Asteroid (285944) 2001 RZ11 passed by the Earth at a distance of about 13 150 000 km (34.21 times the average distance between the Earth and the Moon, or 8.8% of the average distance between the Sun and the Earth), slightly after 3.15 am GMT on Sunday 17 August 2014. There was no danger of the asteroid hitting us, though were it to do so it would have presented a serious threat. (285944) 2001 RZ11 has an estimated equivalent diameter of 990-3100 m (i.e. it is estimated that a spherical object with the same volume would be 990-3100 m in diameter), and an object of this size would be predicted to be capable of passing through the Earth's atmosphere relatively intact, impacting the ground with an energy equivalent to about 45 000-200 000 megatons of TNT (roughly 260 000-12 000 000 times the energy of the Hiroshima bomb). Such an event would result in a crater between 13 and 50 km across, cause devastation on a global scale and would have the potential to affect the climate globally for decades to centuries after the impact event.
The calculated orbit of (285944) 2001 RZ11. JPL Small Body Database Browser.
(285944) 2001 RZ11 was discovered on 10 September 2011 by the Massachusetts Institute of Technology's Lincoln Near Earth Asteroid Research Laboratory in Socorro, New Mexico. The designation 2001 RZ11 implies that it was the 300th asteroid (asteroid Z11) discovered in the first half of September 2001 (period 2001 R), while the designation 285944 implies that it was 285 944th asteroid ever discovered (asteroids are not given this longer designation immediately to avoid naming double or false sightings).
While (285944) 2001 RZ11 occasionally comes near to the Earth, it does not actually cross our orbital path. It has an elliptical 1185 day orbit, at an angle of 53° to the plane of the Solar System, that takes it from 1.08 AU from the Sun (1.08 times the average distance at which the Earth orbits the Sun), slightly outside our orbit, to 3.30 AU from the Sun, (3.30 times the distance at which the Earth orbits the Sun, slightly more than twice the average distance at which the planet Mars orbits the Sun). As a Near Earth Object that remains strictly outside the orbit of the Earth it is classed as an Amor Family Asteroid. This means that close encounters with the Earth do occasionally occur, with the last occurring in August 2001 next predicted for August 2027.
See also...
Asteroid 2014 MP5 passed by the Earth at a distance of about 6 745 000 km (17.54 times the average distance between the Earth and the Moon, or 4.5% of the average distance between the Earth an the...
Asteroid 2014 OF300 passed by the Earth at a distance of 1 468 000 km (3.82 times the average distance between the Earth and the Moon, or 0.98% of the average distance between the Earth and the...
Asteroid (175706) 1996 FG3 is an Apollo Family Near Earth Asteroid (i.e. an asteroid with an orbit that crosses that of the Earth, but which is further from the Sun than the Earth for most of its...
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