The parent asteroid of December's Geminid meteor shower, 3200 Phaethon, is about to make a historically close flyby. Get ready to watch it race across the sky.
We have a fantastic opportunity to see a unique asteroid brighter than it's ever been observed. 3200 Phaethon (FAY-eh-thon), the size of a rural hamlet, will pass within 10.3 million kilometers from Earth at 6 p.m. Eastern Time (23:00 UT) on December 16th. Just before closest approach, it will reach magnitude 10.7, bright enough to track in a 3-inch telescope. And I do mean track. Hold onto your eyepiece! This thing will be scooting along at up 15° per day or 38″a minute (about the distance between Albireo and its companion star) — fast enough to cross the field of view like a slow-moving satellite.
Whenever I see an asteroid or comet move in real time, I'm reminded of a particular 10-km-wide rock that slammed into the Yucatan 65 million years ago and led to the extinction of the dinosaurs. To a T. rex with a telescope, the coming nemesis would have looked like an innocent pinprick of light the night before, inching across the stars just like Phaethon will for us. Only in this instance, we needn't worry about an impact. While classified as a potentially hazardous asteroid, a look ahead shows that Phaeton will keep a safe distance for at least the next four hundred years.
Phaethon distinguishes itself in other ways. It comes closer to the Sun than any other named asteroid, with a perihelion distance of 0.14 AU, or less than half that of Mercury. If lead melts on the innermost planet, where the temperature reaches 800°F (427°C), Phaethon's got it beat by a mile. Its scorched surface tops out around 1200°F (627°C), hot enough for lead to flow like water and aluminum to turn to putty.
Getting roasted by the Sun every 524 days explains Phaethon's other claim to fame as the parent of the Geminid meteor shower. Shortly after it was discovered on October 11, 1983, with the orbiting Infrared Astronomical Telescope (IRAS), American astronomer and comet researcher Fred Whipple noticed that Phaethon's orbit matched that of the Geminids, clinching it as the shower's source.
Most meteor showers originate with comets. When a comet gets near the Sun, solar heating vaporizes dust-rich ice from its nucleus, and the push of sunlight (radiation pressure) blows it back to form a coma and tail. The ejected material forms a trail or stream along the comet's orbit; when the Earth slices through it, the dust slams into the atmosphere and vaporizes, particle by particle, in a meteor shower.
So how do you get dust from an asteroid? An early hypothesis, now discarded, posited that Phaethon might have ice beneath its surface that vaporizes around the time of perihelion. Sounds plausible, but measurements show the asteroid gets too hot for ice to survive either inside or out. Still, you might get blood from a turnip if that turnip is composed of carbonaceous (carbon- and water-rich) minerals like our friend 3200 Phaethon and heated to a temperature high enough to cause the material to break down, crack, and crumble into dust.
If Phaethon was once a traditional comet, it is no longer. These days, it's considered a rock comet. a rare type of object that outgasses rock particles instead of ice.
When you pour boiling water on cold glass, the glass shatters from thermal shock. Similarly, extreme solar heating at the time of perihelion causes rapid expansion of Phaethon's crustal rocks. They fracture and release dust that's carried away by the radiation pressure of sunlight. Experiments have shown that the dust particles are about a millimeter across, approximately the size of the Geminids.
For a brief time during the 2009 perihelion, astronomers recorded a surprise doubling in the asteroid's brightness, likely caused by one of these gritty outbursts. Yet questions remain. A meteor shower requires constant nourishment from its parent to put on a show year after year, decade after decade. Astronomers estimate that to maintain a steady supply of meteor dust, Phaethon would have to flare 10 times as often as observed. Does it shed its skin out of sight, when its in the harsh glare of perihelic sunlight?
Whatever's going on, we're going to have a bang-up December. Not only will Phaethon be brighter than ever, but its appearance coincides with the December 13–14 peak of the annual Geminid meteor shower — a rare pairing! We'll have more on the Geminids next week. For now, our job will be to find and follow this exciting asteroid.
From November 29th through December 3rd, Phaethon brightens from magnitude 14 to 13, but does so handicapped by the glare of the nearly full Moon. Luckily, the Moon's out of the way starting on the 5th, when Phaethon will have reached magnitude 12.6 and be within range of a 6-inch or larger telescope. When brightest from December 12–15, it shines between magnitude 10.7 and 10.9. Because we're seeing the asteroid in full or nearly full phase, Phaethon stays relatively bright for many nights prior and up to closest approach. Soon after, it fades as its phase from our perspective thins to a crescent. By December 19th, it drops back to 13th magnitude and then to 15th magnitude on December 21st.
Phaethon spins once on its axis every 3.6 hours with a +0.4-magnitude variation in brightness that patient observers may be able to detect. Visual observers and astrophotographers should also be on the lookout for brightness flares or the potential appearance of a coma.
Our maps will take you through December 17th. I apologize for their number, but Phaethon covers a lot of ground through a star-rich Milky Way, making detailed maps a necessity. I encourage you to create your own personalized charts, easy to do with sky-mapping programs like Stellarium, MegaStar, and Starry Night. For step-by-step instructions, scroll to the end of this earlier blog.
NASA's 70-meter Goldstone dish will make good use of this apparition to obtain the clearest pictures yet showing Phaethon's shape and surface features by bouncing radio waves off the object and constructing images based on return echoes. Observations are scheduled for 10 days between December 11–21.
If you don't own a telescope and still want a peek at the asteroid, watch it live on Gianluca Masi's Virtual Telescope Project website at these times: December 15th from Arizona starting at 09:00 UT and December 16th from Italy starting at 20:00 UT.
Try to catch Phaethon now because it won't get this bright again until December 2093, when it will pass within 0.02 AU of Earth and brighten to magnitude 9.4.
Additional charts for tracking Phaethon through December 17th: