The spectacular sight which keen South African sky-watchers were privileged enough to witness during the evening of Sunday the 18th of October, from approximately 8:45 to 9:30pm, – a spectacle resembling something out of a big budget UFO movie – turned out to be a great visual demonstration of experimental rocket science in action. From Hermanus to Jo'burg and even Nambia, reports started to flood into the South African Astronomical Observatory's field station in Sutherland, as two bright objects were seen to be passing overhead from South-East to North-West at rapid speed. What made the reports so sensational was the repeated description of the leading object emitting bright, spiralling pulses of light which 'looked like the kind of ripples you get when you throw a stone into a pond' radiating over the whole sky.
Two hours prior to the South African spectacle, at 16:12 GMT, the United Launch Alliance launched the US military Defense Meteorological (weather) Satellite Program (DMSP) from Vandenberg Air Force Base, California, aboard the Atlas V rocket. The flight path of the rocket took it twice around the world in just under 4 hours, with the second sweep of the path taking the rocket across the African continent and into Europe before the rocket was finally sent out of the Earth's orbit over North America (see flight path at http://spaceflightnow.com/atlas/av017/090930groundtrack.html). It was in fact the experiments conducted on Centaur, the upper rocket stage of the Atlas rocket, which were responsible for the eye-catching light show witnessed over Southern Africa.
The main purpose of the rocket's mission was to safely deliver the DMSP to a low-earth orbit, which occurred 18 minutes after launch when the spacecraft separated and the DMSP was released at a height of 827km above ground. After this main objective was completed, the unique characteristics of the launch (namely that the satellite payload was light enough to allow fuel to be left over after completion of its launch) meant that the program could spend the last part of the mission conducting experiments to characterise the performance of their Centaur rocket. The last of these, referred to by ULA as the 'pulsed chilldown demo', was conveniently conducted over South African heads and was responsible for the widely-reported distinctive beams of light radiated in concentric rings from the rocket as it passed overhead.
Fortunately for us, a keen professional outdoor photographer, Mr Mitchell Krog, was out lightening hunting when the Centaur passed over South African skies. Already perfectly set-up for high-resolution fast photography, he was able to capture a stunning series of camera shots which can be found at www.astronomical.co.za/gallery.htm
The last stage of the Centaur's experiments terminated with it dumping its fuel over Europe at ~20:00 GMT. This also didn't fail to go amiss, as reports started to flood in from Europe of a bright object with a bright halo and a comet tail (presumably the rocket's trailing fuel) passing swiftly overhead, and a second bright object (the DMSP) trailing not far behind. Shortly afterwards, the rocket was out of sight, making its way to the Arctic circle and preparing to leave Earth's skies forever. Mission success, score 1 for rocket science!
More information on the mission can be found on these websites:
http://www.spaceweather.com/ (for the European side of the show).