Kuiper Belt Objects: Physical Studies 619
of wide binary pairs, and Goldreich’s mechanism favors the
production of closer pairs. Only the discovery of many more
binaries will allow us to determine whether either of these
mechanisms or some other mechanism is responsible for
the formation of KBO binaries.
11. Mass of the Kuiper Belt
What’s the mass of the entire Kuiper Belt? Gary Bernstein
combined his HST survey for the faintest KBOs with
ground-based telescope surveys for brighter KBOs, and as-
sumed KBOs have an albedo of 0.04 and a density of 1 g
cm−^3 , to estimate a Kuiper Belt mass of∼3 percent of the
Earth’s mass, or about 14 times the mass of Pluto. A major
source of uncertainty in his mass estimate is the uncertainty
in the albedos and densities of KBOs.
It appears that the Kuiper Belt did not always have a mass
of∼3 percent of the Earth’s mass. Specifically, the present
number of KBOs per AU^3 is too small to grow KBOs larger
than∼100 km in diameter by accretion in less time than the
age of the Solar System. Since 1000-km sized KBOs exist, it
is likely that the Kuiper Belt initially had many more KBOs
per AU^3 than today. Calculations by Alan Stern suggest that
the initial Kuiper Belt probably had a mass ten times the
mass of the Earth, and as Neptune grew to a fraction of
its present size, it stirred KBOs from their initial circular
orbits to more eccentric orbits, resulting in frequent dis-
ruptive, rather than accretive collisions especially between
KBOs smaller than 40 to 60 km in diameter. These collisions
probably eroded the Kuper belt mass down to its current
value.
12. New Horizons
Because astronomers can discover and then measure the
physical properties of many KBOs, their work is important
because it gives us a global view of the Kuiper Belt and con-
text for in situ spacecraft measurements. In January of 2006,
NASA’s New Horizons spacecraft departed Earth on a jour-
ney that will culminate in the first flyby of the Pluto-Charon
system in 2015, and hopefully the first flyby of a KBO some-
time before 2020. The $500 million spacecraft weighs only
416 kg (917 lb) and has four instrument packages: (1) a
CCD camera, (2) an ultraviolet, optical, and near-infrared
imaging-spectrometer, (3) a charged particle detector, and
(4) a radio telescope.
These instrument packages will provide in-depth obser-
vations impossible with telescopes on and near the Earth.
For example, if New Horizons comes within a few thou-
sand kilometers to a KBO, it could image the surface of the
KBO with a resolution of 25 m pixel−^1. For comparison,
HST can only image a KBO at 42 AU with a resolution of
about 1200 km pixel−^1.
What kind of surface might the spacecraft image? If New
Horizons visits a small KBO, perhaps it will image a sur-
face with numerous craters, suggestive of an ancient sur-
face bombarded by other small bodies (KBOs and comets)
over the age of the Solar System? On the other hand, if
New Horizons visits a large KBO, perhaps it will see few
craters on the surface, suggestive of some process erasing
older craters. Perhaps the images of a large KBO will show
long linear features in an icy crust, and some roughly round
basins that appear flooded by liquids from the interior, much
like the Voyager spacecraft images of Triton. Perhaps the
spacecraft will catch a geyser erupting, and shooting a plume
of gas and ice above the surface.
There are some problems concerning a New Horizon’s
flyby of a KBO. The spacecraft trajectory is fixed since first it
will fly by Pluto. In addition, the spacecraft has a limited fuel
supply for adjusting its trajectory after the Pluto encounter.
At present, none of the almost 1000 currently-known KBOs
are close to the spacecraft’s trajectory. A flyby of a KBO by
New Horizons depends on discovering a candidate close to
the spacecraft’s trajectory. Perhaps New Horizons will have
enough fuel to visit one of the smaller (50 km diameter) and
more common KBOs. The chances for the spacecraft visit-
ing one of the larger (1000 km diameter) and rarer KBOs
appear slim at the moment.13. Future Work
It is likely that future work on the physical properties of
KBOs and Centaurs will be driven by future state-of-the-
art observatories. The 6-m James Webb Space Telescope
(JWST) near the L2 point will be able to obtain images and
spectra of very large numbers of KBOs and Centaurs from
0.6 to 27μm. It should be possible to measure diameters,
albedos, surface colors, and optical and infrared spectra for
many more objects than possible today. A large increase
in the number of objects with physical property measure-
ments will make it possible to look for statistically significant
correlations between many more physical properties than
possible with today’s telescopes, and thereby better con-
strain the important formation and evolution mechanisms
in the outer Solar System.
Large ground-based telescopes of the future will likely
play a big role in the field too. For example, the Giant Mag-
ellan Telescope (GMT), a configuration of six off-axis 8.4-
m mirror segments around a central on-axis segment that
is equivalent to a filled aperture 21.4 meters in diameter,
and the Thirty Meter Telescope (TMT), a configuration of
more than 700 hexagonal-shaped mirror segments that is
equivalent to a filled aperture 30 meters in diameter, will
make it possible to obtain higher signal precision optical
and infrared spectra than possible with current 10 meter
telescopes. Better spectra and models will make it possi-
ble to map surface concentration of ices (e.g., the CH 4 /N 2