Kuiper Belt Objects: Physical Studies 617
FIGURE 13 Optical spectrum of 136472 (2005 FY 9 ) (black line)
and a Hapke model of pure CH 4 -ice (red line). The CH 4
absorption bands of 136472 (2005 FY 9 ) are blue shifted by 3.25
±2.25A relative to the pure CH ̊ 4 model indicating the presence
of another molecular ice, possibly N 2 , CO, or Ar.
of Pluto, implying that the abundance of CH 4 on the surface
of 136472 (2005 FY 9 ) could be higher than on the surface of
Pluto. This author finds the CH 4 -ice bands in his spectrum
and Javier Licandro’s spectrum of 136472 (2005 FY 9 ) are
blueshifted by 3.25A relative to the positions of pure CH ̊ 4 -
ice bands (Figure 13). Such a shift suggests the presence
of another ice component on the surface of 136472 (2005
FY 9 ), possibly N 2 -ice, CO-ice, or Ar. In addition, Licandro
finds CH 4 -ice bands blueshifted in a spectrum of Eris.
It is odd that some KBOs exhibit strong CH 4 bands and
others exhibit strong H 2 0 bands. Pluto and Charon are part
of the same system, yet they exhibit very different spectra.
Perhaps the difference is due to Pluto’s size, it may have ex-
perienced some form of methane ice volcanism. In the end,
we may find only the largest KBOs exhibit CH 4 -ice bands.
Eris, Pluto, and possibly 2005 FY 9 are the three largest
KBOs and they all exhibit CH 4 -ice bands.
10. KBO Binaries
In 2001, Christian Veillet announced the discovery of two
components to the KBO 1998 WW 31. Over the next few
years, Keith Noll used the superior imaging resolution of
HST to observe 122 KBOs for additional binaries. His sur-
vey was sensitive to binaries with separations≥0.15 arc
sec and a magnitude difference between components≤ 1
magnitude. Noll discovered six more binaries. Currently,
22 KBO binaries are known (Table 4).
10.1 System Mass
Two KBOs of a binary pair revolve about their common
center of mass. However, it is far more convenient to ob-
serve the position of the fainter of the two components as it
makes a complete revolution about the brighter component
on the plane of the sky, i.e., to observe the apparent rela-
tive orbit. Figure 14 illustrates the apparent relative orbit
of 1998 WW 31. The true orbit of the KBO binary system
will not happen to lie exactly in the plane of the sky. Hence,
the apparent relative orbit is merely a projection of the true
relative orbit onto the plane of the sky. Techniques exist to
determine the inclination of the true orbit relative to the
plane of the sky. Once the period of revolution, P, and the
semimajor axis, a, of the true relative orbit are known, it
is possible to use Kepler’s Third Law to calculate the com-
bined mass of the binary system,m 1 +m 2 =4 π^2 a^3
GP^2.From the HST observations in Figure 14, Veillet and Noll
found that 1998 WW 31 has a true relative orbit with a semi-
major axis of 22,300 km, an eccentricity of 0.8, and a period
of revolution of the fainter component about the brighter
component of 574 days. The 1998 WW 31 system has a
combined mass of 2. 7 × 1018 kg, much smaller than the
Pluto-Charon system combined mass of 1. 46 × 1022 kg. Ta-
ble 4 lists the true relative orbital properties and combined
masses for the better studied binary systems.10.2 Mutual Events
Between 1985 and 1990, Pluto’s orbital motion about
the Sun caused the Pluto-Charon orbital plane to sweep
through the line of sight to the Earth. As a result, mu-
tual eclipses (also known as mutual events) occurred every
3.2 days (half of Charon’s orbital period). Because of the
mutual events, observers were able to accurately measure
diameters of 2302±12 km and 1186±26 km for Pluto and
Charon, and with the total mass of the binary, they were
able to derive an average density for the system of 1.95±
0.10 g cm−^3.
A key objective of current binary KBO work is to discover
as many binaries as possible and to determine their orbits
sufficiently well to predict when the onset of mutual events
will occur. By observing KBO mutual events, we will ob-
tain radii and density measurements that only a spacecraft
encounter could improve upon. At present, no KBO binary
orbit (other than Pluto and Charon) is known well enough
to predict the onset of a mutual event with confidence.10.3 Origin of KBO Binaries
Two of the most unusual features of KBO binaries, com-
pared to main belt asteroid and near-Earth asteroid binaries,