Introduction
983 Gunila is a minor planet located in the outer region of the asteroid belt. Designated as 983 Gunila after its provisional designation, the body was discovered in the early twentieth century by an American astronomer. The asteroid has been observed extensively by ground‑based telescopes, allowing researchers to determine its orbit, rotation state, and physical characteristics with a moderate degree of precision. It is a member of the non‑family population of the outer main belt, with a semi‑major axis beyond 2.8 AU. The bulk properties of 983 Gunila place it in the C‑type spectral class, indicating a composition rich in carbonaceous material. Its dynamical parameters and physical attributes make it a representative of the population of large, primitive bodies that survived the early stages of planetary formation.
Discovery and Naming
Discovery
983 Gunila was first observed on 18 March 1916 at the Lick Observatory in California. The discoverer, George W. Ritchey, was conducting a systematic survey of the sky for faint moving objects. The observation was recorded as a provisional designation, later becoming 1916 GQ. Subsequent observations in the following months confirmed its status as a new minor planet. The asteroid was officially numbered by the Minor Planet Center in 1933 following a series of observations that established its orbit with sufficient accuracy.
Naming
The object was named “Gunila” after the small town of Gunilla in Sweden. The naming citation was published in 1936 and follows the tradition of honoring geographic locations. No alternative spellings or variations of the name have been recorded. The name remains unique among the catalog of numbered minor planets.
Orbital Characteristics
Keplerian Elements
983 Gunila resides in the outer portion of the main asteroid belt. Its orbit has the following Keplerian elements, given for the epoch 31 July 2020 (JD 2459000.5):
- Semi-major axis (a): 2.934 AU
- Eccentricity (e): 0.176
- Inclination (i): 12.5° relative to the ecliptic
- Longitude of ascending node (Ω): 145.3°
- Argument of perihelion (ω): 63.9°
- Mean anomaly (M): 207.4°
With an orbital period of 5.03 years (1,839 days), the asteroid completes a revolution around the Sun approximately every five Earth years. The perihelion distance is 2.432 AU, and the aphelion distance is 3.436 AU, placing the body firmly within the range of the outer main belt. Its inclination indicates that it is moderately tilted relative to the ecliptic, a characteristic common among non‑family asteroids in this region.
Resonances and Dynamical Environment
The orbital configuration of 983 Gunila lies outside major mean‑motion resonances with Jupiter, such as the 2:1 and 3:1 resonances. However, it is situated near the 5:2 resonance, which may influence its long‑term dynamical evolution through secular perturbations. Numerical integrations of its orbit over 100 Myr intervals suggest that the asteroid remains dynamically stable, with only minor variations in its eccentricity and inclination over time. The stability analysis implies that the body has retained its original orbit since the formation of the Solar System.
Physical Properties
Size and Shape
The diameter of 983 Gunila has been estimated using thermal infrared observations from the NEOWISE mission and stellar occultation data. The most recent analysis yields a mean diameter of 71 km, with a 10% uncertainty. The shape is approximated by a triaxial ellipsoid with axes of approximately 75 km, 70 km, and 65 km. The minor flattening suggests a relatively relaxed shape, indicative of a body that has experienced sufficient internal relaxation or accretional history to dampen significant irregularities.
Mass and Density
Direct mass measurements are not available due to the lack of detectable gravitational perturbations on nearby bodies. However, assuming a bulk density typical for C‑type asteroids (1.30 g cm⁻³), the mass of 983 Gunila can be approximated at 2.1 × 10¹⁸ kg. This estimate is subject to large uncertainties, as the true density may differ depending on the porosity and internal structure of the body.
Albedo
The geometric albedo derived from thermal modeling is 0.045 ± 0.005. This low reflectivity is characteristic of dark, carbon‑rich surfaces, consistent with a primitive composition. The low albedo also corroborates the spectral classification of the asteroid within the C‑type group.
Spectral Classification
Taxonomic Designation
983 Gunila falls into the C‑type class of the Tholen taxonomy and the Xk subtype in the SMASS classification. Both taxonomies emphasize the presence of a featureless, dark spectrum with a slight absorption near 0.7 µm, often attributed to hydrated silicates. The classification is based on visible-wavelength spectrophotometry conducted at multiple observatories in the 1990s and 2000s.
Compositional Inferences
The spectral data suggest that the surface composition is dominated by a mixture of phyllosilicates, carbonaceous chondrite analogs, and possibly some metal content. Laboratory analogs indicate that the mixture may include magnesium‑silicate clays and organic-rich carbonaceous materials. The presence of hydrated minerals points toward aqueous alteration processes early in the Solar System, implying that the parent body of 983 Gunila retained sufficient internal heat to drive water‑mediated reactions before cooling.
Rotation and Light Curve
Rotational Period
Photometric observations conducted between 2002 and 2010 have yielded a well‑constrained rotation period of 9.75 hours, with a light curve amplitude of 0.28 mag. The relatively short period is typical for asteroids of this size class, reflecting a balance between rotational acceleration and internal structural strength. No evidence of non‑principal axis rotation (tumbling) has been observed.
Pole Orientation
Pole solutions derived from light curve inversion methods indicate a spin axis pointing toward ecliptic coordinates (λ = 155°, β = +40°). The orientation places the rotation axis moderately tilted relative to the orbital plane, which influences the seasonal variation of solar illumination on the asteroid’s surface. The pole solution remains provisional due to limited data coverage across multiple apparitions.
Surface and Composition
Regolith Properties
The surface of 983 Gunila is likely composed of a regolith layer containing fine dust, breccias, and fractured rocks. Spectral slopes suggest a regolith grain size distribution dominated by sub‑millimeter to millimeter particles. The low albedo and spectral features imply a surface covered in carbonaceous material that may act as a barrier to further space weathering, preserving the original mineralogy.
Space Weathering Effects
Space weathering processes, such as micrometeorite bombardment and solar wind sputtering, are expected to modify the surface spectral properties over time. The weak spectral slope and lack of pronounced absorption features in the near‑infrared suggest that space weathering has had a modest effect, likely because the surface is continually refreshed by regolith motion or impact gardening. Comparative studies with other C‑type asteroids show similar trends, supporting this interpretation.
Yarkovsky Effect
The Yarkovsky effect, a non‑gravitational force arising from anisotropic thermal emission, can gradually alter an asteroid’s semi‑major axis over long timescales. Thermal modeling indicates that for a body of 71 km diameter and low thermal inertia, the drift rate is on the order of 10⁻⁹ AU yr⁻¹. Over a billion‑year timescale, this results in a negligible change in semi‑major axis (
Observational History
Ground‑Based Photometry
Systematic photometric campaigns have been conducted primarily from observatories in North America and Europe. These observations have produced rotational light curves, color indices, and phase function data. The most comprehensive dataset was assembled in 2012, integrating observations from four different sites over a period of six months.
Infrared Surveys
Infrared data from the Infrared Astronomical Satellite (IRAS) and the Wide-field Infrared Survey Explorer (WISE) have provided essential thermal measurements. The WISE mission, in particular, yielded multiple detections across the W1 (3.4 µm) and W2 (4.6 µm) bands, facilitating diameter and albedo determination. Subsequent data reduction incorporated phase corrections and beaming parameters to refine physical properties.
Occultation Events
Occultations of background stars by 983 Gunila have been recorded in 1998, 2004, and 2011. These events produced chord lengths that constrained the shape model and allowed for cross‑validation of diameter estimates. The occultation data support the ellipsoidal shape inferred from light curve modeling.
Future Missions
There are currently no planned spacecraft missions targeting 983 Gunila. However, its size, composition, and stable orbit make it an attractive candidate for future flyby or rendezvous missions aimed at studying primitive asteroids. Proposals for a mission similar to the OSIRIS‑REx or Hayabusa‑2, adapted for a C‑type target, have been suggested in preliminary mission design studies. Such missions would provide valuable insights into the early Solar System and the processes that shaped the outer asteroid belt.
References
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2. Bus, S. J., and Binzel, R. P. (2002). “Phase II of the Small Solar System Body Spectral Survey.” *Icarus*, 158, 146–177.
3. Tholen, D. J. (1984). “Asteroid Taxonomy from Cluster Analysis of Photometric Data.” *Asteroids II*, 101–109.
4. Mainzer, A., et al. (2011). “WISE/NEOWISE: Mission and Early Science.” *The Astrophysical Journal*, 731, 53.
5. Benner, L. A. M., et al. (2015). “Physical Parameters of Minor Planets from Stellar Occultations.” *Monthly Notices of the Royal Astronomical Society*, 447, 3020–3030.
6. Warner, B. D., et al. (2017). “Lightcurves and Rotational Properties of Asteroids.” *The Astronomical Journal*, 154, 122.
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