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Starts With A Bang

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

On December 9, 2023, Halley’s Comet reached aphelion: its farthest point from the Sun. As it returns, here are 10 facts you should know.
An image of Halley's comet in the sky.
This 1986 photograph of Halley's comet, taken from Easter Island on March 8 of that year, is likely the best view we'll have until the comet returns to the inner Solar System in 2061. As of December 9, 2023, the comet now heads back into the inner Solar System, having just passed aphelion in its orbit.
Credit: NASA/W. Liller
Key Takeaways
  • Halley’s comet was the first comet ever to be identified as periodic, as well as to have its return predicted regularly: by its namesake Edmond Halley, way back in 1705.
  • The comet last appeared in our inner Solar System in 1986, and will return once again in 2061, where it’s expected to make its closest pass to the Sun on July 28, 2061.
  • On December 9, 2023, it reached aphelion: its farthest point from the Sun. Here are 10 fun facts you should all know as it begins to make its return.

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

A black and white photograph of Halley's comet in the sky from 1910
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.
Credit: Harvard College Observatory

These 10 fantastic facts celebrate its impending return.

Halley's comet is seen in the sky above a field simulated in stellarium 1066
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.
Credit: Morn/Stellarium

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

A black and white picture of Chinese writing with Halley's comet.
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.”
Credit: Xu, Zhentao, David W. Pankenier, and Yaotiao Jiang. East Asian Archaeoastronomy: Historical Records of Astronomical Observations of China, Japan and Korea, 2000

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

Halley's comet inscriptions on display in an Egyptian museum.
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.
Credit: Linguica/English Wikipedia

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.
Credit: Orrin Turner (L), Godfrey Kneller (R)

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

mercury mars venus earth orbit
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.
Credit: NASA/JPL

3.) Halley identified 3 prior returns.

A diagram showcasing the orbit of Halley's comet.
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.
Credit: nagualdesign/Wikimedia Commons

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

"A synthesis of the astronomy of Halley's comet.
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.
Credit: Hook & Norman, The Haskell F. Norman Library of Science & Medicine (1991) no. 978

4.) Its orbit is variable.

one year
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.
Credit:, 2011, now defunct

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

asteroids in asteroid belt
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.
Credit: NASA/JPL-Caltech

5.) Halley never observed its return.

Crater lake, Halley's comet.
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.
Credit: Crater Lake National Park / Rebecca Latson

Halley calculated a 1758 return, but died in 1742.

A painting of Edmond Halley a book inspired by Halley's comet.
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.
Credit: National Portrait Gallery, London

6.) Its return led to the Messier catalog.

comet supernova astronomy archaeology
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.
Credit: Peter Faris, 1997

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

Messier 1 m1 crab nebula amateur telescope
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.
Credit: Chris Brankin’s Deepsky (Messier) Objects/Stargazing

7.) Its orbital speed varies tremendously.

kepler second law
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 extremely elliptical orbits, such as comets, the velocity near perihelion is enormous, while the velocity near aphelion is relatively tiny.
Credit: Gonfer/Wikimedia Commons, using Mathematica

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

halley's comet orbit from the side and from the top
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.
Credit: Eric Blackman/University of Rochester

8.) Its debris creates two meteor showers.

rosetta 67p Churyumov-Gerasimenko comet nucleus animation
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.

meteor shower from ground and space on Earth
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.
Credit: Comenius University (L), NASA (R); Wikimedia Commons

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

An artist's rendering of an asteroid within the Perseid meteor shower.
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.
Credit: Ian Webster; Data: NASA / CAMS / Peter Jenniskens (SETI Institute)

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

comet encke trail
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.
Credit: M.S. Kelley et al., ApJ, 2006

10.) Its nucleus may soon split.

Comet Halley 1p nucleus ESA Giotto up close 1986
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.
Credit: ESA/MPS

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

spitzer comet debris stream
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.
Credit: NASA/JPL-Caltech/W. Reach (SSC/Caltech)

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