Pluto (Dwarf Planet/Plutoid)

Named after the Roman god of the underworld, Pluto discovered on February 18, 1930, and originally considered the ninth "planet," [1] was demoted to the status of "dwarf planet" [2] by the IAU on August 24, 2006, and further redefined as a "plutoid" [3] on June 11, 2008. [4]

It's also official that Pluto is the largest of the dwarf planets (at least for now), its confirmed diameter of 
2,370 kilometers (1,473 miles) making it slightly larger than Eris a distant relative and at one time the supposed title holder.

Pluto has a thin atmosphere, which due to an eccentric orbit 29.66 AU at perihelion and 48.87 AU at aphelion (1 AU being the mean distance between the Sun and Earth approximately 150,000,000 kilometers) forms when frozen methane, nitrogen and carbon monoxide on its surface sublimates into gas upon solar approach, refreezing again as it retreats outward. The mean surface temperature is -229 °Celsius.

Pluto’s composition is still largely unknown, the best guess being a rocky silicate and water ice core surrounded by a layer of water ice in turn covered by a layer of frozen methane.

New Horizons, the first mission in NASA's New Frontiers Program, launched January 19, 2006, reached its closest approach to Pluto,
some 12,500 kilometers (7,750 miles) on July 14, 2015, the first Earth vessel to do so. Armed with a remote-sensing package, its task at Pluto, was to image and examine the atmosphere, geology and morphology of both the dwarf planet and its largest moon as well as grab glimpses of anything else of interest (smaller moons, rings etc.). Due to certain considerations (power etc.) the information gathered was trickled back to Earth over an extended period of time. The last of said data (Pluto flyby) was received by mission control, Laurel, Maryland, on October 25, 2016.

If Pluto is indeed the god of the underworld then this dwarf planet/plutoid is well named, for the chances of their being indigenous life on this frigid orb are slim indeed.

As an interesting aside some of the ashes of Clyde Tombaugh, Pluto’s discoverer, are on board the New Horizons spacecraft.

To date Pluto has five known moons: Charon, Nix, Hydra, Kerberos, (formerly S/2011 (134340) 1 or P4) and Styx (formerly S/2012 (134340) 1 or P5).

[1] A "planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape) and (c) has cleared the neighborhood around its orbit.

[2] A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape), (c) has not cleared the neighborhood around its orbit and (d) is not a satellite.

[3] A "plutoid" is a trans-Neptunian dwarf planet.

[4] Due to its position in the Kuiper belt [5] (sometimes called the Edgeworth-Kuiper belt) Pluto along with Haumea and Makemake has been further designated a Kuiper belt object or KBO by the International Astronomical Union.

[5] The Kuiper belt is a region of the outer Solar System populated by billions of rock-ice objects. It reaches from the orbit of Neptune (30 AU) outward to approximately 55 AU.

* New Horizons is now, with fuel to spare, far beyond Pluto, its next objective a rendezvous with Kuiper belt object (KBO) 2014 MU69. Discovered in June 2014 by astronomers using the Hubble Space Telescope, 2014 MU69 (also known as 1110113Y and nicknamed Ultima Thule) has an estimated (though constantly changing-it may even be a binary) diameter of 19 miles (30 km)roughly a 60th that of Pluto. Relatively primordial with an orbit that has likely not undergone any significant perturbations this classical KBO (also called a cubewano) is thought to be akin to the building blocks of the Kuiper belt's dwarf planets.

Further to the above, a safe and successful flyby of Ultima Thule by New Horizons became reality at 12:33 a.m. ET, January 1, 2019. Confirmation was received at Johns Hopkins University's Applied Physics Laboratory, home to Mission Control just after 10:30 a.m. ET. Images and other data gathered by the spacecraft will be streamed home over the coming months
here's a taste of what's known so far: Ultima Thule is a contact binary, two spheres fused together one three times the size of the other, combined length 21 miles (33 kilometers).

Neither a comet nor an asteroid it's being referred to as a primordial planetesimal. "This thing was born somewhere between 99 and 99.9 percent of the way back to T-zero
. . . It's something that's completely different" says lead scientist Alan Stern.

Other scientists agree and hope that Ultima Thule the most distant celestial object ever explored, a pristine ice and rock relic from the beginnings of the solar system, will eventually give up its secrets and shed a light on its origins.

t will take upward of 20 months for all the captured data from the flyby to be downloaded to Earth—the probe transmits at a glacial speed of only 1 kilobit per second.

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