Dwarf planet Erisis the largest known member of a collection of objects known as the scattered disc, a subset of the larger group, the trans-Neptunian objects (TNOs). It was discovered by a team from Palomar Observatory on January 5th 2005, and was given the “minor planet designation” of 136199. Eris has a larger mass than Pluto by about 27% (and was partly responsible for the reclassification of Pluto by the IAU as a dwarf planet), making it the ninth most massive object orbiting the Sun.
Eris, and its moon Dysnonia, are a very long way away. They orbit in a wild ellipse ranging from 38 to 97 AU from the Sun (about three times farther away than Pluto).
Eris has an appropriate name. In Greek mythology she was the goddess of chaos and discord. Inhabitants of Pluto, if they exist, would surely agree.
1969 – Launch of the USSR’s Venus atmospheric probe Venera 5 from Baikonur Cosmodrome in Kazakhstan. It reached its target on May 16th, and sent back data for almost an hour while descending by parachute through the Venusian atmosphere.
We hurtle way beyond the asteroid belt today, for a change, to celebrate the discovery of TNO (or KBO) 20000 Varuna, first spotted for what it is on November 28th 2000 by Robert S Macmillan, despite appearing on photographic plates dating back to the 1950s.
Varuna is a fairly large classical Kuiper belt object (KBO). Estimates of its size varied widely at the time of discovery from 600 to 1000km. More recent calculations seem to be bringing it down to the lower end of that range, but it still ranks highly in the KBO pecking order.
Varuna has a very rapid rotation (one full turn every 6.34 hours) and a double-peaked light curve. it is thought to be an elongated spheroid, about half as wide again across the equator as from pole to pole.
A recent report in Astrophysical Journal Letters (883 (1)) suggests the possibility of a close-in satellite orbiting Varuna, but there’s nothing conclusive, so we will have to wait and see.
The Hindu deity Varuna, after whom this particular oblate spheroid is named, has similar qualities to the Roman god Neptune, making it a good choice for what at the time was the largest known trans-Neptunian object.
Asteroid 235 Carolina was also discovered today, in 1883. It is one of Johann Palisa‘s collection of 122 asteroids, and came while he was going through (by his standards) a dry patch in his rock hunting career. Having discovered nine in 1882, he “only” managed two in 1883, before hitting his stride again in 1884 with six. Part of the reason for this relative scarcity was probably that Palisa spent a good portion of the year 1883 travelling to watch a total solar eclipse. The spot chosen for the expedition was near to Tahiti, in the chain of coral atolls known as the Line Islands. More specifically . . . .
Pluto‘s biggest moon, Charon, was discovered on June 22nd, 1978 by American astronomer James W Christy at the United States Naval Observatory.
Until recently, Charon, like Pluto, was mostly a collection of blurry smudges on photographic plates, but thanks to the 2015 flyby of NASA’s New Horizons mission, they both suddenly got a lot closer.
At just over 1,000 km wide, Charon is quite small. But because Pluto is also rather diminutive, Charon has a much greater effect on its host than the average satellite, particularly regarding the location of their barycentre, or the point in space about which their two-body system orbits. Whereas the effect of our own Moon is only enough to move the barycentre slightly away from the centre of the Earth and produce a small “wobble” because the relative masses of the two bodies are so different, Charon is so big relative to Pluto that they orbit a point well outside the surface of Pluto. This has led some to argue that the Pluto-Charon system should be considered as a binary system, rather than a dwarf planet/moon arrangement.
Charon’s orbit of Pluto is almost completely circular, with an eccentricity of 0.0002 (our Moon’s is a distinctly wonky 0.05, and Pluto’s eccentricity with regard to the Sun is a positively inebriated 0.25). So the distance of Charon from Pluto can therefore be said to be the same all the time if you round to the nearest kilometre, 17,536. On Pluto there would be no media frenzy for the next “Super Charon”).
Charon is named after the ferryman who the Greeks needed to pay to take their dead to the underworld (ruled by Hades, or Pluto to the Romans). Charon was one of the old “primordial” gods of Greek mythology. His father, Erebus, was the personification of darkness, which probably gave him a head start for landing a job in the underworld.