Asteroid 3200 Phaethon: Geminid Parent at Its Closest and Brightest!

By: Bob King November 29, 2017 30

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.

The orbit of asteroid 3200 Phaethon is highly elongated, much like a comet's. Perhaps it is an extinct comet, or maybe it's a true asteroid that's cracking and shedding rocky bits when closest to the Sun. The 5.1-kilometer-wide Phaethon completes an orbit every 1.4 years.
*Sky & Telescope diagram*

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.

Phases in reverse — This diagram gives an overview of 3200 Phaethon's flight from Auriga through Capricornus in the next few weeks. Its changing phase is shown throughout. Numbers beneath the phases are visual magnitudes at the time. One reason the asteroid fades so rapidly after closest approach is because its phase narrows to a crescent from our perspective. Asteroid size is not to scale.
*Tom Ruen, with additions by the author*

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.

Kissin' cousin? — Compositionally, 3200 Phaethon resembles the larger asteroid 2 Pallas and may possibly be related to it. 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.
*B. E. Schmidt et. all / NASA / ESA*

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.

Phaethon's tail — This closeup of an image of 3200 Phaethon by NASA's STEREO A spacecraft shows a tail extending faintly toward lower left.
*NASA*

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.

Slow start in a thick field — Yes, it's crowded! This map shows the track of 3200 Phaethon from December 6–9, 2017, with positions marked at 0 UT (7 p.m. EST the evening before) and stars plotted to magnitude 13. Click to enlarge and then print out for use at the telescope.
*SkyMap*

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.

Conjunction with Capella — This chart tracks Phaethon from December 9–11 starting at 0 UT. Having Capella nearby is a bonus.
*SkyMap*

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.

On the run! — With Phaethon's speed and brightness increasing, tick marks are shown for every hour on December 11–12. Stars here are plotted to magnitude 11.5.
*SkyMap*

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.

It's round, but that's it! — This radar image, made in December 2007 by the Arecibo dish in Puerto Rico, is the best image we have of Phaethon for the moment.
*Arecibo / Cornell*

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:

December 12–13
December 13–14
December 14–16 (color, B&W)
December 15–17 (color, B&W)

Comments

Russ

December 1, 2017 at 12:59 am

Hi Bob,

This looks to be an interesting apparition to view. Strangely when I use SkySafari on my iPad to plot the course of 3200 Phaethon, I get a path that is significantly different that what is shown on the charts you have supplied. I see Phaethon going right down the middle of the triangle in Triangulum, between Beta and Gamma Triangulum, on November 30th. Your color chart for December 14-16 shows the path considerably north of mine and some 15 days after what I see in SkySafari. I made sure to update the Minor Planet Elements in SkySafari and still get the same course. Now I'm using an iPad-1 with iOS 5.1.1 - yeah, it is ancient by computer standards. So it's possible that with my equipment the software is making an error in its plot. But I haven't found that to be the case prior to this. It is also possible that I've made an error in the use of SkySafari. If that is so, it has escaped my notice.

But this is just a "head-up" to check results. What are others finding in this regard?

In any case Bob, thanks for the interesting article. I hope it all pans out for us.

Best regards,
Russ