April 17 – 17 Thetis

Today’s lump of rock, asteroid 17 Thetis, was discovered in 1852 by yesterday’s birthday boy, Robert Luther.  It was the first asteroid he discovered.

Thetis is a main belt asteroid approximately 90 km across, with an absolute magnitude of 7.76 (apparent magnitude from 9.9 to 13.5).

Not a great deal is known about Thetis, but it is thought to be an “S-type” asteroid (the S stands for stony).   S-types are the second most common asteroids after “C-types” (C = carbonaceous).

Black figure hydria showing Thetis and the Nereids mourning Achilles

Black figure hydria showing Thetis and the Nereids mourning Achilles

Most references to Thetis (goddess of water)  in Greek literature relate in some way to her role as mother of Achilles, the greatest warrior of the Trojan Wars, but she did have some adventures of her own, most notably protecting Zeus from a plot to overthrow him by summoning Briarius, a friendly “Hekatonkheire”, Greek for “hundred-handed one” (and just in case that wasn’t frightening enough, they had fifty heads as well).

2014  –  NASA announced the discovery of Kepler-186F by the Kepler mission, which is using the “transit” method to discover exoplanets.   Kepler had already discovered hundreds of planets, but this was the first Earth-sized planet, orbiting a red dwarf, to be spotted.  The Kepler team believes that red dwarf stars could provide the majority of “habitable zone” planets, and Kepler-186F is on the edge of the host star’s habitable zone, in an orbit similar to that of Mercury.

Kepler-186F, is, unsurprisingly, orbiting a star known as Kepler-186.  This is an “M dwarf” (the Sun is a “G dwarf”) about 500 light years away.  M dwarfs are the most populous type of star in the known universe (7 out of 10 stars fall into this category, even though they can’t be seen by the naked eye). M dwarf stars are much dimmer than the Sun, and smaller, some being only 8% the mass of our star.

Comparison graphic for Kepler-186F (image: NASA)

Comparison graphic for Kepler-186F (image: NASA)

We should probably resist the temptation to get too excited about the possibility of life on Kepler-186F.  It is not known whether it has an atmosphere, and NASA are uncertain as to whether the planet is “tidally locked”, which would be unhelpful to life, or subject to flares from the parent star, which would be fatal.   However, NASA say that the differences between the conditions on Earth and K-186F don’t rule out the possibility of life.

1861  –  Asteroid 67 Asia discovered by Norman Robert Pogson.

1888  –  Asteroid 276 Adelheid discovered by Johann Palisa.  The origin of the name is not known.  There were probably a few Adelheids (and Adelaides) around at the time, but the most high-profile was Princess Adelheid of Hohenlohe-Langenburg, a niece of Queen Victoria.  Unfortunately I can’t find anything spectacular happening in her life in 1888.  Another posibility though, is Princess Helena Adelaide of Schleswig-Holstein-Sonderburg-Glücksburg, born in 1888 (the name Helena had already been taken for an asteroid discovered by  J C Watson in 1868).  Who can say?

1970  –  Splashdown of Apollo 13, following the scariest mission of the entire manned Apollo program.

View of the Moon from Apollo 13 (image credit: NASA).

View of the Moon from Apollo 13 (image credit: NASA).


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