148780 Altjira
A low-density, red classical Kuiper belt binary.
148780 Altjira (provisional designation 2001 UQ18) is a classical Kuiper belt object that appears to be a triple or contact binary double system. It is notable for its nearly equal-sized binary components, making it a true synchronous binary rather than a typical primary-satellite system, and for its exceptionally low bulk density, which suggests a highly porous interior resembling a rubble pile.
Quick Facts
- Minorplanet
- yes
- Background
- #C2E0FF
- Discovery Site
- Deep Ecliptic Survey at Kitt Peak
- Discovered
- 20 October 2001
- Mpc Name
- (148780) Altjira
- Pronounced
- æ · l · ˈ · tʃ · ɪər · ə
- Named After
- Altjira
- Mp Category
- Classical KBO (DES)
- Epoch
- 2025 May 05 (JD 2460800.5)
- Observation Arc
- 7,709 days (21.11 yr)
- Uncertainty
- 4
- Semimajor
- 44.55 AU
Facts from the source article.
Lore & Background
148780 Altjira was named in 2008 after the Arrernte creation deity Altjira (Alchera). It is a binary system consisting of two bodies of nearly equal size: the primary is approximately 246 km in diameter, and the secondary about 222 km. The secondary was discovered on 6 August 2006 from Hubble Space Telescope images. Its orbit has a semi-major axis of 9904±56 km, a period of 139.561±0.047 days, an eccentricity of 0.3445±0.0045, and a retrograde inclination of 35.19°±0.19°. There is indirect evidence that Altjira may be an unresolved hierarchical triple system.
The system's lightcurve is quite flat (Δmag < 0.10), indicating a quasi-spherical body with a homogeneous surface. Thermal modeling gives a combined effective diameter of 331+51−187 km. The bulk density is exceptionally low at roughly 0.3+0.50−0.14 g/cm³, implying highly porous interiors resembling loose rubble piles. Altjira has a relatively high geometric albedo of 0.14±0.05 and exhibits a very red surface color, typical of cold classical Kuiper belt objects.
Reader's Guide
148780 Altjira is significant as one of the least dense known large Kuiper belt objects, with a bulk density of about 0.3 g/cm³. This low density indicates that both bodies are highly porous, likely composed of loose rubble rather than solid ice or rock. Its status as a true synchronous binary—with components of nearly equal size—makes it a key object for studying binary formation and evolution in the Kuiper belt. The flat lightcurve suggests a homogeneous surface, while the very red color aligns with cold classical KBOs. The possibility that Altjira is an unresolved hierarchical triple system, as hinted by indirect evidence, adds to its importance, as only one confirmed hierarchical triple system (47171 Lempo) is known in the Kuiper belt. The system's orbital parameters, including a retrograde inclination, provide constraints on dynamical models of binary KBOs.
Did You Know?
- Altjira's secondary was discovered on 6 August 2006 using the Hubble Space Telescope.
- The system has an exceptionally low bulk density of roughly 0.3 g/cm³, implying a highly porous interior.
Frequently Asked Questions
What is 148780 Altjira?
Altjira is a red-colored classical Kuiper belt object that orbits the Sun well beyond Neptune, originally catalogued under the temporary code 2001 UQ18. Rather than being a single solid body, it consists of two nearly equal-sized components locked in a binary (or possibly triple) configuration.
What makes Altjira's binary system unusual compared to other TNO binaries?
Most trans-Neptunian binaries have one dominant primary with a much smaller companion, but Altjira's two members are close in size at roughly 246 km and 222 km across. That near-equality makes it a true synchronous pair where neither object can be cleanly labelled the 'primary' in the traditional sense.
How dense is 148780 Altjira, and what does that imply about its interior?
Its bulk density is only about 300 kg/m³ (0.3 g/cm³), far below the density of solid rock or even compacted ice. Such an extreme lightness strongly suggests a highly porous, rubble-pile interior held together primarily by mutual gravity rather than material strength.
What are Altjira's size and mass figures?
The primary component measures roughly 246 km in diameter (with a large uncertainty range), while the secondary is about 222 km across; their combined effective diameter is approximately 331 km. The total system mass is estimated at around 4 × 10¹⁸ kg.
Why is 148780 Altjira important to Kuiper belt science?
Its pairing of a near-equal binary architecture with an exceptionally low density provides a critical test case for models of how small outer-solar-system bodies form and remain gravitationally bound. It pushes back against the assumption that most TNOs are monolithic or only moderately porous, highlighting the diversity of internal structures in the Kuiper belt.
More in Trans-Neptunian Objects 1-24
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