Asteroid 2004 RE84 passed by the Earth at a distance of about 5 846 000 km (15.2 times the average distance between the Earth and the Moon, or 3.91% of the distance between the Earth and the Sun), slightly before 3.00 pm GMT on Wednesday 4 October 2017. There was no danger of the asteroid hitting us, though were it to do so it would have presented a significant threat. 2004 RE84 has an estimated equivalent diameter of 75-240 m (i.e. it is estimated that a spherical object with the same volume would be 75-240 m in diameter), and an object at the upper end of this size range would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground directly with an explosion that would be 30 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater over 3.5 km in diameter and devastation on a global scale, as well as climatic effects that would last decades or even centuries.
The calculated orbit of 2004 RE84. Minor Planet Center.
2004 RE84 was discovered on 10 September 2004 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2004 RE84 implies that it was the 2105th asteroid (asteroid E84) discovered in the first half of September 2004 (period 2004 RE84).
2004 RE84 has a 791 day orbital period and an eccentric orbit tilted at an angle of 2.00° to the plane of the Solar System, which takes it from 0.66 AU from the Sun (i.e. 66% of he average distance at which the Earth orbits the Sun, somewhat less the distance at which the planet Venus orbits the Sun) to 2.68 AU from the Sun (i.e. 268% of the average distance at which the Earth orbits the Sun, and almost two times as distant from the Sun as the planet Mars). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). This means that close encounters between the asteroid and Earth are extremely common, with the last having occurred in March 2007 and the next predicted in March 2020. As an asteroid probably larger than 150 m in diameter that occasionally comes within 0.05 AU of the Earth, 2004 RE84 is also classified as a Potentially Hazardous Asteroid.
2004 RE84 also has frequent close encounters with the planets Venus, which it is thought to have last passed in August 1987, and is next predicted to pass in January 2033, and Mars, which it last came close to in December 1982 and is next predicted to pass in July 2074. Asteroids which make close passes to multiple planets are considered to be in unstable orbits, and are often eventually knocked out of these orbits by these encounters, either being knocked onto a new, more stable orbit, dropped into the Sun, knocked out of the Solar System or occasionally colliding with a planet.
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