Sky-watchers out for meteor shower
Sky-watchers have braved freezing temperatures to enjoy a celestial firework display as one of the year's most spectacular meteor showers reached its fiery peak.
Clear skies, which sent the thermometer plunging, ensured a good view of the Geminid meteor shower.
Astronomers said the weather conditions expected were close to perfect for the annual spectacle.
In London just after midnight some very bright meteors streaked across the sky.
At their height, the Geminids could produce between 50 and 100 shooting stars every hour. They might be glowing in multiple colours and include occasional rapid bursts of two or three.
The best time to see the meteors was expected to be at around 2am, when the "radiant" - the point in the sky from which the meteors appear to originate - was almost overhead, next to the constellation Gemini.
But meteors should have been visible throughout the night from around 10pm.
Robin Scagell, vice-president of the Society for Popular Astronomy, said: "It should be a good display, weather permitting - we might not be far off perfect conditions in the UK.
"The constellation is very high in the sky and most of the Moon will have gone away. An average of one comet a minute would be a good rate, and that's possible. You might also get little bursts of activity with two or three together."
The official Geminid "maximum" is at 11am, but daylight will prevent them from being seen then.
Meteor showers occur when the Earth ploughs through clouds of cometary dust. The tiny particles, some no bigger than a grain of sand, burn up brightly as they enter the atmosphere.
The Geminids are unusual in that they are not shed by a classic icy comet but a body that shares characteristics of both comets and asteroids.
Known as 3200 Phaethon, the three-mile-wide object was discovered in 1983 by two British scientists examining Nasa satellite images and initially classified as an asteroid.
But it has an eccentric orbit that looks more like that of a comet than an asteroid and brings it well inside the orbit of Mercury, the closest planet to the Sun, every 1.4 years. Nasa describes it as a "rock comet".
Traditionally asteroids are made of rock and comets mostly of ice.
"It's not as clear-cut as it used to be," said Mr Scagell. "A lot of asteroids are quite icy. There are asteroids that look a bit like comets and comets that look a bit like asteroids."
The Geminid meteor shower itself was first noted in the 1860s. Over time, it has become more intense, with up to 20 comets per hour reported in the 1920s, rising to 50 in the 1930s, 60 in the 1940s and 80 in the 1970s.
Travelling at some 22 miles per second, the meteors burn up about 24 miles above the Earth.
Another unusual feature of the Geminids is that they can shine in different colours. Mostly glowing white, they may also appear yellow, blue, green or red.
Regardless of whether 3200 Phaethon is an asteroid or comet, it is classified as a "potentially hazardous" near-Earth object (NEO).
To be classified as potentially hazardous, an NEO must pass within 4.6 million miles of the Earth. Each year in December, the Earth is less than two million miles from Phaethon's orbit.
At its closest upcoming approach on December 14 2093, the object will be 1,812,640 miles away - quite far enough to be safe.
"It's not any immediate danger to the Earth," said Mr Scagell.