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

| category = Kuiper belt
| epoch = 14 July 2004 (JD 2453200.5)
| semimajor = 6451.398 Gm (43.129 AU)
| perihelion = 6120.810 Gm (40.915 AU)
| aphelion = 6781.985 Gm (45.335 AU)
| eccentricity = 0.051
| period = 103440.6 days (283.20 years)
| speed = 4.53 km/s
| inclination = 17.2°
| asc_node = 97.296°
| arg_peri = 271.631°
| mean_anomaly = 89.673°
| dimensions = 936   km
| mass = ~5.9 kg
| density = ~1 g/cm³
| gravity = 0.15 m/s²
| escape_velocity = 0.39 km/s
| rotation = 0.26432 days (3.17 hours)
| spectral_class =(moderately red) B-V=0.93 V-R=0.64)
| abs_mag = 3.7
| albedo = 0.037
| temperature = ~43 K }}
20000 Varuna ("VAR oo na") is a large classical Kuiper Belt object.

It is named after the Hindu god Varuna. It previously had the provisional designation ' and has been precovered in plates dating back to 1953.

Size

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.

The recent thermal model used by Grundy et al, 2005 estimates the size at 936 km'. This estimate is based on the observations by Jewitt et al, 2001 (900 ) and Lelouch et al, 2002 (1060 ).

Orbit

left

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 50000 Quaoar but more inclined.
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, unknown at this time (2002) for objects so large, (Jewitt & Sheppard, 2002) consider possible models for the shape and density concluding that the most probable interpretation fitting the data is an elongated spheroid (ratio of axis 2:3), composed from a porous mixture of rock, with a mean density around 1g/cm³ (roughly the density of water ice). Since then, another even larger, rapidly rotating (3.9 h) object has been discovered. An elongated shape was also suggested in that case (see references there).

The surface is moderately red (similar to 50000 Quaoar) but exceptionally dark (albedo<0.04) compared with other large classical Kuiper Belt objects indicating it is largely devoid of ice.

References

* Preprint on arXiv.
* Preprint on arXiv.
* Preprint on arXiv.
* PMID 11373669. Reprint on the Author's site.

External links

* Site updated Reretrived on 2006-07-10
* Orbit simulation from NASA JPL site
Varuna