3309 Brorfelde
A binary Hungaria asteroid discovered at Brorfelde Observatory.
3309 Brorfelde, originally labeled 1982 BH, is a roughly spheroidal binary asteroid that belongs to the Hungaria group in the inner main belt. Its diameter is about 4 kilometers. The asteroid was spotted on 28 January 1982 by Danish astronomers Kaare Jensen and Karl Augustesen at the Brorfelde Observatory, located near Holbæk, Denmark. It was named after both the observatory and the nearby village.
This bright, stony object is part of the Hungaria family, the innermost dense cluster of asteroids in the Solar System. Its orbit takes it between 1.7 and 1.9 AU from the Sun, completing one lap every 895 days (roughly 2 years and 5 months). The orbit has a low eccentricity of 0.05 and a relatively high inclination of 21° relative to the ecliptic. No precovery images or earlier identifications exist; the observation arc starts with the discovery itself.
In the SMASS classification, Brorfelde is a common S-type asteroid, meaning it is stony in composition.
Photometric studies between 2005 and 2010, led by Brian Warner and Petr Pravec, produced multiple lightcurves. The best analyses show a rotation period between 2.5041 and 2.5046 hours, with a brightness variation of 0.09 to 0.13 magnitudes—consistent with a nearly spherical shape. These results are rated U=3, superseding earlier, less reliable work by Wiesław Z. Wiśniewski in the 1990s (U=2), Federico Manzini and René Roy in 2005 and 2009 (U=2- and n.a.), and a 2010 Palomar Transient Factory observation that mistakenly reported a period over 9 hours (U=1).
In 2005, photometry revealed Brorfelde is a binary system. Its moon orbits the primary every 18.48 ± 0.01 hours and is roughly 1 kilometer across, based on a secondary-to-primary diameter ratio of 0.26 ± 0.02. Follow-up observations by Brian Warner in January 2014 confirmed a primary rotation of 2.503 hours and a secondary orbital period of 18.51 hours (U=3), with lightcurve plots published online.
NASA's WISE and NEOWISE surveys measured the asteroid at 3.78 kilometers in diameter, with a surface albedo of 0.46 (the most recent result). The Collaborative Asteroid Lightcurve Link, using an absolute magnitude of 14.062, calculates a slightly larger diameter of 3.91 kilometers and a lower albedo of 0.2747.
The minor planet was named to mark the 40th anniversary of the Brorfelde Observatory. It was the first asteroid discovered at that site.
Quick Facts
- Minorplanet
- yes
- Background
- #D6D6D6
- Discovered
- 28 January 1982
- Discoverer
- K. Jensen / K. Augustesen
- Discovery Site
- Brorfelde Obs.
- Mpc Name
- (3309) Brorfelde
- Named After
- Brorfelde Observatory / (discovering observatory)
- Mp Category
- main-beltHungaria
- Epoch
- 4 September 2017 (JD 2458000.5)
- Uncertainty
- 0
- Observation Arc
- 35.25 yr (12,876 days)
- Aphelion
- 1.9143 AU
Facts from the source article.
Lore & Background
3309 Brorfelde is a bright stony asteroid and member of the Hungaria family, which form the innermost dense concentration of asteroids in the Solar System. It orbits the Sun at a distance of 1.7–1.9 AU once every 2 years and 5 months (895 days), with an eccentricity of 0.05 and an inclination of 21° with respect to the ecliptic. On the SMASS taxonomic scheme, it is a common stony S-type asteroid.
Photometric observations between 2005 and 2010 by Brian Warner and Petr Pravec yielded a rotation period between 2.5041 and 2.5046 hours with a brightness amplitude between 0.09 and 0.13 in magnitude, indicating a nearly spheroidal shape. These results superseded earlier observations by Wiesław Z. Wiśniewski in the 1990s, Federico Manzini and René Roy in 2005 and 2009, and the Palomar Transient Factory in 2010, which gave an incorrect period solution of more than 9 hours.
During the 2005 photometric observations, it was revealed that Brorfelde is a binary asteroid. Its moon has an orbital period of 18.48±0.01 hours and measures approximately 1 kilometer in diameter, based on a mean-diameter ratio of 0.26±0.02. Repeated observations by Brian Warner in January 2014 confirmed a period of 2.503 and 18.51 hours for the primary and secondary, respectively.
Reader's Guide
3309 Brorfelde holds significance as a binary asteroid within the Hungaria family, providing insights into the formation and dynamics of small asteroid systems. Its nearly spheroidal shape and low brightness amplitude suggest a relatively uniform body, while the presence of a moon about 1 kilometer in diameter offers a case study for binary asteroid evolution. The asteroid's discovery at Brorfelde Observatory marked the observatory's first minor planet discovery, and it was named for the observatory and the village where it is located on the occasion of the observatory's 40th anniversary. The approved naming citation was published by the Minor Planet Center on 7 September 1987. Measurements from NASA's NEOWISE mission give a diameter of 3.78 kilometers and a surface albedo of 0.46, while the Collaborative Asteroid Lightcurve Link derives an albedo of 0.2747 and a diameter of 3.91 kilometers. The asteroid's observation arc begins with its official discovery observation, as no precoveries or prior identifications were made.
Did You Know?
- 3309 Brorfelde is a binary asteroid with a moon approximately 1 kilometer in diameter.
- It was the first minor planet discovered at Brorfelde Observatory.
- The asteroid has a nearly spheroidal shape, indicated by a low brightness amplitude of 0.09 to 0.13 magnitude.
- Its rotation period is between 2.5041 and 2.5046 hours.
More in Notable Asteroids, Part 2 1-24
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