Active asteroids aresmall Solar System bodies that haveasteroid-like orbits but showcomet-like visual characteristics.[1] That is, they show acoma,tail, or other visual evidence of mass-loss (like a comet), but their orbits remain withinJupiter's orbit (like an asteroid).[2][3] These bodies were originally designatedmain-belt comets (MBCs) in 2006 by astronomersDavid Jewitt andHenry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within themain-belt, whereas the growing population of active asteroids shows that this is not always the case.[2][4][5]
The first active asteroid discovered is7968 Elst–Pizarro. It was discovered (as an asteroid) in 1979 but then was found to have a tail byEric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro.[2][6]
Unlikecomets, which spend most of their orbit at Jupiter-like or greater distances from the Sun, active asteroids follow orbits within the orbit ofJupiter that are often indistinguishable from the orbits of standardasteroids.Jewitt defines active asteroids as those bodies that, in addition to having visual evidence of mass loss, have an orbit with:[3]
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 realSolar System deviating from an idealizedrestricted three-body problem.[3]
The first three identified active asteroids all orbit within the outer part of theasteroid belt.[7]
Some active asteroids display a cometary dust tail only for a part of their orbit nearperihelion. This strongly suggests that volatiles at their surfaces are sublimating, driving off the dust.[10] Activity in133P/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 ofvolatile material helping to expose them tosolar 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 of596 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 (Catalina–PanSTARRS) was discovered independently by two observers byRichard E. Hill using the Catalina Sky Survey's 0.68-m Schmidt telescope and byBryce T. Bolin using the 1.8-m Pan-STARRS1 telescope on Haleakala.[14] The discovery images taken byPan-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 mGran Telescopio Canarias on the island ofLa 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 theSierra Nevada Observatory in Granada on October 12.[15]
By smashing into the asteroid moon of thebinary asteroid65803 Didymos, NASA'sDouble 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.[17][18] Observations show that Dimorphos lost approximately 1 million kilograms after the collision.[19] Impact produced a dust plume that temporarily brightened the Didymos system and developed a 10,000-kilometer (6,200 mi)-longdust tail that persisted for several months.[20][21][22] The DART impact is predicted to have caused global resurfacing and deformation of Dimorphos's shape, leaving animpact crater several tens of meters in diameter.[23][24][25] The impact has likely sent Dimorphos into achaoticallytumbling rotation that will subject the moon to irregulartidal forces by Didymos before it will eventually return to atidally locked state within several decades.[26][27][28]
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.[29] European scientists have proposed a sample-return mission from a MBC calledCaroline to analyse the content of volatiles and collect dust samples.[10]
Asteroid101955 Bennu seen ejecting particles on January 6, 2019, in images taken by theOSIRIS-REx spacecraft
Castalia is a proposed mission concept for a robotic spacecraft to explore133P/Elst–Pizarro and make the firstin situ measurements of water in the asteroid belt, and thus, help solve the mystery of the origin of Earth's water.[64] The lead is Colin Snodgrass, fromThe Open University in the UK.Castalia was proposed in 2015 and 2016 to theEuropean Space Agency within theCosmic Vision programme missions M4 and M5, but it was not selected. The team continues to mature the mission concept and science objectives.[64] Because of the construction time required and orbital dynamics, a launch date of October 2028 was proposed.[64]
On January 6, 2019, theOSIRIS-REx mission first observed episodes of particle ejection from101955 Bennu shortly after entering orbit around thenear-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.[65]
^Snodgrass, Colin; Tubiana, Cecilia; Vincent, Jean-Baptiste; Sierks, Holger; Hviid, Stubbe; Moissl, Richard; Boehnhardt, Hermann; Barbieri, Cesare; et al. (2010). "A collision in 2009 as the origin of the debris trail of asteroid P/2010?A2".Nature.467 (7317):814–6.arXiv:1010.2883.Bibcode:2010Natur.467..814S.doi:10.1038/nature09453.PMID20944742.S2CID4330570.
^Raducan, S. D.; Jutzi, M.; Zhang, Y.; Cheng, A. F.; Collins, G. S.; Davison, T. M.; et al. (March 2023).Low Strength of Asteroid Dimorphos As Demonstrated by the Dart Impact(PDF). 54th Lunar and Planetary Science Conference 2023. Lunar and Planetary Institute. Retrieved4 February 2023.
^Meyer, A. J.; Noiset, G.; Karatekin, Ö.; McMahon, J.; Agrusa, H. F.; Nakano, R.; et al. (March 2023).Tidal Dissipation in Didymos Following the DART Impact(PDF). 54th Lunar and Planetary Science Conference 2023. Lunar and Planetary Institute. Retrieved4 February 2023.
^Tholen, David J.; Sheppard, Scott S.; Trujillo, Chad A. (November 2015). "Evidence for an Impact Event on (493) Griseldis".DPS.47: 414.03.Bibcode:2015DPS....4741403T.
^Ferrín, Ignacio; Hamanowa, Hiromi; Hamanowa, Hiroko; Hernández, Jesús; Sira, Eloy; Sánchez, Albert; Zhao, Haibin; Miles, Richard (September 2012). "The 2009 Apparition of methuselah comet 107P/Wilson–Harrington: A case of comet rejuvenation?".Planetary and Space Science.70 (1):59–72.arXiv:1205.6874.Bibcode:2012P&SS...70...59F.doi:10.1016/j.pss.2012.05.022.S2CID118530975.
^Fernández, Yanga R.; McFadden, Lucy A.; Lisse, Carey M.; Helin, Eleanor F.; Chamberlin, Alan B. (July 1997). "Analysis of POSS Images of Comet–Asteroid Transition Object 107P/1949 W1 (Wilson–Harrington)".Icarus.128 (1):114–126.Bibcode:1997Icar..128..114F.doi:10.1006/icar.1997.5728.
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