Asteroid 2003 HM passed by the Earth at a distance of 6 409 000 km (a little under 17 times the average distance between the Earth and the Moon), slightly before 9.55 am on Sunday 20 April 2014. There was no danger of the asteroid hitting us, though were it to do so it would present a considerable threat. 2003 HM is estimated to have an equivalent diameter of 68-220 m (i.e. a spherical object with the same volume would have a diameter of 68-220 m), and an object towards the upper end of this range would be predicted to be able to pass straight through the Earth's atmosphere and impact the ground, resulting in an explosion over 30 000 times as large as the one caused by the Hiroshima bomb, and leading to the formation of a crater over 3.5 km wide. Such an event would cause devastation over a very wide area, as well as climatic effects that would last for decades.
The calculated orbit of 2003 HM. JPL Small Body Database Browser.
2003 HM was discovered on 23 April 2003 by the Lincoln Near-Earth Asteroid Research (LINEAR) project in New Mexico. The designation 2003 HM implies that it was the 12th object (object M) discovered in the second half of April 2003 (period 2003 H).
2003 HM has a 268 day orbital period, with an elliptical orbit tilted at an angle to the plain of the Solar System which takes in to 0.59 AU from the Sun (59% of the distance at which the Earth orbits the Sun, considerably inside the orbit of the planet Venus) and out to 1.03 AU (3% further away from the Sun than the Earth). This means that close encounters between the asteroid and Earth are fairly common, with the last thought to have happened in October 2011 and the next predicted in August this year (typically 2003 HM has two close encounters with the Earth four months apart, followed by a three year gap before the next such pair). Although it does cross the Earth's orbit and is briefly further from the Sun on each cycle, 2003 HM spends most of its time closer to the Sun than we are, and is therefore classified as an Aten Group Asteroid.
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