May 30 – Messier 12

Today’s main event is not dissimilar to yesterday’s. It happened in 1764, and was the discovery of a globular cluster, Messier 12, (or NGC 6218), by Charles Messier, one day after he discovered globular cluster M10. And at a casual glance, the photograph I’m using today looks remarkably similar to the one I used yesterday. But I’ve had a close look, and they definitely two different balls of stars.

Messier 12 from the Hubble Space Telescope (image credit: NASA / STScl / ESA)
Messier 12 from the Hubble Space Telescope (image credit: NASA / STScl / ESA)

M12 is approximately 75 light years across, at a distance from Earth of about 15,700 light years. It can be located as a faint fuzz in the constellation Ophiuchus with good binoculars, but needs a fairly hefty telescope to bring out detail.

Location of M12 (image created using Stellarium)

I suppose the obvious question, with M10 on the 29th of May and M12 on the 30th is: what about Messier 11? Unfortunately for Charles M11 had already been discovered. He included it in his catalogue, but German astronomer Gottfired Kirch beat him to it by a mere 63 years. M11 is in the constellation of Scutum (the shield), and is commonly known as the “Wild Duck Cluster”.

Extrasolar planet MOA-2007-BLG-192Lb (or just MOA-192b to its friends) was discovered orbiting the low mass red dwarf of the same name (but without the “b”) on May 30, 2008. Spotted by the MOA-II gravitational microlensing survey, it is one of the smaller known extrasolar planets (about 3 times the mass of Earth) and has an appropriately tiny parent star of about 6% the size of the Sun.

1903 – Asteroid 511 Davida discovered in 1903 by R S Dugan and named after astronomer David Peck Todd.

2007 – Saturn’s tiny moon Anthe was first spotted in Cassini images. Anthe may be part of a dynamical family with the moons Methone and Pallene. This is appropriate if true, as they were sisters in mythology, three of the Alkyonides, who threw themselves into the sea when their father was killed by Herakles.

1963 – Happy birthday Helen Sharman, OBE, Sheffield native, chocolate chemist and first Briton in space. She flew on Soyuz flight TM-12 to the Mir space station. It was her only mission.

March 17 – Jim Irwin

March 17th 1930: astronaut Colonel James Benson Irwin, USAF, born in Pittsburgh PA.

James Irwin (image credit: NASA)
James Irwin (image credit: NASA)

In 1971 Irwin, Apollo 15 lunar module pilot, became the eighth man to walk on the Moon, spending over 18 hours on the surface.  He also, on his return, became one of the first people to be grounded, quite literally, for smuggling postage stamps into space.

1852  –  Asteroid 16 Psyche was discovered on March 17th 1852 by Annibale de Gasparis.  Psyche is a large asteroid, about 200 km in diameter, accounting for about 1% of the mass of the entire asteroid belt.  It’s an M-type asteroid, probably mostly nickel and iron.

Cupid and Psyche (Van Dyck)
Cupid and Psyche (Van Dyck)

Psyche is named for a mythological princess, who caught the eye of the god Cupid.  The story is told by Lucius Apuleius in The Golden Ass.

In early 2017 NASA announced plans to  send a probe to Psyche in 2023, as part of their Discovery Program, the main reason being that, as a metallic asteroid, it represents one of the few classes of objects in our neighbourhood that haven’t yet been visited.

1899  –  Saturn’s moon Phoebe discovered by American astronomer W H Pickering.  It, too, is about 200 km in diameter, and may be a captured centaur from the Kuiper belt.  We have some spectacular photographs of Phoebe following the visit of the Cassini spacecraft in 2004.

Phoebe (image credit: NASA)
Phoebe (image credit: NASA)



May 06 – Daphnis

Today in 2005, Saturn’s moon Daphnis was discovered by the Cassini team.

Daphnis (image credit: NASA)
Daphnis (image credit: NASA)

Don’t be fooled by the large object at the bottom of the photograph: that’s Epimetheus, a moon about twelve times wider than Daphnis.  The birthday boy himself is a tiny, barely discernible dot towards the centre of the frame.  You might be surprised by the use of the word “boy” there, assuming Daphnis might be named after a Daphne.  But it wasn’t.  Daphnis was a shepherd (this is a shepherd moon), a very good friend of Pan, and the inventor of pastoral poetry.

Apollo and Daphnis (Perugino).
Apollo and Daphnis (Perugino).

The moon Daphnis, being ridiculously small, wasn’t an easy target to photograph. Fortunately, though, its position in the Keeler Gap within Saturn’s A ring gave it away by causing ripples within the ring. In order to stay within the Keeler Gap, Daphnis has to follow a near-perfect circular orbit. The difference between Daphnis’ perikrone and apokrone (closest approach to and furthest distance from Saturn) is only about 9 km (under 6 miles).

 1896  –  Asteroid 417 Suevia discovered by Max Wolf. Suevia is a K-type/S-type main belt asteroid of approximately 41 km (25.4 miles) diameter. it sweeps serenely round the Sun in an orbit ranging from 363 to 474 billion miles.

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.

March 13 – Uranus

Uranus (named after a primal Greek god of the sky, the son and husband of Gaia, and father of the Titans) was discovered on this day in 1781 by William Herschel.  It’s the 7th furthest planet from the Sun, orbiting at approximately 2.8 billion km, and is often classed as an ice giant with Neptune, as well as being a gas giant. Uranus is the third largest planet by radius (about 25,000km), but only the fourth largest by mass (roughly 14 times as massive as Earth).

Uranus and rings (image credit: NASA).
Voyager 2 image of Uranus and rings (image credit: NASA).

As with the other giant planets, Uranus has a plentiful supply of moons (27 at the  last count).  The largest five are Oberon, Titania, Umbriel, Miranda and Ariel.  None of the Uranian moons are particularly large, with the biggest, Titania, being less than half the size of our Moon.

Uranus also has a ring system.  The rings, of which 13 are known, are dark, narrow, and probably quite young (less than 600 million years old).  They are thought to be the remnants of one or more shattered moons.  The rings can be clustered together in three groups: nine main rings, two dusty ones, and two outer rings, including the brightest, the ε (epsilon) ring.

The atmosphere of Uranus (by which I mean the outer layers) usually presents a blank face to watching humans, and the Voyager photographs painted a picture of a serene world. But recently (August 2014) a collection of enormous bright spots have been observed, showing that giant storms can sometimes flare up on the planet.

1980CALYPSO, one of the smaller of Saturn’s fifty-three named moons, was discovered on this day in 1980.  It’s an irregularly-shaped Trojan, trailing 60 degrees behind the larger moon Tethys at the L5 Lagrangian point, something I’m still planning on writing a blog about (another Trojan, Telesto, occupies a position 60 degrees ahead of Tethys, called the L4).  Calypso is named after a daughter of the Titans Oceanus and Tethys.

1969APOLLO 9, under the command of James McDivitt, splashed down in the North Atlantic on March 13th 1969, after just over 10 days in orbit.

1855  –  Birth of Percival Lowell, proponent of Martian canals, and founder of the Lowell Observatory, one of the oldest in the United States.

Birthday Boy Percival Lowell at the massive 24" Reflector in the observatory which bears his name.
Birthday Boy Percival Lowell at the massive 24″ Reflector in the observatory which bears his name.