The Sky at Night pre­sen­ter looks back at Pluto’s past ahead of its meet­ing with the New Hori­zons probe

Sky at Night Magazine - - A PASSION FOR SPACE - With Mag­gie Aderin-Po­cock Mag­gie Aderin-Po­cock is a space sci­en­tist and co-pre­sen­ter of The Sky at Night

With New Hori­zons rapidly reach­ing its point of clos­est ap­proach to Pluto, it seems a good time for an overview of the dwarf planet’s fas­ci­nat­ing his­tory to date. First known as the hy­po­thet­i­cal Planet X, Pluto’s ex­is­tence was orig­i­nally in­ferred from grav­i­ta­tional per­tur­ba­tions on the or­bits of Nep­tune and Uranus. It was dis­cov­ered, and hailed as a planet, in 1930 by Clyde Tom­baugh. How­ever, es­ti­mates of Pluto’s pro­por­tions were too high; they have been plum­met­ing over the cen­tury and have now set­tled at around one-sixth the mass of the Moon and one-third its vol­ume. At this mass it is un­likely that any or­bital ef­fects would have been ob­served, so it seems that Pluto was ac­tu­ally dis­cov­ered by ac­ci­dent.

Af­ter dis­cov­ery came nam­ing, and a com­pe­ti­tion was set up. It was won by 11-year-old Vene­tia Bur­ney, who sug­gested it should be named af­ter the king of the un­der­world in clas­si­cal mythol­ogy. The dis­cov­ery gen­er­ated much pub­lic in­ter­est and it is claimed that Dis­ney named Mickey Mouse’s ca­nine com­pan­ion Pluto af­ter it.

Ob­ser­va­tions of Pluto have been few and far be­tween. This world can lie 7.5 bil­lion km from Earth, and the align­ment didn’t work for a visit from ei­ther of the Voy­ager space­craft. In 1978 pho­to­graphic plates re­vealed the pres­ence of Charon, Pluto’s largest moon; a com­pan­ion so large that the pair could be con­sid­ered a bi­nary sys­tem. It was not un­til March 1996, some 66 years af­ter the planet’s dis­cov­ery, that the Hub­ble Space Tele­scope was able to take a semi-de­tailed im­age of Pluto’s sur­face.

De­tails trickle in

Hub­ble’s imag­ing cam­paign was con­ducted over 6.4 days – a full ro­ta­tion of the Pluto. The im­ages re­vealed po­lar caps and dark patches over the sur­face. Pluto’s moons Nix and Hydra were both dis­cov­ered in 2005, also us­ing the Hub­ble Space Tele­scope. In Jan­uary 2006 the New Hori­zons space probe was launched to fly past Pluto, then the only world in our So­lar Sys­tem that had yet to be have a robotic vis­i­tor from Earth. In the same year, Pluto was de­moted from planet to dwarf planet – a de­ci­sion that has stuck, though some de­bate still con­tin­ues.

The ob­jec­tives of the New Hori­zons mis­sion re­mained the same: to char­ac­terise and an­a­lyse the sur­face, ge­ol­ogy and at­mos­phere of both Pluto and Charon. It will then look for ad­di­tional satel­lites be­yond the five we now know about be­fore pre­form­ing sim­i­lar tests on other bod­ies in the Kuiper Belt. But as well as look­ing for­ward, New Hori­zons has paid homage to the past. On board the space­craft is an ounce of the ashes of Pluto’s dis­cov­erer Clyde Tom­baugh.

In the near fu­ture we will have a much bet­ter un­der­stand­ing of Pluto and its com­pan­ions, but for now I’m look­ing for­ward to shar­ing the ex­cite­ment of this his­toric mo­ment in an hour-long Sky at Night spe­cial re­port (BBC Four, 20 July, 10pm) di­rect from New Hori­zons mis­sion con­trol dur­ing the flyby.

Hub­ble has re­vealed sea­sonal changes on this far-flung world

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