16525 Shumarinaiko
Binary asteroid in the inner main belt with a synchronous moon.
16525 Shumarinaiko, originally designated 1991 CU2, is a binary asteroid located in the inner main belt, measuring roughly 5 kilometers across. It was found on February 14, 1991, by Japanese astronomers Kin Endate and Kazuro Watanabe at the Kitami Observatory on Hokkaidō, Japan. The asteroid is named after Lake Shumarinai, an artificial lake in northern Hokkaidō created for hydroelectric power in the 1940s, now valued for its scenery. The naming citation was issued by the Minor Planet Center on December 15, 2005.
Orbit and classification place it within the Nysa–Polana complex, a group of overlapping asteroid families near the 3:1 Kirkwood gap, a depleted zone separating the central main belt. It orbits the Sun at distances between 2.1 and 2.7 AU, completing one revolution every 3 years and 9 months (1,358 days). Its orbit has an eccentricity of 0.14 and an inclination of 2° relative to the ecliptic. The first recorded observation was a precovery from the Digitized Sky Survey at Palomar Observatory in March 1950, nearly 41 years before its official discovery.
Physically, Shumarinaiko is assumed to be a stony S-type asteroid. In January 2013, photometric observations by Brian Warner at the Palmer Divide Observatory in Colorado and Dan Coley at DanHenge Observatory in California produced a well-defined rotation period of 2.5932 hours, with a low brightness amplitude of 0.08 magnitude, suggesting a roughly spherical shape. This superseded earlier, less reliable measurements of 2.6425 and 8.8 hours.
During those same 2013 observations, mutual occultation and eclipsing events revealed that Shumarinaiko is a synchronous binary asteroid. Its moon, provisionally designated S/2013 (16525) 1, appears tidally locked with an orbital period of 14.409 hours. The satellite is elongated and measures at least 16% of the primary’s diameter (Ds/Dp <0.16±0.02), corresponding to a diameter of about 830 meters. Over 100 binary asteroids are known in the main belt.
Diameter and albedo estimates vary. The NEOWISE survey gives a diameter of 5.253 kilometers and an albedo of 0.306. The Collaborative Asteroid Lightcurve Link, assuming a standard stony albedo of 0.20, calculates a diameter of 5.66 kilometers based on an absolute magnitude of 13.6.
Quick Facts
- Minorplanet
- yes
- Background
- #D6D6D6
- Discoverer
- K. Endate / K. Watanabe
- Discovery Site
- Kitami Obs.
- Discovered
- 14 February 1991
- Mpc Name
- (16525) Shumarinaiko
- Named After
- Lake Shumarinai / (Japanese lake)
- Mp Category
- main-belt(inner) / Nysa–Polana complex
- Epoch
- 4 September 2017 (JD 2458000.5)
- Uncertainty
- 0
- Observation Arc
- 67.29 yr (24,578 days)
- Aphelion
- 2.7341 AU
Facts from the source article.
Lore & Background
16525 Shumarinaiko is a member of the Nysa–Polana complex, a collection of overlapping asteroid families in the inner main asteroid belt near the Kirkwood gap (3:1 orbital resonance with Jupiter). It orbits the Sun at a distance of 2.1–2.7 AU once every 3 years and 9 months, with an eccentricity of 0.14 and an inclination of 2° with respect to the ecliptic. The body's observation arc begins with a precovery from the Digitized Sky Survey taken at Palomar Observatory in March 1950, almost 41 years prior to its official discovery.
The asteroid is an assumed stony S-type. In January 2013, photometric observations by Brian Warner at the Palmer Divide Observatory in Colorado and Dan Coley at DanHenge Observatory in California yielded a well-defined rotation period of 2.5932 hours with a brightness amplitude of 0.08 magnitude, superseding previous results of 2.6425 and 8.8 hours. The low amplitude suggests a rather spherical shape.
During the same observations, mutual occultation and eclipsing events revealed that Shumarinaiko is a synchronous binary asteroid with an elongated minor-planet moon, provisionally designated S/2013 (16525) 1. The satellite appears tidally locked to its orbital period of 14.409 hours and measures at least 16% of the primary's diameter, translating to approximately 830 meters.
Reader's Guide
Shumarinaiko's significance lies in its binary nature, which provides insights into the formation and evolution of small bodies in the inner asteroid belt. The discovery of its moon in 2013, through photometric observations that detected mutual occultation and eclipsing events, adds to the growing catalog of over 100 known binary asteroids. The satellite's synchronous orbit and tidal locking offer clues about the dynamical processes that shape such systems. The asteroid's membership in the Nysa–Polana complex helps contextualize its origin within a broader family of asteroids sharing similar orbital characteristics. Its well-determined rotation period and low lightcurve amplitude indicate a nearly spherical shape, which is relatively uncommon among small asteroids. The naming after Lake Shumarinai in Hokkaidō, Japan, connects the asteroid to a terrestrial landmark created for hydroelectricity in the 1940s, now valued for its scenery. The precovery from 1950 demonstrates the value of archival astronomical data in extending observational arcs.
Did You Know?
- Shumarinaiko was discovered on Valentine's Day, 14 February 1991.
- Its moon, S/2013 (16525) 1, is tidally locked with an orbital period of 14.409 hours.
- The asteroid's name comes from Lake Shumarinai, an artificial lake in Hokkaidō, Japan.
- A precovery image from 1950 exists, taken 41 years before its official discovery.
Frequently Asked Questions
What is 16525 Shumarinaiko?
It is a binary asteroid in the inner main belt, roughly 5 kilometers across, that completes one orbit around the Sun in about 3 years and 9 months. It belongs to the Nysa–Polana complex and is accompanied by a synchronously rotating moon.
Who discovered 16525 Shumarinaiko and when?
Japanese astronomers Kin Endate and Kazuro Watanabe first observed it on 14 February 1991 from the Kitami Observatory on Hokkaidō. At the time it carried the provisional designation 1991 CU2 before receiving its permanent number.
Why is the asteroid named Shumarinaiko?
The name honors Lake Shumarinai, a hydroelectric reservoir built in northern Hokkaidō during the 1940s that is now appreciated for its scenic beauty. The Minor Planet Center formally published the naming citation on 15 December 2005.
What are 16525 Shumarinaiko's key physical and orbital measurements?
NEOWISE infrared data give it a diameter of about 5.25 km, and it rotates on its axis roughly every 2.6 hours. Its mean orbital period is approximately 1,358 days, keeping it firmly within the inner portion of the main asteroid belt.
What makes 16525 Shumarinaiko stand out among other main-belt asteroids?
Its binary nature—possessing a tidally locked companion moon—distinguishes it from the many single-body members of the Nysa–Polana group. That combination of a relatively large size, an inner-belt orbit, and a confirmed satellite makes it a useful case study for binary-asteroid formation.
More in Notable Asteroids, Part 4 1-24
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