 Uni and Tinia as seen by theHubble Space Telescope | |
| Discovery[1] | |
|---|---|
| Discovered by | Spacewatch (291) |
| Discovery site | Kitt Peak National Obs. |
| Discovery date | 30 October 2002 |
| Designations | |
Designation | (55637) Uni |
Named after | Uni |
| 2002 UX25 | |
| Cubewano (MPC)[2] Extended (DES)[3] | |
| Orbital characteristics[1] | |
| Epoch 5 May 2025 (JD 2460800.5) | |
| Uncertainty parameter 0 | |
| Observation arc | 33.35 yr (12,182 days) |
| Earliestprecovery date | 12 October 1991 |
| Aphelion | 49.291AU |
| Perihelion | 36.716 AU |
| 43.003 AU | |
| Eccentricity | 0.1462 |
| 282.01yr (103,005 days) | |
Averageorbital speed | 4.54 km/s |
| 309.49° | |
| 0° 0m 12.24s / day | |
| Inclination | 19.400° |
| 204.57° | |
| ≈ 5 September 2066[4] ±3 days | |
| 275.27° | |
| Knownsatellites | 1 (Tinia) |
| Physical characteristics | |
| 659±38 km[5] | |
| Mass | (1.25±0.03)×1020 kg[6] |
Meandensity | 0.82±0.11 g/cm3 (assuming equal densities for primary and satellite)[6] 0.80±0.13 g/cm3[5] |
| 0.075 m/s2 | |
| 0.227 km/s | |
| 14.382±0.001 h[7] | |
| Albedo | 0.107+0.005 −0.008[8] 0.1±0.01[5] |
| Temperature | ≈ 43K |
Spectral type | B–V=1.007±0.043[9] V−R=0.540±0.030[9] V−I=1.046±0.034[9] |
| 19.8[10] | |
| 3.87±0.02,[7] 4.0[1] | |
55637 Uni (provisional designation2002 UX25) is a largetrans-Neptunian object that orbits the Sun in theKuiper belt beyondNeptune. It briefly garnered scientific attention when it was found to have an unexpectedly lowdensity of about 0.82 g/cm3.[11] It was discovered on 30 October 2002, by theSpacewatch program.[12]
Uni has anabsolute magnitude of about 4.0,[1] andSpitzer Space Telescope results estimate it to be about 660 km indiameter.[5] The lowdensity of this and many other mid-sized TNOs implies that they have never compressed into fully solid bodies, let alone differentiated or collapsed intohydrostatic equilibrium, and so are not likely to bedwarf planets.[13]
Uni has one known moon,Tinia, discovered in 2005.
Uni wasnumbered (55637) by theMinor Planet Center on 16 February 2003 (M.P.C. 47763).[14] On 1 September 2025, the object was named afterUni, the Etruscan goddess of love and fertility.[15]
Uni has aperihelion of 36.7 AU,[1] which it will next reach in 2065.[1] As of 2020, Uni is 40 AU from the Sun.[10]
TheMinor Planet Center classifies Uni as acubewano[2] while theDeep Ecliptic Survey (DES) classifies it asscattered-extended.[3] The DES using a 10 My integration (last observation: 2009-10-22) shows it with a minimum perihelion (qmin) distance of 36.3 AU.[3]
It has been observed 212 times withprecovery images dating back to 1991.[1]
A variability of the visual brightness was detected which could be fit to aperiod of 14.38 or 16.78 h (depending on a single-peaked or double peaked curve).[16] The light-curve amplitude is ΔM =0.21±0.06.[7]
The analysis of combined thermal radiometry of Uni from measurements by theSpitzer Space Telescope andHerschel Space Telescope indicates an effectivediameter of692 ± 23 km andalbedo of 0.107+0.005
−0.008.[17] Assuming equalalbedos for the primary and secondary it leads to thesize estimates of ~664 km and ~190 km, respectively. If thealbedo of the secondary is half of that of the primary the estimates become ~640 and ~260 km, respectively.[6] Using an improved thermophysical model slightly differentsizes were obtained for Uni and Tinia: 659 km and 230 km, respectively.[5]
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 themethanol compounds on thesurface.[8] It isredder thanVaruna, unlike its neutral-colored "twin"2002 TX300, in spite of similar brightness and orbital elements.
With adensity of 0.82 g/cm3, assuming that the primary and satellite have the samedensity, Uni is one of the largest known solid objects in theSolar System that is less dense thanwater.[11] Why this should be is not well understood, because objects of itssize 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, which was believed to be unlikely given the compactability ofwater ice;[6] this low density thus astonished astronomers.[11] Studies by Grundy et al. suggest that at the lowtemperatures that prevail beyondNeptune, ice is brittle and can support significant porosity in objects significantly larger than Uni, particularly if rock is present; the lowdensity could thus be a consequence of this object failing to warm sufficiently during its formation to significantly deform theice and fill these pore spaces.[18]
| Material | Density (g/cm3) | Notes |
|---|---|---|
| Settled snow | 0.2–0.3 | [19] |
| Slush/firn | 0.35–0.9 | [19] |
| Uni | 0.82 | [6] |
| Glacier ice | 0.83–0.92 | [19] |
| Tethys | 0.984 | [20] |
| Liquid water | 1 | [19] |
| Planetary satellites of |    | |
|---|---|---|
| Dwarf planet satellites of | ||
| Minor-planet moons | ||
| Ranked by size | ||
2025 in space | |||
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Space probe launches |
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| SelectedNEOs | |||
| Discoveries |
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| Comets | |||
| Novae | |||
| Space exploration | Lucy flyby of52246 Donaldjohanson | ||
| Miscellaneous events |
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| Consensus | _(cropped).jpg)  | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Candidate (for TNOs, D+1σ ≥ 700 km or H ≤ 4.0) |
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