Plutos Moon And Mass

When Pluto was spotted, after such a long quest, it hit the headlines worldwide. From its apparent brightness (or perhaps we should say its "faintness") it was obvious Pluto must be small, and some of the euphoria abated. From time to time astronomers would again turn their telescopes towards Pluto, but it was hardly in the news again until 1978 when it was found to have a moon of its own. That moon was given the name Charon (see Figure 14-1). It is about 730 miles in diameter, around half the size of Pluto itself.

FIGURE 14-1. Images of Pluto and its moon, Charon. Ground-based observations demonstrated the latter's existence (top left), but they are only well separated by the Hubble Space Telescope (top right). Between 1985 and 1990 our edge-on alignment to their mutual orbital plane (bottom) led to repeated eclipses, allowing the sizes and combined mass of the two to be evaluated.

FIGURE 14-1. Images of Pluto and its moon, Charon. Ground-based observations demonstrated the latter's existence (top left), but they are only well separated by the Hubble Space Telescope (top right). Between 1985 and 1990 our edge-on alignment to their mutual orbital plane (bottom) led to repeated eclipses, allowing the sizes and combined mass of the two to be evaluated.

The finding of Charon was nice in itself, but also quite a handy thing because it allowed astronomers to determine Pluto's mass properly. Until then all that could be done was the definition of limits on its bulk from various perspectives. Taking characteristic values for the average albedo (the fraction of sunlight reflected), size limits could be calculated. Then, assuming Pluto to be made of rock, or of ice, or of a mixture, possible values for its mass could be calculated. Similarly a maximum value for its mass could be ascribed through the lack of major perturbations of the paths of the outer planets.

To get a better evaluation a probe is needed. That probe might be natural, or artificial. Take the case ofJupiter. The time that the four Galilean satellites take to circuit that planet can be measured, and also the sizes of their orbits. Those two pieces of information—orbit size, orbital period—make it possible to derive the mass ofJupiter, using Kepler's laws of orbital motion. Even if only one such moon existed, the Jovian mass could still be found. Having those four bright satellites makes it a cinch, because you can compare the values obtained using each of them and look for consistency in the result.

Saturn was studied in the same way, especially through its large moon Titan. When the tiny Martian moons Phobos and Deimos were identified in the late nineteenth century it became feasible to reckon the mass of the red planet.

Unfortunately, Venus and Mercury presented long-standing problems because neither has a natural satellite. Using the magnitudes of their mutual orbital perturbations, limits had been placed on their masses, in line with those expected given densities characteristic for rocky bodies with iron cores. Better evaluations awaited visits by space probe to those planets. The mass ofVenus was determined in the 1960s through radio tracking of various spacecraft. Mercury had to wait until the mid-1970s, when NASA's Mariner 10 satellite flew past it three times.

When Charon was discovered it was at last possible to have a stab at Pluto's mass, but the situation was complicated. Firstly, the two objects are close to each other, and far from the Earth. Measuring their separation was therefore extremely difficult, especially through the blurring effect of our atmosphere, although the Hubble Space Telescope improved matters (compare the two upper images in Figure 14-1). Secondly, with a mass ratio of about eight to one, the Pluto—Charon system represents a binary planet (as discussed in the Appendix). Because of these factors, interpreting the orbits to get the mass of Pluto presents difficulties.

The situation was saved by the study of their mutual eclipses. Pluto and Charon rotate every six days about their barycenter with a separation of around 12,200 miles, like a cosmic dumbbell. The scale involved is shown by the lower image in Figure 14-1. The orientation of their axis of rotation is preserved, analogous to a gigantic gyroscope, meaning that at certain times we look edge-on along the plane of their mutual orbit. This means that eclipses will occur, Charon first skimming in front of Pluto, a little over three days later passing behind it. Given the sizes of the objects, one can calculate that such sequences of eclipses will last for about five years, but in episodes separated by 124 years (half the time it takes the Pluto—Charon pair to orbit the Sun).

By a great stroke of fortune, Charon's discovery came with perfect timing, just as a five-year eclipse sequence was about to commence. This ran from 1985 to 1990, allowing astronomers to observe these events and determine a great deal about the double planet. The total intensity of light received by our telescopes was found to drop off by about 20 percent when an eclipse occurred, Charon either dipping behind Pluto or covering part of its disk. The accurate timing of the way in which the brightness varied made it possible to calculate their mutual orbits, and so their combined mass. (Note that I wrote combined mass there, because even these observations have ambiguities, and do not render the individual masses with precision. Charon's bulk seems to lie somewhere between 8 and 16 percent that of Pluto, but we cannot be sure.)

A better idea of the pair's characteristics is unlikely to be obtained until the first spacecraft arrives there. A probe called Pluto Express is on the drawing board. It would certainly need to be an express, using a gravity assist from Jupiter to speed it on its way, because a slow trajectory to distant Pluto would mean that the scientists involved in its planning would have retired before their progeny arrives at its destination. At the time of writing it seems unlikely that we will deliver any space probe to Pluto before 2016.

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