544 Encyclopedia of the Solar System
is called “orbital chaos” by modern dynamicists. This dis-
covery of a formal kind of chaos in Pluto’s orbit does not
imply Pluto undergoes frequent, dramatic changes. How-
ever, it does mean that Pluto’s position is unpredictable on
very long timescales. The timescale for this dynamical un-
predictability has been established to be 2× 107 years by
Jack Wisdom, Gerald Sussman, and their co-workers.
2.2 Pluto’s Lightcurve, Rotation Period, and
Pole Direction
As previously indicated, since the mid-1950s Pluto’s pho-
tometric brightness has been known to vary regularly with
a period of about 6.387 days; more precisely, this period
is 6.387223 days. Despite Pluto’s faintness as seen from
Earth, its period was easily determined using photoelectric
techniques because the planet displays a large lightcurve
amplitude, 0.35 magnitudes at visible wavelengths, which
is equivalent to 38%.
Since at least 1955, it has also been known that Pluto’s
lightcurve is exhibiting an increase in its amplitude with
time. Although the 6.387223 day period is identical to
Charon’s orbit period, Charon’s photometric contribution
is too small to account for the lightcurve’s amplitude. This
in turn implies that the lightcurve’s structure is caused by
surface features on Pluto. Figure 2 shows the shape of the
combined Pluto–Charon lightcurve and its evolution over
the past few decades.
The first study of Pluto’s polar obliquity (or tilt relative to
its orbit plane) was reported in 1973. By assuming that the
variation of the lightcurve amplitude from the 1950s to the
early 1970s was caused by a change in the aspect angle from
which we see Pluto’s spin vector from Earth, it was then de-
termined that Pluto has a high obliquity (i.e., 90± 40 ◦). In
1983, additional observations allowed the obliquity to be re-
fined to 118.5± 4 ◦. Even more recently, the results of the
Pluto–Charon mutual events (or eclipses, see following dis-
cussion) have given a very accurate value of 122± 1 ◦; Pluto’s
corresponding pole position lies near declination − 9 ◦,
right ascension 312◦(equator and equinox of 1950).
It is important to note, however, that torques on the
Pluto–Charon pair cause Pluto’s obliquity to oscillate be-
tween∼ 105 ◦and∼ 130 ◦with an∼3.7× 106 year period.
Thus, although Pluto presently reaches perihelion with its
pole vector nearly normal to the Sun and roughly coinci-
dent with the orbit velocity vector, this configuration is only
coincidental. The pole position executes a 360◦circulation
with a 3.7× 106 year precession period.
2.3 Charon’s Orbit and the System Mass
The discovery that Charon orbits Pluto with a period equal
to Pluto’s rotation period immediately implied the pair has
FIGURE 2 The evolution of Pluto–Charon’s lightcurve over
several decades. (Adapted from R. L. Marcialis, 1988,Astronom.
J. 95 , 941.)reached spin-orbit synchronicity. This is an unprecedented
situation among the planets in the solar system.
Table 2 gives a solution to Charon’s orbital elements ob-
tained from various data. This fit relies on a semimajor axis