Active asteroid

Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show a coma, tail, or other visual evidence of mass-loss, but their orbits remain within Jupiter's orbit. These bodies were originally designated main-belt comets in 2006 by astronomers David C. Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the main-belt, whereas the growing population of active asteroids shows that this is not always the case.
The first active asteroid discovered is 7968 Elst–Pizarro. It was discovered in 1979 but then was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst–Pizarro.

Orbits

Unlike comets, which spend most of their orbit at Jupiter-like or greater distances from the Sun, active asteroids follow orbits within the orbit of Jupiter that are often indistinguishable from the orbits of standard asteroids. David C. Jewitt defines active asteroids as those bodies that, in addition to having visual evidence of mass loss, have an orbit with:

semi-major axis a < a_Jupiter
Tisserand parameter with respect to Jupiter T_J > 3.08

Jewitt chooses 3.08 as the Tisserand parameter to separate asteroids and comets instead of 3.0 to avoid ambiguous cases caused by the real Solar System deviating from an idealized restricted three-body problem.
The first three identified active asteroids all orbit within the outer part of the asteroid belt.

Activity

Some active asteroids display a cometary dust tail only for a part of their orbit near perihelion. This strongly suggests that volatiles at their surfaces are sublimating, driving off the dust. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia. The activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation.
When discovered in January 2010, P/2010 A2 was initially given a cometary designation and thought to be showing comet-like sublimation, but P/2010 A2 is now thought to be the remnant of an asteroid-on-asteroid impact. Observations of 596 Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter.

P/2013 R3

P/2013 R3 was discovered independently by two observers by Richard E. Hill using the Catalina Sky Survey's 0.68-m Schmidt telescope and by Bryce T. Bolin using the 1.8-m Pan-STARRS1 telescope on Haleakala. The discovery images taken by Pan-STARRS1 showed the appearance of two distinct sources within 3" of each other combined with a tail enveloping both sources. In October 2013, follow-up observations of P/2013 R3, taken with the 10.4 m Gran Telescopio Canarias on the island of La Palma, showed that this comet was breaking apart. Inspection of the stacked CCD images obtained on October 11 and 12 showed that the main-belt comet presented a central bright condensation that was accompanied on its movement by three more fragments, A, B, C. The brightest A fragment was also detected at the reported position in CCD images obtained at the 1.52 m telescope of the Sierra Nevada Observatory in Granada on October 12.
NASA reported on a series of images taken by the Hubble Space Telescope between October 29, 2013, and January 14, 2014, that show the increasing separation of the four main bodies. The Yarkovsky–O'Keefe–Radzievskii–Paddack effect, caused by sunlight, increased the spin rate until the centrifugal force caused the rubble pile to separate.

Dimorphos

By smashing into the asteroid moon of the binary asteroid 65803 Didymos, NASA's Double Asteroid Redirection Test spacecraft made Dimorphos an active asteroid. Scientists had proposed that some active asteroids are the result of impact events, but no one had ever observed the activation of an asteroid. The DART mission activated Dimorphos under precisely known and carefully observed impact conditions, enabling the detailed study of the formation of an active asteroid for the first time. Observations show that Dimorphos lost approximately 1 million kilograms after the collision. Impact produced a dust plume that temporarily brightened the Didymos system and developed a -long dust tail that persisted for several months. The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving an impact crater several tens of meters in diameter. The impact has likely sent Dimorphos into a chaotically tumbling rotation that will subject the moon to irregular tidal forces by Didymos before it will eventually return to a tidally locked state within several decades.

Composition

Some active asteroids show signs that they are icy in composition like a traditional comet, while others are known to be rocky like an asteroid. It has been hypothesized that main-belt comets may have been the source of Earth's water, because the deuterium–hydrogen ratio of Earth's oceans is too low for classical comets to have been the principal source. European scientists have proposed a sample-return mission from a MBC called Caroline to analyse the content of volatiles and collect dust samples.

List

Identified members of this morphology class include:

Name	Semi-major axis	Perihelion	Eccentricity	T_Jup	Orbital class	Diameter	Rotation period	Cause	Activity discovery year	Recurrent?
1 Ceres	2.766	2.550	0.078	3.310	main-belt	939.4	9.07	Water sublimation	2014
493 Griseldis	3.116	2.568	0.176	3.140	main-belt	41.56	51.94	Impact	2015
596 Scheila	2.929	2.45	0.163	3.209	main-belt	159.72	15.85	Impact	2011
2201 Oljato	2.174	0.624	0.713	3.299	NEO	1.8	>26	Sublimation	1984
3200 Phaethon	1.271	0.140	0.890	4.510	NEO	6.26	3.60	Thermal fracturing, dehydration cracking, and/or rotational disintegration	2010
6478 Gault	2.305	1.860	0.193	3.461	main-belt	5.6	2.49	Rotational disintegration	2019
	3.159	2.909	0.079	3.197	main-belt	10.38	3.33	Rotational disintegration	2014
65803 Didymos/Dimorphos	1.643	1.013	0.383	4.204	NEO	0.77 / 0.15	2.26	Human-caused impact	2022
101955 Bennu	1.126	0.896	0.204	5.525	NEO	0.48	4.29	Electrostatic lofting, impacts, thermal fracturing, or dehydration cracking	2019
	3.176	2.783	0.124	3.188	main-belt				2023
	2.690	2.068	0.231	3.319	main-belt	0.5			2024
	3.071	2.943	0.042	3.199	main-belt	<0.4			2022
	2.744	1.770	0.355	3.230	main-belt	1.483			2023
	3.062	2.957	0.034	3.201	main-belt				2023
	2.765	2.319	0.161	3.280	main-belt				2023
	3.128	2.451	0.217	3.160	main-belt				2023
	2.696	2.300	0.147	3.351	main-belt				2023
107P/4015 Wilson–Harrington	2.625	0.966	0.632	3.082	NEO	6.92	7.15	Sublimation	1949
133P/7968 Elst–Pizarro	3.165	2.668	0.157	3.184	main-belt	3.8	3.47	Sublimation/rotational disintegration	1996
176P/118401 LINEAR	3.194	2.578	0.193	3.167	main-belt	4.0	22.23	Sublimation	2005
233P/La Sagra	3.033	1.786	0.411	3.081	main-belt	3.0			2010
238P/Read	3.162	2.362	0.253	3.153	main-belt	0.8		Sublimation	2005
259P/Garradd	2.727	1.794	0.342	3.217	main-belt	0.60		Sublimation	2008
288P/	3.051	2.438	0.201	3.203	main-belt	1.8 / 1.2		Sublimation	2011
311P/PanSTARRS	2.189	1.935	0.116	3.660	main-belt	0.4	>5.4	Rotational disintegration	2013
313P/Gibbs	3.154	2.391	0.242	3.133	main-belt	2.0		Sublimation	2003
324P/La Sagra	3.098	2.621	0.154	3.099	main-belt	1.1		Sublimation	2010
331P/Gibbs	3.005	2.879	0.042	3.228	main-belt	3.54	3.24	Rotational disintegration	2012
354P/LINEAR	2.290	2.004	0.125	3.583	main-belt	0.12	11.36	Impact	2010
358P/PanSTARRS	3.155	2.410	0.236	3.134	main-belt	0.64		Sublimation	2012
426P/PanSTARRS	3.188	2.675	0.161	3.103	main-belt	2.4			2019
427P/ATLAS	3.171	2.178	0.313	3.092	main-belt	0.90	1.4	Sublimation/rotational disintegration	2017
432P/PanSTARRS	3.045	2.302	0.244	3.170	main-belt	<1.4			2021
433P/	3.067	2.374	0.226	3.192	main-belt	3.2		Sublimation/rotational disintegration	2021
435P/PanSTARRS	3.018	2.056	0.319	3.090	main-belt				2021
455P/PanSTARRS	3.156	2.193	0.305	3.087	main-belt	<1.6			2017
456P/PanSTARRS	3.165	2.788	0.119	3.125	main-belt	<4.4			2021
457P/2020 O1	2.647	2.329	0.120	3.376	main-belt	0.84	1.67	Sublimation/rotational disintegration	2020
483P/PanSTARRS	3.172	2.449	0.228	3.113	main-belt	<1.8 / <0.8		Sublimation	2016
P/2013 R3	3.033	2.205	0.273	3.184	main-belt	~0.4		Sublimation/rotational disintegration	2013
P/2015 X6	2.755	2.287	0.170	3.318	main-belt	<1.4		Sublimation	2015
P/2016 G1	2.583	2.041	0.210	3.367	main-belt	<0.8		Impact	2016
P/2018 P3	3.007	1.756	0.416	3.096	main-belt	<1.2		Sublimation	2018
P/2019 A3	3.147	2.313	0.265	3.099	main-belt	<0.8			2019
P/2019 A4	2.614	2.379	0.090	3.365	main-belt	0.34			2019
P/2021 A5	3.047	2.620	0.140	3.147	main-belt	0.30		Sublimation	2021
P/2021 R8	3.019	2.131	0.294	3.179	main-belt				2021
P/2022 R5	3.071	2.470	0.196	3.148	main-belt				2022
P/2023 S4	3.134	2.542	0.189	3.185	main-belt				2023
P/2024 L4	2.231	0.672	0.699	3.255	NEO	<0.4		Rotational disintegration?	2024
P/2024 R2	3.138	2.302	0.266	3.104	main-belt				2024