THE ACTIVE ASTEROIDS

“Some carbonaceous chondrite classes (probable fragments of outer-belt asteroids) contain 10%–20% water by weight, bound into hydrated minerals (Jarosewich 1990).”

“Since Whipple (1950), sublimation has been explored in great detail as the driver of mass loss from the classical comets.”

“Even greater interest is added by the possibility that the outer asteroid belt may have supplied part of the volatile inventory of the Earth (Morbidelli et al. 2000).”

“A dynamical simulation indicates that (2201) Oljato has negligible chance of being a captured Jupiter family comet (Bottke et al. 2002).”

“For equal target and projectile densities, the mass of ejecta, me, traveling faster than a given speed, v, can be roughly expressed by a power law me/M = A(v/U )α (5) in which M and U are the impactor mass and speed and A ∼ 0.01 is a constant (Housen & Holsapple 2011).”

“These were called “mainbelt comets” by Hsieh & Jewitt (2006), but here we use the term “active asteroids,” since some of the examples to be considered, while dynamically asteroid-like and showing comet-like properties, are not in the main belt.”

“Near a shadow edge, the photoelectrons produced in sunlight can travel to and stick in unilluminated regions, causing local electric field gradients estimated at E ∼ 10 to 100 V m−1 (Colwell et al. 2007; Farrell et al. 2007).”

“Only a few such objects are currently known (Jewitt et al. 2011; Jewitt 2012); LSST will increase the sample of such objects to ∼100.”

“…events such as cometary outbursts and fragmentation events (Ye et al. 2015; Ishiguro et al. 2016), collisions between asteroids (Snodgrass et al. 2010; Bodewits et al. 2011), unexpected or irregular activity in asteroids (Jewitt 2012; Waszczak et al. 2013; Ye 2017) and Centaurs (Jewitt 2009).”

“This value is probably conservative given that asteroids known as main-belt comets appear to have a much larger water content than this (Jewitt, 2012).”

“Multiple ejection mechanisms have been suggested to explain asteroid activity, including collisions, water-ice sublimation, rotational destabilization, thermal fracturing, and dehydration (8).”

“An alternative to a collision is tidal, thermal and/or spin breakup of a weak planetesimal (e.g., Jewitt 2012).”

“Thermal fracture and cracking due to desiccation shrinkage of hydrated silicates are two processes capable of both producing the dust and ejecting it from the surface (Jewitt & Li 2010; Jewitt 2012; Li & Jewitt 2013).”

“ing from the giant planet region may be conservative, given that main-belt comets may have a much larger water content than this (Jewitt 2012).”

“…at the surface; e.g., Hsieh & Jewitt 2006; Campins et al. 2010; Rivkin & Emery 2010; Licandro et al. 2011; Carry 2012; Hargrove et al. 2012, 2015; Jewitt 2012; Marchis et al. 2012; Takir & Emery 2012; Küppers et al. 2014; Hui & Jewitt 2015) that suggest that icy bodies are abundant in the…”

“Also investigated are connections between surface slope failures on rapidly spinning bodies and the recent observations of “active asteroids”, with multiple apparent shedding events occurring over a relatively brief time span [21, 23, 22, 12].”

“The perihelion temperatures on Phaethon exceed those needed to break-down phyllosilicates (Akai 1992), and are sufficient to induce thermal fracture (Paper I, Jewitt 2012).”

“The combination of asteroid-like orbit and comet-like appearance together qualify R3 as an active asteroid (Jewitt 2012) or, equivalently, a main-belt comet (Hsieh and Jewitt 2006).”

“Processes invoked to explain mass loss from asteroids include sublimation of near surface ice, electrostatic levitation of dust, impact, and rotational instability (Jewitt 2012).”

“This object is now classified as an active asteroid (Jewitt 2012; Hsieh et al. 2015) and was found to be part of the Hygiea outer main belt asteroid family with a fast rotation period.”

“…instance, it has been claimed that the Orgueil CI type chondritic meteorite could be a comet (Gounelle et al. 2006), while comet-like activity ( i.e. water outgassing) is observed for some objects of the asteroid belt (so called “main belt comets” or “active asteroids”) (Bertini 2011; Jewitt 2012).”

“No gas has yet been spectroscopically detected in any of the active asteroids, but the available limits to gas production are consistent with the very low ( 1 kg s−1) mass-loss rates inferred from dust (Jewitt 2012).”

“Jewitt (2012) assumes the particles have radii in the range 0.1µm . a .”

“A survey of recent observations is given in Jewitt (2012); however, since that time, there have been several additional and striking examples of this phenomenon (Jewitt et al. 2013, 2014).”

“Mass loss rates of active asteroids have been estimated to be in the range from ≤ 0.04 kg s−1 (113P/Elst-Pizarro) to ≤ 150 kg s−1 (107P/Wilson-Harrington) (Jewitt 2012).”

“…of each new discovered object will then be essential for first determining the most likely cause of the observed activity (cf. Hsieh et al. 2012a; Jewitt 2012), and then ascertaining the global properties of the population of objects confirmed as MBCs to better understand their implications for…”

“means that in situ spacecraft studies are feasible given present-day technical capabilities. Collectively, MBCs and disrupted asteroids comprise the class of objects known as active asteroids (e.g., Jewitt, 2012). Orbits of small solar system bodies are typically classied as asteroidal or cometary using the Tisserand…”

“bodies known as active asteroids (Jewitt, 2012; Jewitt et al., 2015), which also includes disrupted asteroids, which are objects that exhibit cometlike activity that is produced by non-sublimationdriven effects such as impacts or rotational desta-”

“(8) and (9) for ðq000 pol; u000 polÞ in Eq.”

“Evolving comas or tails around active asteroids (Jewitt 2012) were previously noticeable in data from 1 m class telescopes on sub-km asteroids, with visible magnitude of ∼ 19.”

“…from, their activity, although still not reliably understood, is likely triggered by the impact excavation of subsurface ice (Hsieh, Jewitt & Fernández 2004; Jewitt 2012) because completely exposed surface ice is unstable against sublimation at their heliocentric distances over Gyr time-scales.”

“…et al., 2020), near-Earth asteroids (Delbo et al., 2014; Dombard, Barnouin, Prockter, & Thomas, 2010; Graves, Minton, Molaro, & Hirabayashi, 2019; Jewitt, 2012), and perhaps comets (Alí-Lagoa, Delbo, & Libourel, 2015; El-Maarry et al., 2015; Pajola et al., 2017; Shestakova & Tambovtseva, 1997;…”

“Repetitive activity in the active asteroids, as in the comets, is most simply explained as reflecting the sublimation of near surface ice,although there is no efficient dynamical path from either comet reservoir to the asteroid belt (see Hsieh & Jewitt 2006; Jewitt 2012; Jewitt et al. 2015a).”

“ave been proposed: sublimation of ice, rotational fission, ejection by collisions, electrostatic repulsion, thermal fracture, dehydration stresses, and radiation pressure sweeping (e.g. Bertini 2011, Jewitt 2012, and Jewitt et al. this volume), and at least some of the objects are best explained as experiencing proces…”

“At least two of the ‘‘activated asteroids’’ or ‘‘main belt comets’’ are members of the Themis family, or orbit nearby (Jewitt, 2012).”

“This behavior is naturally explained by thermally-modulated sublimation and extremely difficult to explain as a consequence of any physical disruption (Hsieh et al. 2012a; Jewitt 2012).”

“1 µm) is prevalent in the interstellar medium (e.g. Mathis et al. 1977), while in the solar system the tails of comets and streams coming off the active asteroids can be a mixture of small and large grains (with s from sub-µm to cm), depending on the dust production mechanism (e.g. Jewitt 2012).”

“A subset of asteroids are known to exhibit activity, also called active asteroids (Jewitt 2012; Hsieh 2017; Jewitt & Hsieh 2022).”

“We compared several activity mechanisms (drawn from Jewitt 2012) with the available data.”

“For this work, we find that the terms “active asteroid” (Jewitt 2012), “activated asteroid” (Licandro & Campins 2010), “active main belt object” (Bauer et al. 2012), and the original definition of “main belt comet” (Hsieh & Jewitt 2006) all apply.”

“This limit is comparable to values set spectroscopically on active asteroids (e.g., Jewitt 2012), smaller than the dust production rates inferred in Section 2.2, and 102 to 103 times smaller than commonly measured in short-period comets.”

“…(3) Tisserand parameters with respect to Jupiter TJ>3; TJ describes an object’s orbital relationship to Jupiter by T a a a e a i2 1 cos 1J J 2 J = + -( ) ( ) ( ) where aJ is the semimajor axis of Jupiter, e is the eccentricity, and i is the inclination; see Jewitt (2012) for a thorough treatment.”

“Rotational instability has been proposed as a mechanism causing activity among some asteroids (Jewitt, 2012).”

“Instead, both hypervelocity impact and rotational instability (the latter resulting from YORP- or sublimationdriven spin-up) have been regarded as possible mechanisms (e.g. Jewitt 2012).”

“In a sub-set of the asteroids called main-belt comets (MBCs), the sublimation of excavated volatiles causes transient comet-like activity [6, 7, 8].”

“…in concavities might also explain why water ice was detected on asteroids (24) Themis and (67) Cybele (Campins et al. 2010; Licandro et al. 2011) at heliocentric distances at which water ice is not expected to be stable, and possibly even some of the activated asteroids (see, e.g., Jewitt 2012).”

“Main-belt comets have orbits indistinguishable from main-belt asteroids (TJ > 3) but show comet-like activities owing to sublimation of ice, impacts, and so on (Jewitt 2012; Hsieh & Jewitt 2006).”

“Although the source for such activity is not fully constrained yet (Jewitt 2012), the “Grand Tack” model, which examines the dynamical evolution of the solar system a few million years after the formation of the first solids (Walsh et al. 2011), suggests that some primitive icy objects formed…”

“Extrapolating the results in their Figure 16 suggests Mesc/M2 ≈ fcr(vc/vesc)α , where fcr ≈ 0.01 and α ≈ 1.0–1.5 (for a recent application of this approach to asteroids in the solar system, see Jewitt 2012).”

“profile of all good quality observations between 2002 and 2014. A. Abedin et al. / Icarus 261 (2015) 100–117 103”

“It is thus not a surprise that several socalled main-belt comets were discovered among BCG-type objects (Hsieh & Jewitt 2006, Jewitt 2012).”

“There are several mechanisms that have been proposed to drive mass loss from small bodies, including sublimation of subsurface ices, rotational instability, impact ejection and thermal fracture (see Bertini 2011; Jewitt 2012, for recent reviews of MBC physical properties and activation mechanisms).”

“Despite these lower production rates for any individual asteroid, Main Belt asteroids steadily produce ejecta that contribute to supplying the interplanetary dust complex, although at a share of < 10% (Nesvorný et al. 2010).”

“Both desiccation cracking and thermal fracture are potentially capable of generating dust (Jewitt & Li 2010; Jewitt 2012).”

“…the orbit is that of a main-belt asteroid (semimajor axis 2.290 AU, eccentricity 0.124 and inclination 5.3◦ and Tisserand parameter with respect to Jupiter TJ = 3.582), leading to its classification as an active asteroid (equivalently, a “main-belt comet”; Hsieh and Jewitt 2006, Jewitt 2012).”

“One subgroup of these active asteroids (Jewitt 2012) are the main-belt comets (MBCs), which we define1 as objects in the dynamically-stable main asteroid belt that exhibit a periodic (e.g., ?”

“In some cases, mass loss is the result of discrete near‐instantaneous events due to impacts or rotational instability (Jewitt, 2012; Jewitt et al., 2015).”

“Further evidence for significant water in the Main Belt comes from the so-called Main-Belt Comets (MBCs; Hsieh & Jewitt 2006; Bertini 2011; Jewitt 2012).”

“Another reason why young asteroid families are important is their likely relation 19 with a new class of asteroids identified in recent years, coll e tively known asac20 tive asteroids(Jewitt, 2012).”

“For an elongated strengthless object, the critical period of break-up will happen at a slower spin period (Samarasinha et al. 2004, Jewitt 2012): Pcrit = a/b ( 3π Gρ )1/2 (2) For 62412, a/b ≥ 1.51 and thus Pcrit ≥ 3.5 hours for a density of 2000 kg m −3.”

“A number of mechanisms drive the mass loss, including the likely sublimation of exposed ice, asteroid– asteroid impact, and rotational disruption probably driven by radiation torques (Jewitt 2012; Jewitt et al. 2015).”

“If the weathered material is being removed from the surface in large amounts, we would expect to see a brightening of the asteroid shortly after the dust is released (Jewitt et al., 2015).”

“The discovery of objects exhibiting cometary activity on typical main-belt orbits (the “main-belt comets” or “activated asteroids”: Hsieh and Jewitt 2006) pointed toward the existence of icy bodies in the Themis family and elsewhere within the orbit of Jupiter (Jewitt 2012), underlined by the discovery of absorption b…”

“The rate of MBC discoveries suggests there will be several available during the lifetime of JWST. Searches for gas in MBC comae (as opposed to dust) have placed water outgassing rates of ∼1026 s−1 for several objects (Jewitt 2012).”

“…find a total mass of dust in the presumed Phaethon orbit to be significantly more than it is possible for Phaethon to produce at its current levels (Jewitt 2012), but somewhat less than previously published estimates for the mass of the Geminids (Ryabova 2017), though the latter study does state a…”

“There are active asteroids, also called “main-belt comets”, which spew dust grains when they are close to perihelion (see review by Jewitt 2012), and recently the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-Rex; Lauretta et al. 2017) has even…”

“The main mechanisms that can induce a relevant mass loss for active (comet-like) or inactive asteroids are ice sublimation and impact ejection [Jewitt, 2012].”

“However, the discovery of new asteroid families (Nesvorný et al. 2002) in the main belt, new cometary-like asteroids, asteroid outbursts (Jewitt 2012), or weak cometary activity on NEA (3200) Phaeton (Jewitt et al. 2013) changed this opinion.”

“(1) To qualify as an active asteroid, a body must typically meet three criteria (see Jewitt 2012 for discussion):”

“There are two interesting possibilities to explain this: Either comet 2P/Encke has an origin within the main asteroid belt – recent discoveries (Hsieh & Jewitt 2006; Jewitt 2012) re-enforce this possibility – or there ?”

“The impacts of large bodies on the asteroids should also lead to similar consequences, which is probably what happened SOLAR SYSTEM RESEARCH Vol. 53 No. 4 2019 on asteroid 596 Scheila (e.g., Jewitt, 2012; Kokhirova et al., 2018), but such events are less probable.”

“However, a subpopulation, known as the “active asteroids”, lose mass, generating comae and dust tails that are obvious in optical data (Jewitt 2012).”

“Another obvious source of dynamical coherence among NEOs can be traced back to comets and active asteroids (Jewitt 2012) that produce fragments which initially move in unison.”