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


 

 

December 18 – Epimetheus

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.

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).


 

 

June 16 – 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).


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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August 28 – Enceladus and Alcock

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.

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George Eric Deacon Alcock MBE was born in Peterborough on August 28th 1912, and became a committed observational astronomer from a young age. He was elected a full member of the BAA in 1936, but had become a member of the meteor section at 18 (shortly after Patrick Moore had joined at age 11).

Alcock spent 20 years watching for meteors before deciding to go after a comet. He also found time to get married, fight a war, and discover, independently of Will Hay (yes, the comic actor) the White Spot on Saturn. His method of discovering at first comets, later novae, was really obvious but almost impossible. Simply memorize the positions of a few thousand stars, then see if anything turns up next time you look at them that wasn’t there last time. This system eventually worked extremely well, with Alcock notching up five comets and five novae. And while this might not sound very impressive compared to some of the asteroid hunters we have met in this blog, to put it in context the last British-discovered comet prior to the announcement ofC/1959/Q1(Alcock) had been 60 years before.

Alcck’s search for a comet began as a New Year resolution. He gave himself five years to accomplish the feat (followed by a further five years and a new pair of binoculars when his time ran out). But comets turned out to be like buses, and no sooner had Comet Alcock 1959e, as it would have been known at the time (there’s been a change to the naming system since then), been discovered, three days before his 47th birthday, than along came 1959f, less than a week later. I can’t imagine how the British astronomical community must have felt to be told about two comets in a week after half a century of drought, but I imagine they were pretty chuffed.

After finding another comet Alcock turned his attention to hunting for novae, again prefering visual observation over the photographic method preferred by his rivals. In one way this actually gave him an advantage over them, as anyone who remembers waiting two weeks to get their holiday pictures back from the chemist will understand. Alcock’s first nova was Delphini 1967 (or HRDel) on July 8th 1967. This was closely followed by another, in Vulpecula, in 1968.

As the years went by, the gaps between discoveries widened, but Alcock never gave up. His last comet and nova were on May 3rd 1983 (comet C/1983 H1 IRAS-Araki-Alcock) and March 25th 1991, aged 78 (nova V838 Her).

George Alcock died on December 15th 2000.

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ALSO TODAY . . . .

Asteroid 167 Urda discovered in 1876 by Christian H F Peters.

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August 26 – Voyager 2 reaches Saturn

August 26th 1981 was the date on which Voyager 2 made its closest approach to Saturn. Voyager 1 had already been and gone 9 months previously, despite being launched second, and between them the Voyager twins greatly increased our knowledge of the most beautiful of all the planets. They found, among other things, that (i) the atmosphere is nearly all hydrogen and helium (Saturn would float in water if an appropriately-sized bath could be constructed); (ii) it is a very cold place (-200 to -300°F); (iii) it’s also a bit blowy (wind speeds recorded over 1,100 mph), and (iv) it rotates every 10 hours 39 minutes and 24 seconds.

Saturn's Rings from Voyager 2 (image credit: NASA)

Saturn’s Rings from Voyager 2 (image credit: NASA)

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