949 Hel

949 Hel (prov. designation: A921 EM or 1921 JK) is a dark background asteroid from the outer regions of the asteroid belt, approximately 63 kilometers (39 miles) in diameter. It was discovered by German astronomer Max Wolf at the Heidelberg Observatory on 11 March 1921.[1] The transitional X-type asteroid has a rotation period of 8.2 hours. It was named in memory of the discoverer, after the Norse goddess of the dead, Hel.[3]

949 Hel
Discovery [1]
Discovered byM. F. Wolf
Discovery siteHeidelberg Obs.
Discovery date11 March 1921
Designations
(949) Hel
Pronunciation/ˈhɛl/[2]
Named after
Hel (Norse mythology)[3]
A921 EM · 1952 DN
1954 SN1 · 1921 JK
main-belt [1][4] · (outer)
background [5][6]
Orbital characteristics [4]
Epoch 31 May 2020 (JD 2459000.5)
Uncertainty parameter 0
Observation arc91.34 yr (33,361 d)
Aphelion3.5804 AU
Perihelion2.4255 AU
3.0029 AU
Eccentricity0.1923
5.20 yr (1,901 d)
358.26°
0° 11m 21.84s / day
Inclination10.701°
321.17°
249.54°
Physical characteristics
Mean diameter
  • 60.98±0.74 km[7]
  • 63.494±0.743 km[8]
  • 63.56±4.01 km[9]
  • 69.17±1.4 km[10]
Mass(1.73±0.62)×1018 kg[9]
Mean density
12.86±5.19 g/cm3[9]
8.215±0.001 h[11][12]
9.8[1][4]

    Orbit and classification

    Hel is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements.[5][6] It orbits the Sun in the outer asteroid belt at a distance of 2.4–3.6 AU once every 5 years and 2 months (1,901 days; semi-major axis of 3 AU). Its orbit has an eccentricity of 0.19 and an inclination of 11° with respect to the ecliptic.[4] The body's observation arc begins at Lowell Observatory on 11 January 1931, almost a decade after its official discovery observation at Heidelberg in March 1921.[1]

    Naming

    This minor planet was named from Norse mythology, after Hel, the goddess of the dead and the queen of the underworld. The asteroid's name was proposed by the widow of Max Wolf, who had died two years earlier (RI 1013).[3]

    Physical characteristics

    In the Tholen-like taxonomy of the Small Solar System Objects Spectroscopic Survey (S3OS2), Hel is an X-type asteroid, while in the SMASS-like taxonomy of the S3OS2, it is a Xk-subtype that transitions from the X-type to the uncommon K-type asteroids.[6][13]

    Rotation period

    In September 2016, a rotational lightcurve of Hel was obtained from photometric observations by Pedro Brines and colleges of the Spanish group of asteroid observers (OBAS). Lightcurve analysis gave a rotation period of 8.215±0.001 hours with a brightness variation of 0.13±0.01 magnitude (U=2+).[12] The result supersedes previous observations by French amateur astronomers Laurent Bernasconi and René Roy in December 2001 and February 2004, which gave two tentative periods of 10.862±0.007 and 10.85±0.05 hours with an amplitude of 0.12 and 0.14, respectively.[11][14]

    Diameter, mass and albedo


    According to the surveys carried out by the Japanese Akari satellite, the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE), and the Infrared Astronomical Satellite IRAS, Hel measures 60.98±0.74, 63.494±0.743 and 69.17±1.4 kilometers in diameter, and its surface has a corresponding albedo of 0.063±0.002, 0.058±0.013 and 0.0487±0.002, respectively.[7][8][10] The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0445 and a diameter of 69.11 km based on an absolute magnitude of 9.8.[11]

    Benoit Carry estimates a diameter of 63.56±4.01 kilometers, along with a mass of (1.73±0.62)×1018 kg and a density of 12.86±5.19 g/cm3.[9] Apart from the above mentioned 63.494±0.743 kilometers, the WISE team has also published three more mean-diameters of 52.16±14.00 km and 61.90±18.34 km and 66.742±1.227 km with albedos of 0.06±0.03, 0.05±0.03 and 0.0523±0.0057, respectively.[6][11]

    An asteroid occultation on 27 October 2005, gave a best-fit ellipse dimension of 69.0 × 69.0 kilometers, while a second occultation event gave an ellipse of 61.0 km × 61.0 km on 3 February 2014, with the latter having a better quality rating.[6] These timed observations are taken when the asteroid passes in front of a distant star.[6]

    References
    1. "949 Hel (A921 EM)". Minor Planet Center. Retrieved 14 February 2020.
    2. Noah Webster (1884) A Practical Dictionary of the English Language
    3. Schmadel, Lutz D. (2007). "(949) Hel". Dictionary of Minor Planet Names. Springer Berlin Heidelberg. p. 84. doi:10.1007/978-3-540-29925-7_950. ISBN 978-3-540-00238-3.
    4. "JPL Small-Body Database Browser: 949 Hel (A921 EM)" (2020-01-07 last obs.). Jet Propulsion Laboratory. Retrieved 14 February 2020.
    5. "Asteroid 949 Hel – Proper Elements". AstDyS-2, Asteroids – Dynamic Site. Retrieved 14 February 2020.
    6. "Asteroid 949 Hel". Small Bodies Data Ferret. Retrieved 14 February 2020.
    7. Usui, Fumihiko; Kuroda, Daisuke; Müller, Thomas G.; Hasegawa, Sunao; Ishiguro, Masateru; Ootsubo, Takafumi; et al. (October 2011). "Asteroid Catalog Using Akari: AKARI/IRC Mid-Infrared Asteroid Survey". Publications of the Astronomical Society of Japan. 63 (5): 1117–1138. Bibcode:2011PASJ...63.1117U. doi:10.1093/pasj/63.5.1117. Retrieved 14 February 2020. (online, AcuA catalog p. 153)
    8. Masiero, Joseph R.; Grav, T.; Mainzer, A. K.; Nugent, C. R.; Bauer, J. M.; Stevenson, R.; et al. (August 2014). "Main-belt Asteroids with WISE/NEOWISE: Near-infrared Albedos". The Astrophysical Journal. 791 (2): 11. arXiv:1406.6645. Bibcode:2014ApJ...791..121M. doi:10.1088/0004-637X/791/2/121.
    9. Carry, B. (December 2012). "Density of asteroids". Planetary and Space Science. 73: 98–118. arXiv:1203.4336. Bibcode:2012P&SS...73...98C. doi:10.1016/j.pss.2012.03.009. See Table 1.
    10. Tedesco, E. F.; Noah, P. V.; Noah, M.; Price, S. D. (October 2004). "IRAS Minor Planet Survey V6.0". NASA Planetary Data System. 12: IRAS-A-FPA-3-RDR-IMPS-V6.0. Bibcode:2004PDSS...12.....T. Retrieved 14 February 2020.
    11. "LCDB Data for (949) Hel". Asteroid Lightcurve Database (LCDB). Retrieved 14 February 2020.
    12. Brines, Pedro; Lozano, Juan; Rodrigo, Onofre; Fornas, A.; Herrero, David; Mas, Vicente (April 2017). "Sixteen Asteroids Lightcurves at Asteroids Observers (OBAS) - MPPD: 2016 June-November" (PDF). The Minor Planet Bulletin. 44 (2): 145–149. Bibcode:2017MPBu...44..145B.
    13. Lazzaro, D.; Angeli, C. A.; Carvano, J. M.; Mothé-Diniz, T.; Duffard, R.; Florczak, M. (November 2004). "S3OS2: the visible spectroscopic survey of 820 asteroids" (PDF). Icarus. 172 (1): 179–220. Bibcode:2004Icar..172..179L. doi:10.1016/j.icarus.2004.06.006. Retrieved 14 February 2020.
    14. Behrend, Raoul. "Asteroids and comets rotation curves – (949) Hel". Geneva Observatory. Retrieved 14 February 2020.
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