Pluto 545
TABLE 2 Charon’s OrbitaOrbital Element ValueSemimajor axis,a 19636 ±8km
Orbital period,P 6.387223±0.00002 days
Eccentricity,e 0.0076±0.003
Inclination,i 96.2±0.3◦
Longitudinal perihelion,ω 222.99±0.5◦
Mean anomaly,M 34.84±0.35◦aThese elements are referred to the epoch 17 January 13.0 UT 1993.
See D. J. Tholen and M. W. Buie, 1997, in “Pluto & Charon” (S. A.
Stern and D. J. Tholen, eds.), Univ. Arizona Press, Tucson.determination ofa=19,636±8 km derived from ground-
based andHubble Space Telescopedata; it is statistically
indistinguishable from ground-based results obtained in
the mid-1980s ofa=19,640±320 and ofa=19,558±
153 km.
Based on Charon’s known orbital period and the 19,636
km semimajor axis, the system’s (i.e., combined Pluto+
Charon) mass is 1.47±0.002× 1025 g; this is very small,
just 2.4× 10 −^3 MEarth.
Data from the mutual events showed that unless
Charon’s orbit has a very special orientation relative to
Earth, Charon’s orbital eccentricity is very low. Recently,
HSTobservations have shown that Charon’s orbital eccen-
tricity is nonzero, with a best-estimated value of 0.0076.
The fact that the orbit is not precisely circular indicates
some disequilibrium forces have disturbed it from the zero
value expected from tidal evolution. It is most likely that the
disturbance causing this is generated by occasional close
encounters between the Pluto–Charon system and 100-km
class Kuiper Belt objects (see Section 8), but it may also be
related to perturbations by Nix and Hydra.
3. The Mutual Events
3.1 Background
After Charon’s discovery, the realization that mutual
eclipses between Pluto and Charon would soon occur
opened up the possibility of studying the Pluto–Charon sys-
tem with the powerful data analysis techniques developed
for eclipses between binary stars. Initial predictions by Leif
Andersson indicated that the events could begin as early as
- As Charon’s orbit pole position was refined, however,
the predicted onset date moved to 1983–1986 (this was for-
tuitous because knowledge of the pole could have changed
to indicate that the events had already just ended in the
mid-1970s!). After a multiyear effort by several groups to
detect the onset of these events, the first definitive eclipse
detections were made on 17 February 1985 by Richard
Binzel at McDonald Observatory and were confirmed dur-
ing an event 3.2 days later on 20 February 1985 by David
Tholen at Mauna Kea. These first, shallow events (∼0.01–
0.02% in depth) revealed Pluto and Charon grazing across
one another as seen from Earth.
The very existence of these eclipses proved the hypoth-
esis (by 1985 widely accepted) that Charon was in fact a
satellite, rather than some incredible topographic high on
Pluto. The mutual eclipses persisted until October 1990,
and dozens of events were observed. Important results from
the 1985–1990 mutual events included reconstructed sur-
face “maps” of Pluto and Charon; individual albedos, colors,
and spectra for each object; and improvements in Charon’s
orbit. First, however, was the opportunity to use event tim-
ing to accurately determine the radii of Pluto and Charon.3.2 Radii and Average Density of Pluto and Charon
Prior to the mutual events, the radii of Pluto and Charon
were highly uncertain. Because Pluto and Charon remained
unresolved in terrestrial telescopes (their apparent diame-
ters are each<0.1 arcsec), direct measurements of their
diameters were not available. A well-observed, near-miss
occultation of Pluto in 1965 had constrained Pluto’s radius
to be<3400 km, but no better observations were available
until the mutual events. However, circumstantial evidence
that Pluto was smaller than 3400 km was inferred from the
combination of Pluto’sVastronomical≈14 magnitude and
the 1976 discovery of CH 4 frost (see Section 4.3), which
exhibits an intrinsically high albedo. The small system mass
determined after the discovery of Charon in 1978 strength-
ened this inference, but Pluto’s radius was still uncertain
within the bounds 900–2200 km.
The first concrete data to remedy the situation came
when a fortuitousstellar occultationby Charon was ob-
served on 07 April 1980. The 50 second length of the star’s
disappearance, observed by a 1 m telescope at Sutherland,
South Africa, gave a value for Charon’s radius of 605±
20 km. Improved results from subsequent stellar occulta-
tions yield 603.5±3 km.
As noted earlier, accurate radius measurements for Pluto
resulted from both stellar occultations and from fits of mu-
tual event lightcurves, yielding solutions between 1150 and
1200 km. The range of uncertainty in Pluto’s radius, which is
significantly larger than in Charon’s, results primarily from
uncertainties in Pluto’s atmospheric depth.
The two striking implications of the small radii and com-
parable masses of Pluto and Charon are (1) that Pluto is a
very small planet—even smaller than the seven largest plan-
etary satellites (the Moon, Io, Europa, Ganymede, Callisto,
Titan, and Triton), and (2) that Pluto and Charon form the
only known example of a binary planet (with the system
barycenter outside of Pluto). Based on the radii and the
total mass of the pair, it is possible to derive an average