Heather Smart, of Ashhurst, asks :-

I have been told the Earth gets 100 tonnes heavier each day due to space dust. Is this so?

Duncan Steel (duncansteel.com), a space researcher, author and broadcaster who lives in Wellington, responded.

Yes. The total influx of small lumps of material from space does indeed average about 100 tonnes a day, or 40,000 tonnes per year, but it’s not the same every day: at certain times of year we see meteor showers at night, and these occur when our planet passes through meteoroid streams in orbit around the Sun.

These streams are made up mostly of the rocky grains we call meteoroids, released by comets as sunlight heats them, vaporising some of the ices of which they are composed. Those ices are not just frozen water, but also solid carbon dioxide (‘dry ice’), carbon monoxide, ammonia, methane, and many other organic (i.e. carbon-based) molecules including alcohol.

As the ices turn into vapour, producing the characteristic tails of comets, they sweep out small solid lumps mixed in with them: the meteoroids. These eventually form a loop all around the comet’s orbit, which our planet may run through if the geometry is right. For example in the first week of May each year, and also the third week of October, we see meteor showers that we know are due to pieces of Halley’s Comet.

Many meteoroids originate from comets, then, but also we see random arrivals, many of which seem to be tiny fragments of the asteroids that mostly orbit the Sun between the planets Mars and Jupiter. If asteroids knock into each other bits can be chipped off, and these may migrate inwards and eventually hit the Earth.

Although you can see meteors by eye at night – a synonym for ‘meteor’ is ‘shooting star’ – it is also possible to detect them even during daytime using a suitable radar which bounces radio waves off of the electrons and charged ions in the trail produced as a meteoroid burns up (technically we say ‘ablates’) in the atmosphere at a height typically between 80 and 100 kilometres.

Radar studies of meteors have been carried out at the University of Canterbury since the 1950s. In the 1990s a team there led by Professor Jack Baggaley (and including myself) made the first detection of material from interstellar space (that is, outside of the solar system) reaching the Earth. Don’t let anyone tell you that no space research is done in New Zealand!

A typical shooting star seen by eye is caused by a meteoroid originally the size of a pea or an almond, whereas with radars we can detect meteors caused by far smaller particles, around the dimensions of a sand grain.

From such radar data we know how much space dust hits the Earth each day. Another contributor to our knowledge comes from counting the pits and holes punched in satellites. Between 1984 and 1990 NASA flew a satellite the size of small bus in orbit, called the Long-Duration Exposure Facility. After it was returned to the ground using the space shuttle it was found to be covered with tiny perforations.

Where does all this stuff go? As a meteoroid ablates most of its mass is vaporised as single atoms, but a tiny remnant solid particle may eventually drop to the surface. We call that a micrometeorite. In a typical house it is reckoned that about one in every thousand dust grains is actually a micrometeorite!

Although adding up the masses of all the billions of grains of space dust that arrive each day leads to that value of 100 tonnes, it would not be true to think that the Earth’s mass increases by that amount, because we lose mass too. Some gases, and helium in particular, move up through the atmosphere and eventually are lost into space.

Actually, one could say that helium is our only true non-renewable resource, in that it departs the planet. Natural radioactive decay within terrestrial rocks, for example by uranium, results in the production of new helium atoms, but at a much lower rate than we are losing it from the atmosphere. The main release is through helium vented from oil wells; but some people believe that helium-filled party balloons should be banned because this is another way the gas is being lost forever.