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NASA ScienceCasts - Perseid Meteors vs the Supermoon

Showdown in the Sky: Perseid meteor shower vs the August Supermoon

Scott Sutherland
Meteorologist/Science Writer

Tuesday, August 5, 2014, 12:06 PM - On the night of Sunday, August 10, the full moon that rises will be the biggest and brightest of the year. This Perigee Full Moon, which some call a Supermoon, will be quite the sight to see, but it comes with a down-side.

Each year, towards the end of July, as the Earth moves around the Sun in its orbit, it enters a trail of icy, dusty debris left behind by an interplanetary wanderer - Comet Swift-Tuttle. Ranked as the largest object that makes repeated close passes by Earth, Swift-Tuttle leaves behind a similarly large trail, which is the source of the Perseid meteor shower - the brightest meteor display in our skies, which also has the highest number of meteor fireballs of any other meteor shower we see all year.

Credit: Science@NASA

Technically any of these meteors could be thought of as fireballs, as the bits of debris rocket through the upper atmosphere at over 200,000 km/h, leaving behind long, bright trails. However, from the American Meteor Society, the title 'fireball' is reserved for those larger meteors that 'burn' so bright that they rival the planet Venus, which equates to a magnitude of brighter than -4 (in astronomy, brighter magnitudes have lower numbers, with the bright star Vega being the zero-standard, a full moon being around -12.5 and the Sun being magnitude -26). The word burn is in quotations because most meteoroids don't actually burn. When they slam into Earth's upper atmosphere, they compress the air in front of them. This heats up the air around it into a glowing plasma. However, air is a very poor conductor of heat, so while the meteoroid (the bit of ice, dust or rock) may heat up some, and this can vapourize some of the material on the meteoroid's surface (causing some interesting colours in their tails), they don't actually catch fire and burn. The tiny bits of ice melt, obviously, but most often, meteors 'goes out' because the pieces of debris that cause them slow down enough that they're no longer compressing the air to the point where it will glow. The meteoroid debris then goes on to float around as dust in the upper atmosphere, or larger pieces can fall to the ground (at which point they're called meteorites). Also, contrary to what many might think, meteorites aren't typically hot. Their trip through the upper atmosphere can warm the surface, and the impact can heat them up, but they're actually extremely cold on the inside due to their time in space. Many meteorites discovered right after their fall are found covered in frost!

Some meteoroids (or asteroids if they're bigger than 1 m across) are so large that when the pressures build up against them, as they push against more and more air, there's a noticeable pressure difference between the leading side and the trailing side. Eventually, this pressure difference becomes so great that they can't take it anymore and they explode. These fireballs earn a new title - bolide. The chunk of rock that exploded over Chelyabinsk, Russia, in February 2013, was big enough to actually be called an asteroid (it was roughly 20 metres wide) and it earned the title 'bolide' when it blew apart with the force of a 500 kiloton nuclear bomb.

Bolides are fairly rare, but as the Science@NASA video mentions (along with the graph to the right), there is a good chance at catching the abundant fireballs streaking through the sky over the weekend and through the August 12-13 peak, despite the bright full moon. To get the most out of the shower otherwise, check the skies in the hours before sunrise (after the moon has set) over the next few days, and then in the hours after sunset (before the moon rises) towards the end of next week. That will avoid most of the light pollution from the bright moon.

As always, if you see any fireballs, be sure to let us know (@weathernetwork or @ScottWx_TWN), and if you capture any pictures of the Perseids (meteors or fireballs or both), please send them in to us.

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