Harpalyke.space

Irregular moon of Jupiter (retrograde) · Outer Jovian system · Ananke group

Harpalyke

Jupiter XXII · S/2000 J 5

A grey, four-kilometre fragment of an asteroid that Jupiter captured and a collision shattered.

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Harpalyke at a glance
Object typeIrregular, retrograde moon of Jupiter[1]
GroupAnanke group[1]
DiscoveredNov. 23, 2000, Mauna Kea, Hawaii[2]
DiscoverersS. S. Sheppard, D. C. Jewitt, Y. R. Fernandez, E. Magnier[2]
NamedOctober 2002, after Harpalyce of Greek myth[1]
Size≈ 4.4 km across (radius 2.2 km, assuming albedo 0.04)[2]
ColourGrey, like Praxidike and Iocaste[2]
Mean distance≈ 20.9 million km from Jupiter[3]
Orbital period≈ 617 days[3]
Inclination≈ 148° (retrograde)[3]
Eccentricity≈ 0.24[3]

Orbiting the wrong way

Harpalyke circles Jupiter retrograde — opposite to the planet’s rotation — at an average of about 20.9 million km, tilted around 148°.[3][1] It is one of many small irregular moons on distant, inclined, eccentric orbits.[1]

The Ananke family

Harpalyke shares its orbit type with the larger moon Ananke, making it a member of the Ananke group.[1] Members orbit at 19.2–21.8 million km with inclinations of 144°–156°.[4] NASA describes the group as the remains of a captured asteroid broken by a collision, with Ananke as the largest chunk.[2]

A grey moon

NASA notes Harpalyke is a similar grey to two other Ananke-family moons, Praxidike and Iocaste.[2] Their average colours (B−V 0.77, V−R 0.42) differ from Ananke itself, which sits between grey and light red — one reason some researchers think the Ananke and Pasiphae groups may have been mixed by resonances.[4]

Why so many backwards moons?

Far from a planet, retrograde orbits stay stable over a wider region than prograde ones. That was once thought to explain why Jupiter has so many retrograde moons — though Saturn’s more even mix shows the full story is more complicated.[5]

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Orbit

Harpalyke’s distance from Jupiter ranges from about 15.9 to 25.9 million km over an orbit of roughly 617 days.[3] The orbit changes continuously due to perturbations from the Sun and planets.[1]

Jupiter Callisto (1.88 million km)Himalia (11.44 million km) farthest ≈ 25.9 million km closest ≈ 15.9 million km
Top-down orbit of Harpalyke to scale, computed from JPL orbital elements (a ≈ 20.89 million km, e ≈ 0.239).[3] Callisto and Himalia orbits shown for comparison.[3] Mean orbital elements; the real orbit is tilted steeply and shifts over time, so this top-down view flattens it onto one plane.

The name

In October 2002 the moon was named after Harpalyce, daughter of Clymenus, a tragic figure in Greek myth who in some accounts was also a lover of Zeus.[1]

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How the Ananke group formed

One captured asteroid, one impact, and a family of small backwards moons — including Harpalyke.

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Why do Jupiter’s distant moons go backwards?

Hill spheres, stability and capture — the physics behind Harpalyke’s reversed orbit.

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Harpalyke’s grey colour and what it might mean

A small colour difference that complicates Jupiter’s moon family tree.

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Sources

  1. Harpalyke (moon) — Wikipedia
  2. Harpalyke — NASA Science
  3. Planetary Satellite Mean Elements — NASA JPL Solar System Dynamics
  4. Ananke group — Wikipedia
  5. Hill sphere — Wikipedia

Figures are rounded; where sources differ we note it. Spotted an error? Tell us.