Phaethon

Most meteor showers come from comets. The Geminids, which peak in mid-December and are among the best and most reliable showers of the year, come instead from an asteroid: 3200 Phaethon.^[1] Under perfect conditions, about 120 Geminid meteors an hour can be seen at the peak, each one a fragment of debris from Phaethon's orbit burning up in Earth's atmosphere.^[1]

Discovery and size

Phaethon was discovered on 11 October 1983 by NASA's Infrared Astronomical Satellite, IRAS.^[2] The astronomer Fred Whipple recognized it as the source of the Geminids.^[1] Size estimates range from about 5 km to 6 km, so it is one of the larger potentially hazardous asteroids known.^[1][2] Radar images from the Arecibo Observatory in December 2017 showed a roughly ball-shaped body with a large depression near its equator.^[2] At that pass, on 16 December 2017, Phaethon came within about 10.3 million km, some 27 times the distance to the Moon; it will not come closer until 2093.^[2]

An asteroid that behaves like a comet

Phaethon's elongated orbit, which takes 524 days, carries it well inside the orbit of Mercury, where the Sun heats its surface to about 750 °C.^[3] Near the Sun it brightens, as comets do, although any ice near its surface would have been baked away long ago.^[3] A 2021 study proposed that sodium in its rock vaporizes and vents through cracks, enough to brighten it and to eject small pieces of rock; this would also explain why Geminid meteors contain little sodium.^[3] Scientists still debate whether Phaethon is best described as an asteroid or as a dead or rocky comet.^[1]

A mission to fly past it

Japan's DESTINY+ mission will make a high-speed flyby of Phaethon and study interplanetary dust, which brings carbon and organic matter to Earth and of which Phaethon is one source.^[4] It is aiming for launch in Japan's fiscal year 2028, sharing an H3 rocket with ESA's Ramses spacecraft, and it will first fly past Apophis as a rehearsal.^[4][5]