Meteoroid, meteor, meteorite: one space pebble, three names
Roughly 25 million bits of space rock and dust plunge into Earth's atmosphere every day, adding up to around 15,000 tonnes a year. While drifting through space each is a meteoroid, the streak it makes while burning up is a meteor, and any piece that survives to hit the ground is a meteorite.
Deciding what counts as a meteoroid has taken decades. In 1961 the International Astronomical Union simply called it a solid object in interplanetary space much smaller than an asteroid and much bigger than an atom. A 1995 proposal set the range at 100 micrometres to 10 metres, but once telescopes began spotting asteroids under 10 metres, researchers suggested lowering the ceiling to one metre. In April 2017 the IAU settled on 30 micrometres to one metre, with leeway for anything that produces a meteor. Smaller specks are micrometeoroids or interplanetary dust, and the boundary with asteroids stays fuzzy because it depends on what telescopes can detect.
Most meteoroids are asteroid-belt fragments nudged inward by planetary gravity; others are comet dust, and a few are chips of the Moon or Mars blasted loose by impacts. Nearly all contain nickel and iron from beyond Earth, and they fall into iron, stone and stony-iron types. Stony ones speckled with tiny grains called chondrules are chondrites; those without them, achondrites, usually formed through volcanic activity on their parent bodies. Tracking their trails and light has revealed everything from fluffy clumps a quarter as dense as ice to heavy nickel-iron rocks.
Speeds vary enormously. Near Earth's orbit the fastest travel at about 42 kilometres a second, the Sun's escape speed there, so anything quicker must come from interstellar space. Meeting Earth head-on, as the Leonids do, can yield about 71 kilometres a second, while slow ones like the Phoenicids enter at around 11. In January 2013 a one-metre comet fragment from the Oort cloud struck the air over California and Nevada at roughly 72 kilometres a second and vaporized more than 100 kilometres up.
Bigger visitors are rarer. The largest object hitting Earth on a typical day is probably about 40 centimetres across; in a year, about 4 metres; in a century, about 20. Some 35 to 40 metre-sized objects arrive annually. In October 2008 the 4-metre asteroid 2008 TC3 became the first such body spotted in space and tracked before impact, coming down in remote northern Sudan. On airless worlds like the Moon, these strikes leave lasting craters.
Source: Meteoroid
