Zane White, of Balclutha Primary School, asks :-

On Sunday May 5th, near Clydevale in South Otago, we saw through a hole in the clouds something falling with white smoke-like stuff trailing behind it. It seemed to hit nearby trees. Could it have been space junk?

Duncan Steel, a space researcher, author and broadcaster, responded.

The straightforward answer is yes – but it’s unlikely.

First, some definitions are needed. By “space junk” or “space debris” we usually mean an artificial (man-made) object in space, in orbit around the Earth. Obviously we would not think of functioning satellites – things like the Space Station, the Hubble Space Telescope, and communications satellites – as being “junk”, but apart from those there are in orbit about 3,000 tonnes of old material that no longer has any use. In fact, this space debris is a hazard to our useful satellites, and to our astronauts, because it moves so quickly (typical orbital speeds being about seven kilometres per second, over 20,000 kph). This ever-changing cloud of space junk is estimated to contain well over 100,000 separate pieces bigger than the size of a marble, and the larger fragments are regularly tracked using ground-based telescopes and radars.

Apart from these, spacecraft may also be hit by natural particles: meteoroids and interplanetary dust. Even an object the size of a grain of sand could put a satellite out of action, depending on where it struck. These natural bullets are mostly in orbit around the Sun, and happen to cross the path of Earth in its annual circuit around our local star. Typically they slam into satellites at about 20 kilometres per second, three times faster than the satellite is orbiting our planet.

When a meteoroid enters the upper atmosphere, due to its extreme speed friction quickly causes it to heat up and glow, and we see this as a meteor (or “shooting star”). Usually it evaporates away, just leaving its individual atoms in the air high above us, but occasionally a solid remnant lump may reach the ground intact, and this is termed a “meteorite”. Most meteorites are tiny, a fraction of a millimetre across, but a few are far larger.

From your description it sounds like you saw a bright meteor. This might have been caused by a re-entering fragment of space debris, but it is far more likely that this was due to a natural meteoroid because most days only a few small items of space junk enter the atmosphere, whereas billions of meteoroids do so: about 100 tonnes a day! Quite often a large meteoroid – bigger, say, than a cricket or hockey ball – will leave a trail like that you described, and this might persist in the sky for up to an hour.

People often think that the shooting stars they witness are quite close-by, because they are used to seeing birds, or high-flying aeroplanes. Actually the meteors seen by eye are typically about 60–80 kilometres above the ground, and may be 500 kilometres distant from the viewer. If you did see a meteor it was out over the ocean, a long way away from you.

You did not say what time of day you witnessed this event. If it were after midnight then you might have seen a piece of Halley’s Comet! The reason is that every year, in the first week of May, Earth passes through a loop of meteoroids dropped off by that famous comet along its 76-year orbit around the Sun. These cometary fragments produce a well-known meteor shower, the streaks of light all appearing to emanate from the direction of the constellation Aquarius. However, these all strike our planet on its leading hemisphere, which is why you only see them after midnight.