55637 Uni
Trans-Neptunian object
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55637 Uni (provisional designation 2002 UX25) is a large trans-Neptunian object that orbits the Sun in the Kuiper belt beyond Neptune. It briefly garnered scientific attention when it was found to have an unexpectedly low density of about 0.82 g/cm3.[11] It was discovered on 30 October 2002, by the Spacewatch program.[12]
Uni and Tinia as seen by the Hubble Space Telescope | |
| Discovery[1] | |
|---|---|
| Discovered by | Spacewatch (291) |
| Discovery site | Kitt Peak National Obs. |
| Discovery date | 30 October 2002 |
| Designations | |
Designation | (55637) Uni |
| Pronunciation | /ˈjuːnaɪ/ |
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) |
| Earliest precovery date | 12 October 1991 |
| Aphelion | 49.291 AU |
| Perihelion | 36.716 AU |
| 43.003 AU | |
| Eccentricity | 0.1462 |
| 282.01 yr (103,005 days) | |
Average orbital 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° | |
| Known satellites | 1 (Tinia) |
| Physical characteristics | |
| 659±38 km[5] | |
| Mass | (1.25±0.03)×1020 kg[6] |
Mean density | 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 | ≈ 43 K |
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] | |
Uni has an absolute magnitude of about 4.0,[1] and Spitzer Space Telescope results estimate it to be about 660 km in diameter.[5] The low density of this and many other mid-sized TNOs implies that they have never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are not likely to be dwarf planets.[13]
Uni has one known moon, Tinia, discovered in 2005.
Numbering and naming
Classification
Uni has a perihelion of 36.7 AU,[1] which it will next reach in 2065.[1] As of 2020, Uni is 40 AU from the Sun.[10]
The Minor Planet Center classifies Uni as a cubewano[2] while the Deep Ecliptic Survey (DES) classifies it as scattered-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 with precovery images dating back to 1991.[1]
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).[16] The light-curve amplitude is ΔM = 0.21±0.06.[7]
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+0.005
−0.008.[17] 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.[6] Using an improved thermophysical model slightly different sizes 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 the methanol compounds on the surface.[8] It is redder than Varuna, unlike its neutral-colored "twin" 2002 TX300, in spite of similar brightness and orbital elements.
Composition
With a density of 0.82±0.11 g/cm3, 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.[11] 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.[11] 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.[18]
See also
- 229762 Gǃkúnǁʼhòmdímà – mid-sized trans-Neptunian object with a moon and a similarly low density of 1.0 g/cm3[21]
