| Discovery[1] | |
|---|---|
| Discovered by | M. F. Wolf |
| Discovery site | Heidelberg Obs. |
| Discovery date | 11 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 arc | 91.34yr (33,361 d) |
| Aphelion | 3.5804AU |
| Perihelion | 2.4255 AU |
| 3.0029 AU | |
| Eccentricity | 0.1923 |
| 5.20 yr (1,901 d) | |
| 358.26° | |
| 0° 11m 21.84s / day | |
| Inclination | 10.701° |
| 321.17° | |
| 249.54° | |
| Physical characteristics | |
| Mass | (1.73±0.62)×1018 kg[9] |
Meandensity | 12.86±5.19 g/cm3[9] |
| 8.215±0.001 h[11][12] | |
| 9.8[1][4] | |
949 Hel (prov. designation:A921 EMor1921 JK) is a darkbackground asteroid from the outer regions of theasteroid belt, approximately 63 kilometers (39 miles) in diameter. It was discovered by German astronomerMax Wolf at theHeidelberg Observatory on 11 March 1921.[1] The transitionalX-type asteroid has arotation period of 8.2 hours. It was named in memory of the discoverer, after the Norse goddess of the dead,Hel.[3]
Hel is a non-family asteroid of the main belt'sbackground population when applying thehierarchical clustering method to itsproper orbital elements.[5][6] It orbits the Sun in theouter 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 aneccentricity of 0.19 and aninclination of 11° with respect to theecliptic.[4] The body'sobservation arc begins atLowell Observatory on 11 January 1931, almost a decade after its official discovery observation at Heidelberg in March 1921.[1]
Thisminor planet was named fromNorse mythology, afterHel, the goddess of the dead and the queen of the underworld. The asteroid's name was proposed by the widow ofMax Wolf, who had died two years earlier (RI 1013).[3]
In the Tholen-like taxonomy of theSmall Solar System Objects Spectroscopic Survey (S3OS2),Hel is anX-type asteroid, while in the SMASS-like taxonomy of the S3OS2, it is a Xk-subtype that transitions from the X-type to the uncommonK-type asteroids.[6][13]
In September 2016, a rotationallightcurve ofHel was obtained fromphotometric observations by Pedro Brines and colleges of the Spanish group of asteroid observers (OBAS). Lightcurve analysis gave arotation period of8.215±0.001 hours with a brightness variation of0.13±0.01magnitude (U=2+).[12] The result supersedes previous observations by French amateur astronomersLaurent Bernasconi andRené Roy in December 2001 and February 2004, which gave two tentative periods of10.862±0.007 and10.85±0.05 hours with an amplitude of 0.12 and 0.14, respectively.[11][14]
According to the surveys carried out by the JapaneseAkari satellite, theNEOWISE mission of NASA'sWide-field Infrared Survey Explorer (WISE), and the Infrared Astronomical SatelliteIRAS,Hel measures60.98±0.74,63.494±0.743 and69.17±1.4 kilometers in diameter, and its surface has a correspondingalbedo of0.063±0.002,0.058±0.013 and0.0487±0.002, respectively.[7][8][10] TheCollaborative Asteroid Lightcurve Link derives an albedo of 0.0445 and a diameter of 69.11 km based on anabsolute magnitude of 9.8.[11]
Benoit Carry estimates a diameter of63.56±4.01 kilometers, along with a mass of(1.73±0.62)×1018 kg and a density of12.86±5.19 g/cm3.[9] Apart from the above mentioned63.494±0.743 kilometers, the WISE team has also published three more mean-diameters of52.16±14.00 km and61.90±18.34 km and66.742±1.227 km with albedos of0.06±0.03,0.05±0.03 and0.0523±0.0057, respectively.[6][11]
Anasteroid 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]
