Asteroid (242708) 2005 UK1 passed by the Earth at a distance of about 12 355 300 km (32.2 times the average distance between the Earth and the Moon, or 8.26% of the distance between the Earth and the Sun), slightly after 10.25 am GMT on Monday 12 January 2026. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. Asteroid (242708) 2005 UK1 has an estimated equivalent diameter of 450-1400 m (i.e. it is estimated that a spherical object with the same volume would be 450-1400 m in diameter), and an object of this size would be expected to penetrate the Earth's atmosphere and impact the Earth's surface, causing an explosion with an equivalent energy release of more than 4000 megatons of TNT. An impact of this size would be expected to flatten forests and man-made structures over thousands of kilometres, while producing a crater at least 6 km in diameter and probably significantly larger, as well as causing global climatic effects which would persist for decades if not centuries.
(242708) 2005 UK1 was discovered on 24 October 2005 by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2005 UK1 implies that the asteroid was the 35th object (asteroid K1 - in numbering asteroids the letters A-Z, excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., which means that K1 = 25 + 10 = 25) discovered in the second half of October 2005 (period 2005 K - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).
(242708) 2005 UK1 is calculated to have a 1445 day (3.96 year) orbital period, with an elliptical orbit tilted at an angle of 0.79° to the plain of the Solar System which takes in to 0.79 AU from the Sun (79% of the average distance at which the Earth orbits the Sun) and out to 4.24 AU (4.24 times the distance at which the Earth orbits the Sun, almost three times the distance at which the planet Mars orbits).
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(242708) 2005 UK1 is therefore classed as an Apollo Group Asteroid, which is an asteroid that is on average further from the Sun than the Earth, but which does get closer. As an asteroid possibly larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, (242708) 2005 UK1 is also classified as a Potentially Hazardous Asteroid.
Close encounters between (242708) 2005 UK1 and Earth are fairly common, with the last thought to have happened in April 2018 and the next predicted in December 2029. (242708) 2005 UK1 also has frequent close encounters with other planets, the last being an encounter with Venus in September 1974, and the next predicted being an encounter with Mars in February 2133.
See also...
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