# Satellite of 38628 Huya

The Kuiper belt object 38628 Huya has a single known natural satellite, which as of 2025 has no official designation. Huya and its satellite form a binary system, and are together referred to as the Huya system. The satellite was discovered by a team led by Keith Noll using Hubble Space Telescope images taken on 6 May 2012, and confirmed in reexamination of archival Hubble imagery from 30 June and 1 July 2002. The discovery was reported to the International Astronomical Union and was announced on 12 July 2012.

## Orbit

Measurements of the satellite's position in direct imaging and stellar occultations indicate the satellite orbits Huya with a separation distance of 1,900 km (1,200 mi) and an orbital period of 3.46 days. Compared to other known binary trans-Neptunian objects, the mutual orbit of the Huya system is relatively tight. The satellite's orbit is nearly circular, with a low orbital eccentricity of 0.036. The satellite's orbital inclination with respect to Huya's equator is unknown (due to the unknown axial tilt of Huya), but is presumably very small (coplanar to Huya's equator). The orbital inclination of the Huya system is 66 degrees with respect to the ecliptic, or the plane of the Solar System.
The satellite's orbital elements mentioned above are derived by assuming a Keplerian orbit, which ignores perturbation effects such as precession for simplicity. However, the satellite's Keplerian orbit somewhat deviates from its measured positions in images and occultations; this is most likely explained by precession of the satellite's orbit due to Huya's oblate shape, which has been observed in occultations. Due to the satellite's short orbital period and close distance to Huya, its orbital precession period would likely be less than 5 years.
From the perspective of Earth, the opening angle of the Huya system's mutual orbit is slowly decreasing as Huya system moves along its orbit around the Sun. The Huya system will shift from a pole-on to an edge-on perspective by the year 2033, when the Huya system will enter mutual events season. During mutual events season, Huya and its satellite will take turns eclipsing and transiting each other, producing dips in brightness that last up to ~5 hours and have depths of up to ~0.25 magnitudes. Observations of these mutual events can help refine the Huya system's properties and can reveal the shapes, relative sizes, and surface albedo variations of Huya and its satellite.

## Observations

The satellite is about 1.4 magnitudes dimmer than Huya in visible light. Given the satellite's tight orbit around Huya, it only appears at small angular separations of up to 0.1 arcseconds from Huya.$^{:13}$ This is close to the angular resolution limit of state-of-the-art telescopes such as Hubble and the Keck telescopes with adaptive optics. Because of this, it is challenging to resolve the satellite from Huya in direct imaging; the satellite has only been imaged in 2002 and 2012 by Hubble and in 2021 by Keck. With the sparse number of imaging observations, the orbit of Huya's satellite remained poorly known until it was later detected via stellar occultations in 2021 and 2023.$^{:9}$
As of 2025, the satellite of Huya has been detected via stellar occultation three times, all by chance as the satellite's orbit was not known at these times. Stellar occultations allow for highly accurate measurements of an object's position, which in turn enables the determination of its orbit. The first occultation detection of Huya's satellite occurred on 28 March 2021. The 2021 occultation by the satellite was detected by Ondřejov Observatory in Czech Republic, which reported an eight-second-long dip (corresponding to a chord length of 73 ± 40 km (45 ± 25 mi)) occurring about 3 minutes before the primary occultation by Huya.$^{:10}$ The second occultation by Huya's satellite was observed on 17 February 2023 by Penrose Observatory in Colorado, United States, which reported a nine-second-long dip corresponding to a chord length of 177 ± 35 km (110 ± 22 mi). The third occultation by Huya's satellite was observed on 24 June 2023 by La Palma Observatory in Canary Islands, Spain, which reported an eight-second-long dip corresponding to a chord length of 179 ± 14 km (111.2 ± 8.7 mi). In all cases, the occultation by the satellite was detected at only one location, which prevented the determination of the satellite's shape.

## Physical characteristics

Direct observations of the satellite via stellar occultation place a minimum diameter of 165 km (103 mi), while modeling of the Huya system's thermal emission places a maximum diameter of 243 km (151 mi). If the satellite has the same albedo as Huya, the satellite would be 213 km (132 mi) in diameter. In either case, the satellite is relatively large compared to Huya, being about half of Huya's diameter. The Huya system's satellite-to-primary diameter ratio is intermediate among those of other binary trans-Neptunian objects, which are often found having either equally-sized components or large primary components with small satellites.
With its large size relative to Huya, the satellite is expected to have slowed Huya's rotation down via tidal forces, to the point that both components would be tidally locked to each other. However, observations of Huya's rotational light curve suggest otherwise, instead finding a short rotation period of several hours.$^{:7}$ The present-day non-synchronous rotation of Huya can be explained if the satellite has a low density of around 0.5 g/cm$^3$, which would make it not massive enough to tidally lock Huya's rotation. A similar scenario has been observed in the binary Kuiper belt object 174567 Varda, whose rotation is not tidally locked to its large satellite Ilmarë, though in this case it's been suggested that the system is not old enough for tidal locking.$^{:7}$