2020 SW is a tiny near-Earth asteroid discovered by the Mount Lemmon Survey on 18 September 2020, six days before it made its closest approach to Earth. The asteroid passed within 21,600 kilometres (13,400 mi) from Earth's surface on 24 September 2020 11:13 UT, within the geostationaryaltitude of 36,000 kilometres (22,000 mi). The encounter with Earth perturbed the asteroid's heliocentric trajectory from an Apollo-type orbit to an Aten-type orbit with a semi-major axis within one astronomical unit from the Sun. As a result, the asteroid will not make any close approaches to Earth within 0.01 astronomical units (4 LD) in the next 200 years.
With an estimated diameter roughly 4–9 metres (13–30 ft) across, 2020 SW is comparable to the size of a small school bus. Had it impacted Earth, it would mostly have disintegrated as a fireball during atmospheric entry and might have left a common strewn field. Extensive observations of its brightness during the close encounter show that it is an elongated body with a rapid rotation period of 28.5 seconds.
The asteroid was subsequently listed on the Minor Planet Center's Near-Earth Object Confirmation Page (NEOCP) as C378AD2.[5] Follow-up observations were carried out by four other observatories including Pan-STARRS 1(F51) and the Catalina Sky Survey(V06). Within one day after the asteroid's discovery, the listing was confirmed and publicly announced as 2020 SW on 19 September 2020.[1]
Before the Earth encounter on 18 September 2020, 2020 SW had a more distant Apollo-type orbit crossing the path of Earth. It had a perihelion distance of 0.822 AU and a semi-major axis of 1.015 AU, with an orbital period of 1.02 years. The orbit had an orbital eccentricity of 0.190 and an inclination of 4.2 degrees to the ecliptic.[6]
Based on an magnitude-to-diameter conversion and a measured absolute magnitude of 29.06, 2020 SW measures between 4 and 9 metres in diameter for an assumed geometric albedo of 0.25 and 0.05, respectively.[2][7] Given its small size, 2020 SW can be compared to a small school bus.[10] It is too small to pose any threat to Earth and its nominal orbit is not known to be on an impact trajectory with the planet. Even had it impacted Earth, it would mostly have disintegrated as a fireball during atmospheric entry and might have left a common strewn field.[10]
Shape and rotation
The 2020 SW close encounter with Earth provided an opportunity for astronomers to take detailed measurements of its light curve to determine the asteroid's rough shape and rotation period. On 24 September 2020, within 10 hours before closest approach to Earth, 2020 SW was continually observed by astronomer Peter Birtwhistle at the Great Shefford Observatory(J95) for 2 hours and 51 minutes. He derived a period of 0.0079039±0.0000001 h (28.4540±0.0004 s) and a light curve amplitude of 0.73, indicating an elongated shape with a minimum a/bellipsoidaspect ratio of 1.6. Earlier independent observations of 2020 SW by astronomers at the Northolt Branch Observatories produced similar measurements of the asteroid's light curve, providing a period of 0.00790±0.00001 h (28.44±0.04 s) and an amplitude of 0.72.[3]
^"2020SW Ephemerides". Near Earth Objects – Dynamic Site (Ephemerides at discovery (obs. code G96)). Department of Mathematics, University of Pisa, Italy. Retrieved 23 January 2021.