Every year, our very own planet moves through this cloud, causing those dust particles to come crashing through the atmosphere. Most of them are tiny just centimetres or even millimetres across, but as they burn up 100 kilometres (60 miles) above our heads they leave a bright streak of light. We call this a meteor, although you may know them by their more common name of shooting star. The space between the planets and around Earth's orbit is full of dust, so every night there will be one or two random meteors. But when the Earth travels through the cloudy trail of dust left by a comet such as Swift-Tuttle, there are so many meteors that it is described as a meteor shower. If you have ever seen one. then you'll know that meteor showers are among the most spectacular sights in the entire night sky.
There are many meteor showers each year, some better than others. The dust left by Swift-Tuttle forms the Perseid meteor shower, which is a great summertime meteor shower that runs from 17 July to 24 August each year. However, each meteor shower has a peak, which is a time when the shooting stars falling through the sky occur in their greatest number. For the Perseid meteor shower this peak occurs on 12 and 13 August. Either side of this peak, the number of meteors drops off. You can understand the reason for this if you imagine the trail left by the comet beginning to spread out. The peak coincides with the densest part of the trail and the most active meteor showers can produce more than 100 shooting stars per hour.
Other great meteor showers include the Quadrantids in January with the peak on the 3 and 4 of the month; the Lyrids between 16 and 26 April; the Orionids that peak on 21 October; the Leonids that are at their maximum between 17 and 18 November; and the Geminids, which are at their best on 13 and 14 December. The names of the meteor showers come from the constellations in which they appear to streak from - this is the direction in which Earth is moving through the dust trails. For example, the Perseids streak across the sky from their 'radiant' in Perseus, the Leonids from Leo and the Geminids from Gemini.
Speaking of the Geminids, they are actually unique among meteor showers. All the rest are produced by dust from comets, but the Geminids are produced by dust left by an asteroid, known as 3200 Phaethon. This goes to show that sometimes the lines between asteroids and comets can be blurred. Astronomers have even witnessed some asteroids in the asteroid belt acting like comets, by growing a tail. This is why wrapping up warm, going outside, looking up and counting meteors is scientifically important. Differences in the speed, direction, brightness and colour of meteors can tell us a lot about the nature of the object that produced them. For example. Geminid meteors tend to move more slowly than meteors left by comets and they burn up at a much lower altitude of about 38 kilometres (24 miles) above ground.
Occasionally, a meteor entering the atmosphere is a little larger than the rest. Rather than just leaving the thin streak of a shooting star, they are bigger and brighter, sometimes even brighter than Venus. These are fireballs that are burning up lower in the atmosphere. The brightest fireballs are called bolides - if you're lucky enough to see one, you might even see chunks breaking off, on fire. Meteors around 20 or 30 metres (66 to 98 feet) in size will explosively fragment in the atmosphere, causing an airburst like the dramatic event that occurred over the Russian city of Chelyabinsk in 2013. This exploded 30 kilometres (18 miles) above the ground and the shock wave shattered windows, damaged roofs and sent 1,500 people to hospital with cuts from flying glass.
The biggest meteors can actually reach the ground before they completely disintegrate. When they do, we call them meteorites and over 61,000 of these have been found on Earth. Most of them are chunks of rock smaller than your hand and the most common place to find them are in the white, icy landscape of Antarctica or the barren desert. Here, the charred and pitted black rocks on the ground stand out like a sore thumb. Not all meteorites come from asteroids either - a handful come from the Moon and Mars. Space agencies like NASA talk about launching sample-return missions, where a robot will visit a celestial body and bring back a chunk of the asteroid for examination on Earth. For example, NASA's OSIRIS-REx mission that launches next year will return a sample from an asteroid to Earth.
But meteorites are natural sample return missions, bringing pieces of other celestial bodies to Earth. Of course they're not pristine, having been blasted into space, probably as the result of an impact, before being burned in the atmosphere and landing on the ground on Earth. But they can tell us a great deal about the geology and chemistry of planets and asteroids. There has been some speculation that some of the 132 meteorites from Mars contain evidence for life in the form of microbial fossils. This was a claim made by NASA scientists in the Nineties after examining a meteorite from Mars called ALH 84001, which was found in the Allan Hills region of Antarctica in 1984. Unfortunately, most scientists are now convinced that the microscopic features are not fossils at all, or if they are then they are fossils of microbes from Earth that contaminated the meteorite while it lay on the ice in Antarctica.
Meteorites can also tell scientists plenty about the dawn of the Solar System and the birth of our blue planet Earth. This is because many meteorites represent debris leftover from the distant era when the planets were forming 4.5 billion years ago. They are broadly split into three types: stony meteorites, iron meteorites and a mixture of the two. Stony meteorites, especially a specific type called chondrites, make up the vast majority of meteorites and are the same type of rock that built planets like Earth. They are very primitive, having never really melted and so they preserve the chemical building blocks of the planet-forming disc that surrounded the young Sun. The other type of stony meteorite are called achondntes and these have melted, either in the impacts that blasted them off their original asteroid, or when they were buried deep inside a large asteroid where conditions were hot. So achondrites are special because they tell us about the chemical conditions within large asteroids and the protoplanets similar to those that eventually became the real planets.
Iron meteorites also come from the cores of protoplanets, because that's where all the iron sank to when they formed. Iron meteorites are incredibly hard and dense, but only a twentieth of all meteorites are of this variety. Meanwhile, there are two different types of stony-iron meteorite, called pallasites and mesosiderites. Pallasites are recognisable thanks to their large crystals of a green mineral called olivine. Mesosiderites are more of a jumble of rock and metal, which are made when two asteroids collide in space, the impact fusing different materials together.
On rare occasions, a really big meteor will enter Earth's atmosphere. These are sometimes big enough to blow out large craters, or explode over towns and cities causing harm. A hundred years before the Chelyabinsk meteorite, a similar airburst flattened 80 million trees in Tunguska, a remote region in Siberia. These events worry scientists who fear that one day an asteroid will hit us that could destroy a city or worse, send so much dust into the air that it will block the Sun and end life on Earth. It's a big concern to Queen founder Dr Brian May, who recently lent his support to the Asteroid Day event, to raise awareness about this threat: "30 June was the anniversary of Tunguska in 1908. Not a huge object but it exploded before it hit the ground... which flattened the trees for hundreds of miles around. Now that is a city destroyer, the force of a thousand atom bombs."
To help forewarn us, NASA's Spaceguard programme has found over 90 per cent of asteroids larger than a kilometre in size that come close to Earth. These are the real killers, like the asteroid that wiped out the dinosaurs 65 million years ago. However, there are still millions of asteroids out there that have not been discovered that are smaller than 100 metres (328 feet) but could still do serious damage. This is why Asteroid Day was held on 30 June this year and headlined by Brian May on the anniversary of the Tunguska explosion. It reminded us that we still have much to do to protect ourselves from asteroids. A new space telescope, called Sentinel, is to be launched by the B612 Foundation, which is led by former astronauts dedicated to saving the Earth from asteroids. Comets can also be a danger but, because there are fewer of them, they pose less of a risk. Instead, the Earth is more likely to fly through their tails so we can see spectacular meteor showers in the sky. From shooting stars to fireballs, meteors and meteorites, their origins are all the same, just on vastly different scales.