59 Fun Facts About Mercury

Mercury became the smallest planet when which world was reclassified as a dwarf planet?

Pluto was demoted in 2006 after astronomers found comparable worlds such as Eris, leaving eight planets with Mercury the smallest.

About how long is a year on Mercury?

About 3 months: Mercury laps the Sun every 88 Earth days, so a 30-year-old on Earth would be over 120 in Mercury years.

A sunrise-to-sunrise day on Mercury lasts about twice as long as what?

Its own year: Mercury turns so slowly as it circles the Sun that one side bakes in daylight for a whole Mercury year, then faces a year of night.

Astronomers long believed Mercury kept one face toward the Sun; what did radar reveal instead?

Three spins per two orbits: whenever Mercury was best placed for viewing, it happened to show the same side, which made it look locked.

Despite being closest to the Sun, Mercury is only the second-hottest planet. Which is hotter?

Mercury has almost no atmosphere to trap heat, whereas Venus is wrapped in a thick greenhouse blanket.

Mercury's day side gets hot enough to melt which of these metals?

Lead melts at about 327 °C, and Mercury's equator tops 400 °C by day before plunging to around −170 °C at night.

What is the name of Mercury's largest crater, about a third the diameter of the planet?

Caloris Planitia: lava later flooded the giant basin, burying its original floor beneath volcanic plains.

Which Mercury Seven astronaut became the first American in space?

Alan Shepard flew a 15-minute hop aboard Freedom 7, and a decade later he hit two golf balls on the Moon.

What is the nickname of the hilly region on the exact opposite side of Mercury from its largest impact basin?

The Weird Terrain may have formed when shock waves from the giant impact raced around the planet and converged on the far side, fracturing the ground.

Which science-fiction writer set his short story 'Runaround' on Mercury?

Isaac Asimov's 'Runaround' first spelled out his Three Laws of Robotics: its robot circles a pool of selenium, trapped between two of the laws.

Craters on Mercury are named after what kind of people?

Artists, musicians and authors qualify only once they have been dead for more than three years, which is how Walt Disney came to have a crater there.

Mercury's mountains (montes) are named after what?

The word 'hot' in various languages; Mercury's long cliffs, which rose as the planet shrank, are named for ships of discovery instead.

Roughly what share of Mercury's volume does its metallic core fill?

About 60%: Mercury's iron core fills about 57% of the planet's volume, which makes this small world the second-densest planet.

Mercury has the second-highest density of any planet. Which is first?

Factor out gravitational squeezing and Mercury's raw material is actually denser than Earth's.

What has been confirmed to exist in permanently shadowed craters at Mercury's poles?

Water ice survives on the planet nearest the Sun inside polar craters that sunlight never reaches, and radio telescopes first spotted it in the early 1990s.

Mercury's axis is almost perfectly upright; which planet is tipped so far that it spins on its side?

Uranus is tilted about 98°, perhaps after a collision with an Earth-sized body, while Mercury's axis leans only about 0.03°.

Mercury has the most eccentric orbit of the eight planets; which planet's orbit is the next most oval?

Mars: its oval path defied every circular model, and Tycho Brahe's observations of it revealed that planets travel in ellipses.

What did the first probe to visit Mercury surprisingly discover about the small planet?

A magnetic field, only about 1% as strong as Earth's, yet it can twist with the solar wind's field into 'magnetic tornadoes' that funnel hot plasma down to the ground.

Which was the first spacecraft to fly past Mercury?

Mariner 10 kept working until its attitude-control gas ran out in March 1975, and controllers then switched it off for good.

Which technique did the first probe to reach Mercury pioneer on its way there?

A gravity assist: Luna 3 had borrowed the Moon's pull in 1959, but this probe was the first to swing past one planet to reach another.

Which spacecraft became the first to orbit Mercury?

MESSENGER entered orbit in 2011 and by 2013 had mapped the whole planet, of which the only earlier visitor had seen less than half.

How did the first Mercury orbiter's mission end?

It crashed into the planet's surface in April 2015, after running out of the propellant it needed to hold its orbit.

BepiColombo, the mission to orbit Mercury, is a joint project of ESA and which other agency?

Japan's JAXA built Mio, one of the mission's two orbiters, and the pair travelled to Mercury stacked on a European transfer module.

BepiColombo honours Giuseppe Colombo, whose calculations let the first Mercury probe do what?

Return for repeat flybys: Colombo showed that after its first pass the probe's own orbit round the Sun would bring it back, and it met Mercury three times.

Why did BepiColombo need six flybys of Mercury before it could enter orbit?

To shed speed: a probe falling toward the Sun keeps gaining speed, so each close pass slowed it relative to Mercury until it could be captured.

What was the hidden planet Le Verrier proposed inside Mercury's orbit called?

Vulcan seemed a safe bet, because Le Verrier had already found Neptune by the same method, yet searches over the next half-century never turned it up.

Which theory explained the extra 43 arcseconds per century in Mercury's perihelion advance?

Einstein's general relativity explained it in 1915 as the Sun's mass curving spacetime, removing any need for an unseen planet.

The first transit of Mercury ever observed had been predicted by which astronomer?

Johannes Kepler forecast it in tables published in 1630, but he died that November, less than a year before Mercury crossed the Sun as he had said.

Which US coin was nicknamed for the god Mercury, though its winged head actually shows Liberty?

The dime: Adolph Weinman's winged Liberty was later revived on the American Palladium Eagle, a bullion coin struck since 2017.

Which NASA mathematician did astronaut Glenn insist check the computer's calculations before his flight?

Katherine Johnson checked the numbers, and Glenn, who had said he was ready to go if she called them good, flew a successful mission.

Which observatory's radar first measured Mercury's true rotation period?

Arecibo's giant dish in Puerto Rico overturned the long-held belief that Mercury always keeps the same face turned toward the Sun.

Which nation's scientists were the first to bounce a radar signal off Mercury?

The Soviet Union's team at a deep-space communication centre in Crimea caught Mercury's echo in June 1962, and JPL answered with its own Mercury echo in 1963.

Mercury is named after the Roman god of what?

Commerce and messages: his name may be linked to merx, 'merchandise', and at his May festival merchants sprinkled their goods from his sacred well by Rome's Porta Capena.

Alchemists paired the planet Mercury with the runny metal named after it; which metal did they pair with the Sun?

Gold took the Sun and silver the Moon, while runny quicksilver was matched with the fastest planet, named for a swift god.

The Babylonians linked the planet Mercury with which god of scribes and writing?

Nabu, patron of scribes and wisdom, was believed to write down the fates assigned to gods and people.

Which household instrument long relied on liquid mercury, until toxicity worries led to its sale being restricted?

The thermometer used mercury because it swells evenly as it warms and stays liquid over a wide range, but because the metal is toxic many countries now restrict its sale.

Modern Chinese, Japanese, Korean and Vietnamese names for Mercury literally mean what?

Water star: China's Five Phases system matched each visible planet with an element, pairing Mercury with water and Mars with fire.

Which day of the week did the Romans name after Mercury?

Wednesday: Germanic peoples translated the Latin 'day of Mercury' as Woden's day, perhaps because both gods were linked with eloquence.

Which Project Mercury capsule sank in the Atlantic after its hatch blew open too early?

Liberty Bell 7 flooded and sank while Gus Grissom, his suit losing buoyancy, nearly drowned, and the capsule stayed on the seabed until it was raised in 1999.

Mercury's astronomical symbol is a stylised version of what object?

A caduceus: this two-snake staff is often confused with medicine's one-snake Rod of Asclepius, a mix-up scholars trace to the US Army Medical Corps adopting it in 1902.

Which animal flew a Project Mercury capsule into space before any American astronaut did?

A chimpanzee named Ham flew in January 1961, answering light signals by pushing levers throughout the flight.

The tiny crater Hun Kal marks 20° west on Mercury; its name is a number from which civilisation?

The Maya, who counted in twenties: Hun Kal means 'twenty' in Maya, and a crater was needed because the true zero meridian lay in shadow when the first probe flew past.

Why is Mercury so hard to spot in Earth's sky?

It stays near the Sun, so Mercury is lost in the Sun's glare much of the time and shows only briefly in morning or evening twilight.

Mercury has the largest what among the eight planets, at 7°?

Orbital inclination to the ecliptic: because Mercury's orbit is tilted 7° to ours, the planet usually passes above or below the Sun from our view, so transits are rare.

From some spots on Mercury, what can the Sun appear to do within one day?

Rise, set, then rise again: near Mercury's closest pass to the Sun, its orbital motion briefly outpaces its spin, so the Sun seems to back up in the sky.

NASA engineer Abe Silverstein named Project Mercury; which later NASA programme did he also name?

Apollo: Silverstein also chaired the team that decided the Saturn rocket's upper stages should burn liquid hydrogen.

In Roman myth, the god Mercury was the son of Jupiter and which daughter of the Titan Atlas?

Maia, eldest of the Pleiades; in the Greek telling of the myth, her newborn son crept out of their cave and stole his half-brother's cattle.

What is the chemical symbol of the element mercury?

Hg abbreviates hydrargyrum, Latinised Greek for 'water-silver', a fitting name for a silvery metal that stays liquid at room temperature.

Which of these fish does the US FDA list among the highest in mercury?

Swordfish, a large, long-lived predator, builds up mercury from the smaller fish it eats, so the FDA advises pregnant women and young children to avoid it.

In which red mineral does the element mercury mostly occur in nature?

Cinnabar, ground to powder, gave artists the prized red pigment vermilion, even though, like most mercury compounds, it is toxic.

Which Spanish mine long dominated the world's mercury production?

Almadén supplied Europe's mercury for centuries, but many of the convicts and enslaved workers sent into its tunnels died of mercury poisoning.

Dumping of mercury waste into which bay caused a notorious poisoning disaster?

Minamata Bay: the city of Minamata gave its name to Minamata disease and to a 2013 global treaty to curb mercury pollution.

Mercury poisoning in which line of work is linked with a phrase about madness?

Hatmaking: felt-hat makers breathed mercury vapour from the felting process, and their poisoning is often linked to 'mad as a hatter', though the phrase's origin is disputed.

What did John Glenn's Friendship 7 flight make him the first American to do?

Go into orbit: a false signal that his heat shield had come loose led controllers to keep his retrorocket pack strapped on for re-entry.

Which company was chosen as prime contractor to build the Mercury spacecraft?

McDonnell Aircraft built the Mercury capsules, and NASA then picked the same company to build the Gemini capsule that followed.

Project Mercury's crewed flights rode rockets adapted from which two missiles?

Redstone and Atlas D: the modified Redstone could manage only short suborbital hops, so the orbital missions needed the more powerful Atlas.

When Faith 7 lost power on the last Mercury flight, how did Gordon Cooper steer it home?

By hand: Cooper held the capsule's angle using marks on his window, fired the retrorockets himself and splashed down about four miles from the recovery ship.

Which common tooth filling is traditionally about half mercury?

Silver amalgam is roughly half mercury, and because the metal is toxic its use in fillings is now being phased down.

Mercury is the smallest planet, yet which of Jupiter's moons is even wider?

Ganymede is about 5,270 km across, wider than Mercury, yet it has less than half the smaller planet's mass.