20000 Varuna

This article is about 20000 Varuna, a minor planet. For other uses of the word, see Varuna (disambiguation)

20000 Varuna

Artist's conception of Varuna
Discovery
Discovered by:	R. McMillan (Spacewatch)
Discovery date:	28 November 2000
Orbital characteristics
Epoch 14 July 2004 (JD 2453200.5)
Aphelion	6781.985 Gm (45.335 AU)
Perihelion:	6120.810 Gm (40.915 AU)
Semi-major axis:	6451.398 Gm (43.129 AU)
Eccentricity:	0.051
Orbital period:	103440.6 d (283.20 a)
Avg. orbital speed:	4.53 km/s
Mean anomaly:	89.673°
Inclination:	17.2°
Longitude of ascending node:	97.296°
Argument of perihelion:	271.631°
Physical characteristics
Dimensions:	874 km (avg of thermals)[1]
Mass:	~5.9×1020 kg
Mean density:	0.992 g/cm³[2]
Equatorial surface gravity:	0.15 m/s²
Escape velocity:	0.39 km/s
Sidereal rotation period:	0.13216 d (3.17 h)
Albedo:	0.037-0.21
Temperature:	~43-41 K
Spectral type:	(moderately red) B-V=0.93 V-R=0.64[3]
Absolute magnitude:	3.7

20000 Varuna (pronounced /ˈvɑːrənə/, or less anglicized /ˈvʌruːnə/, from Sanskrit वरुण) is a large classical Kuiper Belt object (KBO). It previously had the provisional designation 2000 WR₁₀₆ and has been precovered in plates dating back to 1953. Under the International Astronomical Union's 2006 draft proposal for the definition of a planet, Varuna would be labelled a dwarf planet if it were proven to be spherical.

Name

Varuna is named after the Hindu god Varuna. He is the god of the sky, rain, oceans and rivers, and the celestial ocean. He is also a god of law and the underworld, especially concerned with the souls of the drowned. He is sometimes considered analogous to Poseidon.

Size

<imagemap> Image:EightTNOs.png|thumb|250px|left|Varuna compared to Eris, Pluto, (136472) 2005 FY₉, (136108) 2003 EL₆₁, Sedna, Orcus, Quaoar, and Earth.

1. Earth

rect 646 1714 2142 1994 The Earth

1. Eris and Dysnomia

circle 226 412 16 Dysnomia circle 350 626 197 (136199) Eris

1. Pluto and Charon

circle 1252 684 86 Charon circle 1038 632 188 (134340) Pluto

1. 2005 FY9

circle 1786 614 142 (136472) 2005 FY9

1. 2003 EL61

circle 2438 616 155 (136108) 2003 EL61

1. Sedna

circle 342 1305 137 (90377) Sedna

1. Orcus

circle 1088 1305 114 (90482) Orcus

1. Quaoar

circle 1784 1305 97 (50000) Quaoar

1. Varuna

circle 2420 1305 58 (20000) Varuna desc none

1. - setting this to "bottom-right" will display a (rather large) icon linking to the graphic, if desired
2. Notes:
3. Details on the new coding for clickable images is here:
4. While it may look strange, it's important to keep the codes for a particular system in order. The clickable coding treats the first object created in an area as the one on top.
5. Moons should be placed on "top" so that their smaller circles won't disappear "under" their respective primaries.

</imagemap> The size of the large KBOs can be determined by simultaneous observations of thermal emission and reflected sunlight. Unfortunately, thermal measures, intrinsically weak for distant objects are further hampered by the absorption of the Earth atmosphere as only the weak ‘tail’ of the emissions is accessible to Earth-based observations. In addition, the estimates are model-dependent with the unknown parameters (e.g. pole orientation and thermal inertia) to be assumed. Consequently, the estimates of the albedo vary resulting in sometimes substantial differences in the inferred size. A recent thermal model estimates the size at 936 ⁺²³⁸₋₃₂₄ km.^[4] This estimate is based on earlier results (900 ⁺¹²⁹₋₁₄₅) ^[5] and (1060 ⁺¹⁸⁰_−-220).^[6]

Orbit

Orbits of Varuna (blue) and Pluto (red).

Varuna is classified as a classical trans-Neptunian object and follows a near-circular orbit with a semi-major axis of ~43 AU, similar to that of Quaoar but more inclined. Its orbital period is similar to Quaoar at 283 Earth years. The graph shows the polar view (top; Varuna’s orbit in blue, Pluto’s in red, Neptune in grey). The spheres illustrate the current (April 2006) positions, relative sizes and colours. The perihelia (q), aphelia (Q) and the dates of passage are also marked. Interestingly, the orbits of Varuna and Pluto have similar inclination and are similarly oriented (the nodes of both orbits are quite close). At 43 AU and on a near-circular orbit, unlike Pluto which is in 2:3 orbital resonance with Neptune, Varuna is free from any significant perturbation from Neptune. The ecliptic view illustrates the comparison of Varuna's near-circular orbit with that of Pluto (highly eccentric, e=0.25), both similarly inclined.

Physical characteristics

Varuna has a rotational period of approximately 3.17 hours (or 6.34 hours, depending on whether the light curve is single or double-peaked). Given the rapid rotation, rare for objects so large, Varuna is thought to be an elongated spheroid (ratio of axis 2:3), with a mean density around 1g/cm³ (roughly the density of water ice).^[7] Examination of Varuna's light curve has found that the best-fit model for Varuna is a triaxial ellipsoid with the axes a,b,c in relations: b/a = 0.63 − 0.80, c/a = 0.45 − 0.52 and a bulk density of 0.992 g/cm³.^[2] Since the discovery of Varuna, another, even larger, rapidly rotating (3.9 h) object (136108) 2003 EL₆₁ has been discovered, also thought to have an elongated shape.^[8] The surface of Varuna is moderately red (similar to Quaoar) but exceptionally dark (albedo<0.04) compared with other large classical Kuiper Belt objects, suggesting that the surface is largely devoid of ice, although small amounts of water ice have been detected on its surface ^[9].

References

External links

• Size and Albedo of Kuiper Belt Object (20000) Varuna. David Jewitt's Home Page. Retrieved on 2005-08-15. Site updated Reretrived on 2006-07-10
• Orbital simulation from JPL (Java) / Ephemeris