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 something to distinguish it from most of the other large, dark, main belt asteroids I’ve been waffling about for the past three years: 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 family of 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 from WISE 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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2 thoughts on “September 09 – Amalthea

  1. Gosh! I hadn’t considered this. This post very is thought-provoking and it also encouraged me to think in more ways.

    Outstanding job. I would appreciate it my blog about DPR Korea.


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