An unpredictable meteor shower will grace our northern skies this Christmas week. Known as the Ursids, they are typically few in number and do tend to be rather faint. But a new moon on the 24th means that the early morning of December 23rd, when they should be at their best, will be dark – and hardy meteor watchers will have a chance to observe this shower. Look to the north – to the constellation of Ursa Minor near the North Star.
The Ursids were first observed around the turn of the last century, but detailed records didn’t really start until the 1945, when a strong display was seen. That year a Czech observer noted 169 meteors per hour, although follow up observations the next two years only saw between 10 and 20 per hour. But an entirely new way of observing- with radio antennae- was used for this meteor shower for the first time. Radio-echo observations became the primary means of studying this shower during in the late 40s and early 50s, with observers at Jodrell Bank in the UK detecting 9 to 20 meteors per hour each year.
This meteor shower is linked to comet 8P Tuttle – a comet that swings into the centre of the solar system every thirteen and a half years. But the highest rates of meteors don’t occur when the comet is nearest to us, but six years later. There was a notable peak in 1986, but it’s not too sure that the peak predicted for 2000 actually occurred. One set of observers noted a lot of meteors, but they were the only ones. The next peak should be in 2016. Two astronomers have explained the six year lag by blaming it on Jupiter! The comet goes out only as far as between the orbits of Jupiter and Saturn and many of the debris particles are affected by Jupiter’s gravity. But if the particles are in the right sort of resonance with Jupiter, when they pass Jupiter’s orbit, the planet isn’t there. This leaves them secure for a while and the stream does not spread out. The result for us is a strong display of meteors when the Earth moves through the narrow stream of comet debris.
Comet Tuttle was studied on its last close pass, when it came within 40 million km of the Earth and was found to have a rather unexpected structure. Comets are often described as dirty snowballs, being made of many ices and rocky particles. But Comet Tuttle isn’t a single snowball– it seems to be made of two different objects, each about 5 km across, rotating around each other every 11 hours. Its unusual structure might support the idea that the early solar system went through a chaotic period about 3.8 billion years ago, when planets migrated from one position to another. This would have disrupted comets and may have led to two of them gently colliding, and they have stayed in touch ever since!
This episode of “What is the Stars?” was first broadcast on Dec 12, 2011, listen live on lyricfm each Monday and Friday at 22:45.