Asteroid 2004 RE84 passed by the Earth at a distance of about 15 154 000 km (39.5 times the average distance between the Earth and the Moon, or 10.1% of the distance between the Earth and the Sun), slightly before 7.10 am GMT on Tuesday 10 March 2020. There was no danger of the asteroid hitting us, though were it to do so it would have presented a considerable threat. 2004 RE84 has an estimated equivalent diameter of 78-250 m (i.e. it is estimated that a spherical object with the same volume would be 78-250 m in diameter), and an object at the upper end of this 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 23 000 times as powerful as the Hiroshima bomb. Such an impact would result in an impact crater 3.8 km in diameter and devastation on a global scale, as well as climatic effects that would last years or even decades.
The calculated orbit of 2004 RE84. Minor Planet Center.
2004 RE84 was discovered on 10 Septemer 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 2021th asteroid (asteroid E84 - in numbering asteroids the letters A-Y, excluding I, are assigned numbers from 1 to 24, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 25, A2 = 49, etc., which means that E84 = 5 + (24 X 84) = 2021) discovered in the first half of September 2004 (period 2004 R.
2004 RE84 has a 791 day (2.17 year) orbital period, with an elliptical orbit tilted at an angle of 2.00° to the plain of the Solar System which takes in to 0.65 AU from the Sun (65% of the distance at which the Earth orbits the Sun, and slightly inside the orbit of the planet Venus) and out to 2.69 AU (269% of the distance at which the Earth orbits the sun and further from the Sun than the planet Mars). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in October 2017 and the next predicted in October 2030. 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). 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 occasional close encounters with the planets Venus, which it last came close to in August 1987 and is next predicted to pass in January 2033, and Mars, it last came close to in December 1982 and is expected to pass again 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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