45 Fun Facts About Halley's Comet

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

Every 74-79 years: that steady beat has let astronomers match the comet to old sky records of its visits going back more than 2,000 years.

Why was Halley's Comet so faint in 1986?

Behind the Sun: at its brightest in February 1986 the comet sat on the far side of the Sun from Earth, and it never came nearer to us than 63 million km.

What does 1P in 1P/Halley record?

First shown to return: when it came back in 1758 as forecast it proved that comets can circle the Sun, so it heads the list of periodic comets as number 1.

What convinced Edmond Halley that three past comets were one?

Similar orbits: the comets of 1531, 1607 and 1682 followed nearly the same path, so in 1705 he forecast a return in 1758.

Why did Halley never see his comet's 1758 return?

He had died in 1742, aged 85, sixteen years before the comet came back on schedule and was given his name.

The comet's on-time return in 1758 was hailed as proof of which scientist's theories?

Isaac Newton: Halley had used Newton's gravity to forecast the return, so when a German amateur spotted the comet on Christmas night 1758 it was cheered as a triumph for Newtonian physics.

French mathematicians warned the comet would run late in 1758 because of what?

The pull of Jupiter and Saturn: Alexis Clairaut, Joseph Lalande and Nicole-Reine Lepaute worked out that the giant planets would hold the comet back until spring 1759, and came within a month of the true date.

Whose 1577 measurements showed comets fly beyond the Moon, not in Earth's air as Aristotle taught?

Tycho Brahe: he found the great comet of 1577 showed no parallax shift against the stars, so it had to lie far beyond the Moon.

What shape is Halley's Comet's orbit?

A long ellipse: the stretched loop keeps it far out in the cold for most of each lap, which is why it disappears for decades and then sweeps back past the Sun.

At its closest to the Sun, Halley's Comet passes between the orbits of which two planets?

Mercury and Venus: at about 88 million km from the Sun it is racing along at roughly 54 km/s, and its ices boil off to form the coma and tail.

At its farthest, about 35 times Earth's distance from the Sun, Halley's Comet is roughly as far out as which body?

Pluto: Halley turns back about 5 billion km out, beyond Neptune and roughly where Pluto orbits, slowing to under 1 km/s before it falls sunward again.

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

It orbits opposite to the planets: anything launched from Earth meets it nearly head-on, which is why the 1986 probes flew past at around 70 km/s, some 250,000 km/h.

Twice a year Earth crosses the trail of dust shed by Halley's Comet, producing what?

Meteor showers: the Eta Aquariids each May and the Orionids each October are both grains of Halley's Comet burning up in Earth's atmosphere.

Halley's 1910 visit was the first time anyone managed to do what to the comet?

Photograph it: photography was only unveiled in 1839, four years after the comet's previous visit, so 1910 was the first chance, and astronomers also split its light that year to read its chemistry.

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

The Oort cloud: this distant shell of icy bodies feeds long-period comets, and the giant planets' gravity is thought to have pulled Halley into its present short orbit.

What nudges Halley's Comet off the schedule that gravity alone would set?

Jets of gas from its surface: as ice boils off the sunlit side the jets act like small rocket engines, a push Fred Whipple explained in 1950 and forecasters now allow for.

Halley's tail can stretch millions of kilometres, but roughly how long is its solid nucleus?

About 15 km: probes found only about a tenth of the peanut-shaped nucleus was giving off gas, yet that small patch feeds the whole vast tail.

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 icy nucleus reflects about as much sunlight as which material?

Coal: the nucleus reflects only a few percent of the sunlight that hits it, because a crust of dark dust covers the ice and leaves it pitch black.

The 1986 flybys broadly confirmed Fred Whipple's famous model of a comet as a what?

Dirty snowball: Whipple proposed in 1950 that a comet is ice mixed with dust, though Halley proved so dusty that some now call it a snowy dirtball instead.

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

Water vapour: about 80% of the gas, because under the crust of dust lies water ice that the Sun turns straight into vapour.

Why is the water in comets like Halley a poor match for Earth's oceans?

It holds too much heavy hydrogen: Halley's water carries far more deuterium than seawater does, so comets of its kind probably did not fill Earth's oceans.

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

Giotto: the European probe swept within about 600 km of the nucleus, far closer than any other craft in the fleet, and sent back close-up pictures of a comet's core.

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

The Soviet Union: Vega 1 and its twin flew in first as pathfinders, and their fix on the nucleus let Europe's probe aim far closer days later.

Japan's first deep-space probe, sent toward Halley in 1985, was named Sakigake, meaning what?

Pioneer: Sakigake was Japan's first craft to leave Earth's pull, and it watched Halley from 7 million km while its twin Suisei went in to about 150,000 km.

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

The Halley Armada: five probes from three space agencies met the comet in March 1986, while other spacecraft watched from a distance.

Which Space Shuttle was carrying an instrument to study Halley's Comet when it was lost with its crew?

Challenger: its final flight in January 1986 carried Spartan Halley, a small satellite meant to record the comet in ultraviolet light.

The first certain record of Halley's Comet, a 'broom star' seen in 240 BC, comes from which country?

China: Greek writers may have seen the same comet around 466 BC, but the Chinese entry for 240 BC is the first that astronomers can tie to Halley for certain.

Halley's visit in 164 BC was recorded on clay tablets, now in the British Museum, by which ancient people?

The Babylonians: their astronomical diaries hold the only surviving record of that visit, and two thousand years on they let astronomers check the orbit computed for the comet.

In AD 66 Josephus described a sword-shaped star, often taken to be Halley, hanging over which doomed city?

Jerusalem: Josephus wrote of a star resembling a sword standing over the city, read afterwards as an omen of its destruction by the Romans.

Romans took Halley's appearance in 12 BC as an omen of the death of Agrippa, right-hand man of which leader?

Augustus: Cassius Dio wrote that a comet hung over Rome for days around the death of Agrippa, the general whose fleet had won him sole power at Actium.

What set Halley's visit in AD 837 apart from every other on record?

Closest to Earth: on 10 April 837 it passed within about 5 million km, some 13 times the Moon's distance, and outshone every star in the night sky.

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

The Bayeux Tapestry: the comet blazes above King Harold and a crowd of fearful Englishmen, months before he died at Hastings and William the Conqueror took the throne.

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

Genghis Khan: the comet of 1222 is sometimes credited with turning his Mongols toward Europe, yet his armies had already swept west into Persia a few years before it appeared.

ESA's Halley probe is named for a painter said to have shown the 1301 comet as what?

The Star of Bethlehem: in his Adoration of the Magi in Padua a fiery comet hangs over the stable, thought to be drawn from the Halley he watched in 1301.

Halley's 1456 visit alarmed Europe as which empire's army besieged Belgrade?

The Ottoman Empire: the comet appeared in 1456 as Ottoman forces invaded Hungary and besieged Belgrade, and Pope Callixtus III ordered special prayers for the city.

Halley's appearance in AD 451 was later said to herald the defeat of which invader at the Battle of Chalons?

Attila the Hun: the comet appeared in the year a Roman and Visigoth alliance checked his invasion of Gaul, and within two years he was dead.

In 1531 Peter Apian used Halley's Comet to show what about comet tails?

They point away from the Sun: the solar wind sweeps gas from the comet's head outward, whichever way the comet itself is moving.

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

Cyanogen: astronomers found the poison in the tail's light, and although Earth crossed the tail on 19 May 1910 the gas was far too thin to harm anyone.

Which American writer was born as Halley's Comet arrived in 1835 and died as it returned in 1910?

Mark Twain: he said in 1909 that he had come in with the comet and expected to go out with it, and he died on 21 April 1910, a day after it rounded the Sun.

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.

What should Halley's Comet shine like in 2061?

The brightest stars: Earth will be far better placed to watch in 2061 than it was in 1986, so one forecast puts the comet near magnitude -0.3.

Which astronomer, then imperial mathematician in Prague, watched the 1607 comet without realising it would return?

Johannes Kepler: he tracked the comet of 1607 and believed comets flew in straight lines, never suspecting that Halley would match it to those of 1531 and 1682.

Which landmark science book did Edmond Halley edit and pay to publish in 1687?

Principia Mathematica: Halley coaxed the book out of its author, edited it and paid the printing costs himself when the Royal Society pleaded it had no money.

On their way to Halley's Comet, the twin Vega probes dropped landers and balloons at which planet?

Venus: each Vega released a lander and a weather balloon there in June 1985, then used the planet's gravity to swing on towards Halley.