984 Gretia

984 Gretia is a stony background asteroid from the central regions of the asteroid belt, approximately 32 km in diameter. It was discovered by German astronomer Karl Reinmuth at the Heidelberg-Königstuhl State Observatory on 27 August 1922 and given the provisional designation . The asteroid was named after Greta, sister-in-law of ARI-astronomer Albrecht Kahrstedt.

Orbit and classification
Gretia is a background asteroid that has not been associated with any known asteroid family. It orbits the Sun in the central main belt at a distance of 2.2–3.4 AU once every 4 years and 8 months (1,714 days). Its orbit has an eccentricity of 0.20 and an inclination of 9° with respect to the ecliptic.

It was first observed as ' at the discovering observatory in 1910, and five years later as ' at the United States Naval Observatory. The body's observation arc begins at Vienna Observatory in September 1922, two weeks after its official discovery observation.

Naming
This minor planet was named after Greta, sister-in-law of Albrecht Kahrstedt (1897–1971), a German astronomer at ARI and director of the institute's Potsdam division, who requested the naming of this asteroid and 1026 Ingrid (daughter of Greta) in a personal letter to the discoverer in February 1926. Kahrstedt himself was honored with the naming of.

The official naming citation was mentioned in The Names of the Minor Planets by Paul Herget in 1955 (H 94). Lutz Schmadel quoted an excerpt of Kahrstedt's letter in his Dictionary of Minor Planet Names (LDS).

Physical characteristics
In the SMASS classification, Gretia is a Sr-subtype that transitions between the common S-type and rare R-type asteroids.

Lightcurves
Since 1997, a large number of rotational lightcurves of Gretia have been obtained from photometric observations. Analysis of the best-rated lightcurves gave a rotation period of 5.778 hours with a maximal brightness amplitude from 0.26 to 0.75 magnitude (U=2–3).

The asteroid's spin axis of (92.0°, 67.0°) and (247.0°, 48.0°) in ecliptic coordinates (λ,&thinsp;β) have also been derived from modeled lightcurves (Q=3).

Diameter and albedo
According to the surveys carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Gretia measures between 31.91 and 36.60 kilometers in diameter and its surface has a high albedo between 0.3566 and 0.4239.

The Collaborative Asteroid Lightcurve Link adopts the results obtained by IRAS, that is, an exceptionally high albedo of 0.4239 and a diameter of 31.91 kilometers with an absolute magnitude of 9.03.