Near-Earth Objects 289
FIGURE 7 Tisserand parameter. The solid line represents the
Tisserand parameter with a value of 3. (Graph provided by Jeff
Bytof, NASA/JPL.)
and co-workers have done extensive computer calculations
to assess the relative effectiveness of these dynamical pro-
cesses. Their calculations suggest that, when considering
NEOs of all sizes, about 15–20% of all NEOs have their
origins as comets. Nearly all of these are currently inactive,
showing no evidence of a coma or a tail. They are comets
disguised as asteroids.
Spacecraft and telescopic measurements of known
comets reveal what characteristics to look for when try-
ing to determine if a given asteroid-like NEO is a comet
in disguise. For example, the inactive surface regions of
comets Halley, Borrelly, Wild 2, and Tempel 1 are very dark
(low albedo) and have gray to reddish colors. Some other
comets go through periods of very low activity, allowing as-
tronomers to clearly see and measure the albedos and col-
ors of the nucleus. All of these measurements consistently
show low albedos (reflecting only about 4% or less of the
incoming light) and gray or reddish colors. When observed
in reflected sunlight, these objects exhibit featureless spec-
tra with no absorption bands due to olivine or pyroxene
(mineral types) on their surfaces.
Knowing the dynamical signature (Tisserand parameter,
T<3), low albedo and gray/red color, allows asteroid-like
NEOs to be identified as extinct comet candidates. A sur-
vey of nearly 50 NEOs residing in orbits havingT<3 con-
ducted by one author (RPB) shows about one half of them
exhibit the low albedo and color characteristics seen for
comet nuclei. From the surveys searching for NEOs, cor-
recting for the fact that for any given size, dark objects will
be more difficult to detect than bright ones, about 30% of
the all NEOs reside inT<3 orbits. If one half of these are
comet-like in their physical characteristics, this suggests up
to 15% of all NEOs are extinct comet candidates. Other re-
searchers find a smaller percentage of 5–15% derived from
simulations of orbital dynamics.
Until 2001 only upper limits on cometary activity were
derived for the extinct cometary candidates. Object 2001
OG 108 has an orbital period of 50 years and inclination al-
most perpendicular to the ecliptic plane, similar to that of
Comet Halley. Upon its discovery, there was no detectable
coma. At a distance of 1.4 AU, the object became active as
it passed through the inner solar system. Its bare nucleus
has the characteristics of cometary nuclei, and when close
to the Sun, it outgases like a comet.3.4 Meteor Shower Associations
The near-Earth objects 2101 Adonis and 2201 Oljato have
orbits similar to those of meteor showers. Adonis is very
difficult to observe and not much is known about it. Oljato,
also a difficult target for telescopes, has intrigued scientists
since it was first observed in 1979. The jury is still out on
whether or not this asteroid is an extinct comet, but the
evidence now seems to suggest that it is asteroidal in its
origin. One thing is certain: The object is not normal even
when considered as an asteroid.
In 1983, Fred Whipple recognized the orbital elements
of an asteroid found by an Earth-orbiting infrared telescope
to be essentially the same as the Geminid meteor shower,
which occurs in mid-December. [SeeInfraredViews of
theSolarSystem fromSpace]. There is little doubt
that this asteroid, now named 3200 Phaethon, is the parent
body of the Geminid meteors. But is Phaethon an extinct
cometary nucleus? The supposition is yes, according to one
line of thought based on similarities of orbital inclinations
and the location of perihelion (longitude of perihelion rel-
ative to the ecliptic) of asteroids and comets compared to
meteor showers. Its reflectance spectrum (see Section 5)
is unlike other comet nuclei, however. There are currently
nine NEOs that have orbital elements that, over the past
5000 years, may be associated with the path of existing me-
teor showers.3.5 Dynamical History
Dynamicists have simulated the pathways that objects
might take from unstable regions of the Asteroid Belt us-
ing computations of dynamical forces acting in the solar
system. In some cases, fragments from asteroid collisions
may be violently cast into these regions of instability. How-
ever, a softer touch may play an even bigger role. Constant