50 Fun Facts About Halley's Comet

Roughly how often does Halley's Comet return to the inner Solar System?

The period drifts because Jupiter and Saturn tug on the orbit; it has run anywhere from 72 to 80 years since 240 BC.

In which year did Halley's Comet last visit the inner Solar System?

It was the worst-placed apparition in 2,000 years, with the comet and Earth on opposite sides of the Sun.

When is Halley's Comet next expected to reach perihelion?

Perihelion is predicted for 28 July 2061, and this time Earth will be on the same side of the Sun, so the show should be far better than 1986.

What is the comet's official designation in the periodic-comet catalogue?

The 'P' means periodic and the '1' means it was the first periodic comet ever recognised.

In which year did Edmond Halley publish the work that showed the comet was periodic?

His Synopsis of the Astronomy of Comets used Newton's new laws to compare the comets of 1531, 1607 and 1682.

Edmond Halley never saw his prediction come true. In what year did he die?

He had personally watched the comet near perihelion in September 1682, decades before he worked out it would return.

Who first spotted the comet on its predicted 1758 return?

He caught it on Christmas Day 1758; the return was the first proof that anything other than planets orbits the Sun.

Which French astronomer first named the comet in Halley's honour, in 1759?

Lacaille is better known for naming 14 southern constellations after scientific instruments.

Which of these people was on the three-person French team that calculated the delayed 1759 return?

Working with Clairaut and Lalande, Lepaute helped compute a 618-day delay caused by Jupiter and Saturn, and got the perihelion date within a month.

Which observation disproved Aristotle's idea that comets were disturbances in Earth's atmosphere?

Parallax showed the comet had to lie beyond the Moon, though many still assumed comets flew in straight lines through the Solar System.

What is the orbital eccentricity of Halley's Comet?

Zero would be a circle and 1 a parabola, so its path is a long, thin ellipse stretching out past Neptune.

Halley's perihelion lies between the orbits of which two planets?

At 0.59 au from the Sun the comet's ices sublimate furiously, producing the coma and tail.

Halley's aphelion, about 35 au from the Sun, is roughly the orbital distance of which body?

Out there it crawls along at under 1 km/s; it reached that farthest, slowest point most recently on 9 December 2023.

What is unusual about the direction Halley's Comet travels around the Sun?

Seen from above the Sun's north pole it moves clockwise, and its orbit is tilted 18 degrees to the ecliptic.

Which two meteor showers are associated with debris from Halley's Comet?

Earth crosses the comet's path twice a year, in early May and again in late October.

How many Halley-type comets (periods of 20-200 years, steep inclinations) had been observed as of 2024?

That compares with 816 Jupiter-family comets, which have shorter periods and shallow orbits.

If Halley was once a long-period comet, where is it thought to have originated?

Jupiter-family comets are instead thought to come from the flatter Kuiper belt just beyond Neptune.

On average, how much do jets of sublimating gas delay Halley's perihelion passage each orbit?

These non-gravitational kicks are why the orbit cannot be integrated accurately much further back than 837 AD.

Roughly how long is the nucleus of Halley's Comet?

For a body that can throw a tail 100 million km long, the nucleus is tiny: about 15 by 8 by 8 km, shaped like a peanut or potato.

Halley's nucleus has an average density of about 0.55 g/cm3. What does that suggest about its structure?

The nucleus is many small pieces held together very weakly, which is why comets so often break apart.

Halley's Comet reflects only about 4% of sunlight. What common material has roughly the same albedo?

Astronomers had expected something more like bare soil at 0.17; instead the nucleus turned out to be pitch black.

Whose 'dirty snowball' model of comets did the 1986 flybys broadly confirm?

The twist was that so much of the surface is dark dust that Halley is arguably a 'snowy dirtball' instead.

Spacecraft found the gas streaming off Halley's nucleus was mostly which substance?

About 80% water vapour, with carbon monoxide and carbon dioxide making up most of the rest.

Why did later measurements rule out Halley-type comets as the main source of Earth's oceans?

The deuterium-to-hydrogen ratio was at first thought to match Earth's oceans, then found to be far higher.

Which spacecraft made the closest pass of Halley's nucleus, on 14 March 1986?

The European Space Agency probe was named after a 14th-century Italian painter who put the comet into a nativity fresco.

Which nation's probe captured the first-ever image of Halley's nucleus, in March 1986?

Vega 1 began returning pictures on 4 March and flew past on 6 March, three days before its twin Vega 2.

What were the two Japanese probes sent to Halley in 1986 called?

Suisei means 'comet' and Sakigake 'pioneer'; they joined the European and Soviet probes at the comet in March 1986.

What informal nickname was given to the fleet of spacecraft that studied the comet in 1986?

Astron, an ultraviolet space telescope, and the International Cometary Explorer also joined in from a distance.

Which Space Shuttle mission carrying a Halley ultraviolet experiment ended in disaster in 1986?

A second comet mission, STS-61-E, was cancelled afterwards and its ASTRO-1 platform did not fly until 1990.

Which instrument first picked up the incoming comet on 16 October 1982?

David Jewitt and G. Edward Danielson used a CCD camera, then a novel instrument, to pick it up.

Which Chinese chronicle contains the first certain record of Halley's Comet, from 240 BC?

About one eighth of all comet sightings in historical records turn out to be Halley.

The only surviving record of the 164 BC apparition was rediscovered in 1984 in which institution's collection?

It survives on two fragmentary Babylonian clay tablets.

Which Armenian king's coins show a crown with a curved-tailed star, possibly Halley's Comet?

Babylonian tablets say the comet was seen 'day beyond day' for a month that year.

Halley's 12 BC apparition was said by Cassius Dio to portend the death of which Roman?

That same appearance has been floated as an explanation for the Star of Bethlehem, though it is a few years too early.

In which year did Halley's Comet make its closest known approach to Earth, perhaps just 0.03 au away?

Its tail may have stretched 60 degrees across the sky, and Emperor Louis the Pious took it as a warning and turned to prayer.

Halley's 1066 apparition is famously depicted on which artwork?

Later that year Harold II died at Hastings, and accounts said the comet looked four times the size of Venus.

According to legend, which conqueror was inspired to march west by the 1222 apparition?

In Korea the comet was reportedly bright enough to see in daylight on 9 September 1222.

Which artist painted the 1301 apparition as the Star of Bethlehem in a chapel fresco?

His Arena Chapel nativity shows the coma, tail and central condensation so accurately that ESA named its 1986 probe after him.

Which pope was later said, almost certainly falsely, to have excommunicated the comet in 1456?

He did order prayers during the Ottoman siege of Belgrade; the excommunication tale was likely invented by an angry 18th-century Frenchman.

Which Ethiopian emperor founded Debre Berhan after seeing a bright light, usually identified as Halley, in 1456?

The name translates as 'City of Light', and he kept it as his capital for the rest of his reign.

Petrus Apianus used the 1531 apparition to prove what about comet tails?

Solar wind and radiation pressure push the tail outward, so a departing comet travels tail-first.

Which toxic gas, detected in the tail in 1910, sparked panic buying of gas masks and 'anti-comet pills'?

Earth passed through the tail on 19 May 1910; scientists insisted it was harmless, and it was.

Which American writer was born two weeks after Halley's 1835 perihelion and died the day after the 1910 one?

He called himself and the comet 'two unaccountable freaks' who came in together and must go out together.

Which Japanese emperor saw the comet in 1986 at age 84, having also seen it as an eight-year-old in 1910?

Very few people get to see Halley twice; the 1835 witnesses interviewed in 1910 were another rare group.

In 1991, at 14.3 au from the Sun, Halley did something comets rarely do that far out. What?

It brightened from magnitude 25 to 19, possibly from hydrogen cyanide polymerising and amorphous ice changing phase.

Where was Halley's Comet imaged in 2003 at magnitude 28.2, the faintest any comet had ever been seen?

The observation was a test run for finding very faint trans-Neptunian objects.

What apparent magnitude is Halley expected to reach at its 2061 return?

That is roughly the brightness of a bright star, compared with a dim +2.1 in 1986; the 2134 pass should be brighter still at -2.0.

Which two earlier comets did Halley match to the 1682 comet in his 1705 Synopsis?

They had been observed by Petrus Apianus and Johannes Kepler respectively; Halley compiled 24 comet observations in all.

What role did Edmond Halley play in the publication of the Principia in 1687?

Halley used Newton's new laws to work out how Jupiter and Saturn perturbed cometary orbits.

What surface temperature did the Vega 1 spacecraft measure on Halley's nucleus?

That is roughly 27 to 127 degrees Celsius, far warmer than the sublimation temperature of the 'dirty ices'.