1757 Porvoo

1757 Porvoo, provisional designation 1939 FC, is a presumably stony asteroid from the inner regions of the asteroid belt, approximately 10 kilometers in diameter. It was discovered on 17 March 1939, by Finnish astronomer Yrjö Väisälä at Turku Observatory on the coast of southwestern Finland.

The asteroid orbits the Sun at a distance of 2.1–2.6 AU once every 3 years and 7 months (1,317 days). Its orbit has an eccentricity of 0.13 and an inclination of 4° with respect to the ecliptic. As no precoveries were taken, and no prior identifications were made, Porvoo's observation arc begins with its official discovery observation.

In the early 1980s, a rotational light-curve of Porvoo was obtained from photometric observations taken by U.S. astronomer Richard P. Binzel using the 0.91- and 2.1-m telescopes at the University of Texas McDonald Observatory. It gave it a well-defined rotation period of 4.89 hours with a brightness variation of 0.30 magnitude (U=3).

According to the surveys carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Porvoo measures 10.03 and 12.81 kilometers in diameter, and its surface has an albedo of 0.049 and 0.073, respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for stony asteroids of 0.20 – contrary to the rather carbonaceous albedo given by the space-based surveys – and calculates a diameter of 6.32 kilometers based on an absolute magnitude of 13.36.

This minor planet was named for Porvoo, Finnish city and municipality located on the southern coast of Finland, and east of the capital Helsinki. Porvoo is one of the six medieval towns in Finland, and is its second oldest city after Turku, location of the discovering observatory. In 1809, at the Diet of Porvoo, the Russian czar confirmed that Finland was annexed to the Russian empire as an autonomous nation. Naming citation was published on 1 August 1980 (M.P.C. 5449).