Picture this. You\u2019re making a quiet start to the weekend when you notice a couple of texts and missed calls. They\u2019re from a reporter. The reporter wants to talk about a \u201cfireball\u201d. That doesn\u2019t sound good. Unless you\u2019re an astronomer, in which case you reply, \u201cWhich fireball?\u201d<\/p>\n
Sure enough, a quick look online turns up videos of a spectacular event over Wellington, a bit before midnight on Friday, January 30.<\/p>\n
Astro people get a fair number of calls about strange things in the sky, so I\u2019ve developed a mental flowchart. The first step is, \u201cWas it moving?\u201d If it was sitting still, there\u2019s a good chance it\u2019s the planet Venus, which shines like a celestial Christmas-tree ornament on a good night.<\/p>\n
Next on the list: \u201cOkay, it was moving, but slowly \u2013 or a streak?\u201d<\/p>\n
Slow-moving objects in the sky have usually been put there by people. For example, in 2017 I got queries about what proved to be a giant Nasa balloon, launched from W\u0101naka, lit up by the setting sun as it sailed through the twilight sky.<\/p>\n
Since it was described as a \u2018fireball\u2019, \u00a0my next question was whether it was \u2018space junk\u2019 (also something we put there, but further up) or a \u2018space rock\u2019 (which, as many children know, but adults often forget, is technically a \u2018meteor\u2019). Space hardware and space rocks make similar streaks in the sky, but they come with different stories, so you need to know what you\u2019re dealing with.<\/p>\n
Only the largest pieces of space hardware make fiery re-entries, and even small items are actively tracked from the ground, so re-entries are usually predicted a few days in advance, even if their exact time and place is harder to pin down. But space rocks \u2013 which come from much further afield and don\u2019t have names and serial numbers \u2013 arrive without notice.<\/p>\n
This case was briefly complicated by a leftover Chinese rocket that was returning to Earth at about the same time that the Wellington fireball appeared. It was big enough to have sparked chatter in the media and it did come down close to New Zealand. The Chinese debris was too far south (and a couple of hours too late) to match the Wellington sighting, but a few early reports conflated the two.<\/p>\n
Once we pin it down as a space rock everyone wants to know a few key things, starting with how big it was and how rare it is. It was likely less than 1m across. A couple of events like this happen every week, somewhere on the planet. But it\u2019s not often you get to see one yourself \u2013 the last one I was asked about was a 2015 event of a similar size over the North Island.<\/p>\n
So, what\u2019s a space rock? And why did this one hit us? We know that solar systems like ours \u2013 sun and planets together \u2013 form from clouds of gas and dust, and space rocks are leftover material that didn\u2019t coalesce into planets. But in addition to putting on a show this means that fireballs can deliver clues about how the solar system was assembled.<\/p>\n