Geoscience Reference
In-Depth Information
1. Introduction
It has long been suspected that short-period comets might have a differ-
ent origin than those of long-periods.
1
,
2
The existence of this Edgeworth-
Kuiper belt was not realized until the discovery of the first (other than Pluto
and Charon) trans-Neptunian object (TNO) 1992 QB
1
by Jewitt and Luu.
3
Currently, more than a thousand of them have been detected. Some of them
are even compatible in size with Pluto.
4
The discovery of Sedna
5
with a
perihelion around 76 AU might indicate the existence of a whole new fam-
ily of TNOs in the outer solar system, where the gravitation of Neptune
plays a less significant role. However, the distribution of comet-size (a few
kilometers) objects is still poorly known. Preliminary results from recent
observation
6
and simulation
7
show the possibility of a broken power law
in the TNO size distribution. Some physical properties can be derived for
the largest TNOs, but certain properties like albedo would be pure spec-
ulation and extrapolation for such small objects. One cannot study those
comets in detail until their orbits bring them closer to us, such as some
of the Centaurs. On the other hand, to explain the current population of
the Jupiter-family comets, which is generally believed to originate from
the trans-Neptunian region, 10
9
-10
10
comets of a size around 1-10 km in
30-50 AU might be required.
6
,
9
Instead of trying to detect the reflected light from those distant small
objects, an occultation survey
8
could probe a comet-size TNO at a few hun-
dreds AU or even farther away. The Taiwan-America Occultation Survey
(TAOS) project
10
-
12
has a design based on this indirect strategy. Some
other, smaller in scope, surveys have been conducted in recent years,
14
,
15
,
26
though convincing statistics describing small comet occultations have yet
to be obtained. However, future improvement in both ground-based and
space-based experiments are expected to provide deeper insight. We briefly
report the status of TAOS in Sec. 2. Related information and details are
available at the TAOS website.
a
Diffraction and the angular size of a tar-
get star
11
,
13
,
14
are two major factors which determine the “detectability”
of a presumed occultation event or the “visibility” of an occultation dip
in a light curve. Occultation is a technique which is capable of detecting
a small comet as far as the inner Oort cloud. A poor distance resolution
may be obtained from the angular size of a target star and the size of the
foreground TNO. Recent analysis
16
suggest that it might be possible to
a
http://taos.asiaa.sinica.edu.tw
