4951 Iwamoto
A stony, synchronous binary asteroid and slow rotator.
4951 Iwamoto (1990 BM) is a stony asteroid from the inner main belt, roughly 5.5 kilometers across. It was discovered on 21 January 1990 by Japanese astronomers Yoshikane Mizuno and Toshimasa Furata at Kani Observatory. The asteroid is a slow rotator, taking 118 hours to complete one spin—far longer than the typical 2.2 to 20 hours for most asteroids. It is also a synchronous binary system: a smaller moon orbits the primary every 4.917 days (the same 118-hour period as the primary’s rotation). Based on the moon’s diameter being at least 76% of the primary’s, the two bodies are estimated at 4.0 and 3.5 kilometers in diameter, though later measurements revised the primary’s size to 5.5 kilometers. The moon’s orbit has a semi-major axis of about 31 kilometers.
The asteroid’s orbit brings it between 1.9 and 2.6 AU from the Sun, with an eccentricity of 0.17 and an inclination of 8 degrees to the ecliptic. It completes one orbit in 3 years and 5 months (1,239 days). Although officially discovered in 1990, it was first observed as 1931 UQ at Lowell Observatory in 1931, extending its observation arc by 59 years.
In the SMASS classification, Iwamoto is an S-type (stony) asteroid. Measurements from NASA’s WISE and NEOWISE surveys give a diameter of 5.192 or 5.515 kilometers and an albedo of 0.218 or 0.186. The Collaborative Asteroid Lightcurve Link, using revised WISE data from Petr Pravec, reports an albedo of 0.1844 and a diameter of 5.528 kilometers, based on an absolute magnitude of 13.74.
Photometric observations from December 2006 to March 2007, conducted at multiple observatories worldwide, yielded a rotation period of 118 hours with a brightness variation of 0.34 magnitude. A second lightcurve in May 2011 by Etienne Morelle and Raoul Behrend confirmed the 118-hour period with an amplitude of 0.38 magnitude.
The asteroid is named after Japanese astronomer Masayuki Iwamoto (born 1954), a discoverer of minor planets at Tokushima Observatory. The naming citation was published by the Minor Planet Center on 5 March 1996.
Quick Facts
- Minorplanet
- yes
- Background
- #D6D6D6
- Discovered
- 21 January 1990
- Discoverer
- Y. Mizuno / T. Furuta
- Discovery Site
- Kani Obs. (403)
- Mpc Name
- (4951) Iwamoto
- Named After
- Masayuki Iwamoto / (Japanese astronomer)
- Mp Category
- main-belt(inner)
- Epoch
- 4 September 2017 (JD 2458000.5)
- Uncertainty
- 0
- Observation Arc
- 85.45 yr (31,210 days)
- Aphelion
- 2.6318 AU
Facts from the source article.
Lore & Background
4951 Iwamoto orbits the Sun in the inner main-belt at a distance of 1.9–2.6 AU once every 3 years and 5 months (1,239 days). Its orbit has an eccentricity of 0.17 and an inclination of 8° with respect to the ecliptic. It was first identified as 1931 UQ at Lowell Observatory in 1931, extending the body's observation arc by 59 years prior to its official discovery observation at Kani.
In the SMASS classification, Iwamoto is a common S-type asteroid. According to the survey carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Iwamoto measures 5.192 and 5.515 kilometers in diameter, and its surface has an albedo of 0.218 and 0.186, respectively. The Collaborative Asteroid Lightcurve Link adopts Petr Pravec's revised WISE-data, that is, an albedo of 0.1844 and a diameter of 5.528 kilometers with an absolute magnitude of 13.74.
From 25 December 2006 to 23 March 2007, photometric observations of Iwamoto were obtained by an international community of photometrists at multiple observatories. Lightcurve analysis gave a rotation period of 118 hours with a brightness variation of 0.34 magnitude. In May 2011, astronomers Etienne Morelle and Raoul Behrend obtained another lightcurve with a concurring period of 118 hours and an amplitude of 0.38 magnitude. With such a long period, Iwamoto is also a slow rotator, as the vast majority of asteroids have a much shorter rotation period of 2.2 to 20 hours.
Reader's Guide
The significance of 4951 Iwamoto lies in its status as a synchronous binary asteroid and slow rotator, which provides insights into the dynamics of asteroid systems. During the photometric observations in 2006/7, it was revealed that Iwamoto (the primary) is a synchronous binary system with a minor-planet moon (the secondary) orbiting it every 4.917 days (or 118 hours, identical to the primary's rotation). Based on the secondary-to-primary mean-diameter ratio of at least 0.76, it was estimated that Iwamoto and its moon measure 4.0 and 3.5 kilometers, respectively; the diameter of Iwamoto has since increased to 5.5 kilometers. The Johnstonarchive estimates that the moon has a semi-major axis of 31 kilometers. The asteroid was named in honor of Japanese astronomer Masayuki Iwamoto (born 1954), a discoverer of minor planets at the Tokushima Observatory. The approved naming citation was published by the Minor Planet Center on 5 March 1996. Its legacy is tied to the collaborative international photometric campaign that revealed its binary nature and extremely long rotation period, contributing to the understanding of asteroid moon formation and slow rotator populations.
Did You Know?
- It is a synchronous binary asteroid with a moon that orbits every 118 hours, identical to the primary's rotation period.
- Its rotation period of 118 hours makes it a slow rotator, as most asteroids rotate in 2.2 to 20 hours.
Frequently Asked Questions
What is 4951 Iwamoto?
It is a stony asteroid residing in the inner portion of the main asteroid belt, with a diameter of roughly 5.5 kilometers. Before receiving its permanent designation, it was tracked under the provisional label 1990 BM.
Who discovered 4951 Iwamoto and where?
Japanese astronomers Yoshikane Mizuno and Toshimasa Furuta identified it on 21 January 1990 using equipment at Kani Observatory in Japan. The observatory is catalogued under minor-planet-center code 403.
Why is 4951 Iwamoto's rotation period unusual?
The asteroid takes approximately 118 hours to complete a single rotation, far exceeding the 2.2-to-20-hour range typical of most known asteroids. This makes it one of the slowest spinners recorded in the belt.
Does 4951 Iwamoto have a moon?
Yes; it is a synchronous binary system in which a smaller companion circles the primary every 4.917 days. The satellite's diameter is estimated to be at least 76 percent of the primary's, an unusually large ratio for a binary pair.
What makes 4951 Iwamoto scientifically interesting?
Its rotation period matches its moon's orbital period exactly, meaning the same hemisphere of the primary always faces the companion. That synchronous configuration, combined with the moon's large relative size, gives researchers a valuable natural laboratory for studying how binary asteroid systems form and maintain their orbits.
More in Notable Asteroids, Part 3 1-24
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