Active asteroid

Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics.^[1] That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupiter's orbit (like an asteroid).^[2][3] These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David 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.^[2][4][5]

Asteroid 596 Scheila displaying a comet-like appearance on 12 December 2010

Dust ejecta and tail from the aftermath of the Double Asteroid Redirection Test's impact on the asteroid moon Dimorphos, as seen by the Southern Astrophysical Research Telescope in 2022

The first active asteroid discovered is 7968 Elst–Pizarro. It was discovered (as an asteroid) 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.^[2][6]

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. Jewitt defines active asteroids as those bodies that, in addition to having visual evidence of mass loss, have an orbit with:^[3]

1. semi-major axis a < a_Jupiter (5.20 AU)
2. 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 (the Tisserand parameter of Jupiter itself) to avoid ambiguous cases caused by the real Solar System deviating from an idealized restricted three-body problem.^[3]

The first three identified active asteroids all orbit within the outer part of the asteroid belt.^[7]

Activity

 

Disintegration of asteroid P/2013 R3 observed by the Hubble Space Telescope (6 March 2014).^[8][9]

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.^[10] Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia.^[2] The activity persists for a month or several^[7] 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.^[7] These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation.^[7]

When discovered in January 2010, P/2010 A2 (LINEAR) was initially given a cometary designation and thought to be showing comet-like sublimation,^[11] but P/2010 A2 is now thought to be the remnant of an asteroid-on-asteroid impact.^[12][13] 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 (Catalina–PanSTARRS) 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.^[14] 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.^[15] 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.^[15]

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.^[16] The Yarkovsky–O'Keefe–Radzievskii–Paddack effect, caused by sunlight, increased the spin rate until the centrifugal force caused the rubble pile to separate.^[16]

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.^[17] European scientists have proposed a sample-return mission from a MBC called Caroline to analyse the content of volatiles and collect dust samples.^[10]

List

Identified members of this morphology class (T_Jup>3.08) include:^[18]: 17

Name	Semi-major axis (AU)	Perihelion (AU)	Eccentricity	TJup	Orbital class	Diameter (km)	Rotation period (hr)	Cause	Activity discovery year	Recurrent?
1 Ceres	2.766	2.550	0.078	3.310	main-belt (middle)	939.4	9.07	Water sublimation^[3]	2014
493 Griseldis	3.116	2.568	0.176	3.140	main-belt (outer)	41.56	51.94	Impact^[19]	2015	✗
596 Scheila	2.929	2.45	0.163	3.209	main-belt (outer)	159.72	15.85	Impact^[20]^[21]^[22]	2011	✗
2201 Oljato	2.174	0.624	0.713	3.299	NEO (Apollo)	1.8	>26	Sublimation^[23]	1984	✗
3200 Phaethon	1.271	0.140	0.890	4.510	NEO (Apollo)	6.26	3.60	Thermal fracturing, dehydration cracking, and/or rotational disintegration^[24]	2010	✓
6478 Gault	2.305	1.860	0.193	3.461	main-belt (inner)	5.6	2.49	Rotational disintegration^[25]^[26]^[27]	2019	✓
(62412) 2000 SY178	3.159	2.909	0.079	3.197	main-belt (outer)	10.38	3.33	Rotational disintegration^[28]	2014	✗
65803 Didymos/Dimorphos	1.643	1.013	0.383	4.204	NEO (Apollo)	0.77 / 0.15	2.26	Human-caused impact	2022	✗
101955 Bennu	1.126	0.896	0.204	5.525	NEO (Apollo)	0.48	4.29	(unknown)^[18]^: 22 Electrostatic lofting, impacts, thermal fracturing, or dehydration cracking	2019	✓
2008 BJ22	3.071	2.943	0.042	3.199	main-belt (outer)	<1.5			2022	✗
107P/4015 Wilson–Harrington	2.625	0.966	0.632	3.082	NEO (Apollo)	6.92	7.15	Sublimation^[29]^[30]	1949	✗
133P/7968 Elst–Pizarro	3.165	2.668	0.157	3.184	main-belt (outer)	3.8	3.47	Sublimation/rotational disintegration^[31]^[32]	1996	✓
176P/118401 LINEAR	3.194	2.578	0.193	3.167	main-belt (outer)	4.0	22.23	Sublimation^[33]	2005	✗
233P/La Sagra (P/2009 WJ50)	3.033	1.786	0.411	3.081	main-belt (outer)	3.0			2010	✗
238P/Read (P/2005 U1)	3.162	2.362	0.253	3.153	main-belt (outer)	0.8		Sublimation^[34]	2005	✓
259P/Garradd (P/2008 R1)	2.727	1.794	0.342	3.217	main-belt (middle)	0.60		Sublimation^[35]	2008	✓
288P/(300163) 2006 VW139	3.051	2.438	0.201	3.203	main-belt (outer)	1.8 / 1.2		Sublimation^[36]	2011	✓
311P/PanSTARRS (P/2013 P5)	2.189	1.935	0.116	3.660	main-belt (inner)	0.4	>5.4	Rotational disintegration^[37]^[38]^[39]	2013	✓
313P/Gibbs (P/2003 S10)	3.154	2.391	0.242	3.133	main-belt (outer)	2.0		Sublimation^[40]	2003	✓
324P/La Sagra (P/2010 R2)	3.098	2.621	0.154	3.099	main-belt (outer)	1.1		Sublimation^[41]	2010	✓
331P/Gibbs (P/2012 F5)	3.005	2.879	0.042	3.228	main-belt (outer)	3.54	3.24	Rotational disintegration^[42]^[43]	2012	✗
354P/LINEAR (P/2010 A2)	2.290	2.004	0.125	3.583	main-belt (inner)	0.12	11.36	Impact^[44]	2010	✗
358P/PanSTARRS (P/2012 T1)	3.155	2.410	0.236	3.134	main-belt (outer)	0.64		Sublimation^[45]	2012	✗
426P/PanSTARRS (P/2019 A7)	3.188	2.675	0.161	3.103	main-belt (outer)	2.4			2019	✗
427P/ATLAS (P/2017 S5)	3.171	2.178	0.313	3.092	main-belt (outer)	0.90	1.4	Sublimation/rotational disintegration^[46]	2017	✗
432P/PanSTARRS (P/2021 N4)	3.045	2.302	0.244	3.170	main-belt (outer)	<1.4			2021	✗
433P/(248370) 2005 QN173	3.067	2.374	0.226	3.192	main-belt (outer)	3.2		Sublimation/rotational disintegration	2021	✓
435P/PanSTARRS (P/2021 T3)	3.018	2.056	0.319	3.090	main-belt (outer)				2021	✗
P/2013 R3 (Catalina–PanSTARRS)	3.033	2.205	0.273	3.184	main-belt (outer)	~0.4		Sublimation/rotational disintegration^[47]	2013	✗
P/2015 X6 (PanSTARRS)	2.755	2.287	0.170	3.318	main-belt (middle)	<1.4		Sublimation^[48]	2015	✗
P/2016 G1 (PanSTARRS)	2.583	2.041	0.210	3.367	main-belt (middle)	<0.8		Impact^[49]	2016	✗
P/2016 J1-A/B (PanSTARRS)	3.172	2.449	0.228	3.113	main-belt (outer)	<1.8 / <0.8		Sublimation^[50]	2016	✗
P/2017 S9 (PanSTARRS)	3.156	2.193	0.305	3.087	main-belt (outer)	<1.6			2017	✗
P/2018 P3 (PanSTARRS)	3.007	1.756	0.416	3.096	main-belt (outer)	<1.2		Sublimation	2018	✓
P/2019 A3 (PanSTARRS)	3.147	2.313	0.265	3.099	main-belt (outer)	<0.8			2019	✗
P/2019 A4 (PanSTARRS)	2.614	2.379	0.090	3.365	main-belt (middle)	0.34			2019	✗
P/2020 O1 (Lemmon–PanSTARRS)	2.647	2.329	0.120	3.376	main-belt (middle)	0.84	1.67	Sublimation/rotational disintegration^[51]	2020	✓
P/2021 A5 (PanSTARRS)	3.047	2.620	0.140	3.147	main-belt (outer)	0.30		Sublimation	2021	✗
P/2021 L4 (PanSTARRS)	3.165	2.788	0.119	3.125	main-belt (outer)	<4.4			2021	✗
P/2021 R8 (Sheppard)	3.019	2.131	0.294	3.179	main-belt (outer)				2021	✗

Exploration

 

Asteroid 101955 Bennu seen ejecting particles on January 6, 2019, in images taken by the OSIRIS-REx spacecraft

Castalia is a proposed mission concept for a robotic spacecraft to explore 133P/Elst–Pizarro and make the first in situ measurements of water in the asteroid belt, and thus, help solve the mystery of the origin of Earth's water.^[52] The lead is Colin Snodgrass, from The Open University in the UK. Castalia was proposed in 2015 and 2016 to the European Space Agency within the Cosmic Vision programme missions M4 and M5, but it was not selected. The team continues to mature the mission concept and science objectives.^[52] Because of the construction time required and orbital dynamics, a launch date of October 2028 was proposed.^[52]

On January 6, 2019, the OSIRIS-REx mission first observed episodes of particle ejection from 101955 Bennu shortly after entering orbit around the near-Earth asteroid, leading it to be newly classified as an active asteroid and marking the first time that asteroid activity had been observed up close by a spacecraft. It has since observed at least 10 other such events.^[4] The scale of these observed mass loss events is much smaller than those previously observed at other active asteroids by telescopes, indicating that there is a continuum of mass loss event magnitudes at active asteroids.^[53]

See also

• Centaur (minor planet)
• Extinct comet

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External links

• Proper elements of active asteroids at Asteroid Families Portal
• Henry Hsieh's page has extensive details on Main-belt comets
• David Jewitt. The Active Asteroids
• Discussion of possible differences in characteristics of the water in MBCs and other comets
• YouTube Interview with David Jewitt (discussion on main-belt comets starts around 9 minutes into video)
• Impact trigger mechanism diagram by David Jewitt
• Comet-like appearance of (596) Scheila
• Jewitt, David (2012). "The Active Asteroids". The Astronomical Journal. 143 (3): 66. arXiv:1112.5220. Bibcode:2012AJ....143...66J. doi:10.1088/0004-6256/143/3/66. S2CID 45208650.
• Hsieh, Henry H.; Yang, Bin; Haghighipour, Nader; Kaluna, Heather M.; Fitzsimmons, Alan; Denneau, Larry; Novaković, Bojan; Jedicke, Robert; Wainscoat, Richard J.; Armstrong, James D.; Duddy, Samuel R.; Lowry, Stephen C.; Trujillo, Chadwick A.; Micheli, Marco; Keane, Jacqueline V.; Urban, Laurie; Riesen, Timm; Meech, Karen J.; Abe, Shinsuke; Cheng, Yu-Chi; Chen, Wen-Ping; Granvik, Mikael; Grav, Tommy; Ip, Wing-Huen; Kinoshita, Daisuke; Kleyna, Jan; Lacerda, Pedro; Lister, Tim; Milani, Andrea; et al. (2012). "DISCOVERY OF MAIN-BELT COMET P/2006 VW 139 BY Pan-STARRS1". The Astrophysical Journal. 748 (1): L15. arXiv:1202.2126. Bibcode:2012ApJ...748L..15H. doi:10.1088/2041-8205/748/1/L15. S2CID 8693844.
• New Comet: P/2012 T1 (PANSTARRS) (Remanzacco Observatory : 16 Oct 2012)
• Ferrín, Ignacio; Zuluaga, Jorge; Cuartas, Pablo (2013). "The location of Asteroidal Belt Comets (ABCs), in a comet's evolutionary diagram: The Lazarus Comets". Monthly Notices of the Royal Astronomical Society. 434 (3): 1821–1837. arXiv:1305.2621. Bibcode:2013MNRAS.434.1821F. doi:10.1093/mnras/stt839. S2CID 118177774.
• P/2013 R3: a Main Belt Comet that is breaking apart. J. Licandro New images obtained with the GTC