Tuesday 1 April 2014

[updated] The bright fireball over Germany of 31 March 2014, 22:34 CEST: an earthgrazing meteor, not a satellite re-entry

[updated 20:55 UT (1/4/2014) to reflect revised fireball duration]

Yesterday evening German astronomical internet fora and my Twitter timeline erupted in a frenzy about a very bright, magnitude -10, west to east moving, very long duration fireball seen over southern Germany near 20:34 UT (22:34 CEST, March 31).

The fireball was widely seen by eye witnesses and captured by a video all-sky station near Ulm. The very spectacular image, by Thomas Tuchan, can be seen here (scroll down in the message list) on the AKM message forum.

As usual, it was science writer Daniel Fischer who was the first to knock on my digital door for an opinion. The question that had popped up, as it does with every long duration slow fireball, was whether this was a meteoric fireball or perhaps a satellite re-entry? In most cases, it is not, although there are exceptions.

My first check in such cases always is with JSpOC to see whether there was a suitable re-entry candidate in the TIP-messages. There was not. This while a re-entering artificial object of this brightness must be a very big object, for which you expect a TIP message.

Next more information came available on the fireball length and duration, notably through Thomas Tuchan's all-sky video image. It shows an almost horizon to horizon event, with a duration of 16 33 seconds. It starts at approximately 15 degrees elevation in Perseus, culminates at 60 degrees North, and ends low on the opposite horizon, at an elevation of about 12 degrees. A span of some 150 degrees!

[Update 20:55 UT: the duration was later revised to 33 seconds]

The very long 150 degree trajectory with a duration of 16 seconds rules out the re-entry of an artificial object. It shows that this was a meteoric fireball, and one that entered the atmosphere at a very shallow angle: a so called Earthgrazing meteor. There are even some examples (most famous one the Grand Tetons fireball of 1972) where such Earthgrazing fireballs left the Earth's atmosphere again!

Satellite re-entries take place between 150 and 50 km altitude. At such altitudes, an earth-orbiting object has a speed of 7.5-7.8 km/s and the resulting apparent angular velocity is about 3 degrees/second for 100 km, and about 5 degrees/seconds for 50 km altitude: but only in the zenith. Lower above the horizon, the angular speed is much less.

I constructed an artificial set of orbital elements for an orbital altitude of 90 km (re-entry in progress) as a test: it takes such an object 1 minute 15 seconds to move from 15 degrees elevation above the western horizon to 15 degrees elevation on the opposite horizon. By contrast, it took the German fireball only 16 33 seconds: i.e. almost a factor two-and-a-half faster.

[Update: the duration was first reported as 16 seconds, later revised to 33 seconds]

All this makes very clear that the German fireball of March 31 was not the re-entry of an artificial object, but a meteoric fireball, most likely an Earth-grazing object of asteroidal origin..

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