Chapter 6
Comet-Like Activity in Related Objects
6.1 Active Asteroids
Several objects in asteroidal orbits have been seen to have dust tails and are therefore
assumed to be losing dusty material. A recent example of this is the 6 km diameter
inner main-belt asteroid, (6478) Gault (Jewitt et al. 2019). In this particular case,
material ejection speeds of 0.15 Æ 0.05 m s
À1 , were derived indicating outflow
speeds around 3 orders of magnitude lower than are typically seen at an active comet.
These “active asteroids” have low inclinations and orbit between about 2 and 4 AU
from the Sun (i.e. a s < a J ) and a Tisserand parameter, T J > 3. Hence, they have
dynamical similarities to main-belt asteroids. One can see in Fig. 6.1, the dynamical
separation of active asteroids from comets.
The activity is not necessarily evidence of comet-like, sublimation-driven, activity from surface ices. The activity seen in some cases may have arisen in ways that
are not strictly comet-like. Jewitt (2012) considered eight different mechanisms
leading to observable mass loss. The more probable candidates are listed in
Table 6.1.
Combinations of these mechanisms together with other phenomena (such as
thermally or impact induced volatile loss) are also conceivable and we have seen
evidence for the influence of several of these mechanisms being active on the
nucleus of 67P even if, in the case of comets, sublimation is the dominant loss
mechanism. In addition to (6478) Gault, asteroidal objects that have shown some
form of mass loss include, 345P/LINEAR, 133P/Elst-Pizarro (asteroid number 7968;
Figs. 6.1 and 6.2) and 107P/Wilson-Harrington (which has the asteroid designation
(4015) Wilson–Harrington).
The loss of material caused by the impact of a small object on a larger asteroidal
body may be visible as a short-lived coma or narrow tail. Jewitt (2012) suggested
this as a possible mechanism for the production of the thin tail of 354P/LINEAR
(also known as P/2010 A2), for example. Impact had previously been suggested as a
source for the activity of 133P/(7968) Elst-Pizarro based on its showing a thin,
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_6
427
Comet-Like Activity in Related Objects
6.1 Active Asteroids
Several objects in asteroidal orbits have been seen to have dust tails and are therefore
assumed to be losing dusty material. A recent example of this is the 6 km diameter
inner main-belt asteroid, (6478) Gault (Jewitt et al. 2019). In this particular case,
material ejection speeds of 0.15 Æ 0.05 m s
À1 , were derived indicating outflow
speeds around 3 orders of magnitude lower than are typically seen at an active comet.
These “active asteroids” have low inclinations and orbit between about 2 and 4 AU
from the Sun (i.e. a s < a J ) and a Tisserand parameter, T J > 3. Hence, they have
dynamical similarities to main-belt asteroids. One can see in Fig. 6.1, the dynamical
separation of active asteroids from comets.
The activity is not necessarily evidence of comet-like, sublimation-driven, activity from surface ices. The activity seen in some cases may have arisen in ways that
are not strictly comet-like. Jewitt (2012) considered eight different mechanisms
leading to observable mass loss. The more probable candidates are listed in
Table 6.1.
Combinations of these mechanisms together with other phenomena (such as
thermally or impact induced volatile loss) are also conceivable and we have seen
evidence for the influence of several of these mechanisms being active on the
nucleus of 67P even if, in the case of comets, sublimation is the dominant loss
mechanism. In addition to (6478) Gault, asteroidal objects that have shown some
form of mass loss include, 345P/LINEAR, 133P/Elst-Pizarro (asteroid number 7968;
Figs. 6.1 and 6.2) and 107P/Wilson-Harrington (which has the asteroid designation
(4015) Wilson–Harrington).
The loss of material caused by the impact of a small object on a larger asteroidal
body may be visible as a short-lived coma or narrow tail. Jewitt (2012) suggested
this as a possible mechanism for the production of the thin tail of 354P/LINEAR
(also known as P/2010 A2), for example. Impact had previously been suggested as a
source for the activity of 133P/(7968) Elst-Pizarro based on its showing a thin,
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_6
427