79360 Sila–Nunam

79360 Sila–Nunam (provisional designation 1997 CS₂₉) is a cold^[9] classical Kuiper belt object (cubewano) and binary system made up of components of almost equal size, called Sila and Nunam, orbiting beyond Neptune in the Solar System. The name of the system is the combined names of the two bodies, Sila and Nunam.^[13]

79360 Sila–Nunam

Sila–Nunam imaged by the Hubble Space Telescope in 2004
Discovery
Discovered by	J. X. Luu D. C. Jewitt C. A. Trujillo J. Chen[1]
Discovery site	Mauna Kea Obs.
Discovery date	4 February 1997
Designations
MPC designation	(79360) Sila–Nunam
Pronunciation	/ˈsiːlə ˈnuːnəm/[2]
Alternative designations	1997 CS29
Minor planet category	TNO (cubewano)[3][4]
Adjectives	Silaupian, Nunaupian[5]
Orbital characteristics[6]
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 3
Observation arc	6940 days (19.00 yr)
Aphelion	44.8452 AU (6.70875 Tm)
Perihelion	43.3862 AU (6.49048 Tm)
Semi-major axis	44.1157 AU (6.59961 Tm)
Eccentricity	0.016536
Orbital period (sidereal)	293.02 yr (107026 d)
Mean anomaly	331.884°
Mean motion	0.00336367°/day
Inclination	2.240951°
Longitude of ascending node	304.34152°
Time of perihelion	≈ 20 October 2055[7] ±3 months
Argument of perihelion	222.597°
Known satellites	1 at 2,777 ± 19 km (1,725 ± 10 mi)[8][9]
Earth MOID	42.3938 AU (6.34202 Tm)
Jupiter MOID	37.9599 AU (5.67872 Tm)
Physical characteristics
Dimensions	≈ 250±30 km (Sila) ≈ 235±28 km (Nunam) (335+41 −42 km combined)[8][9][10]
Mass	1.084 ± 0.022×1019 kg (combined)[9]
Mean density	0.72+0.37 −0.22 g/cm3
Synodic rotation period	300.24 h (12.510 d)
Sidereal rotation period	12.50995 ± 0.00036 d[11]
Geometric albedo	0.086+0.026 −0.017[8]
Temperature	~42 K (−384 °F)
Spectral type	U−B=0.73[8] B−V=1.08[8][12] V−R=0.66[12]±0.04[8] B−R=1.74[8] V−I=1.25±0.03[8] R−J=1.4[8] V−J=2.06±0.03[8] J−H=0.38±0.08[8] V−H=2.45±0.08[8]
Apparent magnitude	21.54–21.78 (2014–2015)
Absolute magnitude (H)	(combined) 5.5,[12] (individual) 6.2 & 6.3 (diff. = 0.12),[8] 5.2[6]

Discovery

Sila–Nunam was discovered on 4 February 1997 by Jane X. Luu, David C. Jewitt, Chad Trujillo, and Jun Chen at the Mauna Kea Observatory, Hawaii, and given the provisional designation 1997 CS₂₉. It was resolved as a binary system in Hubble observations of 22 October 2002 by Denise C. Stephens and Keith S. Noll and announced on 5 October 2005.

Name

The two components are named after Inuit deities. Sila "air" (Iñupiaq siḷa [siʎə], Inuktitut sila) is the Inuit god of the sky, weather, and life force. Nuna "earth" (Iñupiaq amn Inuktitut nuna-m [nunəm])^[14] is the Earth goddess, in some traditions Sila's wife. Nuna created the land animals and, in some traditions, the Inuit (in other traditions Sila created the first people out of wet sand). Sila breathed life into the Inuit.^[6]

Orbit

Sila–Nunam is a dynamically cold classical system (cubewano). It orbits very close to 4:7 mean-motion resonance with Neptune.^[10]

Physical characteristics

In 2010, thermal flux from Sila–Nunam in the far-infrared was measured by the Herschel Space Telescope. As a result, its size, while it was assumed to be a single body, was estimated to lie within the range of 250 to 420 km (155 to 260 mi).^[10] Now that it is known to be a binary system, one body 95% the size of the other, the diameters are estimated to be 243 and 230 kilometres (151 and 143 mi).

Sila–Nunam is very red in visible light and has a flat featureless spectrum in the near-infrared.^[15][16] There are no water ice absorption bands in its near-infrared spectrum, which resembles that of Ixion.^[17]

Sila–Nunam experiences periodic changes in brightness with the full period, which is equal to the orbital binary period (see below). The light curve is double peaked with the secondary period equal to half of the full period. The rotation of both components of the system is synchronously locked with the orbital motion and both bodies are elongated with their long axes pointing to each other.^[11] From 2009 to 2017 Sila–Nunam experienced mutual occultation events.^[9]

Double system

Sila and Nunam are so close in size (within 5%) that they may be thought of as a double cubewano. Sila is approximately 250 km in diameter and Nunam 236 km. Their albedo is about 9%. They orbit at a distance of 2,777 ± 19 km (1,726 ± 12 mi) every 12.51 days:^[9][11]

Semi-major axis:	2,777 ± 19 km
Orbital period:	12.50995 ± 0.00036 d
Eccentricity:	0.020 ± 0.015°
Inclination:	103.51 ± 0.39°

Each has apparently been resurfaced with ejecta from impacts on the other.^[18]