Pluto

Introduction

Discovered in 1930, in the constellation Gemini, by Clyde Tombaugh, this object has had its status as a planet questioned several times the past years. Is Pluto a planet, or is it simply a larger object in the Kuiper-belt? From this questioning rises another question, how large does an object have to be to considered as a planet?
The border has been somewhat diffuse and it needs more clarification. Some would say that when an object is large enough to let its gravity shape it to a planet is enough (about 360 km, depending on the density of the object).

Nevertheless, Pluto is still today considered to be a planet. Some would even call it a double planet since its companion, Charon is so large compared to it. Its about half the size of Pluto and is tidally locked. That is, it spins around its axis (which is one day on charon) in exactly the same time as it completes one orbit around Pluto. Hence it always shows the same face towards the planet. The average distance between the two bodies is about 19 640 km and Charon completes one orbit around Pluto in 6 days 9 hours and 17 minutes.

Quick links

Exploring Pluto (and the Kuiper belt)
Physical characteristics
Pluto's discovery

Charon - Pluto's moon

Pluto's discovery

When studying the orbit of Neptune, astronomers came to the conclusion that there must be yet another planet behind it that is tugging on it. A search for a new planet (sometimes called 'Planet X') had begun.
In 1930, while studying some photographs taken with a few days separation (January 23 - 29), Clyde Tombaugh saw one object moving. He concluded that it must be a planet. Later, it has been found that not only is Pluto smaller than all planets, but it is actually so small that 7 of the solar system moons outsize it (equatorial radius: 1 151 km). The path of Voyager 2 while at Neptune showed that Neptune actually had less mass than was previously thought, which eliminates the "need" for a planet which is tugging on Neptune. Thus the discovery of Pluto was purely coincidental.

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Physical characteristics

As mentioned earlier, Pluto is a very small planet, smaller than any other known in the solar system. It orbits the sun at an average distance of 5.9 billion km. Though the orbit is so eccentric, that during a period of 20 years Pluto is within the orbit of Neptune, making Neptune the planet furthest away from the sun. The last period was 1979 - 1999.
Pluto orbits in a 3:2 orbital resonance with Neptune, which means that for every 2 orbits Pluto makes, Neptune makes 3.
When Pluto is at perihelion (closest to the sun) it has an atmosphere (which was detected when it passed infront of a star), but as it gradually moves further away from the sun, the atmosphere freezes to ice and falls down to the surface. The atmosphere consists of (probably) nitrogen and carbon monoxide, which is in equilibrium with the solid nitrogen and carbon monoxide ices present on the surface.
The pressure of the atmosphere may be between 0.15 - 0.30 Pascal.

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Exploring Pluto (and the Kuiper belt)

The solar system is divided in three parts (the sun excluded): the inner rocky planets (Mercury, Venus, Earth and Mars), the gas giants (Jupiter, Saturn, Uranus and Neptune), and the outermost part: The Kuiper belt objects , which Pluto is a part of. This region of the solar system is left vastly unexplored till today. It contains small icy objects that are the material from which the solar system was created (and, to an extent, from which life was created on Earth). Also, the Kuiper belt contains comets that impact the Earth from time to time. Gaining an understanding of this region could give us more safety.

Till this date, no spacecraft has been sent to Pluto. When Voyager 2 had visited the gas giants, it left Pluto unexplored and headed for interstellar space. It is important to send a probe to explore Pluto in the near future, or we might have to wait for many years before another economic launch window opens up again. For this reason, NASA is planning on sending a spaceprobe, New Horizons in January 2006. It is scheduled to reach Pluto in July 2015. Its mission will be to:

1. Map surface composition of Pluto and Charon.
2. Characterize geology and morphology ("the look") of Pluto and Charon.
3. Characterize the neutral atmosphere of Pluto and its escape rate.
4. Search for an atmosphere around Charon.
5. Map surface temperatures on Pluto and Charon.
6. Search for rings and additional satellites around Pluto.
7. PLUS... conduct similar investigations of one or more Kuiper Belt Objects.

More: New Horizons

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Charon - Pluto's moon

For many decades, Pluto's mass was unknown. It took the discovery of a moon accompanying it to determine its mass with Newton's formulation of Kepler's third law. Charon as the moon was named, was discovered by James W. Christy in July 2, 1978.
Charon's surface, which is mostly covered with water ice is different from Pluto's surface, which is covered with nitrogen ice. They also have a different density: Pluto: 1.8-2.1 g/cm^3, Charon: 1.2-1.3 g/cm^3. The fact that Pluto's axis of rotation and Charon's orbit are highly inclined to the equatorial plane, in combination with the difference in density and surface composition makes some astronomers believe that not only did they both form independently, but that they formed through a similar impact/collision which created the moon/earth. The initial size would have been about 1 600 - 2 000 km in diameter for each object, according to Dr. Robin Canup (assistant director of Southwest Research Institute's® Department of Space Studies). This would also explain why the relative (to Pluto) size of Charon is so large: Our moon's mass if 1% of the Earth's, while Charon's mass is 10-15% of Pluto's.

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Moons: Charon.
Average distance from sun: 5 906 380 000 km.
Equatorial Radius: 1 151 km.
Mass: 1.3 x 10^22 kg, 0.0022% of the earth.
Density: 2 g/cm^3.
Escape Velocity: 1.270 km/s.
Length of day: 6.387 Earth days.
Length of year: 247.92 Earth years.
Mean Orbit Velocity: 4.749 km/s.
Equatorial Inclination to Orbit: 119.61 degrees.
Min./Max. Surface Temperature: -233/-223 °C.
Atmosphere: Methane and nitrogen.
Misc: Discovered by Clyde Tombaugh in 1930

Source: NASA.

Space Art 1: Pluto and Charon. This is a hypothetic view. At their orbit, the light would actually be very dim, and the sun would appear only as a bright star.