A date with Pluto

Ken Tapping, July 8th, 2015

In the sky this week…

  • Venus and Jupiter lie low in the western sky after sunset. Venus is the brightest.
  • Saturn is in the southern sky during the night.
  • The Moon will reach Last Quarter on the 8th.

On 14 July, after a journey taking more than nine years, the New Horizons spacecraft will pass close to the planet Pluto. We will get our first detailed views of that world and its moons. Then the spacecraft will continue on its journey outward, passing by some more distant objects, and then proceed onward into interstellar space.

Pluto was discovered in 1930, as a tiny dot that changed position in the interval between two pictures of the same area of sky. Since then our views of what we used to call the outermost planet in the Solar System never got much better than that. The best images showed a fuzzy disc with darker and lighter blotches on it. That was enough to establish that Pluto has a diameter of about 1100 km, about a quarter of the diameter of the planet Mercury and a third of the diameter of our Moon. In 1978 a moon was discovered, and named Charon, a suitable attendant for the god of the underworld. Since then additional moons have been found, and named Nix, Hydra, Kerberos and Styx. However, Pluto is so far away the only way to learn much more is to have a closer look.

As information accumulated about Pluto, it became clear that there are things casting doubt on it being a planet like the others. Firstly, all the other planets have their orbits lying more or less in the same plane, like marbles rolling on a plate. Pluto’s orbit is a distinct angle to that plane. Secondly all the other planets follow paths around the Sun that do not cross the paths of other planets; they all stay in their own lanes. Pluto on the other hand has an orbit that crosses the orbit of the planet Neptune; its orbit is unusually elongated; its distance from the Sun varies from 29 to 49 times the Earth’s distance from the Sun. Pluto takes almost 250 years to complete one orbit – a Plutonian Year.

The Earth’s orbit is nearly circular, and the seasons are due to our planet’s axis of rotation being inclined, so that in the Northern summer, the Northern Hemisphere is leaning towards the Sun. That means the Southern Hemisphere is leaning away from the Sun, making it winter there. Six months later the situation is reversed. Pluto’s situation is very different from ours. Its seasons are largely driven by its changing distance from the Sun. When it’s at its greatest distance from the Sun its surface temperature is about -240 C. Half a Plutonian Year later, which is 125 of our years, it is at its closest to the Sun, and the temperature rises to a not-very-balmy -220 C. At those temperatures the main gases in our atmosphere would be permanently frozen solid. It has been suggested that Pluto’s surface rocks are made up mainly of solid nitrogen, with some carbon monoxide and methane. There is a thin atmospheric made up of these gases, giving the world a hazy horizon.

Back in the 1930’s, Pluto was believed to be the outermost planet in the Solar System, with the possibility that at some point even more distant planets would be discovered. However now we believe that beyond Neptune the objects fall into a different class. They are small, icy objects that never got to coagulate into planets. Their region of the Solar System is known as the Kuiper Belt, and the bodies known as Kuiper Belt Objects. These are chunks of the original building material used to make the planets, which is why we want a closer look at some of these objects.

Imagine Pluto’s rocky landscape, dimly lit by the distant Sun, which just looks like a particularly bright star. The sky is black and filled with stars. The horizon is hazy. The Earth is so far away it is lost in the Sun’s glare. It is unbelievably cold. That is what our best scientific results and imaginations can do for us at the moment. After 14 July we will have a better picture of what it’s like on Pluto, the most distant object we have yet visited.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mailken.tapping@nrc-cnrc.gc.ca

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