55637 Uni

Numbering and naming

Uni was numbered (55637) by the Minor Planet Center on 16 February 2003 (M.P.C. 47763). On 1 September 2025, the object was named after Uni, the Etruscan goddess of love and fertility.

Classification

Uni has a perihelion of 36.7 AU, which it will next reach in 2065. As of 2020, Uni is 40 AU from the Sun.

The Minor Planet Center classifies Uni as a cubewano while the Deep Ecliptic Survey (DES) classifies it as scattered-extended. The DES using a 10 My integration (last observation: 2009-10-22) shows it with a minimum perihelion (qmin) distance of 36.3 AU.

It has been observed 212 times with precovery images dating back to 1991.

Physical characteristics

A variability of the visual brightness was detected which could be fit to a period of 14.38 or 16.78 h (depending on a single-peaked or double peaked curve). The light-curve amplitude is ΔM = .

The analysis of combined thermal radiometry of Uni from measurements by the Spitzer Space Telescope and Herschel Space Telescope indicates an effective diameter of 692 ± 23 km and albedo of 0.107. Assuming equal albedos for the primary and secondary it leads to the size estimates of ~664 km and ~190 km, respectively. If the albedo of the secondary is half of that of the primary the estimates become ~640 and ~260 km, respectively. Using an improved thermophysical model slightly different sizes were obtained for Uni and Tinia: 659 km and 230 km, respectively.

Uni has red featureless spectrum in the visible and near-infrared but has a negative slope in the K-band, which may indicate the presence of the methanol compounds on the surface. It is redder than Varuna, unlike its neutral-colored "twin" , in spite of similar brightness and orbital elements.

Composition

With a density of , assuming that the primary and satellite have the same density, Uni is one of the largest known solid objects in the Solar System that is less dense than water. Why this should be is not well understood, because objects of its size in the Kuiper belt often contain a fair amount of rock and are hence pretty dense. To have a similar composition to others large KBOs, it would have to be exceptionally porous; this low density initially surprised astronomers. However, studies by Grundy et al. suggest that at the low temperatures that prevail beyond Neptune, ice is quite strong and can support significant porosity in objects significantly larger than Uni, particularly if rock is present; the low density could thus be a consequence of this object failing to warm sufficiently during its formation to significantly deform the ice and fill these interstitial spaces.

References

References

1. [http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=2002UX25 JPL Horizons] Observer Location: @sun (Perihelion occurs when deldot changes from negative to positive. Uncertainty in time of perihelion is [[3-sigma]].)
2. W.M. Grundy, K.S. Noll, M.W. Buie, S.D. Benecchi, D. Ragozzine & H.G. Roe, 'The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp. (229762) 2007 UK. 126)', ''Icarus'' [http://www2.lowell.edu/~grundy/abstracts/2019.G-G.html (forthcoming, available online 30 March 2019)] {{Webarchive. link. (7 April 2019 DOI: 10.1016/j.icarus.2018.12.037,)
3. (7 April 2019). "The Mutual Orbit, Mass, and Density of Transneptunian Binary".
4. Roatsch Jaumann et al. 2009, p. 765, Tables 24.1–2
5. "JPL Small-Body Database Browser: 55637 (2002 UX25)". [[Jet Propulsion Laboratory]].
6. "MPC/MPO/MPS Archive". Minor Planet Center.
7. Rousselot, P.; Petit, J.-M.; Poulet, F.; Sergeev, A. ''Photometric study of Centaur {{mp. (60558) 2000 EC. 98 and trans-neptunian object {{mp. (55637) 2002 UX. 25 at different phase angles'', Icarus, '''176''', (2005) pp. 478–491.[http://adsabs.harvard.edu/abs/2005Icar..176..478R Abstract.]
8. Cowen, Ron. (2013). "Astronomers surprised by large space rock less dense than water". Nature News.
9. Marsden, Brian G.. (2002-11-01). "MPEC 2002-V08 : 2002 UX25". Harvard-Smithsonian Center for Astrophysics.
10. (2009-02-10). "MPEC 2009-C70 :Distant Minor Planets (2009 FEB. 28.0 TT)". Minor Planet Center.
11. Marc W. Buie. "Orbit Fit and Astrometric record for 55637". SwRI (Space Science Department).
12. (2013). "TNOs are Cool: A survey of the trans-Neptunian region. VIII. Combined Herschel PACS and SPIRE observations of 9 bright targets at 70–500 ''μ''m.". Astronomy & Astrophysics.
13. M.E. Brown. (2013). "The density of mid-sized Kuiper belt object 2002 UX25 and the formation of the dwarf planets". The Astrophysical Journal Letters.
14. "(55637) 2002 UX25".
15. "AstDys (55637) 2002UX25 Ephemerides". Department of Mathematics, University of Pisa, Italy.
16. John Stansberry. (2008). "The Solar System Beyond Neptune". University of Arizona press.
17. (October 2012). "Colours of minor bodies in the outer solar system. II. A statistical analysis revisited". Astronomy & Astrophysics.
18. "Typical densities of snow and ice (kg/m³)".
19. (20 June 2017). "The Density of Mid-sized Kuiper Belt Objects from ALMA Thermal Observations". The Astronomical Journal.