(55637) 2002 UX25

Last updated
(55637) 2002 UX25
20131105 2002 UX25 hst.png
2002 UX25 and satellite, as seen by Hubble
Discovery [1]
Discovered by Spacewatch (291)
Discovery site Kitt Peak National Obs.
Discovery date30 October 2002
(55637) 2002 UX25
Cubewano (MPC) [2]
Extended (DES) [3]
Orbital characteristics [1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 2
Observation arc 25.26 yr (9,228 days)
Earliest precovery date12 October 1991
Aphelion 48.491 AU
Perihelion 36.485 AU
42.488 AU
Eccentricity 0.1413
276.95 yr (101,157 days)
Average orbital speed
4.54 km/s
0° 0m 12.96s / day
Inclination 19.484°
≈ 5 September 2066 [4]
±3 days
Known satellites 1
(ø: 190–260 km) [5] [6]
Physical characteristics
Mean diameter
665±29 km [7]
659±38 km [8]
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 [8]
Equatorial surface gravity
0.075 m/s2
Equatorial escape velocity
0.227 km/s
14.382±0.001 h [9]
0.1±0.01 [8]
Temperature ≈ 43 K
B–V=1.007±0.043 [10]
V−R=0.540±0.030 [10]
V−I=1.046±0.034 [10]
19.8 [11]
3.87±0.02, [9] 4.0 [1]

    (55637) 2002 UX25 is a trans-Neptunian object that orbits the Sun in the Kuiper belt beyond Neptune. This TNO briefly garnered scientific attention when it was found to have an unexpectedly low density of about 0.82 g/cm3. [12]


    2002 UX25 has an absolute magnitude of about 4.0, [1] and Spitzer Space Telescope results estimate it to be about 681 km in diameter. [13] The low density of this and many other mid sized TNOs implies that they have likely never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets. [14]

    It was discovered on 30 October 2002, by the Spacewatch program. [15]

    Numbering and naming

    This minor planet was numbered (55637) by the Minor Planet Center on 16 February 2003 ( M.P.C. 47763). [16] As of 2021, it has not been named. [17]


    2002 UX25 (vmag 19.9) as viewed with a 24" telescope UX25-LB1-2009Nov19-06UT.jpg
    2002 UX25 (vmag 19.9) as viewed with a 24" telescope

    2002 UX25 has a perihelion of 36.7  AU, [1] which it will next reach in 2065. [1] As of 2020, 2002 UX25 is 40 AU from the Sun. [11]

    The Minor Planet Center classifies 2002 UX25 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). [18] The light-curve amplitude is ΔM = 0.21±0.06. [9]

    The analysis of combined thermal radiometry of 2002 UX25 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
    . 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 UX25 and its satellite: 659 km and 230 km, respectively. [8]

    2002 UX25 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. [7] It is redder than Varuna, unlike its neutral-colored "twin" 2002 TX300 , in spite of similar brightness and orbital elements.


    With a density of 0.82 g/cm3, assuming that the primary and satellite have the same density, 2002 UX25 is one of the largest known solid objects in the Solar System that is less dense than water. [12] 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, which was believed to be unlikely given the compactability of water ice; [6] this low density thus astonished astronomers. [12] Studies by Grundy et al. suggest that at the low temperatures that prevail beyond Neptune, ice is brittle and can support significant porosity in objects significantly larger than 2002 UX25, 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 pore spaces. [19]

    Density comparison
    Settled snow0.2–0.3 [20]
    Slush/firn 0.7–0.8 [20]
    2002 UX250.82 [6]
    Glacier ice0.83–0.92 [20]
    Tethys 0.984 [21]
    Liquid water1 [20]


    A simulated circular orbit of 210-km-diameter moon at a distance of 4770 km 2002 UX25 moon-orbit.png
    A simulated circular orbit of 210-km-diameter moon at a distance of 4770 km

    The discovery of a minor-planet moon was reported in IAUC 8812 on 22 February 2007. [5] The satellite was detected using the Hubble Space Telescope in August 2005. [5] The satellite was found at 0.16 arcsec from the primary with an apparent magnitude difference of 2.5. [22] It orbits the primary in 8.309±0.0002 days, [9] at a distance of 4770±40 km, yielding a system mass of (1.25±0.03)×1020 kg. [6] [9] The eccentricity of the orbit is 0.17±0.03. [9]

    This moon is estimated to be 210±30 km in diameter. [7] Assuming the same albedo as the primary, it would have a diameter of 190 km, assuming an albedo of 0.05 (typical of other cold, classical KBOs of similar size) a diameter of 260 km. [6]

    Related Research Articles

    Trans-Neptunian object Any object in the Solar System that orbits the Sun at a greater average distance than Neptune

    A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet or dwarf planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (AU).

    28978 Ixion Plutino

    28978 Ixion, provisional designation 2001 KX76, is a large trans-Neptunian object and a possible dwarf planet. It is located in the Kuiper belt, a region of icy objects orbiting beyond Neptune in the outer Solar System. Ixion is classified as a plutino, a dynamical class of objects in a 2:3 orbital resonance with Neptune. It was discovered in May 2001 by astronomers of the Deep Ecliptic Survey at the Cerro Tololo Inter-American Observatory, and was announced in July 2001. The object is named after the Greek mythological figure Ixion, who was a king of the Lapiths.

    20000 Varuna Kuiper belt object

    20000 Varuna, provisional designation 2000 WR106, is a large trans-Neptunian object in the Kuiper belt. It was discovered in December 2000 by American astronomer Robert McMillan during a Spacewatch survey at the Kitt Peak National Observatory. It has an elongated shape due to its rapid rotation. It is named after the Hindu deity Varuna, one of the oldest deities mentioned in the Vedic texts.

    90482 Orcus Trans-Neptunian object and dwarf planet

    90482 Orcus, provisional designation 2004 DW, is a trans-Neptunian dwarf planet with a large moon, Vanth. It has a diameter of 910 km (570 mi). The surface of Orcus is relatively bright with albedo reaching 23 percent, neutral in color and rich in water ice. The ice is predominantly in crystalline form, which may be related to past cryovolcanic activity. Other compounds like methane or ammonia may also be present on its surface. Orcus was discovered by American astronomers Michael Brown, Chad Trujillo, and David Rabinowitz on 17 February 2004.

    (15874) 1996 TL66, provisional designation 1996 TL66, is a trans-Neptunian object of the scattered disc orbiting in the outermost region of the Solar System.

    19521 Chaos

    19521 Chaos is a cubewano, a Kuiper-belt object not in resonance with any planet. Chaos was discovered in 1998 by the Deep Ecliptic Survey with Kitt Peak's 4 m telescope. Its albedo is 0.050+0.030
    , making it, with its absolute magnitude (H) of 4.8, 600+140
    in diameter. It is named after the primeval state of existence in Greek mythology, from which the first gods appeared.

    <span class="nowrap">(55565) 2002 AW<sub>197</sub></span>

    (55565) 2002 AW197 is a classical, non-resonant trans-Neptunian object from the Kuiper belt in the outermost region of the Solar System, also known as cubewano. With a likely diameter of at least 700 kilometers (430 miles), Brown considers it a highly likely dwarf planet candidate. Tancredi notes that photometric observations suggest that it is a spheroid with a high albedo and small albedo spots. However, its low albedo suggests it does not have planetary geology. It is approximately tied with 2002 MS4 and 2013 FY27 (to within measurement uncertainties) as the largest unnamed object in the Solar System and the largest planetoid without a moon. It was discovered at Palomar Observatory in 2002 and has a rotation period of 8.8 hours and a moderately red color.

    <span class="nowrap">(84522) 2002 TC<sub>302</sub></span> Large trans-Neptunian object

    (84522) 2002 TC302, prov. designation: 2002 TC302, is a mid-sized trans-Neptunian object located in the outermost region of the Solar System. It was discovered on 9 October 2002, by American astronomers Mike Brown, Chad Trujillo and David Rabinowitz at the Palomar Observatory in California. The resonant trans-Neptunian object stays in a 2:5 resonance with Neptune. It has a reddish color, a rotation period of 56.1 hours and measures at least 500 kilometers (310 miles) in diameter.

    <span class="nowrap">(55636) 2002 TX<sub>300</sub></span>

    (55636) 2002 TX300 is a bright Kuiper belt object in the outer Solar System estimated to be about 286 kilometres (178 mi) in diameter. It is a large member of the Haumea family that was discovered on 15 October 2002 by the Near-Earth Asteroid Tracking (NEAT) program.

    <span class="nowrap">(307261) 2002 MS<sub>4</sub></span>

    (307261) 2002 MS4 is a large classical Kuiper belt object and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It was discovered in 2002 by Chad Trujillo and Michael Brown, and has precovery images back to 8 April 1954.

    <span class="nowrap">(208996) 2003 AZ<sub>84</sub></span>

    (208996) 2003 AZ84 is a trans-Neptunian object with a possible moon from the outer regions of the Solar System. It is approximately 940 kilometers across its longest axis, as it has an elongated shape. It belongs to the plutinos – a group of minor planets named after its largest member Pluto – as it orbits in a 2:3 resonance with Neptune in the Kuiper belt. It is the third-largest known plutino, after Pluto and Orcus. It was discovered on 13 January 2003, by American astronomers Chad Trujillo and Michael Brown during the NEAT survey using the Samuel Oschin telescope at Palomar Observatory.

    <span class="nowrap">(145452) 2005 RN<sub>43</sub></span> Classical Kuiper belt object

    (145452) 2005 RN43, also written as (145452) 2005 RN43, is a classical Kuiper belt object. It has an estimated diameter of 679+55
    . It was discovered by Andrew Becker, Andrew Puckett and Jeremy Kubica on 10 September 2005 at Apache Point Observatory in Sunspot, New Mexico. Brown estimates that it is possibly a dwarf planet.

    174567 Varda Trans-Neptunian object

    174567 Varda (provisional designation 2003 MW12) is a binary trans-Neptunian planetoid of the resonant hot classical population of the Kuiper belt, located in the outermost region of the Solar System. Its moon, Ilmarë, was discovered in 2009.

    <span class="nowrap">(84922) 2003 VS<sub>2</sub></span> Trans-Neptunian object

    (84922) 2003 VS2 is a trans-Neptunian object discovered by the Near Earth Asteroid Tracking program on 14 November 2003. Like Pluto, it is in a 2:3 orbital resonance with Neptune and is thus a plutino. Analysis of light-curve suggests that it is not a dwarf planet.

    (35671) 1998 SN165, prov. designation: 1998 SN165, is a trans-Neptunian object from the Kuiper belt located in the outermost region of the Solar System. It was discovered on 23 September 1998, by American astronomer Arianna Gleason at the Kitt Peak National Observatory near Tucson, Arizona. The cold classical Kuiper belt object is a dwarf planet candidate, as it measures approximately 400 kilometers (250 miles) in diameter. It has a grey-blue color (BB) and a rotation period of 8.8 hours. As of 2021, it has not been named.

    229762 Gǃkúnǁʼhòmdímà Trans-Neptunian object

    229762 Gǃkúnǁʼhòmdímà, provisional designation 2007 UK126, is a trans-Neptunian object and binary system from the extended scattered disc, located in the outermost region of the Solar System. It was discovered on 19 October 2007 by American astronomers Megan Schwamb, Michael Brown, and David Rabinowitz at the Palomar Observatory in California and measures approximately 600 kilometers (400 miles) in diameter. This medium-sized TNO appears to be representative of a class of mid-sized objects under approximately 1000 km that have not collapsed into fully solid bodies. Its 100-kilometer moon was discovered by Keith Noll, Will Grundy, and colleagues with the Hubble Space Telescope in 2008, and named Gǃòʼé ǃHú.

    50000 Quaoar Cold classical Kuiper belt object

    50000 Quaoar, provisional designation 2002 LM60, is a dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. A non-resonant object (cubewano), it measures approximately 1,121 km (697 mi) in diameter, about half the diameter of Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 4 June 2002. Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar. A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects. In February 2007, Weywot, a synchronous moon in orbit around Quaoar, was discovered by Brown. Weywot is measured to be 170 km (110 mi) across. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son.

    (82075) 2000 YW134, provisional designation: 2000 YW134, is a resonant trans-Neptunian object and binary system, located in the outermost region of the Solar System. It was discovered on 26 December 2000, by astronomers with the Spacewatch survey at Kitt Peak Observatory near Tucson, Arizona. The reddish object stays in a rare 3:8 resonance with Neptune and measures approximately 216 kilometers (130 miles). Its 75-kilometer sized companion was discovered by the Hubble Space Telescope in October 2002. As of 2021, neither the primary body nor its satellite have been named.

    471143 Dziewanna

    471143 Dziewanna, exact:, provisional designation 2010 EK139, is a trans-Neptunian object in the scattered disc, orbiting the Sun in the outermost region of the Solar System.

    <span class="nowrap">(524366) 2001 XR<sub>254</sub></span>

    (524366) 2001 XR254, provisional designation 2001 XR254, is a trans-Neptunian object and binary system from the classical Kuiper belt, located in the outermost region of the Solar System. The cubewano belongs to the cold population and measures approximately 171 kilometers (110 miles). It was first observed on 10 December 2001, by astronomers at the Mauna Kea Observatory, Hawaii. Its 140-kilometer sized companion was discovered by the Hubble Space Telescope in June 2006.


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