3200 Phaethon - Comet or Asteroid ?

[ilan]

Mysterious rock-comet 3200 Phaethon
Deborah Byrd in ASTRONOMY ESSENTIALS | SPACE | December 17, 2017

3200 Phaethon – parent object of the Geminid meteors – is an interesting and mysterious object. It was one of the 1st known bodies in space that blurred the distinction between asteroids and comets.


Radar images of 3200 Phaethon
generated by astronomers at the Arecibo
Observatory on December 17, 2017.
Image via Wikipedia.

The 2018 Geminid meteor shower peaks this week (best morning likely December 14; try December 13, too). This annual shower is always reliable, but was extra special last year because its parent object – a mysterious, blue rock-comet known as 3200 Phaethon – was nearby. When a parent object is nearby, a meteor shower can be extra rich.

This year? 3200 Phaethon is far from Earth, but it’s no less an object of speculation and interest. At a meeting of the Division of Planetary Sciences in Knoxville, Tennessee, earlier this fall, professional astronomers waxed enthusiastic about their studies of 3200 Phaethon in 2017, when it was nearby. They commented that this unusual object was even more enigmatic than they’d previously thought.

What’s so odd about 3200 Phaethon? Let’s start with its blue color. Most asteroids are dull grey to red, depending on the type of material on their surface. Blue asteroids are known, but make up only a fraction of all known asteroids. And Phaethon isn’t just blue. It’s one of the bluest of similarly-colored asteroids (or comets) in the solar system.

Here’s another odd feature of 3200 Phaethon. While comets tend to have more elliptical orbits, asteroid orbits are more circular. 3200 Phaethon’s orbit – which is now exceedingly well known – is highly elongated, reminiscent of some comets. Its orbit crosses the orbits of Mars, Earth, Venus and Mercury.

Plus its orbit brings 3200 Phaethon closer to the sun than any other named asteroid (though some smaller, unnamed asteroids come even closer). At its closest point, Phaethon is only 13 million miles (20.9 million km) from the sun. That’s less than half of Mercury’s closest distance. Its name honors this object’s relationship to the sun. In Greek mythology, Phaethon was the son of the sun god Helios.

3200 Phaethon’s orbit carries it so close to the sun that its surface heats up to about 1,500 degrees Fahrenheit (800 degrees C). That’s hot enough to melt aluminum.

When closest to the sun, 3200 Phaethon releases a tiny dust tail; that’s right, it’s a dust tail for an asteroid, one of only two known so far in our solar system. Scientists have said it’s possible the sun’s heat causes fractures, in much the same way a dry riverbed cracks in the afternoon heat.

Comets are known for their tails. 3200 Phaethon’s dust tail is one of the features of this object that blurs the line traditionally thought to set comets and asteroids apart.

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 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 asteroid number and name: 3200 Phaethon.

Before 3200 Phaethon, scientists linked all known meteor showers to active comets and not asteroids.

Thus 3200 Phaethon surprised them from the beginning, because – while it looked like an asteroid – it appeared to be the source of the annual Geminid meteor shower. Astronomers began calling 3200 Phaethon a comet-asteroid hybrid, an asteroid that behaves like a comet. Later, they began using the term rock-comet.
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An asteroid masquerading as a comet or a comet masquerading as an asteroid? (I like comeroid better than rock-comet.) - ilan