objects influence the motion of the comet. However, it is clearly very useful for
describing most cases.
There are in addition a number of active objects well inside the orbit of Jupiter
with T J > 3. This has led to the description of a new class of object sometimes
referred to as Main Belt Comets (MBCs) although they are perhaps more correctly
referred to as “active asteroids” (Jewitt 2012). We will address these objects in a later
section.
Objects with T J slightly greater 3 are themselves of considerable interest. Duncan
et al. (2004) have pointed out that these objects can experience temporary and
possibly repeated capture by Jupiter. This can result in tidal disruption and breakup through passages within the Roche limit and/or low velocity impact with Jupiter.
The prototype for this behaviour was D/Shoemaker-Levy 9 which had a very close
encounter with Jupiter in 1992 that led to its break-up (Fig. 1.8) and the impact of
more than 20 sub-nuclei with Jupiter 2 years later. These types of events may be
fairly common as indicated by the presence of catenae (chains of impacts) on the
surfaces of the outer Galilean moons (Melosh and Schenk 1993). Figure 1.9 shows
an example (Enki Catana) on Ganymede. It is inferred here that a close approach to
Jupiter led to the break-up of an object which then, almost immediately, impacted
Ganymede as the object fragments were on their outbound path following the Jupiter
encounter.
Of the numbered comets, less than 6% are HTCs (after 1P/Halley itself, 55P/
Tempel–Tuttle is probably the best known) but there is clearly an observational
selection effect caused by their longer periods compared to JFCs. On the other hand,
well over 100 comets fitting the basic criterion (20 < P < 200 years) to be HTCs
have been detected but observed only over one perihelion passage. Several of these
(e.g. more recently discovered objects such as P/2004 A1 and P/2011 S1) have
values of the Tisserand parameter with respect to Saturn, T S , very close to 3 combined with low orbital inclination. This indicates that interactions with Saturn are
likely. As of the end of 2019, 89 comets have been classified unambiguously as
Fig. 1.8 Comet Shoemaker-Levy 9 as observed using the Hubble Space Telescope in May 1994
following a previous close apprach to Jupiter in 1992 (Credit: Dr. Hal Weaver and T. Ed Smith
(STScI), and NASA)
12
1 Light Curves, Orbits, and Reservoirs
describing most cases.
There are in addition a number of active objects well inside the orbit of Jupiter
with T J > 3. This has led to the description of a new class of object sometimes
referred to as Main Belt Comets (MBCs) although they are perhaps more correctly
referred to as “active asteroids” (Jewitt 2012). We will address these objects in a later
section.
Objects with T J slightly greater 3 are themselves of considerable interest. Duncan
et al. (2004) have pointed out that these objects can experience temporary and
possibly repeated capture by Jupiter. This can result in tidal disruption and breakup through passages within the Roche limit and/or low velocity impact with Jupiter.
The prototype for this behaviour was D/Shoemaker-Levy 9 which had a very close
encounter with Jupiter in 1992 that led to its break-up (Fig. 1.8) and the impact of
more than 20 sub-nuclei with Jupiter 2 years later. These types of events may be
fairly common as indicated by the presence of catenae (chains of impacts) on the
surfaces of the outer Galilean moons (Melosh and Schenk 1993). Figure 1.9 shows
an example (Enki Catana) on Ganymede. It is inferred here that a close approach to
Jupiter led to the break-up of an object which then, almost immediately, impacted
Ganymede as the object fragments were on their outbound path following the Jupiter
encounter.
Of the numbered comets, less than 6% are HTCs (after 1P/Halley itself, 55P/
Tempel–Tuttle is probably the best known) but there is clearly an observational
selection effect caused by their longer periods compared to JFCs. On the other hand,
well over 100 comets fitting the basic criterion (20 < P < 200 years) to be HTCs
have been detected but observed only over one perihelion passage. Several of these
(e.g. more recently discovered objects such as P/2004 A1 and P/2011 S1) have
values of the Tisserand parameter with respect to Saturn, T S , very close to 3 combined with low orbital inclination. This indicates that interactions with Saturn are
likely. As of the end of 2019, 89 comets have been classified unambiguously as
Fig. 1.8 Comet Shoemaker-Levy 9 as observed using the Hubble Space Telescope in May 1994
following a previous close apprach to Jupiter in 1992 (Credit: Dr. Hal Weaver and T. Ed Smith
(STScI), and NASA)
12
1 Light Curves, Orbits, and Reservoirs