6478 Gault

6478 Gault, provisional designation 1988 JC1, is a Phocaea asteroid from the inner regions of the asteroid belt, approximately 3.7 kilometers (2.3 miles) in diameter. The likely S-type asteroid was discovered on 12 May 1988, by astronomer couple Carolyn and Eugene Shoemaker at the Palomar Observatory in California. It was named in honor of planetary geologist Donald Gault.[1] In January 2019, it was found that Gault shows cometary activity and that it has multiple tails, making it an active asteroid.[8] It was subsequently realised that it had been active since at least 2013.[9]

6478 Gault
Gault's two comet-like tails seen from Hubble in March 2019
Discovery [1]
Discovered byC. Shoemaker
E. Shoemaker
Discovery sitePalomar Obs.
Discovery date12 May 1988
Designations
(6478) Gault
Pronunciation/ˈɡɔːlt/
Named after
Donald Gault [1]
(American planetary geologist)
1988 JC1 · 1995 KC1
main-belt [1][2] · (inner)
Phocaea [3][4] · MBC
Orbital characteristics [2]
Epoch 27 April 2019 (JD 2458600.5)
Uncertainty parameter 0
Observation arc29.55 yr (10,792 d)
Aphelion2.7514 AU
Perihelion1.8589 AU
2.3051 AU
Eccentricity0.1936
3.50 yr (1,278 d)
289.35°
0° 16m 53.76s / day
Inclination22.811°
183.56°
2023-Jul-04[5]
83.268°
Physical characteristics
Mean diameter
3.7 km (est. at 0.22)[6]
1.794±0.001 h[7]
0.22 (est. family-based)
S (est. family-based)
14.4[1][2]

    Orbit and classification

    Gault is a core member of the Phocaea family (701).[3][4] The large asteroid family consists of nearly 2,000 known stony asteroids, and was named after its largest member, 25 Phocaea. The old family formed up to 2.2 billion years ago and has the highest inclination of all families in the inner asteroid belt. Several of its members are also Mars-crossing asteroids with high eccentricities.[10][11]:23

    It orbits the Sun at a distance of 1.9–2.8 AU once every 3 years and 6 months (1,278 days; semi-major axis of 2.31 AU). Its orbit has an eccentricity of 0.19 and an inclination of 23° with respect to the ecliptic.[2] The body's observation arc begins with its official discovery observation at Palomar in May 1988.[1] It last came to perihelion in January 2020 and will next come to perihelion in July 2023.[5]

    Main-belt comet

    On January 5, 2019, it was discovered that Gault possesses a comet tail, which had not been present in previous images taken during the 2018/19 opposition.[12][8] The asteroid began to break up as its spin accelerated due to the YORP effect, and its rotation speed approached two hours – near the limit of stability for an asteroid. The ejected matter created two dust tails,[7] and the longer one has been estimated at over 800,000 km (500,000 mi) long. Earlier hypotheses of a collision with another asteroid were ruled out as a source of the tail-forming dust, and its two tails were believed to be a result of sudden dust ejections near October 28 and December 30, 2018.[8] It is also possible that solar heating caused sublimation of ice, perhaps beneath the surface in a "pocket," and the force of the ejection of material (after being exposed to solar heating during the rotation of the asteroid) resulted in the spin rate increase. A prior example of such an occurrence is in the literature.[13]

    In April 2019, upon analyzing archive images taken in 2013, 2016 and 2017, it was found that Gault had been perpetually active for at least five years before the discovery, with a tail visible when the asteroid was near its furthest distance from the Sun during the 2013 apparition. If its activity is indeed caused by a rotational breakup, then Gault has remained active far longer than any other object of this type seen before. This indicates that it may represent a new type of object.[9]

    Naming

    This minor planet was named in memory of American planetary geologist Donald Gault (1923–1999), an expert in the field of impact crater forming processes. Gault conducted field experiments and applied his insight to the interpretation of impact data from the Moon, Earth, Mars and Mercury.[1] The official naming citation was published by the Minor Planet Center on 28 July 1999 (M.P.C. 35484).[14]

    Physical characteristics

    Gault's spectral type do resemble a stony S-type asteroid, according its membership to the Phocaea family,[3] but some of the features of the spectrum are more similar to the carbonaceous C-type asteroid class.[7] Based on a generic magnitude-to-diameter conversion,[6] the asteroid measures approximately 3.7 kilometers in diameter, for an assumed family-specific albedo of 0.22,[11]:23 and an absolute magnitude of 14.4.[1] Rotational lightcurves of Gault obtained from photometric observations in 2019 showed a rotation period of approximately 1.79 hours.[7] The body's pole and shape remain unknown.[2][15]

    • Background stars surrounding asteroid 6478 Gault.[16]
    • (6478) Gault imaged on 2019-03-26 with iTelescope's T21
    • JPL Small-Body Database Browser's Orbit Diagram for the Main-belt Asteroid 6478 Gault on 2019-01-01 at 00:00 UTC

    References
    1. "6478 Gault (1988 JC1)". Minor Planet Center. Retrieved 10 January 2019.
    2. "JPL Small-Body Database Browser: 6478 Gault (1988 JC1)" (2017-11-28 last obs.). Jet Propulsion Laboratory. Retrieved 10 January 2019.
    3. "Asteroid 6478 Gault". Small Bodies Data Ferret. Retrieved 10 January 2019.
    4. "Asteroid (6478) Gault – Proper elements". AstDyS-2, Asteroids – Dynamic Site. Retrieved 25 May 2018.
    5. JPL Horizons Observer Location: @sun (Perihelion occurs when deldot changes from negative to positive.)
    6. "Asteroid Size Estimator". CNEOS NASA/JPL. Retrieved 10 January 2019.
    7. Ivanova, Oleksandra; Skorov, Yuri; Luk'yanyk, Igor; Tomko, Dušan; Husárik, Marek; Blum, Jürgen; Egorov, Oleg; Voziakova, Olga (2020). "Activity of (6478) Gault during January 13 - March 28, 2019". arXiv:2005.12030 [astro-ph.EP].
    8. Garner, Rob (28 March 2019). "Hubble Watches Spun-Up Asteroid Coming Apart". NASA. Retrieved 28 March 2019.
    9. C. O. Chandler; J. Kueny; A. Gustafsson; C. A. Trujillo; T. D. Robinson; D. E. Trilling (23 April 2019), Six Years of Sustained Activity from Active Asteroid (6478) Gault, arXiv:1904.10530
    10. Carruba, V. (September 2009). "An analysis of the region of the Phocaea dynamical family". Monthly Notices of the Royal Astronomical Society. 398 (3): 1512–1526. Bibcode:2009MNRAS.398.1512C. doi:10.1111/j.1365-2966.2009.15220.x.
    11. Nesvorný, D.; Broz, M.; Carruba, V. (December 2014). Identification and Dynamical Properties of Asteroid Families. Asteroids IV. pp. 297–321. arXiv:1502.01628. Bibcode:2015aste.book..297N. doi:10.2458/azu_uapress_9780816532131-ch016. ISBN 9780816532131.
    12. Recent CBETs (CBET 4594), Minor Planet Electronic Circular, IAU–Minor Planet Center
    13. Russell, Ray (1986). "Multiple aperture airborne infrared measurements of Comet Halley". ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus (SEE N87-25908 19-90). 250. pp. 125–128. Bibcode:1986ESASP.250b.125R.
    14. "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 10 January 2019.
    15. "LCDB Data for (6478) Gault – Not in Data Base". Asteroid Lightcurve Database (LCDB). Retrieved 10 January 2019.
    16. "Hubble Captures Rare Active Asteroid". www.spacetelescope.org. Retrieved 29 March 2019.

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