(5645) 1990 SP

(5645) 1990 SP is an eccentric and tumbling asteroid, classified as near-Earth object of the Apollo group, approximately 1.7 kilometers in diameter. It was discovered on 20 September 1990, by Scottish–Australian astronomer Robert McNaught at the Siding Spring Observatory in Canberra, Australia. Scientists have said that it has a '1 in 364 billion chance' of colliding with the Earth.

Orbit and classification
The asteroid orbits the Sun at a distance of 0.8–1.9 AU once every 1 years and 7 months (576 days). Its orbit has an eccentricity of 0.39 and an inclination of 14° with respect to the ecliptic.

Close approaches
This near-Earth asteroid has an Earth minimum orbit intersection distance of 0.055 AU, only slightly above the threshold minimum distance of 19.5 lunar distances (0.05 AU) to make it a potentially hazardous object. It also makes close approaches to Mars. On 14 April 1969, it passed the Red Planet at only 0.013 AU.

Precovery
Published by the Digitized Sky Survey (DSS), a first precovery was taken at the discovering observatory in 1974, extending the asteroid's observation arc by 16 years prior to its discovery.

Physical characteristics
The stony S-type asteroid is also characterized as a P-type, based on post-cryogenic observations by the Spitzer Space Telescope, while observations at the NASA Infrared Telescope Facility using its SpeX instrument during a follow-up campaign of the Spitzer-observed objects between 2009 and 2012, gave it a C/X/T spectral type.

Rotation
In April 2002, Czech astronomer Petr Pravec obtained a rotational lightcurve from a photometric observations, which gave a relatively long period of $1.648$ hours with a brightness variation of 0.7 in magnitude (U=2). The observations have also shown that the body is most likely in a tumbling motion.

Diameter and albedo
Estimates for the body's diameter range from 1.6 to 2.2 kilometers with an albedo for its surface between 0.06 and 0.12, according to observations made by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer and by the Spitzer Space Telescope. The Collaborative Asteroid Lightcurve Link takes the revised WISE data – an albedo of 0.0872 and a diameter of 1.65 kilometers – as the best of all available results.