949 Hel
{{Short description|Main-belt asteroid}}
{{Use dmy dates|date=February 2020}}
{{Infobox planet
| minorplanet = yes
| name = 949 Hel
| background = #D6D6D6
| image =
| image_size =
| caption =
| discoverer = M. F. Wolf
| discovery_site = Heidelberg Obs.
| discovered = 11 March 1921
| mpc_name = (949) Hel
| alt_names = A921 EM{{·}}1952 DN
{{mp|1954 SN|1}}{{·}}1921 JK
| pronounced = {{IPAc-en|'|h|ɛ|l}}Noah Webster (1884) A Practical Dictionary of the English Language
| named_after = Hel {{small|(Norse mythology)}}
| mp_category = main-belt{{·}}{{small|(outer)}}
background
| epoch = 31 May 2020 (JD 2459000.5)
| uncertainty = 0
| observation_arc = 91.34 yr (33,361 d)
| aphelion = 3.5804 AU
| perihelion = 2.4255 AU
| semimajor = 3.0029 AU
| eccentricity = 0.1923
| period = 5.20 yr (1,901 d)
| mean_anomaly = 358.26°
| mean_motion = {{Deg2DMS|0.1894|sup=ms}} / day
| inclination = 10.701°
| asc_node = 321.17°
| arg_peri = 249.54°
| mean_diameter = {{plainlist|
| mass = {{val|1.73|0.62|e=18|ul=kg}}
| density = {{val|12.86|5.19|ul=g/cm3}}
| rotation = {{val|8.215|0.001|ul=h}}
| albedo = {{plainlist|
| spectral_type = {{plainlist|
}}
949 Hel (prov. designation: {{mp|A921 EM}} or {{mp|1921 JK}}) is a dark background asteroid from the outer regions of the asteroid belt, approximately {{convert|63|km|mi|abbr=off|sigfig=2|sp=us}} in diameter. It was discovered by German astronomer Max Wolf at the Heidelberg Observatory on 11 March 1921. 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.
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. 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. 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.
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 ({{small|RI 1013}}).
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.
= 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 {{val|8.215|0.001}} hours with a brightness variation of {{val|0.13|0.01}} magnitude ({{small|U=2+}}). 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 {{val|10.862|0.007}} and {{val|10.85|0.05}} hours with an amplitude of 0.12 and 0.14, respectively.
= 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 {{val|60.98|0.74}}, {{val|63.494|0.743}} and {{val|69.17|1.4}} kilometers in diameter, and its surface has a corresponding albedo of {{val|0.063|0.002}}, {{val|0.058|0.013}} and {{val|0.0487|0.002}}, respectively. 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.
Benoit Carry estimates a diameter of {{val|63.56|4.01}} kilometers, along with a mass of {{val|1.73|0.62|e=18|u=kg}} and a density of {{val|12.86|5.19|u=g/cm3}}. Apart from the above mentioned {{val|63.494|0.743}} kilometers, the WISE team has also published three more mean-diameters of {{val|52.16|14.00|u=km}} and {{val|61.90|18.34|u=km}} and {{val|66.742|1.227|u=km}} with albedos of {{val|0.06|0.03}}, {{val|0.05|0.03}} and {{val|0.0523|0.0057}}, respectively.
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. These timed observations are taken when the asteroid passes in front of a distant star.
References
{{reflist|30em|refs=
|type = 2020-01-07 last obs.
|title = JPL Small-Body Database Browser: 949 Hel (A921 EM)
|url = https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2000949
|publisher = Jet Propulsion Laboratory
|access-date = 14 February 2020}}
|title = Dictionary of Minor Planet Names
|url = https://archive.org/details/dictionaryminorp00schm
|url-access = limited
|last = Schmadel | first = Lutz D.
|publisher = Springer Berlin Heidelberg
|chapter = (949) Hel
|page = [https://archive.org/details/dictionaryminorp00schm/page/n97 84]
|date = 2007
|doi = 10.1007/978-3-540-29925-7_950
|isbn = 978-3-540-00238-3}}
|title = 949 Hel (A921 EM)
|work = Minor Planet Center
|url = https://www.minorplanetcenter.net/db_search/show_object?object_id=949
|access-date = 14 February 2020}}
|title = Asteroids and comets rotation curves – (949) Hel
|last = Behrend |first = Raoul
|publisher = Geneva Observatory
|url = http://obswww.unige.ch/~behrend/page3cou.html#000949
|access-date = 14 February 2020}}
|display-authors = 6
|first1 = Joseph R. |last1 = Masiero
|first2 = T. |last2 = Grav
|first3 = A. K. |last3 = Mainzer
|first4 = C. R. |last4 = Nugent
|first5 = J. M. |last5 = Bauer
|first6 = R. |last6 = Stevenson
|first7 = S. |last7 = Sonnett
|date = August 2014
|title = Main-belt Asteroids with WISE/NEOWISE: Near-infrared Albedos
|journal = The Astrophysical Journal
|volume = 791
|issue = 2
|page = 11
|bibcode = 2014ApJ...791..121M
|doi = 10.1088/0004-637X/791/2/121
|arxiv = 1406.6645}}
|display-authors = 6
|first1 = Fumihiko |last1 = Usui
|first2 = Daisuke |last2 = Kuroda
|first3 = Thomas G. |last3 = Müller
|first4 = Sunao |last4 = Hasegawa
|first5 = Masateru |last5 = Ishiguro
|first6 = Takafumi |last6 = Ootsubo
|first7 = Daisuke |last7 = Ishihara
|first8 = Hirokazu |last8 = Kataza
|first9 = Satoshi |last9 = Takita
|first10 = Shinki |last10 = Oyabu
|first11 = Munetaka |last11 = Ueno
|first12 = Hideo |last12 = Matsuhara
|first13 = Takashi |last13 = Onaka
|date = October 2011
|title = Asteroid Catalog Using Akari: AKARI/IRC Mid-Infrared Asteroid Survey
|journal = Publications of the Astronomical Society of Japan
|volume = 63
|issue = 5
|pages = 1117–1138
|bibcode = 2011PASJ...63.1117U
|doi = 10.1093/pasj/63.5.1117
|doi-access= free
}} ([http://vizier.cfa.harvard.edu/viz-bin/VizieR-5?-source=J/PASJ/63/1117/acua_v1&Num=949 online], [https://core.ac.uk/download/pdf/43545172.pdf AcuA catalog p. 153])
|first1 = E. F. |last1 = Tedesco
|first2 = P. V. |last2 = Noah
|first3 = M. |last3 = Noah
|first4 = S. D. |last4 = Price
|date = October 2004
|title = IRAS Minor Planet Survey V6.0
|url = https://sbnarchive.psi.edu/pds3/iras/IRAS_A_FPA_3_RDR_IMPS_V6_0/data/diamalb.tab
|journal = NASA Planetary Data System
|volume = 12
|pages = IRAS-A-FPA-3-RDR-IMPS-V6.0
|bibcode = 2004PDSS...12.....T
|access-date = 14 February 2020}}
|title = LCDB Data for (949) Hel
|publisher = Asteroid Lightcurve Database (LCDB)
|url = http://www.minorplanet.info/PHP/generateOneAsteroidInfo.php?AstInfo=949
|access-date = 14 February 2020}}
|title = Asteroid 949 Hel
|work = Small Bodies Data Ferret
|url = https://sbntools.psi.edu/ferret/SimpleSearch/results.action?targetName=949+Hel
|access-date = 14 February 2020}}
|title = Asteroid 949 Hel – Proper Elements
|publisher = AstDyS-2, Asteroids – Dynamic Site
|url = https://newton.spacedys.com/astdys/index.php?pc=1.1.6&n=949
|access-date= 14 February 2020}}
|first1 = D. |last1 = Lazzaro
|first2 = C. A. |last2 = Angeli
|first3 = J. M. |last3 = Carvano
|first4 = T. |last4 = Mothé-Diniz
|first5 = R. |last5 = Duffard
|first6 = M. |last6 = Florczak
|date = November 2004
|title = S3OS2: the visible spectroscopic survey of 820 asteroids
|url = http://sirrah.troja.mff.cuni.cz/yarko-site/tmp/eos/NEW/spectral_type_figure/s3os2.pdf
|journal = Icarus
|volume = 172
|issue = 1
|pages = 179–220
|bibcode = 2004Icar..172..179L
|doi = 10.1016/j.icarus.2004.06.006
|access-date= 14 February 2020}}
|first1 = B.
|last1 = Carry
|title = Density of asteroids
|work = Planetary and Space Science
|volume = 73
|pages = 98–118
|date = December 2012
|doi = 10.1016/j.pss.2012.03.009
|bibcode = 2012P&SS...73...98C
|arxiv = 1203.4336}} See Table 1.
}}
External links
- [http://www.minorplanet.info/PHP/OneAsteroidInfo.php Lightcurve Database Query] (LCDB), at www.minorplanet.info
- [https://books.google.com/books?id=aeAg1X7afOoC&pg Dictionary of Minor Planet Names], Google books
- [http://obswww.unige.ch/~behrend/page_cou.html Asteroids and comets rotation curves, CdR] – Geneva Observatory, Raoul Behrend
- [http://www.minorplanetcenter.net/iau/lists/NumberedMPs000001.html Discovery Circumstances: Numbered Minor Planets (1)-(5000)] – Minor Planet Center
- {{AstDys|949}}
- {{JPL small body}}
{{Minor planets navigator |948 Jucunda |number=949 |950 Ahrensa}}
{{Small Solar System bodies}}
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