Aaron Taylor, of South Otago High School, asks :-

On Tuesday the 13 of March 2012 at 8:50pm I was sitting in our spa watching, what I believed was a satellite, going slow across the sky (south to north) then all of a sudden it flashed really bright then took off across the sky really fast. What could this be?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

Thank you for providing the date and time of your sighting. The International Space Station (ISS) crossed New Zealand at that time and was widely seen. It was bright! I have no doubt that it was the ISS that you saw. You can get ISS visibility predictions for you locality from http://www.heavens-above.com/.

Most satellites appear to move slowly when they are low in the sky. That is because the satellite is moving more along the line-of-sight. It is coming toward you or going away. When a satellite is high in the sky it appears to move faster. Then it is moving at right-angles to the line-of-sight. You see the same effect with aeroplanes.

Satellites can change brightness quickly, and sometimes make a flash of light. The flash is caused by flat shiny solar panels on the satellite. The panels act like mirrors, making a flash as a reflection of the sun sweeps past us. The ISS has very large solar panels.

Satellites also brighten or fade as they come out of the Earth's shadow or go into it. In the evening satellites will fade when they move into the eastern sky. Alternatively a satellite going west will suddenly appear as it emerges from Earth's shadow. On March 13th the ISS crossed soon after sunset so it stayed visible till it was low in the eastern sky.

Sometimes people get confused when two satellites appear near each other. I don't think such confusion is possible with the ISS, though. It's too bright. A 'two object' confusion can give weird impressions. There have been reports of satellites suddenly changing direction. That isn't possible. What was probably seen was one satellite fade into Earth's shadow just as another was emerging from the shadow nearby. There are a lot of satellites going around, so such coincidences are fairly common.