Friday, November 2, 2018

3200 Phaethon-- An Asteroid or not?

 The Near-Earth Asteroid 3200 Phaethon has -- in addition to being the most frequently misspelled asteroid -- a number of odd traits that call attention to it. 

First of all, it belongs to a rare and distinctive spectral type; it's a B type asteroid.  That suggests a tribal association with the big Belt asteroid 2 Pallas, which in turn implies cold and wet.

Second, Phaethon's orbit is more like that of a comet than that of a typical asteroid: on each trip around the Sun, it dives in to a perihelion passage a mere 0.140 AU from the Sun, far inside Mercury's orbit, and then coasts out to an aphelion distance of 4.025 AU, beyond the main asteroid belt, crossing the orbits of Mercury, Venus, Earth and Mars twice each on every orbit around the Sun.  Each circuit, with eight crossings of planetary orbits, takes 523.5 days

The intensity of sunlight on Phaethon's surface ranges from 50 times the intensity of sunlight at Earth's orbital distance when Phaethon is at perihelion to 6% of normal sunlight at aphelion.  The surface of Phaethon is quite dark, with an albedo (reflectivity) of only 0.1066: more than 89% of the incident sunlight is absorbed.  At perihelion, the daytime temperature can reach a peak of over 1000 Kelvin.  Another oddity of Phaethon is that its color is unusually bluish; we can be quite sure that it is not blue ice!

Phaethon's seemingly hazardous existence is greatly prolonged by the fact that its orbit is significantly tilted out of the plane of the ecliptic, with an inclination of 22.25 degrees relative to the ecliptic.  Generally, while passing across the orbits of the terrestrial planets, Phaethon is safely out of the plane in which they orbit.  The closest it can get to Earth in its present orbit, a hair less than 0.02 AU: 0.02 x 150 million kilometers, gives us 3 million kilometers of clearance when it is at its closest; about 8 times as far away as the Moon.

The orbital inclination of Phaethon spares us from the hazard of a collision with Earth (and the other terrestrial planets) over long time periods- but not permanently.  Phaethon's diameter is about 5.8 km; for comparison, Meteor Crater in Arizona, about 1.2 km in diameter, is the product of a (relatively) low-speed impact of a metallic asteroid with a diameter about 100 times smaller than Phaethon (and thus a volume about 1 million times smaller; allowing for its high density, a mass of about one 3-millionth of the mass of Phaethon).  The impact velocity of Phaethon, because of its extremely eccentric orbit, could easily be twice as high as that of the Meteor Crater impactor, delivering about 4 times as much energy per unit mass. Thus an impact of Phaethon with Earth would deliver roughly one million times as much energy as the Meteor Crater impactor.  Picture an impact crater 120 kilometers in diameter that produced a thick debris blanket over 300 kilometers in diameter.  The dust raised by the impact would throw Earth into an Ice Age for many thousands of years.

In the near future, Phaethon will not and cannot strike Earth.  In the long run, however, there is about a 50% probability that it will hit Earth; 40% chance of hitting Venus, and a few percent each for Mars and Mercury.

Getting a sample of the material of Phaethon would surely be interesting and informative, and nature does provide Earth with samples by natural means: dust expelled from its surface forms the Geminid meteor shower, which strikes Earth in the middle of December.  Unfortunately, the entry velocity is quite high, and acquiring samples of the dust is extremely challenging.  Spacecraft missions to retrieve surface samples from Phaethon are rendered impractical (but not completely impossible) by the body’s high orbital velocity.

So what is Phaethon? Where did it come from, and how did it get in its present orbit?

One clear possibility is that it was a short-period comet that has been stored in its present orbit long enough for solar heating to dispel its near-surface volatiles.  I have seen ludicrous claims that the intense heating during perihelion passages had heated the entire asteroid to the point of baking all the volatiles out of it.  The brief heating events during perihelion passage barely scratch the surface, and can have no effect on the composition of the deep interior.  Phaethon spends virtually its entire existence soaking in the frigid environment of the outer fringes of the Asteroid Belt.

Baby, it's cold out there.

No comments: