Mysterious rock-comet 3200 Phaethon was closest to Earth December 16

This asteroid-comet hybrid is why the Geminid meteors put on a good show in 2017. Info and images of the rock-comet here.

Mike Olason in Denver, Colorado captured the curious rock-comet 3200 Phaethon Saturday night, about 1 hour after Phaethon’s close approach to Earth at 2300 UTC. At the time, Phaethon was about 6.41 million miles from Earth. 

By all reports, the 2017 Geminid meteor shower – which peaked on the mornings of December 13 and 14 – was just grand. That might be because a curious rock-comet known as 3200 Phaethon happened, on the days around the peak, to be gliding through space relatively nearby. This object is thought to be the source of the Geminid meteor shower, and, when a parent object is nearby, a meteor shower can be extra rich. 3200 Phaethon is an unusual object, so no one knew for certain if it would spawn a rich Geminid shower in 2017, but it seems to have done so. Meanwhile, the rock-comet itself came closest to Earth in its 523.5-day orbit last night (December 16, 2017).

See photos: A grand year for the Geminids

Both amateur and professional astronomers have been watching 3200 Phaethon for some weeks, for example, Northolt Branch Observatories in London, England, which created the animation below from images it captured.

Steven Bellavia also produced a video – below. He said he’s had cloudy weather and sub-freezing temperatures. “My fingers still hurt!” he wrote of the night where he captured these images. But capture them he did:

3200 Phaethon swept close to Earth – just 0.069 astronomical units (6.4 million miles, 10.3 million km, 26 lunar-distances) on December 16, 2017 at 23 UTC. Image via Osamu Ajiki (AstroArts)/ Ron Baalke (JPL) / Ade Ashford (AN)/ AstronomyNow.

3200 Phaethon is a very interesting and mysterious object. It was one of the first known bodies in space that blurred the distinction between asteroids and comets. All the other meteors in annual showers are known to be icy debris left behind by comets, after all, but the Geminids are known to come from this strange asteroid-comet hybrid, which some call a rock-comet.

Asteroids are rockier, or more metallic, than comets. Their orbits tend to be more circular, while comets tend to have more elongated orbits. 3200 Phaethon, with its asteroid name, has an orbit that more closely resembles that of a comet than an asteroid. It also has mysterious ejections of dust, and dust tails, and astronomers have said it’s possible that the sun’s heat causes fractures on the surface of 3200 Phaethon, similar to mudcracks in a dry lake bed.

This closeup of an image of 3200 Phaethon by NASA’s STEREO A spacecraft shows a tail extending faintly toward lower left. Image via NASA/

3200 Phaethon was the first asteroid to be discovered via spacecraft on October 11, 1983. Astronomers Simon F. Green and John K. Davies noticed it while searching Infrared Astronomical Satellite (IRAS) data for moving objects. Charles T. Kowal confirmed it optically and said it was asteroid-like in appearance. The object received the provisional designation 1983 TB. Two years later, in 1985, using the convention for naming asteroids, astronomers assigned it its number and name: 3200 Phaethon.

An interesting feature of the orbit of 3200 Phaethon is that it comes very close to the sun, closer than any other named asteroid (though there are numerous unnamed asteroids that do sweep closer to the sun). That’s why it was given the name Phaethon, for the mythological son of the Greek sun god Helios.

Today, it’s classified as a potentially hazardous asteroid, which doesn’t mean it’s a threat to Earth. It just means two things. First, 3200 Phaethon is big – about 3 miles (5 km) wide – big enough to cause significant regional damage if it were to strike Earth. Second, it’s known to make periodic close approaches to Earth. A “close approach” in 2017 means 26 times farther than the moon.

And, by the way, the orbit of 3200 Phaethon is exceedingly well studied, and astronomers know of no upcoming strike by this object in this foreseeable future.

In fact, in December 2017 – according to NASA NEO Earth Close Approaches – there are no fewer than 30 objects that’ll come closer than 3200 Phaethon. The closest of these was 2017 WV12, a 20- to 40-meter-wide object that missed us by about a million miles (1.6 million km) on December 9.

The composition of 3200 Phaethon resembles that of asteroid 2 Pallas. Both are dark, B-type asteroids comprised of materials that have been modified by water. The first in this series of Hubble images is marked with the asteroid’s spin axis (top) and south pole. Image via B. E. Schmidt et. all / NASA / ESA/

3200 Phaethon is far too faint to be visible to the eye during this 2017 close approach. If you want to follow it, you will need a telescope.

AstronomyNow also has 3200 Phaethon charts for telescope users offers this chart of 3200 Phaethon’s track

Bottom line: 3200 Phaethon is a mysterious rock-comet and the source of the Geminid meteor shower. It brushed relatively close to Earth on December 16.

Deborah Byrd