New Zealand's North Island was treated to a spectacular fireworks display around 10pm last night, and reports are consistent with a large meteor or "space rock" hitting the earth's atmosphere. I missed it, but two people in my family saw a bright flash in the sky ("What on earth was that?" "Uh, dunno, what on earth was what?" said the resident scientist. Turns out it was something that was not on earth at all.) 

Fireballs like last night's event are sometimes called bolides and the brightness, reported sonic booms and an explosion at altitude are all consistent with this class of events. Even so, this was a far smaller rock than the one that exploded above Russia (which I blogged about here). A study by Brown et al., published in Nature (420, 294-296, November 2002), found that hundreds of objects roughly half a metre in diameter and packing an energy equivalent to 100 tons of TNT hit the earth every year; last night's event would have been in this size range or a tad smaller. Really big events with an energy similar to the largest nuclear weapons ever tested occur much less often; maybe once every ten thousand years. (The rate of these really big, really rare events can be figured out from the number of rocks we see making a close pass to the earth, not by counting them as they happen.)

  From The flux of small near-Earth objects colliding with the Earth, P. Brown, R. E. Spalding, D. O. ReVelle, E. Tagliaferri and S. P. Worden Nature 420, 294-296(21 November 2002)

 

From The flux of small near-Earth objects colliding with the Earth, P. Brown, R. E. Spalding, D. O. ReVelle, E. Tagliaferri and S. P. Worden Nature 420, 294-296(21 November 2002)

The plot below shows the distribution of big fireballs collated by a NASA study; if you look carefully just one of them sits squarely on top of New Zealand.

NASA study showing detected "fireball" events.

NASA study showing detected "fireball" events.

Bolides are like lotteries – the chances of you winning the big prize are small, but the chances that someone, somewhere will win are pretty good. So if you missed last night's fireball, you will wait a long time before seeing another one. 

Reports that came in from a large part of the North Island suggest that the object was moving roughly north-south. Given that, there is a chance it was a piece of "space junk" rather than an actual meteor; many spacecraft move in orbits taking them from pole to pole. Objects in low earth orbit are routinely tracked from the ground, and if this was a piece of orbiting debris coming back to earth it was easily big enough that its absence will be obvious. 

On the other hand, if this was a space rock it is likely to have been orbiting the sun since the birth of the solar system itself. For 4.6 billion years it led a largely uneventful existence, but the last few seconds of its lifetime were spectacular.