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 14 – Tycho Brahe

Tycho Brahe was born today in 1546 in Denmark. As the child of a wealthy family (he was born in a castle) he was well placed to receive a good education, and took up astronomy while studying law.

Tycho Brahe wearing the Order of the Elephant (and the Moustache of the Walrus)

Tycho Brahe wearing the Order of the Elephant (and the Moustache of the Walrus)

Tycho made several leaps of the imagination, including the development of his own model of the solar system, which he nearly got right. He correctly deduced that the Moon orbits the Earth, and that the planets orbit the Sun. unfortunately he also decided that the Sun must orbit the earth; an easy mistake to make I suppose at that time, if you spend your entire life seeing it cross the sky thousands of times in front of your very eyes.

Tycho was also the first person to decide that novae (they weren’t called that at the time) originated further away than the Moon. The popular view was that the stars were fixed and unchanging, and anything that appeared in the sky had  to be closer than them. But Tycho noticed that a bright new star in Cassiopeia (now called SN1572) did not move against the background stars, and so must be farther away than all the objects that did. It sounds fairly reasonable to us, but in the 16th century the night sky was anything but obvious.


2009 – Launch of the Wide-field Infrared Survey Explorer (WISE) by NASA, on a mission  to map 99% of the sky. So far, it has found more than 33,000 asteroids and comets, over 200 Near-Earth Asteroids (including 43 potentially hazardous ones), the most luminous galaxy in the known universe, and numerous brown dwarfs.


2013  –  the Chinese Chang’E 3 mission lands on the Moon.

The Chang'E Lunar Lander, imaged by the Yutu rover (image: China National Space Administration)

The Chang’E Lunar Lander, imaged by the Yutu rover (image: China National Space Administration)

 


 

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 08 – Adrastea

Adrastea, the second moon out from the “surface” of Jupiter, was discovered by analysis of Voyager 2 images in 1979 by David C Jewitt and G Edward Danielson.  A subsequent photograph is available (the Voyager one was of a tiny speck) taken by the Galileo spacecraft, and here it is, looking more like an out-of-focus lemon than a moon . . . .

Adrastea (image credit: NASA)

Adrastea (image credit: NASA)

Adrastea is like the Earth’s Moon, in that it always keeps one face pointing towards the parent planet, but it has an unusual orbital period of 7 hours 9 minutes, which is about two hours less than one Jovian day, even though Jupiter has the fastest rotation of all the planets (which accounts for its easily visible bulge).  Only a very few moons do this (three are known, the others being Metis and Mars’ moon Phobos).  As I mentioned two days ago, we can tell without looking it up that Adrastea will have a prograde orbit, because it ends with an “a”.

Metis and Adrastea share another unusual fact in common.  They orbit too close for comfort to their parent, meaning that at some point in the future they will impact the planet.

In Greek mythology, Adrasteia was the nymph who had to nurse Zeus and hide him from his father, Kronos.  Her name means “inescapable”.


1695   –   Death of Christian Huygens.


1959   –   Launch of Explorer 6.


2011   –   Final launch of shuttle Atlantis (flight number STS-135).


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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September 09 – Amalthea

Amalthea, named after the foster-mother of Zeus, is Jupiter’s third moon (counting outwards), and the largest of the inner satellites. It was the last to be discovered by an astronomer standing staring up a telescope rather than taking photographs to peruse later. The astronomer was Edward Emerson Barnard (of Barnard’s Star fame) and he discovered Amalthea on September 9th 1892 with the 36 inch refractor at Lick Observatory in California.

Amalthea is roughly ellipsoidal in shape (a bit like a rugby ball), about 250 km long and 140 km wide, and orbits Jupiter with the long axis always pointing towards the planet (as with our own Moon, known as tidal locking). It is mostly reddish in colour, but patches of green have been seen. The surface is widely cratered.

Amalthea from Voyager 1 (image credit: NASA)

Amalthea from Voyager 1 (image credit: NASA)

Amalthea would be a fantastic place from which to view Jupiter. The giant planet occupies 46° in the sky, or about a quarter of it. You would have to careful not to jump up and down with the excitement, though, as the escape velocity of 0.06km/s means that if you did jump up, the “and down” part wouldn’t happen.

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Asteroid 56 Melete , despite being another large, dark, main belt asteroid, does have somthing to distinguish it from most of the other large, dark, main belt asteroids I’ve been waffling about fot the past six months. It’s a P-type. These asteroids are typically found in the outer reaches of the main belt, and have a low albedo with a reddish spectrum. They are thought to have organic silicates in their make-up, and possibly even water ice.

This particular P-type asteroid was discovered from Paris by Hermann Goldschmidt on September 9th, 1857. It was named after one of the three Boeotian muses of Greek mythology (her name means ‘ponder’, so she is the muse of meditation), and although I’ve been looking all over the place I have been completely unable to find her likeness on any painting, drawing, etching, frieze, fresco or vase, which is annoying. I shall ponder where to try next.

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Asteroid 61 Danaë  –  discovered September 9th 1860. Danaë is a large, rocky S-type asteroid of about 80 km diameter in the main belt, rotating every eleven and a half hours. It was discovered from Paris by Hermann Goldschmidt, but named by Robert Luther after the mother of Perseus.  The father was, as usual, Zeus, who impregnated her in the guise of a shower of golden rain (no comment).

Danaë, in playful mood.

Danaë, in playful mood.

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Asteroid 189 Phthia  is another notch on the tripod for Christian Heinrich Friedrich Peters (he was here yesterday with 116 Sirona, and it’ll be his birthday later in the month). Phthia is a rocky S-type asteroid, about 40 km wide, in the main belt. It was first spotted on September 9th 1878 from Clinton, New York. Phthia is named after a place: it was the name of an area of southern Thessaly, in Greece, founded by Achilles’ grandfather Aiakos, and was home to the Myrmidons, who fought on the winning side in the Trojan War.

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Asteroid 297 Caecilia  –  September 9th 1890. A main belt asteroid of about 40 km diameter, orbiting the Sun every 5.6 years. I have so far been unable to find any reference to the origin of this name. It was discovered by Auguste Charlois on the same day as 298 Baptistina (see below), another asteroid with a mysterious name.

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Asteroid 298 Baptistina, discovered by Auguste Charlois on September 9th 1890, has a more colourful past than most. It is the head of the Baptistina familyof asteroids, all of which share a similar orbit and are thought to have a common origin in a much larger body that was destroyed in a collision. For a while it was thought that this event resulted in the creation of a fragment that hit Earth 65 million years ago and wiped out the dinosaurs. Recently though, data fromWISE has given a date for the collision that destroyed Baptistina of about 80 million years ago. This is too recent, as the resulting fragment would have needed far longer to reach Earth and collide with us than the 15 million years available.

Baptistina, as mentioned earlier in today’s offering, is another of those pesky rocks whose name refers to a person, place or event about which we appear to have no knowledge.

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August 29 – Margaret and 74 Galatea

Margaret (or Uranus XXIII) is, so far as I know, the only prograde irregular satellite among Uranus’ collection of, at last count, 27. It was discovered on August 29th, 2003 by Scott S Sheppard and David Jewitt using the 9.3m Subaru telescope at Mauna Kea, Hawaii.

A further satellite of Uranus was also confirmed on this day, but as the first sighting of S/2001 U2 (now known as Ferdinand) was an unconfirmed glimpse on August 13th 2001 we shall say no more about it for the next 50 weeks.

Margaret is named after a minor character in Shakespeare’s Much Ado About Nothing. She is the servant or chamber-maid of Hero, the beautiful daughter of Leonato.

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74 Galatea (discovered August 29th, 1862) was the third of five asteroids to be discovered by the German astronomer Ernst Wilhelm Leberecht Tempel (comets were more his thing – he found an impressive 21!).

Galatea is a large, dark, C-type main belt asteroid, and if it seems as though every asteroid I mention fits that description it’s because 75% of all known asteroids are C-types, and the main belt contains 93% of all the numbered minor planets.

Pygmalion and Galatea (by Falconet)

Pygmalion and Galatea (by Falconet)

Two possibilities exist for the choice of the name Galatea. Ovid tells us on the one hand that it was the name of the ivory statue carved by the sculptor Pygmalion, with which he fell in love. But on the other hand he also uses the name to describe a nereid (sea nymph) whose lover, the river spirit Acis was killed by a boulder thrown by Galatea’s jealous suitor, Polyphemus the cyclops. Ovid omits to discuss what kind of aim a cyclops would have.

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August 25 – Cupid and Mab

Today in 2003  –  Cupid and Mab, moons of Uranus, both of which had been too dim to see on Voyager photographs, discovered by Mark R Showalter and Jack J Lissauer using the Huble Space Telescope. Cupid was named after the character in Timon of Athens, and Mab after the queen of the fairies who is mentioned in Romeo and Juliet (and as a long-time fan I would also refer you to The Fairy Feller’s Master Stroke on the album Queen II).  Mab has unusual hobbies. She drives her chariot (which I believe is made from an acorn) up people’s noses to enable her to influence their dreams, and she is thought to decide who gets infected by herpes simplex.

The best picture I could find of Mab.

The best picture I could find of Mab.

Neither Cupid nor Mab could be described as impressive bodies.  Cupid measures about 18 km in diameter, while Mab is thought to be about 24 km.

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Asteroid 84 Klio discovered by R Luther in 1865.  Clio (or Klio or Kleio) was the muse of history.  A daughter of Zeus and Mnemosyne, she had one son, to whom she gave the butchly masculine name Hyacinth. Her own name is derived from the verb kleô, meaning to celebrate or make famous.

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1889 – Asteroid 287 Nephthys discovered by C F H Peters, and is the last of his incredible haul of 48 asteroids.  Nephthys is a large, S-type main belt asteroid, and for a change is named after a character from Egyptian mythology, the daughter of Nut and Geb, and sister of Isis.

Nephthys

Nephthys

I’m going to have to do some digging on Peters, because I find it hard to believe that after 28 years of tracking the things, he doesn’t have an asteroid named after him.  I’ve found two so far named after people called Peters, and he isn’t either of them.

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Voyager 2 makes its closest approach to Neptune on August 25th, 1989.  This was the end of a bit of a purple patch for NASA.  Photographs from the outer planets had enthralled the inhabitants of this one for more than a decade, and Neptune didn’t disappoint.  Voyager 2 was able to get some great shots of the planet, including the “Great Dark Spot” which seems to have subsequently vanished. There was also time for a visit to Triton, Neptune’s volcanically active largest moon, thought to be a captured Kuiper belt object.

Neptune and Triton from the departing Voyager 2 (image credit: NASA)

Neptune and Triton from the departing Voyager 2 (image credit: NASA)

Although they didn’t know it at the time, this fly-by marked the point at which every planet in the solar system had been visited (back in the day they still had Pluto on the list).

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