Namaka (moon)

• 1. Introduction

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  Namaka is the smaller, inner moon of the dwarf planet Haumea, a body so faint that from Earth it shines at just 1.5% of Haumea's brightness. For years after its discovery, astronomers could barely separate its light from its parent. Namaka's angular separation from Haumea is less than 1 arcsecond, invisible without a high-resolution telescope. Somewhere in that faint smudge of light sits a small, irregularly shaped world. Scientists suspect it tumbles with no fixed axis at all, in a way that is difficult to predict from one year to the next. How do you study a body you can barely see and cannot predict? And what does its erratic orbit reveal about a violent event 4.4 billion years ago? Something struck Haumea hard enough to fling pieces of it into space.

• 2. Discovery And Name

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  The Keck II telescope at Mauna Kea captured the images on the 1st of March, the 28th of May, and the 30th of June 2005. Hidden in that high-resolution data was a faint companion no one had noticed before. Michael E. Brown and the W. M. Keck Observatory's adaptive optics team spotted it that fall. It came months after the same team had found Haumea's brighter, outer moon Hiiaka, in January 2005. The new moon had escaped notice for so long simply because it was fainter and sat closer to Haumea.

  The Central Bureau of Astronomical Telegrams announced the discovery on the 1st of December 2005. At first, the moon carried only the provisional designation S/2005 2, marking it as the second moon of Haumea found that year. Brown nicknamed it "Blitzen," after one of Santa Claus's reindeer, continuing a run of Christmas-themed nicknames he had given the Haumea system.

  The International Astronomical Union made the name official on the 17th of September 2008, confirming Haumea, Hiiaka, and Namaka together as figures from Hawaiian mythology. In that tradition, Nāmaka is a water spirit born from the body of the fertility goddess Haumea. Brown's team had proposed the names two years earlier, in September 2006, as tribute to the Hawaiian location where they found the moons.

  None of that naming explained why Namaka's orbit behaves so strangely, tilted and stretched in ways that would take years more to untangle.

• 3. A Precarious Orbit

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  Namaka circles Haumea once every 18.3 days, at an average distance of 25,500 km, a wide arc but a strange one. Its path is elliptical rather than circular, with an eccentricity of about 0.22. The orbit is also tilted by roughly 13 degrees relative to both Haumea's equator and Hiiaka's orbital plane.

  Two forces constantly reshape that path. Hiiaka's gravity pulls at Namaka from farther out, while Haumea's elongated, non-spherical shape creates a variable gravitational field close in. Together they cause Namaka's eccentricity and inclination to shift over time, along with a slow precession of the orbit's orientation.

  Namaka completes 2.689 orbits for every one that Hiiaka completes, a ratio close to a clean 8:3 fraction. In that resonance, Hiiaka finishes 3 orbits for every 8 Namaka completes. Simulations suggest this resonance let Hiiaka transfer its own orbital eccentricity and inclination onto Namaka. That process likely played out over the past few hundred million years. Nobody knows exactly when the resonance began, or whether it still holds today.

  That same gravity doesn't stop at shaping Namaka's path. It reaches all the way to Haumea's own spin and the faint ring that surrounds it.

• 4. A Gravitational Tug

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  Namaka's gravity reaches all the way to Haumea itself, gently torquing the dwarf planet's rotation axis into a slow precession of less than 1 degree. That precession takes the same amount of time as Namaka's own nodal precession, a period of several decades.

  Haumea's ring feels barely any pull from Namaka at all. The moon is too small and orbits too far from the ring to move it much. Namaka's tilted orbit could, in theory, disperse the ring's particles or shift its inclination. But Haumea's own flattened shape exerts a far stronger pull, one that cancels out any such effect.

  Calculating that tug is one thing. Actually catching Namaka in the act, faint and close as it is, has proven to be a much harder problem.

• 5. Observing Namaka

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  On the 16th of March 2025, Namaka passed directly in front of a distant star and briefly blocked its light. It is the only stellar occultation by Namaka ever recorded. Astronomers had also expected Namaka to eclipse and occult Haumea sometime between 2009 and 2011, based on models of its orbit.

  No such eclipse or occultation was detected during that window. Later recalculations showed the original predictions had been close in timing, just offset from when the events actually occurred.

  When it is visible at all, Namaka appears about 3.7 times fainter than Hiiaka, roughly 27% as bright as its larger sibling. Spotting it apart from Haumea requires a high-resolution telescope such as the Hubble Space Telescope.

  Every one of those measurements feeds into a harder question: how big is Namaka, and what is it actually made of?

• 6. Weighing A Faint Moon

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  Namaka's diameter is poorly constrained, but visible-light and thermal-infrared observations, combined with assumed values for albedo and density, point to a figure near 150 km. At that size, Namaka would be roughly half the diameter of Hiiaka. A more precise measurement would require catching Namaka as it occults a background star.

  Namaka's mass, determined by measuring how its orbit and Hiiaka's tug at each other, is roughly ten times less than Hiiaka's, about 10% of it. It is also about 3,000 times less massive than Haumea, or roughly 0.03% of Haumea's mass. If Namaka's diameter really is 150 km, that mass implies a low bulk density of about 0.65. Such a low density would suggest Namaka's interior is highly porous, similar to other small trans-Neptunian objects.

  Namaka's surface reflectivity, or geometric albedo, has never been directly measured, but estimates place it somewhere between 50% and 80%. Like Haumea and Hiiaka, that surface appears to be made mostly of fresh water ice.

  A moon this small and this porous would be expected to behave predictably as it spins. Namaka does not.

• 7. A Chaotic Spin

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  In 2008, Hubble Space Telescope observations caught Namaka's brightness fluctuating by 0.3 magnitudes, a pattern that pointed to an elongated, irregular shape. Follow-up Hubble observations in 2009 and 2010 found no significant periodic fluctuation at all.

  That silence leaves two possibilities open. Either Namaka's rotation axis is tilted at a steep angle, or the moon spins slowly, with a rotation period longer than a single day.

  Namaka orbits close enough to Haumea that the dwarf planet's tides are expected to have slowed its spin considerably. A slow rotation, combined with Namaka's eccentric orbit, would trigger multiple spin-orbit resonances driven by those same tides. Simulations predict that Namaka's axial tilt and rotation period could shift unpredictably from year to year, much like Saturn's chaotically rotating moon Hyperion.

  Confirming that chaotic spin will take a long stretch of high-resolution observations, the same patient kind of watching that has driven every finding about this moon so far.

• 8. A Violent Origin

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  Seventy-seven to eighty-two million years after the Solar System itself formed, two large Kuiper belt objects of similar size are thought to have collided obliquely. They merged into a single, rapidly rotating body that gradually deformed into an elongated shape, becoming Haumea.

  That giant-impact picture explains Haumea's fast rotation and unusually high bulk density well. It fails, though, to explain the existence of Haumea's moons or its family of icy Kuiper belt objects sharing similar orbits. An impact energetic enough to match Haumea's properties should have flung fragments away faster than Haumea's own escape velocity.

  Researchers now favor a gentler mechanism instead: rotational fissioning. It likely occurred roughly 80 million years after the initial impact, or 147 to 162 million years after the Solar System formed. A 2022 study led by Jessica Noviello and collaborators proposed that Haumea kept growing its rocky core after the impact. As that core grew, conservation of angular momentum gradually sped up Haumea's spin.

  Centrifugal force at Haumea's equator eventually grew strong enough to fling icy surface material into orbit, forming a disk around the dwarf planet. That process cost Haumea about 3% of its original mass and 14% of its original angular momentum.

  Namaka's low density today suggests that the material in that disk came together gently rather than violently. Some of the moons born from the disk scattered each other and escaped the Haumea system entirely, becoming independent Haumea family Kuiper belt objects. The rest of the material settled into orbit as Namaka, Hiiaka, and Haumea's ring.

  Namaka and Hiiaka most likely started out with circular, coplanar orbits close to their present positions around Haumea. Tidal interactions with Haumea, together with gravitational nudges from passing trans-Neptunian objects, gradually reshaped those orbits into the tilted, elliptical paths Namaka follows today.