What is Phaethon: a comet or an asteroid?
Galina Ryabova wonders why the celestial object behind the Geminids seems so unlike those that produced other showers
DINTERVIEWED BY PAUL SUTHERLAND
ecember sees the return of one of the strongest and most reliable meteor showers of the year: the Geminids. Meteor showers are usually produced by the dust ejected by comets as they are warmed on their long elliptical orbits around the Sun. The parent comets for most important showers seen during the year have been identified, such as comet 109P/Swift-Tuttle for the summer Perseids.
The parent body of the Geminids was only identified in 1983, spotted in images taken by an infrared satellite called IRAS. Named Phaethon, the object came as a surprise because, unlike other meteoroid progenitors, it had more in common with a class of space rock called the Apollo asteroids. So is Phaethon a comet or an asteroid?
We don’t know a lot about Phaethon. It has a diameter of about 5km and rotates rapidly in a period of around 3.6 hours, compared to a median rate for all asteroids of 8.5 hours. What is really interesting is that it rotates on its side, because its spin axis is angled only 2º above its orbital plane. It is classified as an F- or B-type asteroid, making it a stony type of rock called a carbonaceous chondrite, and we know its shape is not spherical.
The problematic perihelion
This asteroid has very small perihelion distance, meaning it gets very close to the Sun, to a distance of only about 21 million km, and 2,000 years ago it got to less than 19 million km. No comet could survive for very long so close to the Sun. But perhaps a comet could be captured into this orbit, lose its volatiles (those chemical compounds with low boiling points), and so be turned into asteroid. There are asteroids known with even smaller perihelion distances than Phaethon. A quick search of the NASA JPL Small-Body Database reveals 31 of them.
Following its discovery in 1983, Phaethon at first showed no trace of activity. However it brightened Prof Galina Ryabova, of the Institute of Applied Mathematics and Mechanics at Tomsk State University, Russia, is one of the world’s leading authorities in meteor science and a keen supporter of professionalamateur collaboration via the International Meteor Organization. by a factor of about three in June 2009 and May 2012, for a couple of days after perihelion. Later a cometlike tail was discovered on the asteroid images both in 2009 and 2012. David Jewitt and Jing Li, of the University of California, Los Angeles, observed this brightening via the STEREO-A spacecraft, and concluded that dust production resulting from thermal decomposition and thermal fracture of the asteroid was the most probable explanation.
The evidence is overwhelming that Phaethon is the parent body for the Geminids. And while we don’t know much about Phaethon, the Geminids is one of the most studied meteoroid streams, so we may use observations of the shower it produces every December to make some conclusions about its origin. I use mathematical modelling, computer simulation in other words, to demonstrate how the stream was generated and subsequently evolved, then compare my model with the actual observations. The stream structure seems to show its cometary origin. The activity curve of its shower has two distinct peaks, and this is a feature that supports a cometary ejection model. I believe that Phaethon was a comet, captured on its present orbit. It lost volatiles at a catastrophic rate, over just one to three orbital revolutions, and is now an asteroid.
I have studied this stream for more than 35 years, but there is still more to be discovered. We have already made the most of old observational data. Now we need more ‘high end’ observational data, especially to give us individual meteoroids’ orbits. Amateur astronomers can still play an important part in observing this shower.
My latest scientific paper on the Geminids is based on visual observations of the 2004 shower when a total of 29,077 Geminid meteors were detected in 612 hours of observing by amateur observers all over the world. But what I would really like to see, because it could answer many questions, is a space mission to Phaethon.