The month of December offers us the most intense meteor shower of the year. These are the Geminids, so-called because they appear to emanate from the constellation of Gemini the Twins. At the peak of the shower, over 120 meteors – two meteors every minute – can be spotted under a clear, moonless sky away from light pollution.
This year the peak is expected on the night of the 13th to the 14th of December, however Geminid activity should remain high during the preceding (12th/13th) as well as the following (14th/15th) nights. The presence of a waxing gibbous moon during this period will tend to wash out the fainter meteors, though this is unlikely to spoil the show owing to the Geminids being generally bright.
To astronomers, this shower is a bit of an oddball. Meteors showers are thought to be debris from comets where, in many cases, the parent comet is known. For instance, the magnificent Perseids in August are in this way associated to comet 109P/Swift-Tuttle. The Perseids are sometimes referred to as the “Tears of St Laurence” on account of the Feast Day of St Laurence falling on the 10th of that month, two days before the peak of the shower.
But for a long time since the Geminids’ discovery in 1862, the shower was considered an “orphan”, lacking a recognised parent comet. Then in 1983, British astronomer Simon Green and colleagues identified a new solar system object in data obtained by the Infra Red Astronomy Satellite (IRAS for short). The object received the designation 1983 TB and, when astronomers calculated its orbit, they found that it moved in an unusual path that took it far inside the orbit of the planet Mercury and as close as 13 million miles from the surface of the Sun. Subsequently, the object was given the name Phaethon after the son of Helios, the sun god in greek mythology who granted Phaethon his wish to drive Helios’ chariot across the sky for a single day. This turns out tragically as Phaethon loses control of the horses drawing the chariot, eventually forcing Zeus to strike down Phaethon with a bolt of lightning lest the youth’s wayward antics prove catastrophic for the Earth below.
Fairly quickly after its discovery, it was noticed that asteroid Phaethon and the Geminid shower move in essentially identical orbits around the Sun, pointing to Phaethon as the parent body. However, intense scrutiny of Phaethon in subsequent years showed no evidence of the sort of activity normally expected of comets: no gaseous coma surrounding the nucleus and no tail; to all intents and purposes, this object appeared asteroidal.
Recently, new observations from the STEREO sun-orbiting probes showed that Phaethon tends to brighten ever so slightly as it goes through its closest approach to the Sun and also revealed the existence of a faint tail, earning Phaethon the appellation of “rock comet”. In addition, astronomers have found Phaethon’s colour to be bluish, in contrast to most other solar system asteroids which are either red or neutral grey. In fact, the best colour match to Phaethon seems to be a group of carbon-rich asteroids between Mars and Jupiter which are also thought to contain volatile compounds like water.
Current wisdom points to the intense solar heating, generating temperatures as high as 800 C on Phaethon – hot enough to melt aluminium – somehow acting to carry dust off Phaethon’s surface and causing the brightenings as well as the tail, though the precise mechanism is as-yet undetermined. In any case, the estimated amount of dust is tiny and nowhere near enough to populate a dense meteor stream like the Geminids.
To solve the mystery of the Geminids, scientists are now pinning their hopes on DESTINY+, a probe built by the Japanese space agency JAXA and slated for launch in 2024. The probe will give us our first close-up look at Phaethon in 2028, measure the composition of the dust coming off Phaethon and look for signs of recent mass shedding on the asteroid’s surface – “recent” here meaning some thousands of years ago – that could account for the high particle density within the Geminid stream.
The August Perseids are sometimes referred to as the “Tears of St Laurence” on account of the Feast Day of St Laurence falling on the 10th day of that month, two days before the peak of the shower. If you are fortunate to spot some Geminid meteors streaking across the night sky this coming weekend or early next week, you could think of them as the tears of Phaethon who, like his mythological namesake, did not appreciate the power of the Sun and suffered the consequences.