Pluto has the largest, most eccentric and most highly-inclined orbit of any of the known planets. Because of this large eccentricity, Pluto, when near perihelion, is actually closer to the Sun than Neptune can ever be; this will next occur in 1989. The highly-inclined orbit takes Pluto as much as a billion and a quarter kilometers above the plane of the ecliptic – a distance almost as great as Saturn’s average distance from the Sun. Because of this there is no chance of a catastrophic collision with Neptune. Calculations of the motions of Neptune and Pluto in their orbits have been made taking account of the perturbations which result from their mutual gravitational attraction. These calculations show that Neptune and Pluto never come closer together than 16.7 astronomical units even though the closest points in the two orbits are much nearer to each other than this. It is interesting to note that this minimum distance between Pluto and Neptune is more than that between Pluto and Uranus, which is 10.6 astronomical units.

R.A.Lyttleton has suggested that Pluto may be an escaped satellite of Neptune. The orbit of Neptune’s satellite, Triton, is unusual for such a large satellite as it is highly inclined and Triton’s orbital motion is retrograde. Lyttleton and G.P.Kuiper have sug¬gested that Pluto and Triton were both in orbit around Neptune until an interaction took place that sent Pluto into orbit around the Sun and Triton into its present orbit. This theory is not wholly satisfactory since it is difficult to see how Pluto could end up in an orbit which does not closely approach that of Neptune.

The angular diameter of Pluto is only about a quarter of a second of arc which corresponds to a diameter of 5860km, but this could be in error by as much as 50 per cent. A more accurate value could be obtained if Pluto occulted a star. Such an occultation would be a rare event and none has been observed; a near miss occurred in 1965 and this showed that Pluto’s diameter is at most 6800km.

The mass is obtained by measuring Pluto’s perturbations of Neptune. These are very small because of Pluto’s low mass and hence are difficult to measure. Although Pluto was only discovered in 1930, earlier observations of what were thought to be stars have now been shown to have been prediscovery records of Pluto. These date back to 1846 but since then Pluto has made only half a revolution of the Sun, and satisfactory mass determinations cannot be made from less than a complete orbit. Account must also be taken of the perturbations of other planets, particularly Saturn and Uranus, and a small error in the mass of either of these would cause a large error in the derived mass of Pluto. The best value for the mass of Pluto is 0.11 times that of the Earth but this is subject to a very large uncertainty. This mass, together with a diameter of 5860 km gives a density of 6200kgm-3, but-this value is also highly uncertain.

The rotation period of Pluto was first measured in 1955 by observing the regular changes in its brightness. The latest value for the period is 0.3874 days. This period is quite constant but the mean magnitude and the range both vary. Over a 20-year period the mean V magnitude, after allowing for the varying distances from the Earth and the Sun, has decreased by 0.20 and the amplitude of the variations has increased from 0.11 to 0.22 magnitudes. If Pluto has bright polar caps, a patchy equatorial region and a highly-inclined rotation axis, then such a change would result as Pluto’s motion around its orbit caused it to present a varying aspect to the Earth.

No atmosphere has been detected on Pluto and it may not have one. The surface temperature must be around 40 or 50K and at such a low temperature molecules such as carbon dioxide, water and ammonia would lie fro/en on the surface. The surface gravity is probably sufficiently low for the light gases hydrogen and helium to escape from the planet altogether. Minute amounts of gaseous methane or nitrogen could be present but these would be almost entirely frozen on the surface. The heavier inert gases such as neon and argon could form a permanent atmosphere but they are difficult to detect spectroscopically.

Because the radius of Pluto is so poorly known, it is pointless to convert its brightness at various wavelengths into albedo values. The measurements do show that Pluto reflects more of the incident red and blue light than of the green light. Similarly the lack of accurate knowledge of the density means that no meaningful studies of the internal structure are possible. A large magnetic field is unlikely, since Pluto is small and rotates slowly. The slow rotation will also result in considerable differences between day-side and night-side temperatures with a maximum of perhaps 50 K.

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