1.5.1 Studies of Weakly-Active and Dormant Comets
Early exploration by ¨Opik (1963) speculated that a significant fraction of NEAs are of cometary origin. In subsequent decades, it became more common for some asteroids to be found to ex- hibit cometary activity and be reclassified as comets. Although some of these reclassifications were due to cometary nuclei discovered initially beyond the typical water-ice sublimation line (∼ 3 AU) and later becoming active as they approached the sun, most cases are associated with JFCs that are weakly active (and therefore the cometary features can be difficult to ob- serve). Though one may speculate that the weak activity may simply due to the small sizes of the nuclei (e.g. sub-km nuclei), surprisingly, most measured weak-active cometary nuclei are comparable in sizes to their normally-active counterparts (Ye et al., 2016).
The earliest known weakly-active comet is 28P/Neujmin 1. It has been studied extensively partly due to its large size (10–15 km versus a few km for typical JFC nuclei, c.f. Lamy et al.,
1.5. PreviousStudies 25
2004) making it easier to observe. It is found that the fraction of active surface area is∼0.1% or
100 times lower than typical JFCs (Campins et al., 1987). Spectroscopic studies suggested that the nucleus of 28P/Neujmin 1 is highly evolved and is similar to the Trojan asteroids (Campins et al., 2007).
Another well-known object that is likely approaching its dormancy is 107P/(4015) Wilson- Harrington. Readers may immediately notice the dual comet-asteroid designation of this ob- ject. 107P/Wilson-Harrington displayed a diffuse tail when it was discovered in 1949 but has appeared asteroidal since its recovery in 1979 (Fern´andez et al., 1997). It may be a near-dead comet with intermittent activity, although this argument is weakened by the fact that the de- tection of cometary activity is old and unrepeated, as well as the fact that it has aTJ= 3.08 is
slightly beyond the classicalTJ=3 boundary (Jewitt et al., 2015).
Since weakly-active comets are faint, diffuse and often require observations with either large telescopes, sensitive detectors or long integrations, it is unsurprising that the known pop- ulation of weakly-active comets remained tiny until the 2000s, when a number of modern NEO surveys were put into operation (Figure 1.10). By creating a debiased near-Earth JFC model, Fern´andez & Morbidelli (2006) predicted that there are as much as 103NEJFCs that are larger
than∼ 100 m. For comparison, there are ∼ 500 known JFCs at this size or larger as the time of writing (mid 2016), which implies a significant number of undetected comets. Since the coverage of NEO surveys are fairy frequent and complete now (one full-sky coverage every
∼2 weeks), it is likely these comets are undetected primarily due to their faintness.
Dormant comets are even more difficult to directly study than weakly-active comets, as they do not display any cometary features. As such, dynamical methods of isolating potential dormant comets have proved to be very useful. Different authors (e.g. Levison & Duncan, 1997; Bottke et al., 2002; Fern´andez & Morbidelli, 2006) have arrived at two major conclusions: (1) there are at least as many dormant JFCs as active JFCs; and (2) there is a small but non- negligible chance for main-belt asteroids to achieve JFC-like orbits via certain mean-motion resonances, which presents extra challenges for observational studies. While observational
V
Figure 1.9 Comet 107P/(4015) Wilson-Harrington at discovery (1949 November 19). The comet is marked by an arrow. The plate was taken by the 48-inch Oschin Telescope at Palomar Observatory appropriated toB-band. The image has been enhanced by the European Southern Observatory (ESO) photographic laboratory at Garching.
1.5. PreviousStudies 27
Figure 1.10 The distribution of total cometary absolute magnitudes (M1) versus the year dis-
covered. The magnitude data is retrieved from the JPL Small-Body Database on 2016 June 16. It can be seen that most comets with M1 >15 were found after about the year 2000.
method is difficult, there are signs that a large number of dormant comets exist. Fern´andez et al. (2002) showed that objects on comet-like orbits do, in general, possess comet-like (i.e. low) albedo, than those on asteroid-like orbits.
As individual objects (either weakly-active comets or dormant comet candidates) are usu- ally difficult to observe, most studies to-date focus at the general characteristics of the popula- tion. However, our understanding of this topic would not advance without close examination of a sufficient number of specific objects.
1.5.2 Meteor Studies
Meteor observation provides a unique perspective to look into the properties of the dust that originated from the associated parent bodies. Drummond (1982) pioneered this topic by look- ing at theoretical meteor radiants from NEAs and comparing them against contemporary and medievalfireball observations. Drummond used Southworth & Hawkins (1963)’sD-criterion
(and its variant) to quantify orbital similarity between meteoroid stream and the parent, a tech- nique that was followed by many later studies. A handful of associations were speculated upon; however, as the author pointed out, the lack of precise meteor orbits hampered the confidence of the proposed associations.
The recognition of asteroid (3200) Phaethon as the parent body of the annual Geminid meteor shower in 1983 sparked interest in searching for meteor showers that trace to dormant comet candidates and other ordinary asteroids. Systematic searches were conducted by several parties (see the reviews by Jenniskens, 2008a,b) of which a few dozen linkages were proposed. As meteor showers are produced by dust ejection, meteor observations can provide a strong constraint on previous dust production of the parent body. However, the lack of precise meteor orbits continued to be a limiting factor (e.g. Jopek et al., 2002) until very recently, when a number of extensive video networks are established and data interpretations just begun. The problem is further complicated by a variety of ejection mechanisms (sublimation, tidal or ther- mal disruption, collision, etc.) the details which are still not well understood. The secular evolution of meteoroid streams as well as the challenges in accurately measuring stream orbits (the latter of which is essential in comparing the similarities between the orbit of the stream and the parent) all make the problem difficult to solve. For example, questions still exist over the formation of the Geminid stream, some 30 years after Phaethon has been identified as the parent body of this stream.
Using meteor observation to study weakly-active comets is an exciting but largely un- charted territory. NEJFC 15P/Finlay has been suspected to be a near-death comet due to the absence of the Finlayid meteor shower (Beech et al., 1999), though this theory has been neither confirmed nor disproved. Jenniskens (2006) has noted a few predicted showers from several weakly-active or inactive bodies including JFC 209P/LINEAR, one of the most weakly-active comets found to-date.
1.6. Questions forThisThesis 29