July 2015 – Pluto and the New Horizons Spacecraft

Heavens Above!

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to highlight a particular phenomenon in this month’s night sky. Last month, we showcased the the great observatories of space. This month, we will discuss the flyby of the planet pluto by the New Horizons spacecraft.

The Realm of Pluto

The only major body to be added to our list of planets during the 20th century, Pluto was first observed by the late astronomer Clyde Tombaugh during his term at the Lowell Observatory in the American southwest. The notion of a planet beyond Neptune had been suggested as early as the 1840’s due to its peculiar motion, but until Tombough’s observations in February of 1930 no further planet had been found. Coming from the Greco-Roman mythology, Pluto was the god of the underworld – fitting for such a far-off place. Pluto remained a planet until 2006, it was downgraded to a dwarf planet.

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Fig 1. Pluto discovery plates (credit: Lowell Observatory Archives)

The ruling was made by the International Astronomical Union, which is the highest administration for the field of astronomy. Now this decision wasn’t without good reason, and I hope to convince you why. Pluto is unlike the other planets. For starters, it’s more like an asteroid than a planet. The inner planets (Mercury, Venus, Earth, and Mars) all have rocky surfaces and are fairly small. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are essentially giant globes of cold gas. Pluto has the characteristics of an inner planet, but the location of an outer planet – which causes a bit of a stir!

As it turns out, this is not the only strike against Pluto. In fact, we know of dozens of similar objects scattered throughout the solar system. For example, the dwarf planet Ceres which orbits between Mars and Jupiter in the asteroid belt. Pluto itself has moons which could just as equally be called dwarf planets. When the International Astronomical Union met to decide the fate of Pluto, nine planets was never on the table. It was either eight planets … or fifteen and growing. They laid down their decision, by stating that Pluto is not a planet namely because it has not “cleared the neighbourhood” around its orbit – AKA it still has asteroids and debris nearby – something with the Earth and the rest of the planets do not.

New Horizons

Nearly a century later, we still have a very tenuous understanding of this mysterious and far-off world. Our best photographic efforts have returned hardly a clue. In fact, if you were to look at your own hand with the resolution of the best image of Pluto that we have, you would struggle to make out the spaces between your fingers!

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Fig 2. Maps of Pluto (credit: Hubble Space Telescope)

In 2001, our journey to Pluto began. NASA had green-lighted the New Horizons project with the hopes of a flyby of the Plutonian surface. Five years later, an Atlas V rocket launched the probe off towards the far reaches of the solar system at roughly 59,000 km/h – the fastest spacecraft ever launched. Indeed, it has taken ten years and 7,500,000,000 kilometres for New Horizons to reach Pluto (and watch it be demoted along the journey!).

And just now, in July 2015, the New Horizons probe made its maiden pass of the the dwarf planet Pluto, carrying aboard it seven scientific instruments … and a small portion of Clyde Tomboughs ashes. The standing goals of the project are to map the surface composition of Pluto and its moon Charon, characterise it’s brief atmosphere, record surface temperatures, and report on any abnormalities and geographical features. In short, to discover Pluto for more than just a dot on a telescope lens.

So keep your eyes out for updates and releases from NASA as well as ESA. This is the first time in the history of our species when we will have mapped all “nine” planets, and known our home neighbourhood in all it’s beauty!

Coming Next Month: Astronomy of the Renaissance

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