The Moon orbits at about 384,400 km. An asteroid that passes inside that line makes a close approach in astronomical terms, and the largest objects ever recorded doing so are listed below. Sizes come from the objects' brightness and are approximate; distances are measured between Earth's centre and the asteroid at closest approach, so the altitude above the surface is about 6,400 km less.
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2019 OK — about 100 m, 25 July 2019, 72,000 km. The largest known object to pass this close. It was discovered by the ASAS-SN survey less than a day before closest approach, arriving from the direction of the Sun where survey telescopes do not look, and it crossed at roughly 24 km/s. An object that size striking land would release several megatons; over open ocean the effects at the surface would be much smaller.
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2004 FH — about 30 m, 18 March 2004, 49,100 km. It passed above the ring of geostationary satellites, which sit at 35,786 km, but close enough that it crossed from one side of the Moon's orbit to the other in 31 hours. Discovered three days earlier by the LINEAR survey, it was the closest pass of a catalogued object at the time and the first to be tracked continuously through the encounter.
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2009 DD45 — about 20 m, 2 March 2009, 72,000 km. Roughly the size of a small office block, and comparable to the object that exploded over Tunguska in 1908. It was found six days before the flyby — early enough for observatories to measure its orbit and rule out an impact on the next pass.
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2012 TC4 — about 15 m, 12 October 2017, about 50,150 km. The first near-miss used as a rehearsal. Astronomers had tracked the asteroid in 2012 and predicted its 2017 return to within a few hundred kilometres; observatories worldwide were asked to observe it, and the exercise tested the warning network end to end. NASA and ESA put the flyby at 43,780 km above the surface, which is outside geostationary orbit and about an eighth of the way to the Moon.
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2020 VT4 — about 5 to 11 m, 13 November 2020, 6,700 km. The closest confirmed non-impacting pass on record: it crossed roughly 370 km above the South Pacific, lower than the International Space Station orbits, and nobody saw it until 15 hours later. At that size it would have done no more than produce a bright fireball.
One upcoming pass is already well characterised. Asteroid 99942 Apophis, about 370 m across, will pass roughly 31,000 km from Earth on 13 April 2029 — close enough to be visible to the naked eye and close enough that Earth's gravity will bend its orbit. It will not hit, and the flyby is the best opportunity yet to study how a large asteroid responds to being pulled by a planet.