555 Norma, provisional designation 1905 PT, is a background asteroid from the outer regions of the asteroid belt, approximately 33 kilometers in diameter. It was discovered on 14 January 1905, by German astronomer Max Wolf at Heidelberg Observatory in southwest Germany.[17] The asteroid was named after the title character of Bellini's opera Norma.[3]
Classification and orbit
Norma is a background asteroid, located near the region occupied by the Themis family, a prominent family of outer-belt asteroids with nearly coplanar ecliptical orbits. It orbits the Sun at a distance of 2.7–3.7AU once every 5 years and 8 months (2,078 days). Its orbit has an eccentricity of 0.15 and an inclination of 3° with respect to the ecliptic.[1] The body's observation arc begins at Heidelberg in March 1911, more than six years after its official discovery observation.[17]
Physical characteristics
Spectral type
In the SMASS classification, Norma is a B-type asteroid. These types of asteroids have a featureless surface that displays magnesium-rich silicates, which likely accounts for the relatively high albedo as an outer-belt asteroid. Norma surface consists of more than 50% amorphous magnesium pyroxenes based on data collected with the Subaru Telescope.[15]
Photometry
In April 2007, a first rotational lightcurve of Norma was obtained from photometric observations by French amateur astronomer Pierre Antonini. Lightcurve analysis gave a rotation period of 30.6 hours with a brightness variation of 0.2 magnitude (U=2).[14] However more recent observations by two American astronomers have since superseded this result.
In December 2011, Robert Stephens at the Santana Observatory (646) obtained a lightcurve that gave a period 19.55 hours and a brightness amplitude of 0.06 magnitude (U=2+), while Frederick Pilcher measured a period of 19.508 hours with an amplitude of 0.25 at the Organ Mesa Observatory (G50) in November 2016.[12][13]
The Collaborative Asteroid Lightcurve Link (CALL) agrees with IRAS, and derives an albedo of 0.0528 and a diameter of 40.02 kilometers based on an absolute magnitude of 10.8. CALL also assumes Norma to be a C-type asteroid (rather than a B-type) due to its derived low albedo and the general spectral type of the Themis family.[4]
