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It is black out there, very black. In between the specks of light called stars and in between the galaxies there is just a lot of black. That may be true to our eyes but it is not true at all. In the darkness of space in the black bits there is a giant monster called dust.

Okay, okay dust isn’t a monster but it got your attention didn’t it? You probably think that dust is a boring subject but it is far from that if you take the time to look closely at it (very closely as dust tends to be quite small). Calling the particles dust gives the wrong impression in some respects and the right impression in others. Dust particles are the very basis of our solar system as it may have been a cloud of dust that produced the Sun and eventually the planets.

Dust particles are mostly less than 1 mm in diameter. They are a mixture of many elements and compounds but the most abundant being silicates and glassy nodules with perhaps a few sulphides, metals and other minerals and carbonaceous material. They were once a curse to astronomers as large clumps of gas tended to get in the way when they were scouring the sky for things that astronomers tend to scour the sky for.

Dust particles were discovered by observing the zodiacal light that appears in the night sky extending up from the vicinity of the Sun. This light is produced by the dust particles scattering and reflecting the sun’s light and causing a faint glow. This was a big clue as to the existence of interplanetary dust.

There are three main sources of interplanetary dust these being cometary emissions, particles from asteroids and dust particles from impacts around the solar system mainly in the Kuiper belt and asteroid belt. The asteroid belt lies between Mars and Jupiter and the Kuiper belt lies just beyond Neptune which is a source for comets. Another source for comets but much, much further out is the Oort cloud. Most of the comets in the Kuiper belt have short orbital periods (time taken for one complete revolution around its orbit) and so their effect is much more than the orbits of comets from the Oort cloud which are thought to take tens of thousands of years but obviously are quite hard to measure!

Comets are quite important to us not only because they could impact the Earth but because they leave a trail of particles. A comet is made up of mostly dirty, icy rubble that tends to break off and leave a trail of dust behind it. The trail of debris follows the orbit of the comet and forms a tube of dust particles which is called a meteoroid stream.

All active comets produce a meteoroid stream and I expect you can imagine that when the Earth passes through a meteoroid stream the meteoroid stream will have an effect on the Earth. As the meteoroids enter into the atmosphere the atoms of the meteoroids get excited and heat up. A byproduct of this is the production of light and this produces a streak of light. This effect is probably most well known as a meteorite shower and some spectacular effects can be seen if you’re in the right place at the right time. 50 meteor showers occur every year. If you fancy your luck in catching a meteorite shower (well worth the effort) then try these viewing tips. If you just can’t be bothered and fancy the easy option then take a look at the video below.

Dust particles aren’t just in the wake of comets but crop up all over the solar system. This is because of three processes, orbital evolution, the Poynting-Robison effect and radiation pressure.

Orbital evolution isn’t just restricted to dust particles but also to any other body in the solar system. This is the effect of the gravity of the planets, especially the giant planets such as Jupiter and Saturn, on the orbits of the dust particles. It’s quite simple really, if the particle gets too close it gets pushed around.

Radiation pressure is the force that light exerts on the particle. Light is made up of photons and these have momentum. When they hit the particle they transfer momentum to the particle and give it a little shove. This may not sound plausible but when you think of how small the particle is it shouldn’t be such a shock. This force over quite a bit of time can push the particle out of the solar system.

the Poynting-Robison effect is related radiation pressure. It has an almost opposite effect as radiation pressure does. Each particle has a slight motion and 90° away from its main path and this means that the photon can come from the side (extremely and very slightly). This has the effect of slowing down the particle in its orbit and after many orbits the particle slowly spirals in to the inner solar system. Very large particles are affected by this but most effect is seen on the very small particles and a 10 Micro metre size particle can take 10,000 years to reach the inner solar system which is not that long really when you think that the solar system is a 4.56 billion years old.

Particles as mentioned before are the building blocks of the solar system and the universe and can give us quite a light display on certain nights. Hopefully after learning a bit about dust particles you will never think of dust in the same way again or perhaps you will?

Here’s a podcast on planetary dust made by astronomy cast and the lazy way of seeing shooting stars.

[podcast]http://www.weirdwarp.com/wp-content/uploads/2010/04/planetary dust podcast.mp3[/podcast]