December 24 – Discovery of Saturn’s Moon, Rhea (1672)

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

 

 

December 18 – Discovery of Saturn’s moon, Epimetheus (1966)

Saturn’s moon Epimetheus, discovered today in 1966 by Richard Walker, is a non-identical twin.  It, and its larger brother, Janus, which had been discovered three days previously by Audouin Dollfus, fly round Saturn in almost identical orbits (their mean orbital radii differ by a mere 50km or so).  Obviously, at the time of discovery, with only  the evidence of Earth-based telescopes to go by, this caused a certain amount of confusion among astronomers trying to work out where the supposed new single moon was.  At the time it was thought, also fairly obviously,  highly unlikely that two moons would share the same orbit without hitting each other.

Epimetheus from the Cassini Orbiter (image: NASA/JPL/SSI)
Epimetheus from the Cassini Orbiter (image: NASA/JPL/SSI)

It is possible that at some time in the past Epimetheus and Janus were part of a single larger body, but if that is the case it is likely to have been a very long time ago.  Both are heavily cratered, and their surface features display softening of their edges by the accumulation of dust, suggesting nothing much has changed there for some time.

Epimetheus was, in Greek mythology, a Titan (nowadays the name of a rather larger Saturnian moon), brother of Prometheus (also a moon of Saturn).  The pair were sons of Iapetus (another Saturnian satellite).  Epimethius was, according to some myths, married to Pandora (and guess what she gave her name to?).

Epithemius was given the job, shortly after the creation of the Earth, of finding a positive trait to give to each of the newly designed animals.  Unfortunately he ran out of good characteristics with one animal remaining, which you’ve probably already guessed was man.  That’s when Prometheus came along and gave  man the gift of fire, after which it all went downhill.

Epimetheus (3rd left) meets Pandora for the first time.
Epimetheus (3rd left) meets Pandora for the first time.

As it happens, though, they do share the orbit, and they don’t hit each other.  One moon does catch the other up every four years, but interactions between their gravities causes the inner moon to move faster (which causes it to move to a higher orbit) and the outer moon to slow down, causing its own orbit to drop a little.  So they actually exchange places, which is why I’m not going to call them the fifth and sixth moons, because they are each neither and both.


Launch, in 1958, of SCORE, the world’s first communications satellite (Signal Communications by Orbiting Relay Equipment).  The launch, from Cape Canaveral, was via an Atlas-B 10B rocket.

December 15 – Discovery of Saturn’s Moon, Janus (1966)

J R Hind discovered today’s main belt asteroid, 23 Thalia, from Hyde Park, London, on December 15th, 1852 (I’d like to see him try that nowadays). Thalia is an S-type asteroid of about 107 km diameter, located between the 3:1 and 5:2 Kirkwood gaps.

In Greek mythology, Thalia, daughter of Zeus and Mnemosyne, with a name derived from the verb “to flourish”, was the muse of comedy and pastoral poetry. She may or may not (depending on which source you believe) have been the mother of the Corybantes, attendants to the Great Mother of the Gods, and associated with particularly orgiastic rites.


We also have a moon today.  The discovery of Janus, one of the inner Saturnian satellites, is attributed to Audouin Dollfus, who first observed it on December 15th 1966. Three days later, Richard Walker also observed an object in the right place but at the wrong time, which caused confusion for a while, but was eventually found to be another moon, Epimetheus, which shares an orbit with Janus.

Janus, photographed by the Cassini probe (image credit: NASA / JPL / SSI)
Janus, photographed by the Cassini probe (image credit: NASA / JPL / SSI)

Janus is the two-faced Roman god of beginnings, entrances, gates, etc. You should thank him the next time an automatic door opens for you. Janus is also one of the select group of deities after whom a month (January) is named, and strangely he has no Greek counterpart.


1965  –  launch of San Marco 1 by Italy. Being their first satellite, the Italians wisely did not fill it with lots of expensive equipment. It did, though, contain a couple of experiments to study the ionosphere, the layer of the atmosphere stretching from about 60 km to 1,000 km, a region you need to know about if you’re planning to become a space-faring nation, needing to send radio messages over great distances.


2000  –  Death of George Alcock, aged 88, hunter of novae and comets. I believe he found five of each (remarkable for south-eastern England), some of them from indoors using binoculars, and even occasionally through double glazing1!  His eyesight must have been unbelievable.


2014  –  Voyager 1 crossed the termination shock.


 

2015  –  Launch of Expedition 46 to the International Space Station.  This caused considerable press interest in my homeland (in fact I’m going to call it a frenzy) because in addition to Russian cosmonaut Yuri Malenchenko and American astronaut Tim Kopra, the three-man crew contained Tim Peake, the first Briton to float into the ISS (I was going to say “set foot aboard” the ISS, but I’ve seen the footage, and feet don’t feature much).  Because of the  numbering system they use at the ISS when crews overlap, these three also formed part of Expedition 47.

ISS Expedition 46 Patch
ISS Expedition 46 Patch

As a supporter of Port Vale FC, I was distraught to discover that one of Tim Peake’s tasks whilst on this mission was to unveil a flag featuring the name of our local rivals, Stoke City.  I’ve gone off him a little.


 

1   Journal of the British Astronomical Association, vol.111, no.2, p.64-66

October 25 – Discovery of Saturn’s moon, Iapetus (1671)

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 of Greek mythology, as per John Herschel’s suggestion that they be given the names of the mythological siblings of Kronos (the Greek equivalent of Saturn). Iapetus is sometimes credited with being a distant ancestor of the human race, and the story goes that each of his four sons (Atlas, Prometheus, Epimetheus and Menoetius) was responsible for a particular fault in mankind. Thanks, Iapetus.


September 17 – Discovery of Saturn’s Moon, Mimas (1789)

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 Cassinihas 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.


ALSO TODAY . . . .

1976 – 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.


September 16 – Discovery of Saturn’s Moon, Hyperion (1848)

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 unusual 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, and they were the children of the old gods 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 of facts 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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August 28 – Discovery of Enceladus (1789)

Enceladus was discovered on this day in 1789 by William Herschel. This 500km diameter moon of Saturn has been proven recently to have the most impresive volcanoes in the Solar System. They are cryovolcanoes, so what comes out is cold (in this case water), and study of them has shown that they have been able to project water as far as Saturn itself, as well as providing enough ice to make up the parent planet’s “E ring”.

Enceladus (image credit: NASA)
Enceladus (image credit: NASA)
Enceladus venting, probably as a result of hydrothermal activity. (Image: NASA/JPL/SSI)
Enceladus venting, probably as a result of hydrothermal activity. (Image: NASA/JPL/SSI)

Enkelados was one of the Gigantes, children of Uranus and Gaia. This particular one, for his sins, was wounded in battle by Athene and buried under Mount Etna. Being buried under a volcano was a common fate for the assorted monsters and giants defeated by the Olympian gods. I suppose it was a handy method for explaining away subterranean rumblings.

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ALSO TODAY . . . .

Asteroid 167 Urda discovered in 1876 by Christian H F Peters.
George Eric Deacon Alcock MBE was born in Peterborough, 1912.

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July16 – Launch of Apollo 11 (1969)

I’ll keep this quick, but as you may have heard, July 16th 1969 is a fairly important day in spaceflight history.  It’s the day on which Neil A Armstrong, Michael Collins and Edwin “Buzz” Aldrin decided to get away from it all.  Their Saturn V rocket (SA-506), the fifth manned Apollo mission, blasted off from the Kennedy Space Centre at about half past one in the afternoon (Staffordshire Time) and may be the reason I have the vaguest recollection of my infant school gathering after dinner to watch a launch on the school television.

Launch of Apollo 11 (image credit: NASA)
Launch of Apollo 11 (image credit: NASA)

So it’s 2019, and 50 years since the launch, and the media is abuzz with stuff about every tiniest aspect of the mission, so I’m not going to try to compete.


Also today, in 1990, Mark R Showalter, using old frames from Voyager 2, discovered Saturn’s walnut-shaped moon, Pan, in the Encke Gap of the A Ring (the outermost of the main bright rings).

Thanks to the Cassini probe, we now have images beyond the wildest dreams of Voyager scientists:

Pan, imaged by Cassini (credit: NASA/JPL-Caltech/Space Science Institute)

Pan, of course, invented the pan pipes. He was a pretty hot musician all round, but a little big-headed. The story goes that he challenged Apollo to a musical duel. Pan was good, but Apollo was better. Only one person listening to the contest believed that Pan had won. This was Midas (he of the golden touch). So annoyed was Apollo at this lack of musical taste that he changed Midas’ ears into something more appropriate: those of a donkey.

The Judgement of Midas (Peter Paul Rubens)
The Judgement of Midas (Peter Paul Rubens)

Now, correct me if I’m incorrect, but in today’s artistic offering, is Pan playing his own invention upside-down?


This post originally published in 2015.  Updated 2017, and again in 2019. 

June 16 – Discovery of Proteus

Today’s anniversary is the discovery of Neptune‘s second largest moon, Proteus, found by analysis of Voyager 2 snapshots taken over a period of time leading up to June 16th 1989.  So, while June 16th isn’t the actual discovery date,    it’s as close as we’re likely to get.  It is thought that Proteus wasn’t formed at the same time as Neptune, but is a by-product of the capture of Triton.

Proteus is approximately 418 km in diameter (about 260 miles) and orbits Neptune close to the equatorial plane at a distance of a little over 117,000 km.  But aside from this, and the fact that it is dark and heavily cratered, almost nothing else is known about it.

Proteus (from Voyager 2). Image credit: NASA.
Proteus (from Voyager 2). Image credit: NASA.

Proteus is named after a shape-changing sea god, son of Poseidon, the Greek god whose Roman equivalent is Neptune.  Neptune’s moons are generally named after children or other associates of Poseidon (Triton, for example, was his other son).


ALSO TODAY:

2010: Launch of Soyuz TMA-19.

 

June 01 – Discovery of Saturn’s moons Methone and Pallene

Today in 2004, two new moons were discovered around Saturn by the Cassini Imaging Team at NASA. They are now named Methone and Pallene, in keeping with John Herschel’s suggestion that Saturnian moons should be named after the close family members of Kronos (the Greek equivalent to the Roman Saturn).

Methone is really small. It’s about two miles (3 km) in diameter, and orbits every 24 hours, over 120,000 miles above the planet, between the orbits of Mimas and Enceladus, and may well have been part of one of them at some time in the past.

Methone, photographed by Cassini (image credit: NASA).
Methone, photographed by Cassini (image credit: NASA).

Pallene is also diminutive, but at three miles wide (4 km) it is the larger of the Alkyonides group comprising itself, Methone and Anthe (the smallest).


1966 – Launch of Surveyor 1, on its way to the Oceanus Procellarum (Ocean of Storms) to check out the Moon’s surface prior to the launch of the Apollo program.


1990 – Launch of the ROSAT (Röntgensatellit) x-ray and UV telescope from Cape Canaveral.