130 Elektra

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130 Elektra is a large asteroid in the outer part of the main asteroid belt. It is a main-belt asteroid with three small moons, making it a quadruple asteroid system—the only known quadruple system in the Solar System’s main belt.

Discovery and naming
- Discovered on February 17, 1873, by Christian Peters at Litchfield Observatory in New York.
- Named after Electra, a figure in Greek mythology.
- Not to be confused with Electra, a star in another context.

What Elektra is like
- Type and surface: Elektra is a G-type (and sometimes classified as a Ch-type in other systems) asteroid, suggesting a surface similar in composition to Ceres and containing organic materials. It shows signs of having undergone aqueous alteration, and its spectrum points to a surface with carbon-rich materials.
- Size and shape: Elektra is a large, irregular body with an elongated shape. It has a mean diameter of about 199 km (roughly 262 by 205 by 164 km) and a flattening of about 0.43.
- Mass and density: Its mass is around 6.4 to 7.0 × 10^18 kilograms, giving a density of about 1.3 g/cm^3.
- Rotation and tilt: Elektra spins every about 5.225 hours (5.224663 hours, with a small uncertainty) and has a high axial tilt of about 156 degrees, meaning its rotation axis is nearly perpendicular to the plane of the Solar System.
- Orbit around the Sun: It takes about 5.53 Earth years to orbit the Sun, with an average distance (semi-major axis) of about 3.13 AU. Its path is somewhat oval (eccentricity ~0.209).

Satellites: three moons around Elektra
- Beta (S/2003 (130) 1): The largest and outermost moon, about 6 km in diameter. It circles Elektra at roughly 1,300 km with a 5.3-day orbital period. Its orbit is somewhat eccentric (about 0.08) and inclined about 160 degrees to the celestial equator.
- Gamma (S/2014 (130) 1): The second satellite discovered, about 2 km in diameter. It orbits at about 500 km from Elektra with a period of roughly 1.19 days. Its orbit is nearly circular (eccentricity around 0.03) and inclined about 156 degrees to the celestial equator.
- Delta (the inner satellite, sometimes called S/2014 (130) 2): The innermost moon, about 1.6 km across. It orbits at about 344 km with a period of about 0.68 days (16 hours). Its orbit is highly eccentric (around 0.33) and inclined about 38 degrees relative to Elektra’s spin axis. This moon was difficult to detect because of Elektra’s glare, and it was identified only after careful image processing of data from the VLT-SPHERE instrument.

Key implications
- All three moons have spectra similar to Elektra, supporting the idea that they are fragments from a disruptive collision that broke apart Elektra.
- The system is the first quadruple asteroid system ever imaged in the main belt and, as of recent studies, Elektra has the most moons of any main-belt asteroid.
- The moons are faint and close to the bright primary, so observing them requires the largest telescopes and advanced adaptive optics and image processing.

Occultations and observations
- Elektra has been observed occulting stars several times since 2007, including a notable event on April 21, 2018, when many observers across Europe recorded the star’s dimming. These events help map Elektra’s shape and size, reinforcing the peanut-like appearance inferred from other data.

Overall, 130 Elektra is a large, irregularly shaped main-belt asteroid with a low density and a fast, tilted rotation. It hosts three small moons that are likely debris from a past collision, making Elektra a unique and dynamically interesting quadruple asteroid system.