sky relative to the Sun can vary significantly dependent on the Moon-Earth distance at that moment.

At perihelion, it reaches ~55 km/s. At aphelion, it’s under 1 km/s.

As shown at aphelion (late 2023/early 2024), Halley’s comet traces out a highly elliptical path. At its closest approach to the Sun, it passes interior to the orbit of Venus and just outside of Mercury’s orbit, while at aphelion, its farthest distance, it’s more comparable to Plutonian distances.

8.) Its debris creates two meteor showers.

Comet 67P/Churyumov-Gerasimenko was imaged many times by the ESA’s Rosetta mission, where its irregular shape, volatile and outgassing surface, and cometary activity were all observed. In the future, when the Sun heats up and swells into a red giant, objects in the asteroid belt, Kuiper belt, and possibly even the inner Oort cloud will be heated so severely that their volatile ices will be evaporated away.

Both May’s Eta Aquariids and October’s Orionids arise from Halley’s comet.

A view of many meteors striking Earth over a long period of time, shown all at once, from the ground (left) and space (right). If a comet’s path crosses Earth’s orbit twice, its debris stream can create up to two meteor showers per year.

9.) It’s not the most common comet type.

Each year, Earth passes through the debris stream of comet Swift-Tuttle, creating the visual phenomenon known as the Perseid meteor shower. Comet Swift-Tuttle remains the single most dangerous object known to humanity, and is a Halley-type comet, taking 133 (between 20 and 200) years to complete an orbit.

Only 116 Halley-type comets are known, versus 656 Jupiter-family comets.

This map shows the debris stream of Comet Encke, a short-period comet in orbit around the Sun and the parent of the Taurid meteor shower. The white area indicates where the debris stream is densest and corresponds to the comet’s nucleus, but debris has been stretched out and extended all across the entirety of the comet’s orbit. Comets with periods shorter than 20 years are far more numerous than the Halley-type comets, with periods between 20 and 200 years.

10.) Its nucleus may soon split.

The last time Halley’s Comet visited the inner Solar System was 1986, when the ESAs Giotto probe flew by it and took this photograph from a distance of just 2000 km. The Sun, to the left, heats the comet’s nucleus which leads to offgassing and the release of dust.

This low-density, rubble pile iceball is losing mass quickly, and may soon decompose.

As they orbit the Sun, comets and asteroids typically break up over time, with debris between the chunks along the path of the orbit getting stretched out to create debris streams. These streams cause meteor showers when the Earth passes through that debris stream. This image taken by Spitzer along a comet’s path shows small fragments outgassing, but also shows the main debris stream that gives rise to the meteor showers that occur in our Solar System.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.