1100 Arnica , provisional designation , is a Koronian asteroid from the outer regions of the asteroid belt, approximately 17 kilometers in diameter. It was discovered by Karl Reinmuth at the Heidelberg Observatory in 1928 and named after the herbaceous plant Arnica (aster; daisy). The asteroid is likely of stony composition and has a rotation period of 14.535 hours. Discovery Arnica was first observed as at the Heidelberg-Königstuhl State Observatory in December 1904. It was officially discovered on 22 September 1928, by German astronomer Karl Reinmuth at Heidelberg in southwest Germany. On 14 October 1928, it was independently discovered by astronomers Friedrich Schwassmann and Arthur Wachmann at the Bergedorf Observatory in Hamburg. The Minor Planet Center does not recognize these independent discoverers. Orbit and classification Arnica is a member of the Koronis family (), an outer belt asteroid family with nearly co-planar ecliptical orbits. The family consist of nearly 6,000 known members and is named after its parent body 158 Koronis. It orbits the Sun in the outer asteroid belt at a distance of 2.7–3.1 AU once every 4 years and 11 months (1,803 days; semi-major axis of 2.90 AU). Its orbit has an eccentricity of 0.07 and an inclination of 1° with respect to the ecliptic. The body's observation arc begins at Heidelberg in October 1918, almost 10 years prior to its official discovery observation. Close asteroid approaches Arnica occasionally makes close approaches to other main-belt asteroids. It will pass close to 88 Thisbe three times before the year 2200. On 21 February 2043, Arnica will be from Thisbe. On 31 March 2112, it will be from Thisbe. Its closest approach to Thisbe will occur on 17 May 2181, when its distance to Thisbe will be . It will also approach 7 Iris and 16 Psyche, coming within and on 28 November 2104 and 16 July 2199, respectively. Physical characteristics Arnica is an assumed stony S-type asteroid, which is also the overall spectral type for members of the Koronis family. Rotation period Several rotational lightcurves of Arnica have been obtained from photometric observations since 2003. Analysis of the best-rated lightcurve gave a rotation period of 14.535 hours with a consolidated brightness amplitude between 0.09 and 0.28 magnitude (). Diameter and albedo According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Arnica measures 16.894 and 17.234 kilometers in diameter and its surface has