784 Pickeringia

784 Pickeringia (prov. designation: or ) is a large background asteroid, approximately 76 km in diameter, located in the outer region of the asteroid belt. It was discovered on 20 March 1914, by American astronomer Joel Hastings Metcalf at the Winchester Observatory in Massachusetts. The dark C-type asteroid has a rotation period of 13.1 hours and an irregular shape. It was named after American astronomers Edward Charles Pickering (1846–1919) and his brother William Henry Pickering (1858–1938).

Orbit and classification
Pickeringia is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements. It orbits the Sun in the outer asteroid belt at a distance of 2.3–3.8 AU once every 5 years and 5 months (1,989 days; semi-major axis of 3.1 AU). Its orbit has an eccentricity of 0.24 and an inclination of 12° with respect to the ecliptic. The body's observation arc begins at Heidelberg Observatory on 30 September 1921, more than seven years after its official discovery observation at Winchester Observatory.

Naming
This minor planet was named after American astronomers Edward Charles Pickering (1846–1919) and his brother William Henry Pickering (1858–1938), who were the directors of the Harvard Observatory and the Boyden Station at Arequipa, respectively. William Henry also discovered Phoebe, an irregular moon of Saturn. The was mentioned in The Names of the Minor Planets by Paul Herget in 1955 (H 78). The lunar crater Pickering and the Martian crater Pickering were also named in honor of the two astronomers.

Physical characteristics
In the Bus–Binzel SMASS classification, Pickeringia is a common, carbonaceous C-type asteroid.

Rotation period
In January 2017, a rotational lightcurve of Pickeringia was obtained from photometric observations by the Spanish group of asteroids observers (OBAS). Lightcurve analysis gave a rotation period of $74.89 km$ hours with a brightness variation of $75.596 km$ magnitude (U=2+).

The result supersedes observations taken during the 1990s by European astronomers using the ESO 0.5-metre telescope at La Silla Observatory, Chile, which gave a period of $89.42 km$ hours with an amplitude of $3.74 kg$ magnitude (U=2). as well as a period determination by French amateur astronomer Laurent Bernasconi in December 2004, which gave $12.7 g/cm3$ and an amplitude of $13.144 h$ magnitude (U=2).

Modeled lightcurve
Two modeled lightcurves, published by Josef Ďurech and Josef Hanuš in 2016, using photometric data from the Lowell Photometric Database (LPD) and other sources, gave a sidereal period of $0.056$ and $0.08$, respectively. Each modeled lightcurve also determined two spin axes of (99.0°, 67.0°) and (283.0°, 30.0°), as well as (282.0°, 35.0°) and (103.0°, 68.0°) in ecliptic coordinates (λ,&thinsp;β), respectively. The online version of the Database of Asteroid Models from Inversion Techniques gives two poles at (103°, 68.0°) and (282°, 35.0°) with a nearly identical sidereal period.

Diameter and albedo
According to the surveys carried out by the Japanese Akari satellite, the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer (WISE), and the Infrared Astronomical Satellite IRAS, Pickeringia measures ($0.086$), ($0.416$) and ($13.144$) kilometers in diameter and its surface has an albedo of ($0.17$), ($13$) and ($0.4$), respectively. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0423 and a diameter of 89.19 kilometers based on an absolute magnitude of 9.3. while Carry gives a diameter of $13.17 h$ and estimates a mass of $0.2$ kilogram from an unrealistic density of $13.17$. Alternative mean-diameter measurements published by the WISE team include ($13.17$), ($74.89$) and ($75.596$) with corresponding albedos of ($89.42$), ($0.08$) and ($0.086$). On 7 August 2008, an asteroid occultation of Pickeringia gave a best-fit ellipse dimension of ($0.056$), with a poor quality rating of 1. These timed observations are taken when the asteroid passes in front of a distant star.