December 24 – Rhea

Saturn’s second-largest moon, Rhea, was discovered on this day in 1672 by Giovanni Domenico Cassini, who had presumably finished his Christmas shopping early.

Rhea from the Cassini probe (image: NASA / JPL / SSI)

Rhea from the Cassini probe (image: NASA / JPL / SSI)

Like our own Moon, Rhea always keeps one side, the one you can’t see in this photo, facing towards the parent planet, and from the above view of Rhea’s cratered anti-Saturnian surface, this does look to be a place very much like the Moon we are familiar with down here.  But appearances can be deceptive.  For a start, it’s a lot smaller than the moon, and if it soft-landed on the Earth it would fit quite comfortably inside the borders of Zaïre (in this country people usually use Wales as the benchmark for large areas, but I felt like a change).  Secondly, although it looks nice and rocky, Rhea is thought to be comprised of as much as three-quarters ice, and only one quarter rock (studies of Rhea’s inertia suggest that it doesn’t even posses a rocky core).

Rhea is named after a Titaness of some standing, the daughter of Uranus and Gaia.  As mother of Zeus and Hera, she was grandmother or great-grandmother to almost all the Olympian gods and goddesses worth mentioning.

Rhea

Rhea

There are several suggestions doing the rounds concerning the origin of the name Rhea.  It may derive from the word for “flow”, or it could have a root in a much older word for “powerful”.  But the guy who named the large, flightless, South American bird after her was almost certainly thinking of a derivation of the Greek έρα, meaning “ground”.

Another Rhea

Another Rhea


 

 

October 25 – Iapetus

Iapetus, the third largest moon of Saturn, was discovered on October 25th, 1671, by Giovanni Cassini, and is a weird old place for a couple of reasons. Firstly, it looks like two different moons, depending on whether you view it from the leading or trailing hemisphere, and secondly because of a pronounced ridge around the equator that gives the moon the appearance of a walnut.
The colour difference is really obvious. It was first suggested by Cassini himself, who noticed that he could only see Iapetus when it was on Saturn’s western side. Various theories have been put forward to explain this two-tone look, with the latest being to invoke thermal segregation , as a result of Iapetus’ very long rotation period (79 Earth days). This might cause one side to be brighter than the other, with Iapetus’ tidally locked rotation being the reason it will always look darker from Earth when on one side of the planet.

Iapetus from the Cassini spacecraft (image credit: NASA)

Iapetus from the Cassini spacecraft (image credit: NASA)

The equatorial ridge has proved equally baffling, with two formation theories currently being pondered: (i) the result of much faster rotation at some point in the past, and (ii) the collapse of a ring.

Cassini view of Iapetus' equatorial ridge (image credit: NASA)

Cassini view of Iapetus’ equatorial ridge (image credit: NASA)

Iapetus was named after one of the Titans, as per John Herschel’s suggestion that they be given the names of the mythological siblings of Kronos (the Greek equivalent of Saturn).


 

 

October 24 – Ariel and Umbriel

We have a double-header today.  Two moons of Uranus, Ariel and Umbriel, were discovered on October 24th 1851 by Bolton-born William Lassell, brewer and keen amateur astronomer, who was in the fortunate position of being able to use his not inconsiderable wealth to fund the construction of impressive telescopes.

Let’s take a quick look at Ariel first, a reasonably large moon of just over 579 km radius, orbiting its parent at a distance of about 190,000 km.  It travels at a fantastic pace, completing an orbit of the vast planet below it in two and a half Earth days.

Ariel viewed from Voyager 2 (image credit: NASA)

Ariel viewed from Voyager 2 (image credit: NASA)

Of all the moons of Uranus, the surface of Ariel appears to be the youngest, evidenced by the lack of older, larger impact craters, and the presence of recent geologic activity.  It is also the brightest of the large Uranian moons, and is tidally locked (keeps the same face to the planet at all times).  Ariel has only been visited up close and personal by one mission, Voyager 2, and as far as I know there are no plans to make a return journey just yet.

If you’ discover a moon of Uranus you have two choices from which to choose a suitable name: Alexander Pope’s The Rape of the Lock, or the plays of William Shakespeare.  At the sugestion of Lassell, Ariel was chosen by Sir John Herschel, son of Sir William Herschel, the discoverer of Uranus.  It’s a good choice, as it falls into both categories.

Now we move outwards slightly to Umbriel, which gets its name from that of a sprite in Pope’s narrative poem.  It too was chosen by John Herschel.

View of Umbriel from Voyager 2, 1986 (image credit: NASA)

View of Umbriel from Voyager 2, 1986 (image credit: NASA)

Umbriel is the third farthest major moon from Uranus (Ariel is the second), and is the darkest of the major Uranian satellites.  We don’t yet know why Umbriel is so much darker than similarly sized bodies in orbit around Uranus, but I’m sure somebody’s working on it somewhere.  With a radius of 584 km, it is only slightly larger than Ariel, and, like it’s neighbour, it’s no slouch, travelling at the impressive rate of 16,800 km/hour, meaning a year on Umbriel takes about four and a half Earth days.

For, that sad moment, when the Sylphs withdrew
And Ariel weeping from Belinda flew,
Umbriel, a dusky, melancholy sprite,
As ever sully’d the fair face of light,
Down to the central earth, his proper scene,  
Repair’d to search the gloomy Cave of Spleen.

(The Rape of the Lock, Alexander Pope, 1714)


1601   –   Death of the Danish astronomer, Tycho Brahe, whose use of the title De Nova Stella to announce the supernova of 1572 led to the word nova being applied to all such phenomena.


1890   –   First meeting of the British Astronomical Association at the Hall of the Society of Arts in London. Unlike the already established Royal Astronomical Society, the BAA was open to female members, and even allowed them onto its council. One can only speculate of course, but I assume the RAS had somehow decided that night vision was improved by ownership of testicles.


1964   –   Launch of Kosmos 49 (DS-MG 2) to test orientation systems and study the magnetosphere.  The launch was from the Kapustin Yar site near Volgograd (previously known as Stalingrad). The mission was a short one, and the satellite’s orbit decayed on August21st the year after launch.


2004   –   Discovery of Saturn’s extremely tiny moon, Polydeuces by the Cassini Imaging Science team.


October 10 – Triton

Discovered on October 10th 1846 by William Lassell, (who was living and working at that time in West Derby, Liverpool), the Solar System’s biggest retrograde moon Triton, aka Neptune I, is, at 1,353 km radius (approx) the largest satellite of Neptune.

Triton from Voyager 2 (image: NASA)

Triton from Voyager 2 (image: NASA)

Lassell wasn’t daft.  The massive Neptune itself had only been discovered 17 days previously, so where else was he going to be looking for moons?

Current thinking is that Triton is a captured Kuiper Belt Object.  It has a thin atmosphere of mostly nitrogen, with a little methane mixed in, and is, like our own Moon, locked in a synchronous orbit, meaning it always keeps the same face pointing toward its parent (or possibly in this case “adoptive parent”).

Triton (image credit: NASA/JPL-Caltech)

Triton (image credit: NASA/JPL-Caltech)


1868  –  Discovery of asteroid 106 Dione by J C Watson.  Not to be confused with the Saturnian moon of the same name, this Dione is a main belt, G-type asteroid of around 147 km diameter.  The name is a reference to a titaness of Greek mythology, whom some sources call the mother of Aphrodite.


1874  –  Discovery of asteroid 139 Juewa, also by J C Watson.  This main belt asteroid of about 160km wide was discovered by Watson during a visit to China.  He was there to view the transit of Venus on December 9th, but couldn’t resist bagging an asteroid while he waited.  The name was chosen by Prince Gong (or Kung), son of the Emperor.

Prince Gong of the Qing Dynasty.

Prince Gong/Kung of the Qing Dynasty.

The photograph shows a 40-ish year old Gong, looking about 60. It was taken by the Scottish photographer John Thomson.


September 30 – 81 Terpsichore

Asteroid 81 Terpsichore, discovered on September 30th 1864 by Ernst Wilhelm Tempel, is a large (about 120 km diameter), dark, “C” type (carbonaceous) main-belt asteroid.  Terpsichore is named after my least favourite muse, the one in charge of dancing.  Her name means “delight in dancing“, an alien concept to myself.  According to the epic poem the Dionysica of Nonnus, she was the mother of the Sirens.

Terpsichore by Antonio Canova (Cleveland Museum of Art)

Terpsichore by Antonio Canova (Cleveland Museum of Art)


The Jovian moon Themisto, discovered on September 30th 1975 by Charles Kowal and Elizabeth Roemer, is a rather small, irregular moon, orbiting about halfway between Jupiter’s large, well known Galilean moons and the rather less famous Himalia Group of prograde irregular companions. The best photographs currently available on the Internet show Themisto as a white pinhead on a black background, so I haven’t bothered “borrowing” one.

Themisto was named after a character from Greek mythology, the subject of a tragedy by Euripides which has unfortunately been lost. In the play, Themisto accidentally kills her own children, believing them to be the offspring of her husband’s first wife. When she realises what has happened, Themisto does what the star of any tragedy would be expected to do, and kills herself as well. The Greek stage at an average drama festival was littered with more bodies than an entire season of Murder She Wrote.

Co-discoverer Elizabeth Roemer also has two asteroids to her name, the magnificently named 1930 Lucifer and 1983 Bok. Charles T Kowal does even better, with 19 minor planets discovered between 1970 and 1981, including 2060 Chiron, the first of the centaurs. His collection of astronomical trophies also boasts a few comets, and Leda, another Jovian satellite.


ALSO TODAY . . .

Asteroid 191 Kolga was discovered today in 1878 by C H F Peters, and named in honour of the daughter of a Norse sea God. The name means “chilling wave”.

Chilling Wave

Chilling Wave

Two years later to the day, Johann Palisa added asteroid 219 Thusnelda to his collection. Thusnelda is an S-type main belt asteroid of approximately 38 km diameter. The name comes from a Germanic princess captured in AD 15 by the Roman general Germanicus Julius Caesar , adoptive son of the Emperor Tiberius, and father of Caligula.


July 06 – Lysithea

Lysithea, discovered today in 1938 by Seth B Nicholson at Mount Wilson, is the second smallest of the Himalia group of Jupiter‘s moons (the others being Himalia, Leda and Elara).  This small group of moons, all about 11 million km from Jupiter, are similar in appearance and behaviour, and are therefore thought to have a common origin, possibly a C- or D-class asteroid.

Lysithea is named after a daughter of Oceanus, and was one of Zeus’ many conquests.  And apart from these scanty facts there is just about nothing else I can find out about her.  Newly discovered Jovian moons are all now named after lovers of Jupiter (Zeus) and as a rule end in an ‘a’ or an ‘o’ if they are prograde (orbit in the same direction as the planet) or an ‘e’ if they are retrograde (orbit in the opposite direction to the planet).

Being a mere 18 km across, and 778 million km away, there are no great photographs of Lysithea available.  I was tempted to draw a black square and put a single spot of white on it, but I resisted.


May 01 – Nereid

Neptune’s third largest moon, Nereid, was discovered on May 1st, 1939, by Gerard P Kuiper, using the 83-inch Otto Struve Telescope at Mcdonald Observatory, Texas. It is about 170 km in diameter, and has a day lasting just 11.5 hours, but a year almost identical to ours, at 360 days.

Not Voyager 2's greatest photograph, but the best we have of Nereid (image: NASA).

Not Voyager 2’s greatest photograph, but the best we have of Nereid (image: NASA).

Nereid has a prograde rotation, and a very eccentric orbit, which takes it from 853,000 miles at periapsis to 5,999,000 miles at apoapsis. This has led astronomers to believe it is either a captive asteroid (or Kuiper Belt Object) or has had a previously more normal orbit changed by the capture of Triton.

The Nereids after whom this moon is named were the fifty daughters of Nereus and Doris. They were sea nymphs who attended Neptune (he married one of them, Amphitrite, who bore his son Triton.

 

March 25 – Titan

Titan, Saturn’s largest moon, and the second largest moon in the Solar System (behind Ganymede, which is only ever so slightly bigger), was discovered on March 25th 1655 by the Dutch astronomer Christiaan Huygens.  As the first moon to be discovered around Saturn there was no immediate pressure on Huygens to find an impressive name for it, so he settled for Luna Saturni (Saturn’s moon).  It wasn’t until Cassini discovered a further four Saturnian moons that a naming system became an issue, and even then the solution wasn’t particularly imaginative (“Saturn IV” to start off with, then “Saturn VI” after a couple more were found).  It was John Herschellson of the more famous William, who came up with the name Titan, as well as the names of the other six saturnian moons known at the time.

Titan, with Tethys in the background (image: NASA)

Titan, with Tethys in the background (image: NASA)

Titan, as you can see from the picture below, is shy, and doesn’t like to show us a great deal of surface detail.  It is the only moon in the Solar System with a substantial atmosphere, so dense in fact that the surface pressure is about half as great again as on Earth.  It is also suspected of having the potential to support microbial life, making it a very tempting place for Earthlings to visit.

Titan from the Cassini Spacecraft (false colour image in Ultraviolet and Infrared). Image credit: NASA / JPL / Space Science Institute.

Titan from the Cassini Spacecraft (false colour image in Ultraviolet and Infrared). Image credit: NASA / JPL / Space Science Institute.

In the hierarchy of Saturnian moons, Titan is right at the top of the pile.  It has a mass of 1.34 x 1023 kg (that’s about twice the mass of our own lightweight moon) which makes it far and away the biggest, accounting for 96% of the combined mass of all Saturn’s satellites.


1928  –  Jim Lovell (commander of Apollo 13) born today in Cleveland, Ohio. Captain James Lovell, USN, is a veteran of four space flights (he was the first man to achieve the feat) totalling 29 days: Gemini 7, Gemini 12, Apollo 8, and Apollo 13. He is also the only person to fly to the Moon twice without landing on it.


September 17 – Enterprise vs The Death Star

Not really, but it’s a good headline.

Mimas, or Saturn I, (sometimes known as the “Death Star” for reasons obvious to anyone who’s ever seen one of the original Star Wars films) was discovered on September 17th 1789 by William Herschel. It’s a heavily cratered, 396 km wide, low density moon with one really obvious surface feature: the 130 mile wide crater Herschel. It’s hard to imagine how Mimas managed to survive the impact of a body capable of forming such a gigantic hole, and there are fractures on the opposite side of the moon that may indicate it nearly didn’t.

Mimas from Cassini. (Image credit: NASA / JPL)

Mimas from Cassini. (Image credit: NASA / JPL)

Mimas has proved irresistible to a succession of spacecraft visiting Saturn and its environs. Pioneer 11, Voyager 1 and Voyager 2 all flew past, and Cassini has taken some very close shots from less than 10,000 km away.

Mimas, like the other six satellites of Saturn known at the time, was named after one of the Gigantes (giants) of Greek mythology. The names were suggested by John Herschel, son of William.

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ALSO TODAY . . . .

 Space Shuttle Enterprise (OV-101), the first of her kind, was rolled out of her construction plant for a photo opportunity with Gene Roddenberry and the cast of Star Trek. The phrase “roll out” is particularly apt in this case, as Enterprise was intended for testing in Earth’s atmosphere only, and so was built without engines.

1930 – Birth of Thomas P Stafford, veteran of Gemini and Apollo missions (21 days in space), commander of the second manned mission to orbit the Moon.

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September 16 – Hyperion

Hyperion is an unusual moon of Saturn.  It was discovered on September 16th 1848 by three astronomers: William Lassell of Liverpool (on his own), and Americans William Cranch Bond and his son George Phillips Bond (together).  The Bonds spotted the moon first, but Lassell got published first, so all three are credited.  They are honored together in the form of the only named feature on Hyperion, a prominent ridge called the Bond-Lassell Dorsum.

Hyperion (image credit: NASA/JPL/Space Science Institute)

Hyperion (image credit: NASA/JPL/Space Science Institute)

It’s an odd place, Hyperion.  You can tell from the photograph that it isn’t a regular shape, suggesting it may be the remnant of a larger body destroyed by an impact at some point long ago, and appears to be a bit like a bath sponge (i.e. mostly holes) or the papery wasp’s nest I extracted from our bird box last autumn following a Summer spent creeping up to it with an insect spray and running away as fast as possible after giving them a quick blast in the entrance.  Latest calculations suggest that as much as 40% of the moon’s volume is empty space, and what solids there are may turn out on closer inspection to be mostly water ice, with a sprinkling of frozen methane and carbon dioxide, and only a small percentage of rock.  Hyperion is the second largest moon in the Solar System to have an irregular shape, and is the largest with a “chaotic” (wobbly) rotation.

Hyperion was first visited by Voyager 2 in August 1981, where its odd shape and rotation were the subject of much head-scratching.  Voyager only got to within about 300,000 miles though, so it was left to the Cassini mission to reveal the full oddness of the moon’s surface when it got to a mere 628 miles (1010 km) in September 2005.  Cassini took today’s  first photograph.

In Greek mythology, Hyperion was a Titan.  There were twelve of them, children of Gaia and Uranus.  Hyperion married Theia, his sister (as usual) and had three children with her: Helios (the Sun), Selene (the Moon) and Eos (the Dawn).  So today’s second photograph (aren’t you lucky?) is the Horse of Selene from the east pediment of the Parthenon (now in the British Museum).  I first saw this sculpture when I was seventeen, on a sixth form trip to London.  If I’d paid more attention to my teacher, Mr Perry, at the time I could probably tell you all sorts about it, but alas.

The Horse of Selene.

The Horse of Selene.

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ALSO TODAY  . . . . 

C-type main belt asteroid 105 Artemis was discovered on September 16th 1868 by J C Watson, and named after the Greek goddess of the hunt.

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