February 18 – Discovery of the “Planet” Pluto (1930)

I feel sorry for Pluto. When I was younger it was the ninth biggest planet in the solar system, but unfortunately is now (probably) only the second biggest “dwarf planet” following the discovery of minor-planet 136199 Eris by the Palomar Observatory on January 5th 2005. This discovery encouraged an acceleration of the debate over whether or not Pluto should ever have been called a planet, the result being that the IAU published their Definition of a Planet in the Solar System on August 24th 2006. So I suppose that’s the day on which Pluto stopped being a planet, and became a Kuiper Belt Object. And as if that weren’t damage enough for Pluto’s image, there is still a debate going on as to whether Pluto and Charon should be re-classified as a binary system. At the moment Charon is a moon of Pluto; but the centre of their combined mass doesn’t lie within either body, so strictly speaking it should get higher billing.

On the upside, Pluto does get its own Disney character, and lends its name to both plutoids (anything beyond the orbit of Neptune that has managed to attain a roughly spherical shape) and plutinos (anything in the above group that orbits the Sun twice in the same time it takes Neptune to make it round three times).

Pluto was discovered on February 18th 1930 by Clyde Tombaugh from Illinois. Tombaugh was a prolific discoverer of variable stars, and also of asteroids, many of which he found while searching for Pluto. But Pluto will always be his claim to fame, and following his death in 1997 a small quantity of his ashes were sent on their way out there aboard the New Horizons probe, which arrived at the ex-planet on July 14th 2015 to a blaze of publicity, and began sending back fabulous snapshots.

Pluto from the New Horizons probe (image credit: NASA/APL/SwRI)
Pluto from the New Horizons probe (image credit: NASA/APL/SwRI)

And not before time, as you can see from the second photograph (below); because close-ups of Pluto from the enormous distance of the Earth don’t reveal a great deal of detail.

Pluto and Charon (image: NASA)
Pluto and Charon, pre-New Horizons (image: NASA)

As well as the aforementioned Charon, Pluto has four other moons: Nix, Hydra, Kerberos and Styx. All moons of Pluto are named in accordance with the convention that they are mythological, and have some association with their parent (Hydra, for example, was the nine-headed guardian of an entrance to the underworld).

Surface detail of Pluto from New Horizons (image credit: NASA)
Surface detail of Pluto from New Horizons (image credit: NASA)

As I just mentioned, Pluto is a Kuiper Belt object (KBO). The Kuiper belt, (rhymes with sniper, not kipper, although I have to admit that I do find the concept of a kipper belt rather appealing) or, to give it its full name the Edgeworth-Kuiper Belt, stretches from about 30 to 50 AU from the Sun, and contains an enormous number of mostly smallish bodies (trans-Neptunian Objects, or TNOs) left over from the formation of the Solar System.

A Selection of KBOs (image: NASA)
A Selection of KBOs (image: NASA)

Why Edgeworth gets edged out in popular literature while Kuiper gets the kudos is something I might know more about by the time his birthday comes around (Feb 26th, 1880). Neither of them correctly predicted what the belt was like anyway, so it’s anybody’s guess.

Anyhoo, there are thought to be as many as 100,000 TNOs within the belt with a diameter of over 100km (which is why I say “smallish” not “small”) and, because Pluto is now a member, everybody knows the name of at least one.

Results from New Horizons are changing our view of Pluto for ever. It now see that a crust of water ice might be acting in place of a bedrock, supporting mountains made of frozen nitrogen and methane. It also seems that Pluto might, somehow, be still geologically active. We need to visit again!


February 18th, 1977 was the day on which the space shuttle Enterprise made her first “attached” flight. Strictly speaking, I suppose, this wasn’t the maiden flight of a shuttle, as Enterprise was securely strapped to the back of a specially adapted Boeing 747 for the duration. I now know that she didn’t have any engines or a heat shield, and was therefore incapable of actually flying in space, but at the time I was young(er), and mightily impressed (and it was the first time a shuttle’s wheels had been higher than the hangar roof, so it counts). I was even more impressed when Enterprise was flown, again attached to the 747, over the family home at Brown Edge, Staffordshire, six years later as part of a promotional tour of Europe, drumming up satellite launching business for NASA.


January 05 – Discovery of Eris (2005)

Dwarf planet Eris is 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 - Troublemaker
Eris – Troublemaker

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.


 

November 28 – Discovery of KBO 20000 Varuna (2000)

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.

Varuna pacifying Sri Rama.
Varuna pacifying Sri Rama.

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 . . . .

Caroline Island, Kiribati (image credit: NASA)
Caroline Island, Kiribati (image credit: NASA)

June 22 – Discovery of Pluto’s Moon, Charon (1978)

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.

 

True colour image of Charon (Image credit: NASA New Horizons).

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 up close (image credit: NASA New Horizons).

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”).

Pluto and Charon from just under 4 million miles (6m km). Image:NASA New Horizons

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.

Caron and some guests, as depicted by Ukrainian artist Alexander Dmitrievich Litovchenko